PROTEIN KINASE REGULATION

Abstract: A method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leu116, Val80 and/or Lys111 of full-length mouse PKA, wherein the ability of the compound to inhibit, promote or mimic the interaction of the said hydrophobic pocket-containing protein kinase with an interacting polypeptide is measured and a compound that inhibits, promotes or mimics the said interaction is selected, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr. The protein kinase may be PDK1, PKB, SKG or p70 S6 kinase. A method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket as defined above, for example PDK1, comprising the steps of (1) determining the effect of a test compound on the protein kinase activity of the said protein kinase, and/or a mutant thereof, and (2) selecting a compound capable of modulating the protein kinase activity of the said protein kinase to different extents towards (i) a substrate that binds to the said hydrophobic pocket of the said protein kinase (hydrophobic pocket-dependent substrate) and (ii) a substrate (such as PKB) that does not bind, or binds to a lesser extent than the first said substrate (hydrophobic pocket-independent substrate), to the said hydrophobic pocket of the said protein kinase.

Description: PROTEIN KINASE REGULATION The present invention relates to regulation of protein kinases. Stimulation of cells with insulin and growth factors generates the second messengers Ptdlns (3, 4, 5,) P3 and Ptdlns (3, 4) P 2 (Leevers et al (1999) Curr Opin Biol 11, 219-225) which induce the activation of certain members of the AGC subfamily of protein kinases that include protein kinase B (PKB) (Shepherd et al (1998) Biochem J 333: 471-479; Alessi & Downes (1998) Biochem Biophys Acta 1436, 151-164), p70 S6 kinase (S6K) (Proud C G (1995) Trends in Biochem Sci 21, 181-185; Pullen & Thomas (1998) FEBS LETT 410, 78-82), serum and glucocorticoid- induced kinase (SGK) (Kobayashi & Cohen (1999) Biochem J 339, 319- 328; Park et al (1999) EMBO J 18, 3024-3033) and protein kinase C (PKC) isoforms (Mellor & Parker (1998) Biochem J, 332, 281-292). These kinases can then mediate many of the effects of insulin and growth factors by phosphorylating key regulatory proteins (reviewed in Shepherd et al (1998) Biochem J 333, 471-479; Alessi & Downes (1998) Biochem Biophys Acta 1436, 151-164 and Alessi & Cohen (1998) Curr Opin Genet Dev 8, 55-62). The interaction of Ptdlns (3, 4, 5) P 3 with the PH domain of PKB causes PKB to translocate to the plasma membrane where it is activated by phosphorylation of two residues, namely Thr308 and Ser473. Both of these residues need to be phosphorylated for maximal activation and their phosphorylation in vivo is prevented by inhibitors of phosphatidylinositol (PI) 3-kinase (Shepherd et al (1998); Alessi & Downes (1998)). Thr308 lies in the activation loop of the kinase domain while Ser473 is located C- terminal to the catalytic domain, in a region that displays high homology between different AGC family members. Importantly, p70 S6K (Pearson et al (1995) EMBO J 14, 5278-5287), PKC isoforms (Mellor & Parker (1998)) and SGK (Kobayashi & Cohen (1999); Park et al (1999)) also possess residues lying in sequences equivalent to Thr308 and Ser473 of PKB, whose phosphorylation is necessary for activation of these kinases in vivo. Ser473 of PKB and the equivalent residues of p70 S6 kinase, PKC and SGK lie in a hydrophobic motif: Phe-Xaa-Xaa-Phe-Ser/Thr-Phe/Tyr distinct from the sequences surrounding Thr308. The protein kinase termed 3-phosphoinositide-dependent protein kinase- 1 (PDK1) plays a central role in activating AGC subfamily members (reviewed in Belham et al (1999) Current Biol 9, R93-R96; Peterson & Schreiber (1999) Current Biol 9, R521-524]). PDK1 phosphorylates PKB at Thr308 (Alessi et al (1997) Curr Biol 7, 261-269; Alessi et al (1997) Curr Biol 7, 776-789; Stokoe et al (1997) Science 277, 567-570; Stephens et al (1998) Science 279, 710-714) and the equivalent residues on PKC isoforms (LeGood et al (1998) Science, 281, 2042-2045; Chou et al (1998) Curr Biol 8, 1069-1077; Dutil et al (1998) Curr Biol 8, 1366- 1375), p70 S6 kinase (Alessi et al (1998) Curr Biol 8, 69-81; Pullen et al (1998) Science, 279, 707-710) and SGK (Kobayashi & Cohen (1999); Park et al (1999)). Cyclic AMP-dependent protein kinase (PKA) is also phosphorylated by PDK1 at the equivalent residue (Thrl97) and this is required for PKA activity (Chen et al (1998) Proc Natl Acad Sci, USA 95, 9849-9854). However, unlike the other members of the AGC subfamily of protein kinases discussed above, PKA does not possess a residue equivalent to Ser473 of PKB. Instead, its amino acid sequence terminates with the sequence Phe-Xaa-Xaa-PheCOOH corresponding to the first part of the hydrophobic motif Phe-Xaa-Xaa-Phe-Ser/Thr-Phe/Tyr that surrounds Ser473 (the "PDK2" phosphorylation motif). Nevertheless, this C-terminal region of PKA plays an important role as its mutation or deletion greatly diminishes activity (Etchebehere et al (1997) Eur J Biochem, 248, 820-826). Recently, we discovered that the kinase domain of PDKl interacts with a region of Protein kinase C-Related Kinase-2 (PRK2) termed the PDKl Intracting Fragment (PIF). This converts PDKl from a form that phosphorylates PKB at Thr308 to a form that phosphorylates PKB at both Thr308 and Ser473 (Balendran et al (1999) Current Biology 9, 393-404). PIF contains a hydrophobic sequence motif (Phe-Xaa-Xaa-Phe-Asp-Tyr) similar to that found in PKB, except that the residue equivalent to Ser473 is an Asp. Mutation of any of the conserved aromatic residues in this motif or mutation of the Asp residue to either Ala or Ser greatly weakens the interaction of PIF with PDKl, indicating that PIF-binds to PDKl via these residues (Balendran et al (1999). p70 S6 kinase (p70 S6K or S6K) is activated by insulin and growth factors and mediates the phosphorylation of the 40S ribosomal protein S6 (Proud (1995). Trends in Bioch. Sci 21, 181-185). This enables efficient translation of mRNA molecules containing a polypyrimidine tract at their 5' transcriptional start sites (Lane et al (1993) Nature 363,170-172). p70 S6K also phosphorylates unknown proteins to permit progression through the Gl phase of the cell cycle (Jefferies et al (1997) EMBO J. 16, 3693- 3704). p70 S6K is activated by insulin and growth factors, through a PI3-kinase dependent pathway, and becomes phosphorylated on at least 7 Ser/Thr residues in response to these agonists. The phosphorylation of two of these residues namely Thr252 and Thr412 on the longer splice variant of the α-isofoπn (Thr229 and Thr389 on the shorter splice variant) appear to make the most important contribution to the activation of p70 S6K (Pearson et al (1995) EMBO J. 14, 5278-5287; Pullen & Thomas (1998) FEBS LETT.410, 78-82; Weng et al (1998) J. Biol. Chem.273, 16621- 16629). Phosphorylation of Thr252 alone or mutation of Thr412 to glutamic acid to mimic phosphorylation of this residue, results in a small activation of p70 S6K. In contrast, phosphorylation of both residues or phosphorylation of Thr252 in the T412E mutant of p70 S6K results in large activation of expressed p70 S6K, showing that phosphorylation of Thr252 and Thr412 leads to a synergistic activation p70 S6K (Weng et al (1998) J. Biol. Chem.273, 16621-16629; Alessi et al (1998) Curr. Biol. 8, 69-81). The residues surrounding Thr252 and Thr412 of p70 S6K are highly conserved in all AGC family members and phosphorylation of the residues equivalent to Thr252 and Thr412 of p70 S6K is necessary for activation and/or stability of these kinases in vivo (Belham et al (1999) Current Biol. 9, R93-R96), as discussed above. Thr412 is located C-terminal to the catalytic domain, and the residues surrounding Thr412 lie in a Phe-Xaa- Xaa-Phe-Ser/Thr-Phe/Tyr consensus motif. As discussed above, 3-phosphoinositide dependent protein kinase-1 (PDKl) can phosphorylate p70 S6K at Thr252 in vitro and in transfection experiments. Phosphorylation of p70 S6K by PDKl in vitro is independent of the presence of Ptdlns(3,4,5) P 3 , and activation is increased greatly if the non catalytic carboxy terminal tail of p70 S6K is deleted and if Thr412 is mutated to an acidic residue Recently, we made the surprising observation that PDKl can be converted from a form that phosphorylates Thr308 of PKB alone (the residue equivalent to Thr252 in p70 S6K) to a form that phosphorylates both Thr308 and Ser 473 (the residue equivalent to Thr412 in p70 S6K) through interaction with a region of Protein Kinase C-Related Kinase- 2(PRK2), termed the PDKl Interacting Fragment (PIF) (Balendran et al (1999) Curr Biol 9(8), 393-404; GB 9906245.7, filed 19 March 1999). We identify and characterise a hydrophobic pocket at a position equivalent to a hydrophobic pocket on the small lobe of protein kinase A (PKA) on the small lobe of the kinase domain of protein kinases other than PKA, for example PDKl, and identify polypeptides that interact with the hydrophobic pocket. We identify the effect of such polypeptides on the protein kinase activity and stability of such protein kinases. We identify assays and protein kinase substrates that can be used to identify drugs that activate or inhibit the activity of a protein kinase by interacting with the hydrophobic pocket. A first aspect of the invention provides a method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leull6, Nal80 and/or Lyslll of full-length mouse PKA, wherein the ability of the compound to inhibit, promote or mimic the interaction of the said hydrophobic pocket- containing protein kinase with an interacting polypeptide is measured and a compound that inhibits, promotes or mimics the said interaction is selected, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr. The residue immediately C-terminal of the Phe/Tyr-Xaa-Xaa-Phe/Tyr sequence may be any residue. Preferably, the interacting polypeptide comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa- Phe/Tyr, wherein Zaa represents a negatively charged amino acid residue. Also preferably, the interacting polypeptide may have the C-terminal sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-COOH, preferably Phe-Xaa-Xaa- Phe-COOH, or Phe/Tyr-Xaa-Xaa-Phe/Tyr-(X) n - COOH, preferably Phe- Xaa-Xaa-Phe-(X) n -COOH, wherein n is between 1 and 20, 15, 10, 5, 4, 3 or 2, preferably between 1 and 4, most preferably 4. Each amino acid X is any amino acid residue, preferably glycine. Thus, it is preferred that the interacting polypeptide has the C-terminal sequence Phe-Xaa-Xaa-Phe- (Gly) 4 -COOH. The interacting polypeptide preferably does not have the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Ser/Thr-Phe/Tyr. When the interacting polypeptide, for example comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr is not part of the same polypeptide chain as the protein kinase, it is preferred that the interacting polypeptide has fewer than about 400, 380, 350, 300, 250, 200, 150, 100, 80, 50, 40 or 30 amino acids. When the hydrophobic pocket-containing polypeptide is PDKl, it is preferred that the interacting polypeptide is not full length PKB or SGK (phosphorylated or unphosphorylated forms) or other known naturally occurring substrate of PDKl, for example PKCζ. The negatively charged amino acid residue Zaa may be, for example, an aspartate, glutamate, phosphorylated serine (phosphoserine), phosphorylated threonine (phosphothreonine) or phosphorylated tyrosine (phosphotyrosine) residue, or a negatively charged non-naturally occuring residue. It is preferred that Zaa is an aspartate, glutamate, phosphoserine or phosphothreonine residue, still more preferably an aspartate or glutamate residue. It is preferred that the first residue in the sequence corresponding to any of the above consensus sequences is a phenylalanine residue. Phenylalanine is found in this position in naturally occuring polypeptides in which a said consensus sequence has been identified. It may also be preferred that the fourth residue in the sequence corresponding to any of the above consensus sequences is a phenylalanine residue. Phenylalanine and tyrosine are both (separately) found in this position in naturally occuring polypeptides in which a said consensus sequence has been identified. Preferred interacting polypeptides in which the residue immediately C- terminal of the Phe/Tyr-Xaa-Xaa-Phe/Tyr amino acid sequence is not a negatively charged amino acid residue may comprise the amino acid sequence FEGFA or FAGFS. The hydrophobic pocket-containing protein kinase may be the protein kinase termed 3-phosphoinositide-dependent protein kinase 1 (PDKl). Alternatively, it may be Serum and Glucocorticoid stimulated protein kinase (SGK), Protein Kinase B (PKB), Protein Kinase A (PKA), p70 S6 kinase, p90 RSK, PKC isoforms (for example PKCα, PKCδ, PKCζ), PRK1, PRK2, MSK1 or MSK2. Hydrophobic pocket-containing protein kinases and their EMBL database accession numbers are listed in Table I and shown in Figures 15 and 16. All AGC family protein kinases may be hydrophobic pocket-containing protein kinases, as defined above. In addition to the protein kinases shown in Figures 15 and 16, rhodopsin and G-protein coupled receptor protein kinases, for example, also have a hydrophobic pocket as defined above and the residue equivalent to Lys76 of mouse PKA is a lysine residue. The term PDKl as used herein includes a polypeptide (a PDKl polypeptide) comprising the amino acid sequence identified as PDKl in Alessi D.R et al (1997) Curr. Biol. 7: 261-269, Alessi D.R et al (1997) Curr. Biol. 7: 776-789, Stokoe D et al (1997) Science 277: 567-570 or Stephens L et al (1998) Science 279: 710-714, or a variant, fragment, fusion or derivative thereof, or a fusion of a said variant or fragment or derivative, for example as described in W098/41638, incorporated herein by reference. It is preferred that the said PDKl polypeptide is a protein kinase. It is preferred that the said PDKl polypeptide is a protein kinase that is capable of phosphorylating a threonine residue that lies in a Thr- Phe-Cys-Gly-Thr-Xaa-Glu-Leu consensus motif (where the underlined Thr corresponds to the threonine that is phosphorylated by PDKl and Xaa is a variable residue), and preferably that is capable of phosphorylating PKB, for example PKBα, at residue Thr308. The rate at which the said PDKl polypeptide is capable of phosphorylating a threonine residue as described above may be increased in the presence of PtdIns(3,4,5)P 3 or PtdIns(3,4)P 2 but it will be appreciated that this is not essential. The said polypeptide may be capable of phosphorylating the equivalent residues to Thr308 of PKBα on PKC isoforms (LeGood et al (1998) Science 281: 2042-2045; et al (1998) Curr. Biol. 8: 1069-1077; Dutil et al (1998) Curr. Biol. 8:1366-1375), p70 S6 kinase (Alessi et al (1998) Curr. Biol. 8: 69- 81; Pullen et al (1998) Science 279, 707-710), SGK (sequence given in Webster et al (1993) Mol. Cell. Biol. 13, 1031-2040; equivalent residues identified in US application no 112217 filed on 14 December 1998; GB 9919676.8, filed on 19 August 1999, and Kobayashi & Cohen (1999)) and PKA (Cheng et al (1998) Proc. Natl. Acad. Sci. USA 95: 9849-9854). It may further be preferred that the substrate specificity and/or other characteristics of the said PDKl polypeptide in vitro may be substantially as reported in Alessi D.R et al (1997) Curr. Biol. 7: 261-269, Alessi D.R et al (1997) Curr. Biol. 7: 776-789, Stokoe D et al (1997) Science 277: 567-570 or Stephens L et al (1998) Science 279: 710-714. The terms SGK, PKB, PKA, p70 S6 kinase, p90 RSK, PKCα, PKCδ, PKCζ or PRK2, for example, as used herein include a polypeptide (a SGK, PKB, PKA, p70S6 kinase, p90 RSK, PKCα, PKCδ, PKCζ or PRK2 polypeptide) comprising the amino acid sequence identified as a SGK, PKB, PKA, p70 S6 kinase, p90 RSK, PKCα, PKCδ, PKCζ or PRK2, respectively, in the relevant EMBL database records indicated in Table I. Table I Activation or T- AGC Accession Loop Hydrophobic number Motif consensus: TFCGTxxYxAPD FxxFSY L E YTF TFCGTPEYLAPE FPQFSY (Y15056) PKBα PKBβ TFCGTPEYLAPE FPQFSY (P31751) PKBγ TFCGTPEYLAPE FPQFSY (AF135794) SGK1 TFCGTPEYLAPE FLGFSY (AAD41091) SGK2 TFCGTPEYLAPE FLGFSY (AF169034) SGK3 TFCGTPEYLAPE FLGFSY (AF169035) PKCα TFCGTPDYIAPE FEGFSY (4506067) PKCβl TFCGTPDYIAPE FAGFSY (4506069) PKCβll TFCGTPDYIAPE FEGFSF (P05127) PKCγ TFCGTPDYIAPE FGGFTY (P05129) PKCδ TFCGTPDYIAPE FAGFSF (5453970) PCKζ TFCGTPNYIAPE FEGFEY (4506079) PKCi TFCGTPNYIAPE FEGFEY (4506071) PRK1 TFCGTPEFLAPE FLDFDF (AAC50209) PRK2 TFCGTPEFLAPE FRDFDY (AAC50208) p70-S6Kα TFCGTIEYMAPE FLGFTY (AAA36410) p70-S6Kβ TFCGTIEYMAPE FLGFTY (4506739) p90-RSKl SFCGTVEYMAPE FRGFSF (138556) p90-RSK2 SFCGTVEYMAPE FRDFSF (P51812) p90-RSK3 STCGTIEYMAPE FRGFSF (CAA59427) MSK1 SFCGTIEYMAPD FQGYSF (AAC31171) MSK2 SFCGTIEYMAPE FQGYSF (AAC67395) PKA TLCGTPEYLAPE FSEF ( 1 ) (P22612) PDKl SFVGTAQYVSPE ( 2 ) (AF017995) Table π. Alignment of the amino acid sequences surrounding the T- loop and the hydrophobic motif of AGC kinases. All the sequences and accession numbers are from human proteins. The underlined residues correspond to those that become phosphorylated. Footnotes: (1) the PKA protein terminates at this position (2) PDKl does not possess a hydrophobic motif. It is particularly preferred, although not essential, that the variant or fragment or derivative or fusion of the PDKl , or the fusion of the variant or fragment or derivative has at least 30% of the enzyme activity of full- length human PDKl with respect to the phosphorylation of full-length human PKBα or SGK1 on residue Thr308 in either the presence or absence of PtdIns(3,4,5)P 3 or PtdIns(3,4)P 2 . It is more preferred if the variant or fragment or derivative or fusion of the said protein kinase, or the fusion of the variant or fragment or derivative has at least 50%, preferably at least 70% and more preferably at least 90% of the enzyme activity of PDKl with respect to the phosphorylation of PKBα or SGK1. However, it will be appreciated that variants or fusions or derivatives or fragments which are devoid of enzymatic activity may nevertheless be useful, for example by interacting with another polypeptide. Thus, variants or fusions or derivatives or fragments which are devoid of enzymatic activity may be useful in a binding assay, which may be used, for example, in a method of the invention in which modulation of an interaction of PDKl (as defined above) with a interacting polypeptide, for example an interacting polypeptide comprising the amino acid sequence motif Phe/Tyr-Xaa-Xaa-Phe/Tyr, for example Phe/Tyr-Xaa-Xaa-Phe/Tyr- Zaa-Phe/Tyr, for example Phe/Tyr-Xaa-Xaa-Phe/Tyr-Asp/Glu-Phe/Tyr or Phe/Tyr-Xaa-Xaa-Phe/Tyr-PhosphoSer/PhosphoThr-Phe/Tyr is measured. It is preferred that the variant or fragment or derivative or fusion of the said hydrophobic pocket-containing protein kinase, or the fusion of the variant or fragment or derivative comprises a hydrophobic pocket in the position equivalent to the hydrophobic pocket of (mouse) PKA that is defined by residues including Lys76, Leull6, Val80 and/or Lyslll of full-length mouse PKA, as discussed further below. Equivalent preferences apply to a variant or fragment or derivative or fusion of the SGK, PKB, PKA, p70 S6 kinase, p90 RSK, PKCα, PKCδ, PKCζ or PRK2 (for example), or the fusion of the variant or fragment or derivative, with the substitution in relation to SGK, PKB and p70S6 kinase of the peptide substrate Crosstide (GRPRTSSFAEG), or for PKB and SGK of the peptide substrate RPRAATF; the subsitution in relation to PKA of the peptide substrate Kemptide (LRRASLG); the substitution in relation to PKC isoforms and PRKl/2 of histone HI; and the substitution in relation to MSK1/2 or p90-RSKl/2/3 of CREBtide (EILSRRPSYRK). By "variants" of a polypeptide we include insertions, deletions and substitutions, either conservative or non-conservative. In particular we include variants of the polypeptide where such changes do not substantially alter the activity of the said polypeptide, for example the protein kinase activity of PDKl , as described above. By "conservative substitutions'' is intended combinations such as Gly, Ala; Nal, He, Leu; Asp, Glu; Asn, Gin; Ser, Thr; Lys, Arg; and Phe, Tyr. It is particularly preferred if the PDKl (or SGK, PKB, PKA or p70 S6 kinase or other hydrophobic pocket-containing kinase as defined above) variant has an amino acid sequence which has at least 65% identity with the amino acid sequence of PDKl referred to above (or the sequence for SGK (including SGK1, 2 and 3), PKB, PKA or p70 S6 kinase, for example, as appropriate, referred to above), more preferably at least 70%, 71 %, 72%, 73% or 74%, still more preferably at least 75%, yet still more preferably at least 80%, in further preference at least 85%, in still further preference at least 90% and most preferably at least 95% or 97% identity with the amino acid sequence defined above. It is still further preferred if the PDKl (or SGK, PKB, PKA or p70 S6 kinase or other hydrophobic pocket-containing kinase, as defined above) variant has an amino acid sequence which has at least 65% identity with the amino acid sequence of the catalytic domain, particularly the residues forming the hydrophobic pocket, of PDKl (or, for example, SGK, PKB, PKA or p70 S6 kinase) in the appropriate sequence referred to above, more preferably at least 70%, 71 %, 72%, 73% or 74%, still more preferably at least 75%, yet still more preferably at least 80%, in further preference at least 83 or 85%, in still further preference at least 90% and most preferably at least 95% or 97% identity with the amino acid sequence defined above. It will be appreciated that the catalytic domain of a protein kinase-related polypeptide may be readily identified by a person skilled in the art, for example using sequence comparisons as described below. The percent sequence identity between two polypeptides may be determined using suitable computer programs, for example the GAP program of the University of Wisconsin Genetic Computing Group and it will be appreciated that percent identity is calculated in relation to polypeptides whose sequence has been aligned optimally. The alignment may alternatively be carried out using the Clustal W program (Thompson et al (1994) Nucl Acid Res 22, 4673-4680). The parameters used may be as follows: Fast pairwise alignment parameters: K-tuple(word) size; 1, window size; 5, gap penalty; 3, number of top diagonals; 5. Scoring method: x percent. Multiple alignment parameters: gap open penalty; 10, gap extension penalty; 0.05. Scoring matrix: BLOSUM. It is preferred that the PDKl (or, for example, SGK, PKB, PKA or p70 S6 kinase) is a polypeptide which consists of the amino acid sequence of the protein kinase PDKl (or, for example, SGK, PKB, PKA or p70 S6 kinase as the case may be) sequence referred to above or naturally occurring allelic variants thereof. It is preferred that the naturally occuring allelic variants are mammalian, preferably human, but may alternatively be homologues from parasitic or pathogenic or potentially pathogenic organisms. Examples of such organisms and homologues, and of uses of modulators of such homologues are given in US patent application No 60/112,114, filed on 14 December 1998, and applications claiming priority therefrom, or in Casamayor et al (1999) Curr Biol 9, 186-197. The PDKl may also be a polypeptide with the amino acid sequence of residues 51 to 404 of full-length human PDKl; this may comprise the protein kinase domain of PDKl, as described in Example 2. The PDKl (or SGK, PKB, PKA or p70 S6 kinase) may also be Myc epitope-tagged or His-tagged, as described in Example 1. The p70 S6 kinase, for example, may have a His tag at its N-terminus and/or may lack the carboxy terminal 104 residues (p70 S6K-T2; see Example 1). The PDKl or SGK may be a Saccharomyces cerevisiae homologue, for example Pkhl or Pkh2 (PDKl homologues) or Ypkl or Yrk2 (SGK homologues), as described in Casamayor et al (1999) Curr Biol 9, 186-197. It is preferred that the PDKl (or, for example, SGK, PKB, PKA or p70 S6 kinase) is a polypeptide that is capable of interacting with a polypeptide comprising the amino acid sequence motif Phe/Tyr-Xaa-Xaa-Phe/Tyr, preferably Phe-Xaa-Xaa-Phe/Tyr, more preferably Phe-Xaa-Xaa-Phe, still more preferably Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr or Phe/Tyr-Xaa- Xaa-Phe/Tyr-COOH, for example the polypeptide PIF or PIFtide, as defined below. Further preferences for the said polypeptide are as given above in relation to the interacting polypeptide. The capability of the said PDKl (or, for example, SGK, PKB, PKA or p70 S6 kinase) polypeptide with regard to interacting with or binding to a polypeptide, for example a polypeptide comprising the amino acid sequence motif Phe/Tyr-Xaa-Xaa-Phe/Tyr, for example Phe/Tyr-Xaa- Xaa-Phe/Tyr-Zaa-Phe/Tyr or Phe/Tyr-Xaa-Xaa-Phe/Tyr-COOH, may be measured by any method of detecting/measuring a protein/protein interaction, as discussed further below. Suitable methods include methods analagous to those discussed above and described in Example 1 or Example 2, for example yeast two-hybrid interactions, co-purification, ELISA, co-immunoprecipitation and surface plasmon resonance methods. Thus, the said PDKl (or SGK, PKB, PKA or p70 S6 kinase) may be considered capable of binding to or interacting with a polypeptide, for example a polypeptide comprising the amino acid sequence motif Phe/Tyr- Xaa-Xaa-Phe/Tyr, for example Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr, Phe/Tyr-Xaa-Xaa-Phe/Tyr-Asp/Glu-Phe/Tyr or Phe/Tyr-Xaa-Xaa- Phe/Tyr-PhosphoSer/PhosphoThr-Phe/Tyr if an interaction may be detected between the said PDKl polypeptide and the said interacting polypeptide by ELISA, co-immunoprecipitation or surface plasmon resonance methods or by a yeast two-hybrid interaction or copurification method, for example as described in Example 1 or Example 2. It is preferred that the interaction can be detected using a surface plasmon resonance method, as described in Example 1 or 2 and in Balendran et al (1999), supra and GB9906245.7, supra. The interacting polypeptide may be immobilised on the test surface, for example it can be coupled through amino groups to a SensorChip CM5™, according to the manufacturer's instructions, or a biotinylated polypeptide can be bound to an avidin coated SensorChip SA. The protein kinase (at concentrations between, for example 0 and between lOμM and l.OμM, for example 2μM) is then injected over the surface and steady state binding deteπnined in each case. From these measurements a I j can be determined. It is preferred that the interaction has a I j of less than 8μM, more preferably less than 5μM, 2μM, lμM, 500nM, 300nM, 200nM or lOOnM, for example about 150nM. The I j of the interaction determined between GST-PDK1 and PIF may be about 150nM. Alternatively, a K,, can be determined for a polypeptide in competition with the immobilised polypeptide. The protein kinase (for example at a concentration of 0.5μM) is mixed with free polypeptide (for example, at concentrations between 0 and 3μM) and the mixture injected over the immobilised polypeptides. The steady state binding is determined in each case, from which the K,j of the interaction can be determined using the Cheng-Prescott relationship. Alternatively, the interaction may be expressed in terms of an observed response or relative observed responses, measured in terms of mass of protein bound to the surface, as described in Example 2. For example, the polypeptide may be immobilised by amino coupling to a SensorChip CM5 and each protein kinase (for example different mutated protein kinases, as discussed below) for example at a concentration of l.OμM, -injected over the immobilised polypeptide. Alternatively, the polypeptide may be immobilised on a SA SensorChip and each protein kinase, for example at a concentration of 40nM injected over the immobilised polypeptide. The steady state response for each protein kinase is determined, for example expressed in Response Units (RU). 1000RU corresponds to 1 ng/mm 2 of protein bound to the surface. A response of less than 10RU may indicate that no interaction has taken place. A response of at least 10RU may indicate that the immobilised and injected molecules interact with each other. It will be appreciated that the above methods may be used to determine whether a particular polypeptide is an interacting polypeptide in respect of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leul l6, Val80 and/or Lyslll of full-length mouse PKA, for example a naturally occuring said hydrophobic pocket- containing protein kinase. By "protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Nal80 and/or Lyslll of full-length mouse PKA" is meant a polypeptide having an amino acid sequence identifiable as that of a protein kinase catalytic domain, and further having a predicted or determined three-dimensional structure that includes a hydrophobic pocket corresponding to the region indicated in Knighton et al (1991) Science 253, 407-414 for PKA as interacting with C-terminal amino acids of full-length PKA, for example Phe348 and/or Phe351 , as discussed in Example 2. The hydrophobic pocket in PKA is in the small lobe of the catalytic domain and does not overlap with the ATP or peptide substrate binding sites on PKA. Residues Lys76, Nal80, Lyslll and/or Leullό in a hydrophobic pocket of PKA may interact with residues Phe347 and Phe350 at the C-terminus of full length mouse PKA (Uhler et al (1996) PNAS USA 83, 1300-1304). It is preferred that the protein kinase has identical or conserved residues that are equivalent to Lys76, Nal80, Lyslll and/or Leull6 of mouse PKA, more preferably at least Lys76 and Leullό of mouse PKA, most preferably an identical residue equivalent to Lys76. Thus, for example, the protein kinase may have a Lys residue at the position equivalent to Lys76 of PKA and/or a Leu residue at the position equivalent to Leullό of PKA. Lysll5 and Leul55 of PDKl, for example, are equivalent to Lys76 and Leullό, respectively, of PKA. It is preferred that the protein kinase does not have an Ala at the position equivalent to Lys76 and/or a Ser, Asp or Glu at the position equivalent to Leullό of PKA. The protein kinase may have a Nal residue at the position equivalent to Leullό of PKA, as in PRK1 and 2 (see Figures 15 and 16), or an He residue. The protein kinase may have a non-conserved residue at the position equivalent to Lyslll, for example a glutamine residue and/or at the position equivalent to Nal80. Figures 15 and 16 shows an alignment of examples of protein kinases having a hydrophobic pocket at the position equivalent to the said hydrophobic pocket of PKA. The residues equivalent to Lys76, Val80, Lyslll and Leullό of full length mouse PKA are indicated. A Lys residue is present at the position equivalent to Lys76 of mouse PKA in all of the aligned sequences. An amino acid sequence may be identifiable as that of a protein kinase catalytic domain by reference to sequence identity or similarities of three dimensional structure with known protein kinase domains, as known to those skilled in the art. Protein kinases show a conserved catalytic core, as reviewed in Johnson et al (1996) Cell, 85, 149-158 and Taylor & Radzio-Andzelm (1994) Structure 2, 345-355. This core folds into a small N-terminal lobe largely comprising anti-parallel β-sheet, and a large C-terminal lobe which is mostly α-helical. A deep cleft at the interface between these lobes is the site of ATP binding, with the phosphate groups near the opening of the cleft. Protein kinases also show conserved sequences within this catalytic core, and the residue equivalent to a given residue of, for example, PKA, may be identified by alignment of the sequence of the kinase with that of known kinases in such a way as to maximise the match between the sequences. The alignment may be carried out by visual inspection and/or by the use of suitable computer programs, for example the GAP program of the Umversity of Wisconsin Genetic Computing Group, which will also allow the percent identity of the polypeptides to be calculated. The Align program (Pearson (1994) in: Methods in Molecular Biology, Computer Analysis of Sequence Data, Part II (Griffin, AM and Griffin, HG eds) pp 365-389, Humana Press, Clifton). The comparison of amino acid sequences or three dimension structure (for example from crystallography or computer modelling based on a known structure) may be carried out using methods well known to the skilled man, as detailed below and as described in Example 2. MAP kinase, MEK1, Cdk2 and Erk2 (for example) are not protein kinases having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Nal80 and/or Lyslll of full-length mouse PKA. MEK1, Cdk2 and ERK2 have histidine, glutamine and proline, respectively at the residue equivalent to Lys76 of full-length mouse PKA ie not lysine or a conservative substitution, and do not interact with a Phe-Xaa-Xaa-Phe amino acid sequence. MEK1, Cdk2 and ERK2 may have a larger hydrophobic pocket which interacts with an amino acid sequence motif (which may be Phe-Xaa-Phe-Pro) which is not Phe-Xaa-Xaa-Phe. Thus, these protein kinases do not have a hydrophobic pocket in the position equivalent to the said hydrophobic pocket of protein kinase A. The interacting polypeptide may interact with the said hydrophobic pocket of the protein kinase. Thus, it is preferred that the interacting polypeptide interacts with the protein kinase but interacts less strongly with the protein kinase in which one or more residues forming the said hydrophobic pocket is mutated, preferably to a non-conserved amino acid. Most preferably, the mutated residues are the residues equivalent to residues Lys76, Nal80, Lyslll and/or Leullό in the hydrophobic pocket of PKA that is defined by residues including Lys76, Leullό, Nal80 and/or Lyslll of full-length mouse PKA. It is particularly preferred that the residue at the position equivalent to residue Lys76 of PKA is mutated to an Ala and/or that the residue at the position equivalent to Leullό of PKA is mutated to a Ser, Asp or Glu. It will be appreciated that the interacting polypeptide may interact with additional regions of the protein kinase. For example, it may interact (for example via the acidic residue or group in the preferred amino acid sequence indicated above) with a residue equivalent to Gln35 of PKA (in the N-terminal non-catalytic region of PKA), which appears to form a hydrogen bond with the C-terminal carboxylate group of Phe350, when the C-teπninus of PKA interacts with the hydrophobic pocket of PKA. The interaction may be measured by any of the methods discussed above. In particular, it may be measured using surface plasmon resonance, as discussed above and in Example 1 and 2. It is particularly preferred that the relative strength of interaction with the protein kinase and the mutated protein kinase is determined by measuring the relative steady state responses, as described above. It is preferred that the response (expressed in RUs) for the unmutated protein kinase is at least 2, 5, 10, 30, 50, 80, 100, 200 or 500 times the response for the mutated protein kinase. The interacting polypeptide, for example comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr may be part of the same polypeptide chain as the protein kinase. For example, PKA comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, and SGK, PKB and p70 S6 kinase comprise the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr wherein Zaa is phosphoserine or phosphothreonine. Thus, the interaction may be an intramolecular interaction, for example in which the hydrophobic pocket (of the protein kinase domain of the polypeptide) and the interacting portion of the polypeptide, for example a portion of the polypeptide comprising a Phe/Tyr-Xaa-Xaa-Phe/Tyr sequence, within a single polypeptide chain, interact. Alternatively, two or more such polypeptide chains may form a dimer or multimer through intermolecular interactions between, for example, the hydrophobic pocket of one polypeptide chain and the interacting portion of a second polypeptide. Intramolecular interactions can be measured by techniques known to those skilled in the art, including cross-linking studies, structural studies and fluorescence resonance energy transfer (FRET) methods, in which changes in separation between fluorophores, for example attached to different parts of a molecule, can be measured. A polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa- Phe/Tyr-Ser/Thr-Phe/Tyr may interact with a said hydrophobic pocket of a protein kinase with different affinity depending upon the phosphorylation state of the Ser/Thr residue. Thus, the polypeptide may interact with the hydrophobic pocket more strongly when phosphorylated on the Ser/Thr residue than when not so phosphorylated. In the absence of phosphorylation, the interaction may be substantially undetectable using one or more of the methods described above or may be about 2, 5 or 10- fold weaker than when phosphorylated. Thus, for example, an intra- or intermolecular interaction between the SGK, PKB or p70 S6 kinase protein kinase domain and the portion comprising the sequence Phe/Tyr- Xaa-Xaa-Phe/Tyr-Ser/Thr-Phe/Tyr may occur substantially only when the said sequence is phosphorylated on the Ser/Thr residue. The interaction may modulate, for example increase, the activity and/or stability of the protein kinase domain (or entire polypeptide). It is preferred that the interacting polypeptide, for example comprising the amino acid sequence motif Phe/Tyr-Xaa-Xaa-Phe/Tyr, is a polypeptide that is capable of binding PDKl and inhibiting its activity towards p70 S6 kinase in substantially the same way as a polypeptide with the amino acid sequence EDNKKHPFFRLIDWSALMDKKNKPPπPTIRGREDVSNFDDEFTSEA PILTPPREPRILSEEEQEMFRDFDYIADWC (termed PIF) or (GST)- EDVKKHPFFRLIDWSALMDKKNKPPFIPTIRGREDVSNFDDEFTSEA PILTPPREPRILSEEEQEMFRDFDYIADWC (termed GST-PIF) or REPRILSEEEQEMFRDFDYIADWC (termed PIFtide) as described in Example 1, wherein GST represents a glutathione S-transferase portion, as known to those skilled in the art, and the sequence corresponding to the amino acid sequence motif is underlined. Alternatively or in addition, it is preferred that the interacting polypeptide, for example comprising the amino acid sequence motif Phe/Tyr-Xaa-Xaa- Phe/Tyr, is a polypeptide that is capable of binding PDKl and increasing its activity towards (ie phosphorylation of the underlined residue of) KTFCGTPEYLAPEVRR (termed T308tide) in substantially the same way as a polypeptide with the amino acid sequence EDVKKHPFFRLIDWSALMDKKNKPPFIPTIRGREDVSNFDDEFTSEA PILTPPREPRILSEEEQEMFRDFDYIADWC (termed PIF) or (GST)- EDVKKHPFFRLIDWSALMDKKVKPPΠPTIRGREDVSNFDDEFTSEA PILTPPREPRILSEEEQEMFRDFDYIADWC (termed GST-PIF) or REPRILSEEEQEMFRDFDYIADWC (termed PIFtide) as described in Example 2. The three-letter amino acid code of the IUPAC-IUB Biochemical Nomenclature Commission is used herein, with the exception of the symbol Zaa, defined above. In particular, Xaa represents any amino acid. It is preferred that Xaa and Zaa represent a naturally occuring amino acid. It is preferred that at least the amino acids corresponding to the consensus sequences defined above are L-amino acids. By modulation of the protein kinase activity is included inhibition or an increase in the protein kinase activity. The protein kinase activity of PDKl that is modulated may be phosphorylation of the underlined residue in a polypeptide with the amino acid sequence Thr/Ser-Phe-Cys-Gly-Thr-Xaa-Glu-Leu ("PDKl" activity). Alternatively or in addition, the modulated activity may be phosphorylation of the underlined residue in a polypeptide with the amino acid sequence Phe-Xaa-Xaa-Phe-Ser/Thr-Phe/Tyr ("PDK2" activity). The polypeptide may be, for example, a PKB, SGK, p70 S6 kinase, PKC or (in relation only to phosphorylation of the underlined residue in a polypeptide with the amino acid sequence Thr/Ser-Phe-Cys-Gly-Thr-Xaa- Glu-Leu) PKA polypeptide. The protein kinase activity of PKA that is modulated may be phosphorylation of the underlined residue in a polypeptide with the amino acid sequence Arg-Arg-X-Ser-Y, wherein X is any small residue and Y is a large hydrophobic group. Substrates of PKA include the transcription factor CREB, which is phosphorylated on Serl33, and the polypeptide BAD, which is phosphorylated on Serll2 and Serl55. The protein kinase activity of PKB, SGK or p70 S6 kinase that is modulated may be phosphorylation of the underlined residue in a polypeptide with the amino acid sequence Arg-Xaa-Arg-Xaa-Xaa-Ser/Thr. The polypeptide may be Glycogen Synthase Kinase 3 (GSK3), 40 S ribosomal subunit S6, BAD, 6-phosphofructo-2-kinase, phosphodiesterase3b, human caspase 9, endothelial nitric oxide synthase or BRACAl. A compound identified by a method of the invention may modulate the ability of the protein kinase to phosphorylate different substrates, for example different naturally occuring polypeptides, to different extents. The compound may inhibit the protein kinase activity in relation to one substrate but may increase the protein kinase activity in relation to a second substrate, for example as discussed in Example 2. For example, the protein kinase activity may be modulated to a different extent for PKB when compared with SGK, p70 S6 kinase and/or PKC. It will be appreciated that the modulatory, for example inhibitory action of a compound found to bind (or inhibit binding of a polypeptide or compound) to the protein kinase may be confirmed by performing an assay of enzymic activity (for example PDKl and/or PDK2 protein kinase activity) in the presence of the compound. The said interacting polypeptide may be derivable from PRK1, PRK2, a PKC isoform, for example PKCζ, PKCα or PKCδ, PKA, SGK, p70 S6 kinase or PKB, preferably from the C-teπninal portion of PKA, PRKl, PRK2, PKCα, PKCδ or PKCζ. The said interacting polypeptide may be derivable from PRK2 by proteolytic cleavage, for example by Caspase 3, as described in Balendran et al (1999), supra. Thus, the interacting polypeptide may comprise or consist essentially of the amino acid sequence from residue 701 to the C-terminus of PRK2. This may correspond to the C-terminal 77 amino acids of PRK2, termed the PDKl-Interacting Fragment (PIF; see Balendran et al (1999), supra). The PIF region of PRK2 may lie immediately C-terminal to the kinase catalytic domain of PRK2. The polypeptide may comprise or consist essentially of the amino acid sequence of residues 960 to 984 of PRK2 (termed Region B) or the equivalent region of PRKl, PRKl, PKBα, p70S6 kinase, PKB, SGK, a PKC isoform, for example PKCζ or PKCα, or PKAβ as shown in Figure 7E. The polypeptide may comprise or consist of the C-terminal 223 or C-terminal 62 amino acids of PKA, as described in Example 2 and shown in Figure 7. PKC isoforms are described, for example, in Mellor & Parker (1998) Biochem J 332, 281- 292. A polypeptide that comprises an amino acid sequence that corresponds to the consensus sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr- Ser/Thr-Phe/Tyr may interact with PDKl (1) when the serine or threonine residue is phosphorylated, so that the polypeptide then comprises an amino acid sequence that corresponds to the consensus sequence Phe/Tyr-Xaa- Xaa-Phe/Tyr-PhosphoSer/PhosphoThr-Phe/Tyr, or (2) if the serine or threonine residue is replaced by an aspartate or glutamate residue. PKCδ comprises an amino acid sequence corresponding to the consensus sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Ser/Thr-Phe/Tyr (see Figure 15) and may interact with PDKl when unphosphorylated. The said interacting polypeptide may comprise or consist essentially of the sequence REPRILSEEEQEMFRDFDYIADWC or REPRILSEEEQEMARDFDYIADWC or REPRILSEEEQEMFGDFDYIADWC. The said interacting polypeptide may further comprise the sequence EDNKKHPFFRLIDWSALMDKKNKPPFIPTIRGREDNSNFDDEFTSEA PILTPP (see Balendran et al (1999), supra and GB 9906245.7). The said interacting polypeptide may comprise or consist essentially of a variant of a sequence indicated above. Preferably, in such a variant, the residues that correspond to the amino acid sequence Phe/Tyr-Xaa-Xaa- Phe/Tyr in the sequence indicated above are unchanged, or, if changed, still have the sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr. It is preferred that the residues within about 2, 5 or 10 amino acids C- or N- terminal of the said sequence are also unchanged. It is preferred that the interacting polypeptide has fewer than about 400, 380, 350, 300, 250, 200, 150, 100, 80, 50, 40 or 30 amino acids. The said interacting polypeptide may comprise a GST portion, as described in Examples 1 and 2. This may be useful in purifying and/or detecting the said interacting polypeptide. The said interacting polypeptide may be biotinylated or otherwise tagged, for example with a 6His, HA, myc or other epitope tag, as known to those skilled in the art. A further aspect of the invention provides a said interacting polypeptide immobilised on a surface of an article suitable for use as a test surface in a surface plasmon resonance method. The surface may be a SensorChip™ surface, for example a SensorChip CM5™ or SA SensorChip™ surface. It is preferred that the interacting polypeptide is not PIF or PIFtide. It is further preferred that the interacting polypeptide comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr and does not comprise the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr or Phe/Tyr-Xaa-Xaa- Phe/Tyr-Ser/Thr-Phe/Tyr. It is preferred that the interacting polypeptide has fewer than about 400, 380, 350, 300, 250, 200, 150, 100, 80, 50, 40 or 30 amino acids. The ability of the compound to inhibit or promote the interaction of the said protein kinase with the interacting polypeptide may be measured by detecting/measuring the interaction using any suitable method and comparing the interaction detected/measured in the presence of different concentrations of the compound, for example in the absence and in the presence of the compound, for example at a concentration of about lOOμM, 30μM, lOμM, 3μM, lμM, 0.1 μM, 0.01 μM and/or 0.001 μM. Suitable methods include methods analagous to those discussed above and described in Example 1 or Example 2, for example yeast two-hybrid interactions, co-purification, ELISA, co-immunoprecipitation and surface plasmon resonance methods. A further aspect of the invention provides a method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA), wherein the effect of the said compound on the rate or degree of phosphorylation of a substrate polypeptide of the said hydrophobic pocket-containing protein kinase by the said hydrophobic pocket-containing protein kinase in the presence of an interacting polypeptide is determined, and a compound that modulates the said rate or degree of phosphorylate is selected, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa- Xaa-Phe/Tyr and is comprised in a separate polypeptide chain to the hydrophobic pocket-containing protein kinase, and wherein the substrate polypeptide has fewer than 400 amino acids, still more preferably fewer than 380, 350, 300, 250, 200, 150, 120, 100, 80, 70, 60, 50, 40, 30, 25 or 20 amino acids. The effect of the compound may be determined by comparing the rate or degree of phosphorylation of the said substrate polypeptide by the said hydrophobic pocket-containing protein kinase in the presence of different concentrations of the compound, for example in the absence and in the presence of the compound, for example at a concentration of about lOOμM, 30μM, lOμM, 3μM, lμM, 0.1 μM, 0.01 μM and/or 0.001 μM. The substrate polypeptide may comprise a portion that is the interacting polypeptide, for example that comprises the amino acid sequence Phe/Tyr- Xaa-Xaa-Phe/Tyr. Thus, the substrate polypeptide may comprise non- overlapping interacting and substrate portions. The substrate polypeptide may comprise (1) an interacting portion, for example comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr and (2) a substrate portion comprising a consensus sequence for phosphorylation by a protein kinase having a hydrophobic pocket in the position equivalent to the said hydrophobic pocket of Protein Kinase A (PKA), for example PDKl, PKB, SGK, p70 S6 kinase or PKA, for example the sequence Thr/Ser-Phe-Cys- Gly-Thr-Xaa-Glu-Leu, Phe-Xaa-Xaa-Phe-Ser/Thr-Phe/Tyr, Arg-Arg-X- Ser-Nal or Arg-Xaa-Arg-Xaa-Xaa-Ser/Thr. It is preferred that the amino acid sequences indicated in relation to the said substrate and interacting portions are separated by between about 1 and 100 to 150 amino acids, preferably between about 5 and 50, still more preferably between about 10 and 30 amino acids, for example about 26 amino acids. A further aspect of the invention provides a substrate polypeptide as defined above wherein the amino acid sequence indicated in relation to the said substrate and interacting portions are separated by between about by between about 1 and 100 to 150 amino acids, preferably between about 5 and 50, still more preferably between about 10 and 30 amino acids, for example about 26 amino acids. Thus, if the hydrophobic pocket-containing protein kinase is PDKl, the substrate polypeptide may comprise the sequence KTFCGTPEYLAPEV (substrate portion) and, for example, the sequence EPRILSEEEQEMFRDFDYIADWC (interacting polypeptide portion, for example hydrophobic pocket binding portion). The substrate polypeptide may, for example, comprise or consist of the sequence KTFCGTPEYLAPEVRREPRΓLSEEEQEMFRDFDYIADWC. Alternatively, the substrate portion and the interacting portion may be present on separate polypeptide chains, ie as separate substrate polypeptide and interacting polypeptide. Thus, if the hydrophobic pocket-containing protein kinase is PDKl, the substrate polypeptide may comprise or consist of the sequence KTFCGTPEYLAPEV, and the interacting polypeptide may comprise or consist of the sequence EPRILSEEEQEMFRDFDYIADWC . It will be appreciated that the compound may interact with PDKl or with the said interacting polypeptide or with both. A further aspect of the invention provides a method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA wherein the effect of the said compound on the rate or degree of phosphorylation of a substrate polypeptide of the said hydrophobic pocket-containing protein kinase by the said hydrophobic pocket-containing protein kinase is determined, and a compound that modulates the said rate or degree of phosphorylation is selected, wherein the effect of the compound is determined in the absence (including substantial absence) of an interacting polypeptide, wherein an interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, and wherein the substrate polypeptide has fewer than 400 amino acids, still more preferably fewer than 380, 350, 300, 250, 200, 150, 120, 100, 80, 70, 60, 50, 40, 30, 25 or 20 amino acids. Thus, the substrate polypeptide and the hydrophobic pocket-containing protein kinase do not comprise an interacting polypeptide that interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr. The compound may mimic the effect of the interaction of an interacting polypeptide (that interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa- Phe/Tyr) with the protein kinase. The effect of the compound may be determined by comparing the rate or degree of phosphorylation of the said substrate polypeptide by the said hydrophobic pocket-containing protein kinase in the presence of different concentrations of the compound, for example in the absence and in the presence of the compound, for example at a concentration of about lOOμM, 30μM, lOμM, 3μM, lμM, O.lμM, O.OlμM and/or O.OOlμM. Most preferably, the protein kinase is PDKl and the substrate polypeptide consists of or comprises the amino acid sequence KTFCGTPEYLAPEV or KTFCGTPEYLAPEVRR. A compound that mimics the effect of an interacting polypeptide on PDKl may increase the rate or extent of phosphorylation of such a substrate polypeptide by PDKl . By "mimic the effect of the interaction of the said interacting polypeptide with the protein kinase" is meant that the compound has a quantitative or qualitative effect on the hydrophobic pocket-containing protein kinase, for example on its protein kinase activity or stability, that is the same as an effect of the interacting polypeptide on the protein kinase, for example on its protein kinase activity or stability, as discussed in Examples 1 and 2. For example, the interacting polypeptide PIF increases the rate at which PDKl phosphorylates the polypeptide KTFCGTPEYLAPEVRR; a mimic of PIF may increase the rate at which PDKl (in the absence of PIF) phosphorylates the said polypeptide. The protein kinase and interacting polypeptide may form a complex, which may be detected in a cell-free system, for example by BiaCore measurements, as described in Examples 1 and 2. The ability of the compound to inhibit or promote the formation or stability of the complex may be determined by exposing the protein kinase and/or interacting polypeptide and/or complex of the protein kinase and interacting polypeptide to the compound and deteπnining any change in the affinity, extent or stability of the interaction in the presence of the compound. The estimated equilibrium dissociation constant of the association between GST-PIF and His-tagged PDKl may be 600nM. The estimated dissociation constant I j between His-PDKl and an immobilised and biotinylated 24 residue synthetic peptide corresponding to Region B above (PIF) detected using Surface Plasmon Resonance measurements was 800 nM, or 1.5μM. It is preferred that the said protein kinase, interacting polypeptide and/or, where appropriate, substrate polypeptide, is a recombinant or synthetic polypeptide. It is further preferred that the said protein kinase, interacting polypeptide and/or, where appropriate, substrate polypeptide is substantially pure when introduced into the method of the invention. By "substantially pure" we mean that the protein kinase or interacting polypeptide or substrate polypeptide is substantially free of other proteins. Thus, we include any composition that includes at least 30% of the protein content by weight as the said protein kinase or interacting polypeptide or substrate polypeptide, preferably at least 50%, more preferably at least 70%, still more preferably at least 90% and most preferably at least 95% of the protein content is the said protein kinase or interacting polypeptide or substrate polypeptide. Thus the substantially pure protein kinase or interacting polypeptide or substrate polypeptide may include a contaminant wherein the contaminant comprises less than 70% of the composition by weight, preferably less than 50% of the composition, more preferably less than 30% of the composition, still more preferably less than 10% of the composition and most preferably less than 5 % of the composition by weight. The substantially pure said protein kinase or interacting polypeptide or substrate polypeptide may be combined with other components ex vivo, said other components not being all of the components found in the cell in which said protein kinase or interacting polypeptide or substrate polypeptide is naturally found. The said protein kinase, for example PDKl (or SGK, PKB, p70 S6 kinase or PKA), and said interacting polypeptide may be exposed to each other and to the compound to be tested in a cell in which the said protein kinase and the said interacting polypeptide are both expressed, as described in Examples 1 and 2. The protein kinase may be the endogenous protein kinase or it may be a protein kinase expressed from a recombinant construct. Similarly, the said interacting polypeptide may be endogenous or it may be expressed from a recombinant construct, as described in Example 1. The said protein kinase and the said interacting polypeptide are not exposed to each other in a cell in which the said protein kinase and the said interacting polypeptide are both naturally expressed. The said protein kinase and the said interacting polypeptide are not both endogenous polypeptides to the cell in which the said protein kinase and the said interacting polypeptide are exposed to each other. A complex may also be detected by coimmunoprecipitation or copurification experiments, or using fluorescence resonance energy transfer (FRET) techniques (for example using fusion proteins comprising fluorescent proteins, for example green, blue or yellow fluorescent proteins (GFPs; YFPs, BFPs, as well known to those skilled in the art)), for example in material from cells in which the protein kinase (as defined above) and the said interacting polypeptide are coexpressed, as described in Examples 1 and 2. A further aspect of the invention provides a compound (termed an interacting compound) capable of modulating the protein kinase activity of a hydrophobic pocket-containing protein kinase as defined above wherein the compound inhibits the interaction of the said protein kinase with an interacting polypeptide, wherein the interacting polypeptide interacts with the hydrophobic pocket of the said protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, wherein the compound does not comprise a polypeptide having the amino acid sequence Phe/Tyr- Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr and is not PKA or PKCδ. A further aspect of the invention provides a compound (termed an interacting compound) capable of modulating the protein kinase activity of a hydrophobic pocket-containing protein kinase as defined above, wherein the compound modulates the rate or degree of phosphorylation of a substrate polypeptide of the said hydrophobic pocket-containing protein kinase by the said hydrophobic pocket-containing protein kinase in the absence (including substantial absence) of an interacting poiypeptide, wherein an interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr- Xaa-Xaa-Phe/Tyr, and wherein the substrate polypeptide has fewer than 400 amino acids, still more preferably fewer than 380, 350, 300, 250, 200, 150, 120, 100, 80, 70, 60, 50, 40, 30, 25 or 20 amino acids. A further aspect of the invention provides a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, wherein the compound modulates the rate or degree of phosphorylation of a substrate polypeptide of the said hydrophobic pocket-containing protein kinase by the said hydrophobic pocket-containing protein kinase in the presence of an interacting polypeptide, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr and is comprised in a separate polypeptide chain to the hydrophobic pocket-containing protein kinase, and wherein the substrate polypeptide has fewer than 400 amino acids, still more preferably fewer than 380, 350, 300, 250, 200, 150, 120, 100, 80, 70, 60, 50, 40, 30, 25 or 20 amino acids. The compound may be or comprise a polypeptide having the C-terminal sequence Phe-Xaa-Xaa-Phe-COOH, or Phe/Tyr-Xaa-Xaa-Phe/Tyr-(X) n - COOH, preferably Phe-Xaa-Xaa-Phe-(X) n -COOH, wherein n is between 1 and 150, 100, 60, 50, 30, 20, 15, 10, 5, 4, 3 or 2, preferably between 1 and 4, most preferably 4. Each amino acid X is any amino acid residue, preferably glycine. Thus, it is preferred that the polypeptide has the C- teπninal sequence Phe-Xaa-Xaa-Phe-COOH or Phe-Xaa-Xaa-Phe-(Gly) 4 - COOH. The polypeptide may consist of or comprise contiguous residues derivable from PKA. For example, it may comprise the C-terminal about 223, 220, 200, 180, 160, 140, 120, 100, 80, 70, 65, 63, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10 or 5 amino acids of PKA, or a variant or fusion thereof that has the C-terminal sequence Phe-Xaa-Xaa-Phe-COOH or Phe/Tyr-Xaa-Xaa-Phe/Tyr-(X) n -COOH. It will be appreciated that the polypeptide may comprise a covalent modification, for example it may be modified by biotinylation ie comprise a biotin group. A further aspect of the invention provides a compound identifiable by the method of the invention (termed an interacting compound), provided that the compound is not a polypeptide having the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr and is not full length PKA. The compound may be, for example, a compound selected on the basis of, or designed to have, as well known to those skilled in the art, a three- dimensional conformation that may be similar to that of an interacting polypeptide, for example comprising the amino acid sequence Phe/Tyr- Xaa-Xaa-Phe/Tyr, for example Phe/Tyr-Xaa-Xaa-Phe/Tyr-COOH, as discussed above. A further aspect of the invention provides a mutated protein kinase, wherein the protein kinase before mutation has a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, and wherein one or more residues forming the hydrophobic pocket of the protein kinase is mutated. It is preferred that the said protein kinase is not PKA. The said protein kinase may be, for example, SGK, PKB, p70 S6 kinase or PDKl, preferably PDKl. It is preferred that the mutated residue(s) are the residues equivalent to residue Lys76, Val80, Lyslll and/or Leullό in the hydrophobic pocket of PKA. It is particularly preferred that the residue at the position equivalent to residue Lys76 of PKA is mutated to an Ala and/or that the residue at the position equivalent to Leullό of PKA is mutated to a Ser, Asp or Glu. The equivalent residues of are indicated for several protein kinases in Figure 15. The mutated protein kinase may be useful in determining whether a polypeptide or compound interacts with the hydrophobic pocket of the unmutated protein kinase, as discussed above and shown in Examples 1, 2 and 3. For example, the abilities of a compound (including polypeptide) to bind to the mutated and unmutated protein kinase, or to modulate the activity of the protein kinase towards one or more substrates of the protein kinase, may be measured and compared. A further aspect of the invention provides a preparation comprising a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, and a second, interacting compound (encompassing an interacting polypeptide), wherein the interacting compound interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, wherein the said preparation further comprises a substrate polypeptide as defined above and does not comprise all of the components found in a cell in which said protein kinase or compound is naturally found. A further aspect of the invention provides a preparation comprising a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, and a second, interacting compound, wherein the interacting compound interacts with the hydrophobic pocket of the protein kinase, wherein the said preparation does not comprise all of the components found in a cell in which said protein kinase or compound is naturally found, and wherein when the protein kinase is PDKl, the interacting compound is not a polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr. The interacting compound may be an interacting polypeptide as defined above. Preferences for the interacting polypeptide and protein kinase are as given above. It is preferred that an interacting polypeptide does not comprise the sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Ser/Thr-Phe/Tyr. Thus, the preparation may be substantially free of polypeptides with which the protein kinase or compound is present or associated in a cell other than a said interacting polypeptide. The compound may be a compound of the invention that mimics the effect of an interacting polypeptide on the protein kinase. Thus, we include any composition that includes at least 30% of the protein content by weight as the said protein kinase or interacting polypeptide or (if appropriate) substrate polypeptide (ie in combination), preferably at least 50%, more preferably at least 70%, still more preferably at least 90% and most preferably at least 95% of the protein content is the said protein kinase or interacting polypeptide or (if appropriate) substrate polypeptide. Thus, the invention also includes preparations comprising the said protein kinase, the said interacting compound, for example polypeptide, and the said substrate polypeptide (if appropriate), and a contaminant wherein the contaminant comprises less than 70% of the composition by weight, preferably less than 50% of the composition, more preferably less than 30% of the composition, still more preferably less than 10% of the composition and most preferably less than 5% of the composition by weight. The invention also includes a preparation comprising the said protein kinase and the said interacting compound, for example polypeptide, and the said substrate polypeptide (if appropriate) when combined with other components ex vivo, said other components not being all of the components found in the cell in which said protein kinase and/or interacting compound, for example polypeptide, and/or substrate polypeptide is naturally found. A further aspect of the invention provides a method of phosphorylating a substrate polypeptide for a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, wherein a preparation according to the preceding aspect of the invention is used. The substrate polypeptide comprises the appropriate consensus sequence for phosphorylation by a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, for example PDKl, PKB, SGK, p70 S6 kinase or PKA, for example the sequence Thr/Ser-Phe-Cys-Gly-Thr-Xaa-Glu-Leu, Phe-Xaa- Xaa-Phe-Ser/Thr-Phe/Tyr, Arg-Arg-X-Ser-Val or Arg-Xaa-Arg-Xaa-Xaa- Ser/Thr. When the protein kinase is PDKl, the substrate polypeptide may be PKB, for example PKBα, SGK, p70S6 kinase, PKA or a PKC isoform. When the protein kinase is p70 S6 kinase, the substrate may be ribosomal 40S subunit S6. When the protein kinase is PKB, SGK or p70S6 kinase, the substrate may be glycogen synthase kinase 3 (GSK3), BAD, 6- phosphofructo-2-kinase, phosphodiesterase 3b, human caspase 9, endothelial nitric oxide synthase or BRCA, for example BRCA2. It will be appreciated that the method may be carried out in the presence of a phosphoinositide, for example PIP 2 or PtdIns(3,4,5)P 3 (PIP 3 ). The said PIP 2 or PIP 3 may increase the rate or extent of phosphorylation of the underlined residue in a substrate polypeptide with an amino acid sequence corresponding to the consensus sequence Phe-Xaa-Xaa-Phe-Ser/Thr- Phe/Tyr and/or of the residue corresponding to the underlined residue in the consensus sequence Thr/Ser-Phe-Cys-Gly-Thr-Xaa-Glu-Leu, for example by PDKl. The substrate may be PKB, for example PKB comprising a functional (ie phosphoinositide-binding) PH domain. A further aspect of the invention provides a method of phosphorylating p70 S6 kinase on the residue equivalent to Thr412 of full length human p70 S6 kinase wherein the said p70 S6 kinase is exposed to PDKl. A further aspect of the invention provides the use of PDKl in a method of phosphorylating p70 S6 kinase on the residue equivalent to Thr412 of full length human p70 S6 kinase. The p70 S6 kinase has a serine or threonine residue at the position equivalent to Thr412 of full length human p70 S6 kinase. The p70 S6 kinase is preferably a naturally occuring p70 S6 kinase, for example full length human p70 S6 kinase, or a fragment or fusion thereof, or a fusion of a fragment thereof, for example as described in Example 1. The p70 S6 kinase and/or the PDKl are preferably recombinant p70 S6 kinase or PDKl, still more preferably recombinant p70 S6 kinase or PDKl expressed in a bacterial, yeast or mammalian cell. The method may be performed in vitro or in a cell. A further aspect of the invention provides a method of identifying a compound that modulates the activation and/or phosphorylation of p70 S6 kinase on the residue equivalent to Thr412 of full length human p70 S6 kinase by PDKl wherein the activation and/or phosphorylation of p70 S6 kinase on the residue equivalent to Thr412 of full length human p70 S6 kinase by PDKl is measured in the presence of more than one concentration (for example in the presence or absence) of the compound. A further aspect of the invention is a compound identified or identifiable by the said method. A further aspect of the invention provides the use of an interacting polypeptide as defined above, for example comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr or an interacting compound of the invention in a method of stabilising a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, wherein the protein kinase is exposed to the compound or polypeptide. Stabilisation may be measured by measuring the TM 50 value. The TMso value is the temperature at which heating for two minutes produces a 50% reduction in protein kinase activity (measured using any appropriate substrate) compared with the protein kinase activity before such heating, as described in Example 2. An increase in the TM 50 value, for example by at least about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 15° C indicates stabilisation. A further aspect of the invention provides a method of modulating in a cell the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, wherein a recombinant interacting polypeptide is expressed in the cell, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or has the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr. Preferences for the protein kinase and interacting polypeptide are as indicated above. For example, GST-PIF may be expressed in a cell, as described in Example 1 and 2. The GST-PIF may inhibit the phosphorylation of p70 S6 kinase by PDKl. Suitably, the method comprises the steps of providing a recombinant polynucleotide suitable for expressing the interacting polypeptide in the cell, providing the recombinant polynucleotide in the cell, and exposing the cell to conditions under which the cell expresses the interacting polypeptide from the recombinant polynucleotide. A further aspect of the invention provides a polypeptide which comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, wherein said polypeptide does not comprise the amino acid sequence Phe/Tyr-Xaa-Xaa- Phe/Tyr-Zaa-Phe/Tyr or Phe/Tyr-Xaa-Xaa-Phe/Tyr-Ser/Thr-Phe/Tyr and is not full-length PKA. The polypeptide may have the C-terminal sequence Phe-Xaa-Xaa-Phe-COOH, or Phe/Tyr-Xaa-Xaa-Phe/Tyr-(X) n -COOH, preferably Phe-Xaa-Xaa-Phe-(X) n -COOH, wherein n is between 1 and 200, 150, 100, 50, 30, 20, 15, 10, 5, 4, 3 or 2, preferably between 1 and 4, most preferably 4. Each amino acid X is any amino acid residue, preferably glycine. Thus, it is preferred that the polypeptide has the C- terminal sequence Phe-Xaa-Xaa-Phe-COOH or Phe-Xaa-Xaa-Phe-(Gly) 4 - COOH. The polypeptide may consist of or comprise contiguous residues derivable from PKA. For example, it may comprise the C-teπninal about 223, 220, 200, 180, 160, 140, 120, 100, 80, 70, 65, 63, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10 or 5 amino acids of PKA, or a variant or fusion thereof that has the C-terminal sequence Phe-Xaa-Xaa-Phe-COOH or Phe/Tyr-Xaa-Xaa-Phe/Tyr-(X) n -COOH. PKA sequences are shown in Figures 15 and 16 and in the database records indicated in Figure 1. The sequence of PKAα, for example, is also shown in Maldonado et al (1988) Nucl Acids Res 16(16), 8189-8190 (Accession no 4506055). Thus, for example, the polypeptide may comprise or consist essentially of the C- terminal 223 or 63 amino acids of PKA, for example human or mouse PKA. The polypeptide may be useful as an interacting polypeptide, as defined above. The said polypeptide of the invention may comprise a GST portion, as described in Examples 1 and 2. This may be useful in purifying and/or detecting the said polypeptide. A further aspect of the invention provides a polynucleotide encoding a polypeptide or mutated protein kinase of the invention. A still further aspect of the invention provides a recombinant polynucleotide suitable for expressing a polypeptide or mutated protein kinase of the invention. A yet further aspect of the invention provides a host cell comprising a polynucleotide of the invention. A further aspect of the invention provides a method of making a polypeptide or mutated protein kinase of the invention, the method comprising culturing a host cell of the invention which expresses said polypeptide or mutated protein kinase and isolating said polypeptide or mutated protein kinase. It will be appreciated that the said polypeptide of the invention that comprises the amino acid sequence Phe/Tyr-Xaa-Xaa- Phe/Tyr, may be isolated as a complex with an endogenous protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, for example PDKl expressed in the cell or with a recombinant said protein kinase expressed in the cell. A further aspect of the invention provides a polypeptide or mutated protein kinase obtainable by the above method. The interacting polypeptide as defined above may have up to about 950, 900, 800, 700, 600, 500, 400, 300, 200, 100, 80, 70, 60, 50, 40, 30, 20, 18, 16, 15, 14, 12, 10, 8 or 7 amino acids residues. It will be appreciated that the polypeptide may comprise a covalent modification, for example it may be modified by biotinylation ie comprise a biotin group. Such a peptide may be useful in the methods of the invention, for example in altering the enzymic activity of a protein kinase, for example PDKl in vitro or in vivo. The above polypeptides may be made by methods well known in the art and as described below and in Example 1 or 2, for example using molecular biology methods or automated chemical peptide synthesis methods. It will be appreciated that peptidomimetic compounds may also be useful. Thus, by "polypeptide" or "peptide" we include not only molecules in which amino acid residues are joined by peptide (-CO-NH-) linkages but also molecules in which the peptide bond is reversed. Such retro-inverso peptidomimetics may be made using methods known in the art, for example such as those described in Meziere et al (1997) J. Immunol. 159, 3230-3237, incorporated herein by reference. This approach involves making pseudopeptides containing changes involving the backbone, and not the orientation of side chains. Meziere et al (1997) show that, at least for MHC class JJ and T helper cell responses, these pseudopeptides are useful. Retro-inverse peptides, which contain NH-CO bonds instead of CO-NH peptide bonds, are much more resistant to proteolysis. Similarly, the peptide bond may be dispensed with altogether provided that an appropriate linker moiety which retains the spacing between the Cα atoms of the amino acid residues is used; it is particularly preferred if the linker moiety has substantially the same charge distribution and substantially the same planarity as a peptide bond. It will be appreciated that the peptide may conveniently be blocked at its N- or C-terminus so as to help reduce susceptibility to exoproteolytic digestion. Thus, it will be appreciated that the interacting polypeptide, for example which comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr to which a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, may be exposed may be a peptidomimetic compound, as described above. A further aspect of the invention is a cell containing a recombinant nucleic acid suitable for expressing a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, and a recombinant nucleic acid suitable for expressing a second polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, wherein when the said protein kinase is PDKl, the said second polypeptide is not PIF, as defined above, and does not comprise the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Ser/Thr- Phe/Tyr. Recombinant polynucleotides suitable for expressing a given polypeptide are well known to those skilled in the art, and examples are described in Examples 1 and 2. It will be appreciated that a recombinant nucleic acid molecule may be suitable for expressing the protein kinase and the second polypeptide comprising the amino acid sequence Phe/Tyr- Xaa-Xaa-Phe/Tyr. The cell is preferably a mammalian or insect cell. A further aspect of the invention is a method of making a preparation comprising a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, and a second, interacting compound, wherein the interacting compound interacts with the hydrophobic pocket of the protein kinase, wherein the said preparation does not comprise all of the components found in a cell in which said protein kinase or compound is naturally found, and wherein when the protein kinase is PDKl, the interacting compound is not a polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr, wherein the said protein kinase and the said interacting polypeptide are co-expressed in a cell as defined in the above aspect of the invention. The protein kinase and the interacting polypeptide may be separated from other cellular components, for example using methods discussed above or in Examples 1 and 2. A further aspect of the invention is a preparation obtainable by the above method of the invention. An antibody reactive towards p70 S6 kinase or a fragment or fusion thereof that is phosphorylated on the residue equivalent to Thr412 of the longer splice variant of human α-isoform of p70 S6 kinase, but is not reactive with p70 S6 kinase or a fragment or fusion thereof that is not phosphorylated on the said residue equivalent to Thr412, is described in Example 1 and is available from Upstate Biotechnology Inc., 199 Saranac Avenue, Lake Placed, NY, USA. A similar antibody is available from New England Biolabs (UK) Ltd, Knowl Piece, Wilbury Way, Hitchin, Herts, SG4 OTY. The antibody may react with the peptide SESANQVFLGFTYVAPSV (corresponding to residues 401 to 418 of the said longer splice variant) in which the underlined residue is phosphorothreonine. Methods of preparing such an antibody are given in Example 1. Antibodies reactive towards the said polypeptides may be made by methods well known in the art. In particular, the antibodies may be polyclonal or monoclonal. Suitable monoclonal antibodies which are reactive towards the said polypeptide may be prepared by known techniques, for example those disclosed in "Monoclonal Antibodies: A manual of techniques ' " , H Zola (CRC Press, 1988) and in "Monoclonal Hybridoma Antibodies: Techniques and Applications", SGR Hurrell (CRC Press, 1982). Techniques for preparing antibodies are well known to those skilled in the art, for example as described in Harlow, ED & Lane, D "Antibodies: a laboratory manual" (1988) New York Cold Spring Harbor Laboratory. It will be appreciated that the invention provides screening assays for drugs which may be useful in modulating, for example either enhancing or inhibiting, the protein kinase activity of a protein kinase (for example, the protein kinase activity towards a particular substrate) having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, for example 4δ PDKl, SGK, PKB, PKA or p70 S6 kinase, for example the PDKl or PDK2 activity (as discussed above) of PDKl . The compounds identified in the methods may themselves be useful as a drug or they may represent lead compounds for the design and synthesis of more efficacious compounds. The compound may be a drug-like compound or lead compound for the development of a drug-like compound for each of the above methods of identifying a compound. It will be appreciated that the said methods may be useful as screening assays in the development of pharmaceutical compounds or drugs, as well known to those skilled in the art. The term "drug-like compound" is well known to those skilled in the art, and may include the meaning of a compound that has characteristics that may make it suitable for use in medicine, for example as the active ingredient in a medicament. Thus, for example, a drug-like compound may be a molecule that may be synthesised by the techniques of organic chemistry, less preferably by techniques of molecular biology or biochemistry, and is preferably a small molecule, which may be of less than 5000 daltons. A drug-like compound may additionally exhibit features of selective interaction with a particular protein or proteins and be bioavailable and/or able to penetrate cellular membranes, but it will be appreciated that these features are not essential. The term "lead compound" is similarly well known to those skilled in the art, and may include the meaning that the compound, whilst not itself suitable for use as a drug (for example because it is only weakly potent against its intended target, non-selective in its action, unstable, difficult to synthesise or has poor bioavailability) may provide a starting-point for the design of other compounds that may have more desirable characteristics. A further aspect of the invention is a kit of parts useful in carrying out a method, for example a screening method, of the invention. Such a kit may comprise a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, for example PDKl, SGK, PKB, PKA or p70 S6 kinase, and an interacting polypeptide, for example a polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr and not comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa- Phe/Tyr or Phe/Tyr-Xaa-Xaa-Phe/Tyr-Ser/Thr-Phe/Tyr. It may further comprise a substrate polypeptide, as defined above. Preferences for the protein kinase, substrate polypeptide and interacting polypeptide are as indicated above. The kit may comprise a mutated protein kinase of the invention. It will be understood that it will be desirable to identify compounds that may modulate the activity of the protein kinase in vivo. Thus it will be understood that reagents and conditions used in the method may be chosen such that the interactions between, for example, the said protein kinase and the interacting polypeptide, are substantially the same as between the human protein kinase and a naturally occuring interacting polypeptide comprising the said amino acid sequence. It will be appreciated that the compound may bind to the protein kinase, or may bind to the interacting polypeptide. The compounds that are tested in the screening methods of the assay or in other assays in which the ability of a compound to modulate the protein kinase activity of a protein kinase, for example a hydrophobic pocket- containing protein kinase, as defined above, may be measured, may be compounds that have been selected and/or designed (including modified) using molecular modelling techniques, for example using computer techniques. A further aspect of the invention provides a method of selecting or designing a compound that modulates the activity of a hydrophobic pocket- containing protein kinase as defined above, the method comprising the step of using molecular modelling means to select or design a compound that is predicted to interact with the said hydrophobic pocket-containing protein kinase, wherein a three-dimensional structure of a compound is compared with a three-dimensional structure of the said hydrophobic pocket and/or with a three-dimensional structure of an interacting polypeptide, as defined above, and a compound that is predicted to interact with the said hydrophobic pocket is selected. Thus, the three-dimensional structure of a compound may be compared with the three-dimensional structure of an interacting polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr. In particular, the structure of the compound may be compared with the structure of the portion (the interacting portion) of the interacting polypeptide that interacts with the hydrophobic pocket, as discussed above and in Example 2, for example the Phe/Tyr-Xaa-Xaa-Phe/Tyr portion of the interacting polypeptide. A compound that mimics the structure of the interacting polypeptide, preferably the interacting portion of the polypeptide, still more preferably the features of the interacting portion that interact with residues of the protein kinase that define the hydrophobic pocket, ie residues equivalent to Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, may be selected. The three-dimensional structure of a compound may be compared with the three-dimensional structure of the hydrophobic pocket. A compound that can interact with the hydrophobic pocket, in particular residues equivalent to Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA, in a similar manner (for example similar separation and/or type of interaction ie hydrophobic or ionic, and/or similar cumulative energy of interaction) to an interacting polypeptide may be selected. Methods of assessing the interaction are well known to those skilled in the art. The three-dimensional structures that are compared may be predicted three-dimensional structures or may be three-dimensional structures that have been determined, for example by techniques such as X-ray crystallography, as well known to those skilled in the art. The three- dimensional structures may be displayed by a computer in a two- dimensional form, for example on a computer screen. The comparison may be performed using such two-dimensional displays. The following relate to molecular modelling techniques: Blundell et al (1996) Stucture-based drug design Nature 384, 23-26; Bohm (1996) Computational tools for structure-based ligand design Prog Biophys Mol Biol 66(3), 197-210; Cohen et al (1990) J Med Chem 33, 8δ3-δ94; Navia et al (1992) Curr Opin Struct Biol 2, 202-210 . The following computer programs, for example, may be useful in carrying out the method of this aspect of the invention: GRID (Goodford (19δ5) J Med Chem 28, 849-857; available from Oxford University, Oxford, UK); MCSS (Miranker et al (1991) Proteins: Structure, Function and Genetics 11, 29-34; available from Molecular Simulations, Burlington, MA); AUTODOCK (Goodsell et al (1990) Proteins: Structure, Function and Genetics 8, 195-202; available from Scripps Research Institute, La Jolla, CA); DOCK (Kuntz et al (1982) J Mol Biol 161, 269-288; available from the University of California, San Francisco, CA); LUDI (Bohm (1992) J Comp Aid Molec Design 6, 61-78; available from Biosym Technologies, San Diego, CA); LEGEND (Nishibata et al (1991) Tetrahedron 47, 8985; available from Molecular Simulations, Burlington, MA); LeapFrog (available from Tripos Associates, St Louis, MO); Gaussian 92, for example revision C (MJ Frisch, Gaussian, Inc., Pittsburgh, PA ® 1992); AMBER, version 4.0 (PA Kollman, University of California at San Francisco, ® 1994); QUANTA/CHARMM (Molecular Simulations, Inc., Burlington, MA ® 1994); and Insight II/Discover (Biosym Technologies Inc., San Diego, CA ® 1994). Programs may be run on, for example, a Silicon Graphics™ workstation, Indigo 2 ™ or IBM RISC/6000™ workstation model 550. The selected or designed compound may be synthesised (if not already synthesised) and tested for its effect on the relevant hydrophobic pocket- containing protein kinase, for example its effect on the protein kinase activity. The compound may be tested in a screening method of the invention. A further aspect of the invention is a compound identified or identifiable by the above selection/design method of the invention. A still further aspect of the invention is a compound (or polypeptide or polynucleotide) of the invention for use in medicine. The compound (or polypeptide or polynucleotide) may be administered in any suitable way, usually parenterally, for example intravenously, intraperitoneally or intravesically, in standard sterile, non-pyrogenic formulations of diluents and carriers. The compound (or polypeptide or polynucleotide) may also be administered topically, which may be of particular benefit for treatment of surface wounds. The compound (or polypeptide or polynucleotide) may also be administered in a localised manner, for example by injection. The compound may be useful as an antifungal (or other parasitic, pathogenic or potentially parasitic or pathogenic organism) agent. A further aspect of the invention is the use of a compound (or polypeptide or polynucleotide) as defined above in the manufacture of a medicament for the treatment of a patient in need of modulation of signalling by a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, for example PDKl, SGK, PKB or p70 S6 kinase, for example insulin signalling pathway and/or PDKl/PDK2/SGK/PKB/p70 S6 kinase/PRK2/PKC/PKA signalling. The patient may be in need of inhibition of a said hydrophobic pocket-containing kinase in an infecting organism, for example the patient may have a fungal infection for which treatment is required. The compound may inhibit the infecting organism's (for example fungal) hydrophobic pocket-containing protein kinase, but may not inhibit the patient's equivalent hydrophobic pocket-containing protein kinase. A further aspect of the invention is a method of treating a patient in need of modulation of signalling by a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and or Lyslll of full-length mouse PKA, for example PDKl, SGK, PKB or p70 S6 kinase, for example insulin signalling pathway and/or PDKl/PDK2/SGK/PKB/p70 S6 kinase/PRK2/PKC/PKA signalling, wherein the patient is administered an effective amount of a compound (or polypeptide or polynucleotide) as defined above. A compound that is capable of reducing the activity of PKC, for example PKCβ, PRKl or 2, PKA, PDKl (ie the PDKl and/or the PDK2 activity), PKB, SGK or p70 S6 kinase may be useful in treating cancer. PDKl, for example via PKB and/or SGK, may be capable of providing a survival signal that protects cells from apoptosis induced in a variety of ways (reviewed in Cross et al (1995) Nature 378, 785-789 and Alessi & Cohen (1998) Curr. Opin. Genetics. Develop. 8, 55-62). Thus, such compounds may aid apoptosis. Reduction of the activity of PDKl, PKB, SGK and/or p70 S6 kinase may promote apoptosis and may therefore be useful in treating cancer. Conditions in which aiding apoptosis may be of benefit may also include resolution of inflammation. A compound is capable of increasing the activity of PDKl, PKB, SGK or p70 S6 kinase may be useful in treating diabetes or obesity, or may be useful in inhibiting apoptosis. Increased activity of PDKl, PKB, SGK or p70 S6 kinase may lead to increased levels of leptin, as discussed above, which may lead to weight loss; thus such compounds may lead to weight loss. For example, such compounds may suppress apoptosis, which may aid cell survival during or following cell damaging processes. It is believed that such compounds are useful in treating disease in which apoptosis is involved. Examples of such diseases include, but are not limited to, mechanical (including heat) tissue injury or ischaemic disease, for example stroke and myocardial infarction, neural injury and myocardial infarction. Thus the patient in need of modulation of the activity of PDKl, PKB, SGK or p70 S6 kinase may be a patient with cancer or with diabetes, or a patient in need of inhibition of apoptosis, for example a patient suffering from tissue injury or ischaemic injury, including stroke. Thus, a further aspect of the invention provides a method of treating a patient with an ischaemic disease the method comprising administering to the patient an effective amount of a compound identified or identifiable by the screening methods of the invention. A still further invention provides a use of a compound identifiable by the screening methods of the invention in the manufacture of a medicament for treating an ischaemic disease in a patient. Thus, a further aspect of the invention provides a method of treating a patient with an ischaemic disease the method comprising administering to the patient an effective amount of a compound identifiable by the screening methods of the invention. If the patient is a patient in need of promotion of apoptosis, for example a patient with cancer, it is preferred that the compound of the invention that is used in the preparation of the medicament is capable of reducing the activity of PDKl, PKB, SGK or p70 S6 kinase. If the patient is a patient with diabetes or a patient in need of inhibition of apoptosis, for example a patient with ischaemic disease, it is preferred that the compound of the invention that is used in the preparation of the medicament is capable of increasing the activity of PDKl , PKB, SGK or ρ70 S6 kinase. The invention further provides a method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, for example PDKl, comprising the steps of (1) determining the effect of a test compound on the protein kinase activity of the said protein kinase, and/or a mutant thereof, and (2) selecting a compound capable of modulating the protein kinase activity of the said protein kinase to different extents towards (i) a substrate that binds to the said hydrophobic pocket of the said protein kinase (hydrophobic pocket-dependent substrate) and (ii) a substrate (such as PKB) that does not bind, or binds to a lesser extent than the first said substrate (hydrophobic pocket-independent substrate), to the said hydrophobic pocket of the said protein kinase. It is preferred that the protein kinase is PDKl. Preferences indicated above apply to this and following aspects of the invention as appropriate. A compound that inhibits the protein kinase activity of the said protein kinase (for example PDKl) to a greater extent towards the hydrophobic pocket-dependent substrate than towards the hydrophobic pocket- independent substrate may be selected. When the protein kinase is PDKl, the hydrophobic pocket-dependent substrate may be SGK, PRK2, S6K1 or PKCζ. The hydrophobic pocket- independent substrate may be PKB. A further aspect of the invention provides a method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and or Lyslll of full-length mouse PKA (for example PDKl), comprising the step of deteraύning the effect of the compound on the protein kinase activity of, or ability of the compound to bind to (1) the said protein kinase mutated at a residue defining at least part of the said hydrophobic pocket of the protein kinase, for example the residue equivalent to lysine 76 of full-length mouse PKA. The method may further comprise determining the effect of the compound on the protein kinase activity of, or ability of the compound to bind to, the protein kinase (for example PDKl) which is not mutated at the said residue defining at least part of the said hydrophobic pocket of the protein kinase. When the protein kinase is PDKl, it may lack a functional PH domain (ie it may lack a PH domain capable of binding a phosphoinositide) . The effect of the compound on the rate or degree of phosphorylation of a hydrophobic pocket-dependent substrate may be determined. A compound may be selected that decreases the protein kinase activity of the said protein kinase, for example PDKl, towards a hydrophobic pocket- dependent substrate and does not affect or increases the protein kinase activity towards a hydrophobic pocket-independent substrate, for example PKB when the kinase is PDKl . An activator of PDKl may niimic insulin and may be useful in treating diabetes or obesity, and may protect cells from apoptosis. A further aspect of the invention provides a kit of parts useful in carrying out a method according to the preceding aspect of the invention, comprising (1) a mutated protein kinase as defined above and/or the protein kinase which is not a mutated said protein kinase as defined above (2) a hydrophobic pocket-dependent substrate and a hydrophobic pocket- independent substrate of the said protein kinase. A further aspect of the invention provides the use of a compound capable of inhibiting to a different extents the rate or degree of phosphorylation by a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA (for example PDKl), of a hydrophobic pocket-dependent substrate than of a hydrophobic pocket-independent substrate of the protein kinase, in the manufacture of a medicament for the treatment of a patient in need of inhibition to different extents of (1) phosphorylation of a hydrophobic pocket-dependent substrate of the said protein kinase and (2) phosphorylation of a hydrophobic pocket-dependent substrate of the said protein kinase. Preferably the protein kinase is PDKl . The compound may be an interacting polypeptide or compound as discussed above. For example, the compound may be PIF when the protein kinase is PDKl . It is preferred that the compound inhibits to a greater degree the rate or degree of phosphorylation by the protein kinase (for example PDKl) of (1) a hydrophobic pocket-dependent substrate of the protein kinase than (2) a hydrophobic pocket-independent substrate of the protein kinase. When the protein kinase is PDKl, the compound may be used to treat diabetes or cancer. The invention will now be described by reference to the following Examples and Figures: Figure Legends Figure 1. PIF prevents PDKl from phosphorylating p70 S6K in vitro. GST-p70 S6K lacking the C-terminal 104 amino acids (GST-p70 S6KT2) (lμg) was incubated for 30 min at 30°C with Mg [γ 32 P] ATP and GST- PDK1 (50nM) in the presence or absence of either wild type (wt) GST- PIF or D97δA GST-PIF (1.5μM), or the indicated PIF peptides (4 μM) in a final volume of 20 μl. The reactions were terminated by making the solutions 1 % in SDS, the samples subjected to SDS-polyacylamide gel electrophoresis, and the phosphorylation assessed by autoradiography of the gel. The position in the gel where GST-p70 S6KT2 (73 kDa) migrates is indicated with an arrow. The only other 32p_ι a belled protein on the gel which is not shown, corresponded to autophosphorylation of PDKl and contained ~10 fold lower levels of 32p_ ra dioactivity than that of the GST- p70 S6T2 phosphorylated by PDKl in the absence of PIF. A high amount of PDKl is used in this experiment to achieve a near maximal phosphorylation of GST-p70 S6T2. If the reactions were carried out at a 10-fold lower concentration of PDKl under conditions where the phosphorylation of GST-p70 S6T2 by PDKl is linear with time and the amount of substrate used, then PIF still prevented the phosphorylation of GST-p70 S6T2 (data not shown), wt indicates wild type and PSer indicates phospho-serine. The results of a duplicate experiment for each condition are shown, and similar findings were obtained in five separate experiments. Figure 2. PDKl phosphorylates p70 S6K at Thr412 in vitro and this is inhibited by PIF. 0.5 μg of either wild type GST-p70 S6K-T2 (wt), T252A-GST-p70 S6K-T2 (252A) or T412A-GST-p70 S6K-T2 (412A) were incubated for 90 min at 30°C with MgATP in the presence or absence of wild type (wt) or kinase-dead (kd) GST-PDK1 expressed in either 293 cells or bacteria in the presence (+) or absence (-) of the wild type PIF peptides (4 μM) in a final volume of 20 μl. The reactions were terminated by making the solutions 1% in SDS, the samples were subjected to SDS-polyacylamide gel electrophoresis, and the phosphorylation of p70 S6K at Thr412 was assessed by immunoblotting with the T-412P antibody. Similar results were obtained in three separate experiments. Figure 3. PIF inhibits p70 S6K activation and phosphorylation at Thr252 and Thr412. 293 cells were co-transfected with constructs expressing the wild type (wt) full length HA-p70 S6K (A) or the full length HA-T412E p70 S6K (B) with either GST-PIF, GST-F977A-PIF, or GST. 24h post transfection the cells were serum starved for lόh and them stimulated for 40 min with 100 nM IGF1. The cells were lysed and HA- p70 S6K was immunoprecipitated and assayed as described in methods. Protein from each lysate (10 μg for the HA blots or 20 μg for the T412-P blot) was electrophoresed on a 10% SDS/polyacrylamide gel and immunoblotted using HA-antibody or the T412-P antibody. The T412-P antibody was incubated with either the synthetic peptide (10 μg/ml) corresponding to residues 401 to 418 of p70 S6K phosphorylated at Thr412 (phosph-412E peptide), or the unphosphorylated peptide (de- phospho-Thr412 peptide). The T412-P antibody consistently recognises a protein termed "non specific band" in cell lysates which migrate at (75 kDa) derived from non transfected and tansfected cells. The intensity of this band does not change with IGF1. It is not co-immunoprecipitated with HA-p70 S6K (date not shown). The HA-p70 S6K activities shown are the average + SEM for a single experiment carried out in triplicate. Similar results were obtained in 8 separate experiments (A) and 2 experiments (B). The immunoblotting was carried out in 3 separate experiments with similar results. Figure 4. PIF does not inhibit PKBα activation or its phosphorylation at Ser473. 293 cells were co-transfected with constructs expressing the wild type full length HA-PKBα with either GST-PIF or GST. 24h post transfection the cells were serum starved for 16h and then stimulated for 15 min with 100 nM IGF1. The cells were lysed and HA-PKBα was immunoprecipitated and assayed as described in Methods. Protein from each lysate (10 μg) was elecfrophoresed on a 10% SDS/polyarylamide gel and immunoblotted using HA-antibody or the S473-P antibody. The HA- PKBα activities shown are the averages _+ SEM for a single experiment carried out in triplicate, similar results were obtained in 3 separate experiments. Figure 5. A kinase-dead PDKl inhibits p70 S6K activation and phosphorylation at Thr252 and Thr412. 293 cells were co-transfected with constructs expressing the wild type (wt) full length HA-p70 S6K (A) full length (A) full length HA-252A p70 S6K (B), full length HA-412A p70 S6K (C) or full length HA-412E p70 S6K (D) with either wild type PDKl, a kinase-dead (kd) mutant or PDKl or the empty pCMV5 vector. 24h post transfection, the cells were serum starved for 16h and then stimulated for 40 min with 100 nM IGF1. The cells were lysed, wild type and mutant forms of HA-p70 S6K immunoprecipitated and assayed as described in methods. The HA-252A p70 S6K or the HA-412A p70 S6K are essentially inactive under all conditions as reported previously [6] (data not shown). Protein from each lysate (10 μg for the HA blots or 20 μg for the T412-P blot) was electrophoresed on a 10% SDS/polyacrylamide gel and immunoblotted using HA-antibody or the T412-P antibody. The T412-P antibody was incubated in the presence of the dephosphorylated peptide corresponding to residues 401 to 41δ of p70 S6K. The HA-p70 S6K activities shown are the average +_ SEM for a single experiment carried out in triplicate. Similar results were obtained in at least 3 separate experiments. Comparable results to the HA and T412-P blots shown here also obtained in at least 3 separate experiments. Figure 6. Quantitative analysis of the binding of PDKl to p70 S6K. Surface Plasmon Resonance measurements were carried out on a BiaCore instrument as described in the Methods. His-PDKl was injected at the indicated concentrations over (A) 2000 RUs of p70 S6K-T2 (closed squares), or 412Ep70 S6K-T2 (closed circles) which was immobolised by amine coupling to a CM5 Sensorchip. Experiments carried out in the presence of either 10 μM wild type PIF peptide (hexagons) or 10 μM D97δA mutant PIF peptide (triangles) are indicated. The responses at steady state binding were recorded. All data are single determinations from a representative experiment that was repeated at least 3 times with similar results. The data on the binding of wild type p70 S6K to PDKl concentrations above 2 μM is not shown, as our analysis of the date suggested that non-specific protein binding was contributing to part of the observed binding response. Figure 7. Two hybrid interaction of PDKl and wild type and mutant C-terminal fragment of PKA (A) The Y190 yeast strain was transformed with vectors expressing PDKl fused to the Gal4 DNA binding domain (GBD), together with vectors encoding either PIF or the wild type or indicated mutants of a C-terminal fragment of PKA (PKACT residues 129-350) fused to a Gal4 activation domain (GAD). As a control, yeast were also co-transformed with the GBD domain alone and the GAD domain alone. The yeast were grown overnight at 30°C and galactosidase filter lift assays performed at 30°C for 4h. An interaction between GBD-PDK1 and GAD-PKAcT induces the expression of β- galactosidase which is detected as a blue colour in the filter lift assay. (B) Alignment of the amino acid sequence of the C-terminal 77 amino acids of PKA with the equivalent region of AGC subfamily kinases indicated. Identical residues are denoted by white letters on black background, and similar residues by grey boxes. The aromatic residues in the hydrophobic motif are indicated by arrows. Figure 8. C-terminal Phe-Xaa-Xaa-Phe residues of PKA interact with a hydrophobic pocket on the PKA kinase domain, predicted to be conserved in PDKl. (A) Ribbon structure of the PKA/PKI/ATP tenary complex [33: Example 2]; PKI and the ATP molecule are indicated. The C-terminal Phe 347 and Phe 350, are indicated. The position of phospho- Thr 197 (the PDKl phosphorylation site) in the T-loop is indicated. (B) Detailed structure of the hydrophobic pocket on the kinase domain of PKA that interacts with the C-terminal Phe-Xaa-Xaa-Phe residues of PKA. Lys76 (equivalent of Lysll5 in PDKl), Leu 116 (equivalent of Leul55 in PDKl), Phe347 and Phe350, and certain amine residues are shown. (C) The structure of the PDKl kinase domain was modelled as described in methods. The region of PDKl equivalent to the hydrophobic pocket of PKA termed the PIF-binding pocket is shown. Residues predicted to be involved in binding to PIF are highlighted. (D) Alignment of the amino acid residues of PDKl around the PIF-binding pocket and the equivalent region of PKA. Identical residues are denoted by white letter on black background and similar residues by grey boxes. Residues on PKA which interact with the C-terminal Phe-Xaa-Xaa-Phe motif are marked with an asterisk. Figure 9. Effect of mutation of conserved residues in the PIF-binding pocket of PDKl on the ability to interact with PIF. 293 cells were transiently transfected with DNA constructs expressing GST-PIF and either wild type Myc-PDKl or the indicated mutants of PDKl. 36 h post transfection the cells were lysed and GST-PIF purified by affinity chromatography on glutathione-Sepharose beads. 2μg of each protein was electrophoresed on a 10% SDS/polyacrylamide gel and either stained with Coomassie blue (A and E) or immunoblotted using an anti Myc antibody to detect Myc-PDKl (B and F). To establish that the wild type PDKl and mutant proteins were expressed at a similar level, lOμg of total cell lysate was electrophoresed on a 10% SDS/polyacrylamide gel and immunoblotted using anti-Myc antibodies (C and G). Duplicates of each condition are shown. Similar results were obtained in 3 to 5 separate experiments. (D) Surface Plasmon Resonance measurements were carried out in a BiaCore instrument as described in the Methods to measure the interaction of wild type and mutant GST-PDK1 preparations with the 24 residue synthetic peptide whose sequence encompasses the PDKl binding site on PIF termed PIFtide [24: Example 2]. PIFtide was immobilised on a SA SensorChip and wild type (wt) or the indicated mutants of PDKl were injected at a concentration of 40nM. All data are single determinations from a representative experiment that was repeated at least 3 times with similar results. For clarity, the bulk, refractive index changes associated with the beginning and end 10 seconds of the injection have been removed. Figure 10. Leul55 mutants of PDKl do not interact with either PIF or the C-terminal fragment of PKA in the two hybrid system. The Y190 yeast strain was transformed with vectors expressing the wild type PDKl or the indicated mutants of PDKl fused to the Gal4 DNA binding domain (GBD) together with vectors encoding for the expression of either the 77 C-terminal residues of PRK2 (PIF) or the C-terminal fragment of PKA (PKA CT residues 129-350 fused to a Gal4 activation domain (GAD). As a control yeast were also co-transformed with vectors expressing the GAD and GBD domains only. The yeast were grown overnight at 30°C and galactosidase filter lift assays performed at 30° for 4h. An interaction between GBD-PDK1 and either GAD-PIF or GAD-PKA CT induces the expression of β-galactosidase which is detected as a blue colour in the filter lift assay. Figure 11. Phosphorylation of Thr308 of PKB by wild type and PIF- binding pocket mutants of PDKl. Wild type or mutants forms of GST- PDK1 were expressed in 293 cells and purified by affinity chromatography on glutathione-Sepharose beads. Each GST-fusion protein (0.2 ng) was incubated for 30 min at 30°C with GST-S473D- PKBα and MgATP in the presence or absence of phospholipid vesicles containing 100 μM phosphatidycholine. 100 μM phosphatidyylserine and 10 μM 5/z-l-stearoyl-2-arachidonoyl-D-PtdIns (3, 4, 5) P3 and the increase in specific activity of GST-S473D-PKBα was determined relative to a control incubation in which the PDKl was omitted (average for 6 determinations, three independent experiments). The basal activity of GST-S473D-PKBα was 1.5 U/mg. Under the conditions used it was verified that the activation of GST-473D-PKBα was proportional to the amount of PDKl added to the assay (data not shown), (-indicates PDKl was omitted. Figure 12. PDKl is activated and stabilised through its interaction with PIFtide. (A) GST-PDKl activity was measured in the presence of increasing concentrations of wild type (wt) PIFtide (closed circles) or a mutant D978A PIFtide (open circles) using the synthetic peptide substrate termed T30δtide, as described in Material and Methods. The data was fitted to a hyperbola using the Kaleidagraph™ software. The connection needed to obtain 50% activation of PDKl was 0.14 μM for wt-PIFtide and 1.1 μM for D97δA-PIFtide. The assay shown was performed in triplicate and there was less than 5% difference between each assay. Similar results were obtained in 2 further experiments (B)- The wild type GST-PDKl (circles) or the L155D mutant of GST-PDKl (squares) was incubated in the presence (closed symbols) or absence (open symbols) of 100 μM PIFtife and then for 2 min at the indicated temperatures, rapidly brought to 0°C (see Materials and Methods), and 2 min later assayed at 30 °C for 10 min using T30δtide as substrate. The activity of PDKl obtained by incubation at 30°C was taken as 100%. The assay shown was performed in duplicate with similar results obtained in two separate experiments. Figure 13. Effect of PIFtide on PDKl pocket mutants. Wild type and the indicated mutants of GST-PDKl were assayed with T30δtide either in the absence (dotted bars) or in the presence of 2 mM PIFtide (dashed bars), or 35 μM PIFtide (filled bars). Under the conditions used the phosphorylation of T30δtide by PDKl was linear with time (data not shown). (A) shows the PDKl mutants which are activated in the absence of PIFtide, and (B) the mutants which are activated by high concentrations of PIFtide. The assay was performed in triplicate with less than 10% difference between triplicate samples. Similar results were obtained in 3 separate experiments. Figure 14. PDKtide is a vastly superior substrate for PDKl than T308tide because it interacts with the PIF-binding pocket of PDKl. (A) His-PDKl was assayed for activity using as substrate the indicated concentration of either PDKtide (open triangles) or T308tide (open circles). (B) HIS-PDK1 was assayed for activity in the presence if PDKtide (25 μM closed triangles) or T308tide (100 μM closed circles) in the presence of the indicated concentrations of PIFtide. The assay was performed in triplicates with less than 5 % difference between the triplicate samples. Similar results were obtained in 3 separate experiments. Figure 15. Alignment of AGC protein kinase family members. The residue equivalent to Lys 76 of mouse PKA (or residue Lys77 of human PKAα) is indicated. The residues equivalent to ValδO, Lyslll and Leullό of mouse PKA are also indicated. The position of the hydrophobic motif Phe/Tyr-Xaa-Xaa-Phe/Tyr is indicated by double lines. Figure 16. Alignment of further AGC protein kinase family members. The residue equivalent to Lys 76 of mouse PKA (or residue Lys77 of human PKAα) is indicated. The residues equivalent to ValδO, Lyslll and Leullό of mouse PKA are also indicated. The position of the hydrophobic motif Phe/Tyr-Xaa-Xaa-Phe/Tyr is indicated by double lines. 6δ Figure 17. Protein used a substrates of PDKl . Figure 18. Phosphorylation and activation of substrates by PDKl PIF pocket mutant Leul55Glu. GST-PDKl and GST-PDKl L155E were tested for their ability to phosporylate and activate the different substrates. PDKl L155E is known to disrupt the hydrophobic PIF pocket. Substrates (0.6 μM) were incubated in vitro in the absence or in the presence of PDKl or PDKl L155E. Activation of substrates was assessed by further incubating the reaction mixture with [γ- 32 P]ATP and the peptide substrate Crosstide. Activation of the substrate protein kinase is observed as a difference between the activity without or in the presence of the stated concentration of PDKl . Phosphorylation of the substrates was quantified by perfoπning the phosphorylation reaction in the presence of [γ- 32 P]ATP, separating the products of the reaction by SDS-PAGE followed by phosphoimager analysis. Parallel experiments were blotted with antibodies that specifically detect the phosphorylated form of the 256 site on S6K1, 252 site on SGK1 and 308 site on PKB. Immunoblots to detect the phosphorylated form of the hydrophobic motif site of S6K1 and PKB under these conditions did not reveal any band (not shown). Under the conditions used, phosphorylation of substrates by PDKl were linear with time and amount of enzyme. Experiments were performed in duplicates at least two times. The results shown correspond to one particular experiment. Duplicates within one experiment did not vary more than 10%, usually less than 5% . Substrates tested were (A) Baculovirus expressed His-tag S6K1 T2 and S6K1 T2 412E, (B) GST-SGK1 and GST-SGK1 422D previously dephosphorylated with PP2A, (C) GST-FL-PKB and GST-FL-PKB 473D, (D) GST-PKB-ΔPH and GST-PKB-ΔPH. Figure 19. Effect of PIFtide on the in vitro phosphorylation and activation of PDKl substrates. Substrates (0.6 μM) were incubated in vitro with GST-PDKl as indicated in the presence or absence of PIFtide (2). Figure 20. Effect of PIFtide on the activation of S6K1 and SGK1 by PDKl PIF pocket mutants (155A, 115A, 119A, 150A). GST-PDKl, GST- PDKl L155E, 155A, 115A, 119A and 150A were tested for their ability to activate His-S6K1 412E and GST-SGK1 422D. The phosphorylation and activation of substrates was performed as described in Fig. 17. When PIFtide was included in the reaction, it was pre-incubated on ice for " 15 min until the reaction was initiated with the addition of ATP-Mg. Figure 21. Interaction of S6K1 and SGK1 with PDKl. 293 cells were transiently transfected with DNA constructs expressing GST, GST-PDKl wt or PDKl L155E together with constructs expressing either wild type or the indicated mutants or truncations of HA tagged S6K1 (A) or wild type GST-ΔN-SGK1 or 422D mutant. 36 h post transfection the cells were lysed and GST fusion protein was purified by affinity chromatography on glutathione-Sepharose beads. Aliquots were electrophoresed on a 10% SDS-polyacrylamide gel, stained with Coomassie Blue, or immunoblotted using an anti-FLAG antibody to detect FLAG-S6K1 and anti-Myc antibody to detect Myc-PDKl. To establish that the wild type and mutant proteins were expressed at similar levels, 10 mg of total lysate was electrophoresed on a 10% SDS-polyacrylamide gel and immunoblotted using the indicated antibodies. Duplicates of each condition are shown. Similar results were obtained in two different experiments. Figure 22. Model for PDKl specificity. Over-expression of substrates of PDKl in 293 cells produces protein kinases which are constitutively phosphorylated (PKCζ and PRK2), others that are phosphorylated and activated within 1-2 minutes of IGF1 stimulation (PKB), and others that are phosphorylated and activated after 10-40 minutes of exposure to the same stimulus (S6K and SGK). Thus, there should be a mechanism that ensures this particular specificity. Here we depict a possible model for the phosphorylation of PDKl substrates that is supported by the results here presented and is in agreement with published observations. In this model, PDKl activity needs not be regulated. Rather, modifications on substrates other than PKB could allow the direct interaction of the C-teπninal hydrophobic motif of these kinases with the PIF binding pocket of PDKl. Interaction with its substrates by this means would be the regulatory step ensuring the temporal and spacial specificity of PDKl. After synthesis, PKCζ would be in a conformation that enables its direct interaction with PDKl and hence it is constitutively phosphorylated []. In 293 cells the overexpression of PRK2 leads to a similarly active (phosphorylated) enzyme, but it is suggested that the interaction of PRK2 with PDKl could be regulated by Rho {]. SGK and S6K must be modified by phosphorylation in order to allow the interaction with PDKl, which prompts their phosphorylation and activation. The main interaction between PKB and PDKl is likely to be dependent on PtdIns(3,4,5)P3 possibly by lipid mediated co-localisation. In PKB interaction, a minor role could be played by PDKl PIF pocket, since ΔPH-PKB phosphorylation is dependent on the hydrophobic motif - PIF binding pocket interaction. Example 1: Evidence that PDKl phosphorylates p70 S6 kinase in vivo at Thr412 as well as Ser252. Abbreviations: PKB, Protein kinase B;PtdIns, Phosphatidylinositol;PI3- kinase, Phosphoinositide 3-kinase; PH, pleckstrin homology; RSK, Ribosomal S6 kinase; MSK, Mitogen and Stress Stimulated kinase; 1,2- SAD-PtdIns(3,4,5)P 3 ,sn-l-stearoyl-2-arachidonoyl-D- PtdIns(3,4,5)P 3 ;C 16 -PtdIns(3,4,5)P 3 ,^-l,2 di-palmitoyl-D- PtdIns(3,4,5)P 3 ;C 16 -RdIns(3,4)P 2 ,57i-l,2 di-palmitoyl-D-PtdIns(3,4)P 2 . In this Example, we demonstrate that PDKl expressed in cells, for example 293 cells or bacteria, is capable of phosphorylating p70 S6 kinase at Thr412 in vitro. We find that PDKl bound to PIF is no longer able to interact with or phosphorylate p70 S6 kinase in vitro at either Thr252 or Thr412. The expression of PIF in cells prevents IGFl from inducing the activation of the p70 S6 kinase and its phosphorylation at Thr412. Overexpression of PDKl in cells induces the phosphorylation of p70 S6 kinase at Thr412 in unstimulated cells, and a catalytically inactive mutant of PDKl prevents the phosphorylation of p70 S6K at Thr412 in IGF1- stimulated cells. These observations provide further evidence that PDKl is one of the kinases that regulates the activation of p70 S6 kinase, and the first evidence that PDKl mediates the phosphorylation of p70 S6 kinase at Thr-412 in cells. Experimental Procedures Materials The peptides used to assay PKBα, (RPRAATF) [23] p70 S6K (GRPRTSSFAEG) [24] and the peptides used to raise and purify the T412-P antibody were synthesised by Dr G. Blomberg (University of Bristol, U.K). Protein G-Sepharose, glutøthione Sepharose and CHX- Sepharose were purchased from Pharmacia; Protease-cocktail tablets from Roche, tissue culture reagents, IGFl and microcystin-LR were from Life Technologies; sensorChips CM5 and SA were from BiaCore AB; biotinylated reagent and secondary antibodies coupled to horse radish peroxidase were from Pierce. Antibodies. The phospho-specific antibody recognising p70 S6K phosphorylated at Thr412 was raised in sheep against the peptide SESANQVFLGFTYVAPSV (corresponding to residues 401 to 418 of the longer splice variant of human α-isoform of p70 S6K), in which the underlined residue is phosphothreonine. The antibody was affinity purified on CH-Sepharose covalently coupled to the phosphorylated peptide. The antibodies were then passed through a column coupled to the non-phosphorylated peptide and the antibodies that did not bind to this column were selected. Monoclonal antibodies recognising the HA or Myc epitope were purchased from Boehringer Mannheim, the monoclonal antibody recognising GST was purchased from Sigma and used to verify the level of expression of GST-PIF in cells, white rabbit polyclonal antibodies recognising the 18 C-terminal residues of PRK2/PIF were purchased from SantaCruz Biotechnology. Preparation of insect cell His-p70 S6K. p70 S6K with a His-epitope tag at its N-terminus lacking the carboxy terminal 104 residues is termed p70 S6K-T2. In order to prepare wild type and the mutant 412E-p70 S6K-T2 the cDNA for these constructs was amplified by PCR from the pMT2 vector encoding these forms of p70 S6K [6] using the following oligonucleotides:5'- AGG ATC CAC CAT GCA CCA TCA CCA TCA CCA TAT GAG GCG AGC AAG GAG GCG G-3' and 5'-GCG GCC GCT CAA CTT TCA AGT AC A GAT GGA GCC-3'. The PCR products were then subcloned into the BamHl/Notl sites of the pFASTBAC 1 vector and this vector was used to generate recombinant baculovirus using the Bac-to-Bac system (Life Technologies Inc). The resulting viruses, encoded p70 S6K-T2 or 412E-p70 S6K-T2 with an N-terminal hexahistidine sequence, and was used to infect Sf21 cells (1.5 x 10 6 /m) at a multiplicity of infection of 5. The infected cells were harvested 72 h post-infection and the His-p70 S6K proteins purified by Ni 2+ /NTA (nitrilotriacetic acid)-agarose chromatography as described previously for PKBβ [25]. They were then dialysed against 50 mM Tris/HCl pH 7.5, 0.1 mM EGTA, 0.27 M sucrose, 0.03% (by vol) Brij-35, 0.15 (by vol) 2-mercaptoethanol, 1 mM benzamidine and 0.2 mM phenylmethylsuphonyl fluoride, snap frozen in aliquots and stored at -80°C. p70 S6K-T2 or 412E p70 S6K-T2 were both recovered with a yield of 60 mg/litre of infected Sf21 cells and were > 90%homogeneous as judged by polyacrylamide gel electrophoresis followed by Coomassie Blue staining. Phosphorylation of GST-p70 S6K by PDKl. GST-PIF and GST- D97δA-PIF were expressed in human embryonic kidney 293 cells, purified on glutathione-Sepharose, and the very small amount of endogenous PDKl associated with GST-PIF was removed by immunoprecipitation with a PDKl antibody [22]. Phosphorylation of GST-p70 S6K-T2 by PDKl was carried out as described previously [7] except that PDKl was incubated with the indicated concentration of GST- PIF or PIF peptide for 10 min on ice prior to initiation of the assay with g[ γ 32pATP]. The wild type GST-p70 S6K-T2, and the mutant T252A- GST-p70 S6K-T2, T412A GST-p70 S6K-T2 proteins were expressed in 293 cells and purified as described previously [7]. Wild type and catalytically inactive GST-PDKl was expressed either in 293 cells [26] or in E.coli [27] Transient Transfection Experiments. The DNA constructs encoding for the wild type and mutant forms of HA-p70 S6K in the pMT2 vector used in this study have been described previously [6]. The constructs encoding wild type HA-PKBα [25]; wild type Myc-PDK [26] and a catalytically inactive mutant of Myc-PDKl, (in which Lys 111 and Asp223 are both mutated to Ala) termed kinase-dead PDKl [26], were all in the pCMV5 vector. The constructs used to express GST-PIF and the mutant GST- F977A-PIF [22] are in the pEBG2T vector. The empty pEBG2T vector was used to express GST protein in control experiments. DNA constructs used in this study were purified from bacteria using the Qiagen plasmid Mega kit according to the manufacturer's protocol. 293 cells cultured on 10 cm diameter dishes in Dulbecco's Modified Eagle's Medium containing 10% (by vol) foetal bovine serum, were transfected with 2μg of DNA construct encoding either wild type or mutant HA-p70 S6K or HA-PKBα, and 10 μg of DNA construct encoding either GST-PIF, GST-F977A-PIF, GST, Myc-PDKl, kinase-dead Myc- PDKl , or the empty pCMV5 vector using a modified calcium phosphate method [2δ] . 24h post transfection the cells were deprived of serum for 16 h, and exposed to IGFl (100 nM) for the time indicated. The cells were lysed in 1 ml of lysis buffer (50 mM Tris/HCl pH 7.5, ImM EDTA, ImM EGTA, 1 % (by vol) Triton X-100, ImM sodium orthovanadate, 10 mM sodium β-glycerophosphate, 50 mM NaF, 5 mM sodium pyrophosphate, lμM microcystin-LR, 0.27 M sucrose and protease cocktail tablets), cleared by centrifugation, and 50 μg of protein was subjected to immunoprecipitation with anti HA monoclonal antibody. The protein concentrations of the lysates were determined by the Bradford method. Kinase assays. The HA-p70 S6K or HA-PKBα immunoprecipitates were washed and assayed for kinase activity using the peptide Crosstide (GRPRTSSFAEG) as described previously for PKBα [2δ]. One unit of activity, U, was that amount which catalysed the phosphorylation of lnmol of substrate in one minute. Immunoblotting for dephosphorylated and Thr412-phosphorylated p70 S6 kinase. Cell lysates were made 1% SDS and the indicated amounts of protein were subjected to SDS/Polyacrylamide gel electrophoresis, subsequently transferred to nitrocellulose and immunoblotted using the indicated monoclonal antibody or the T412-P phospho-specific antibody (0.4 μg/ml) in 50 mM Tris/HCl pH7.5, 0.15M NaCl, 0.5% (by vol) Tween and 10% (by mass) skimmed milk. Detection was performed using the enhanced chemiluminescence reagent (Amersham Pharmacia Biotech). Surface plasmon resonance measurements of PDKl binding to p70 S6 kinase. p70 S6K-T2 and T412Ep70S6K-T2 mutant were amine coupled to a CM5 sensor chip (BIAcore AB) according to the manufacturer's instructions. The indicated concentrations of His-PDKl was injected over the chip at a flow rate of 30 μl/min and the steady-state binding determined, in the presence or absence of PIF peptide. The apparent equilibrium dissociation constant (K^) for the binding of His-PDKl to p70 S6 kinase was determined by fitting the increase in steady-state binding upon increased PDKl concentration to a rectangular hyperbola using SigmaPlot 4 (SPSS Inc). The measure of response in our experiments is termed RU; 1000 RU= 1 ng/mm 2 of protein bound to the surface. Results Phosphorylation of p70 S6K by PDKl is inhibited by PIF. PDKl binds with submicromolar affinity to a region of Protein Kinase C-Related Kinase-2 (PRK2), termed the PDKl -Interacting Fragment (PIF) [22]. PIF is situated C-terminal to the kinase domain of PRK2, and the binding of this region of PRK2 to PDKl is mediated by a consensus motif similar to that encompassing Thr412 of p70 S6K, except that the residue at this position is Asp (Asp97δ), rather than Thr or Ser. In Fig 1, we demonstrate that PDKl when complexed to either GST-PIF or a 24 residue synthetic peptide whose sequence encompasses the PDKl binding site on PIF (PIFtide), was unable to phosphorylate GST-p70 S6K-T2 (a deletion mutant of p70 S6K which lacks the C-terminal 104 residues) in vitro. In a parallel experiment it was verified that PDKl complexed to GST-PIF or the PIF peptide, was able to phosphorylate PKB at both Thr30δ and Ser473 to near stoichiomefric levels (data not shown) as reported previously [22]. GST-p70 S6K-T2 was used as a PDKl substrate (Fig 1) rather than the full length p70 S6K which is very poorly phosphorylated by PDKl in vitro [7,8]. Truncation of the C-terminal 104 residues of p70 S6K is likely to be benign, as p70S6K-T2 when expressed in cells possesses indistinguishable properties to the full length protein as it is still activated by insulin and growth factors in a rapamycin and wortmannin sensitive manner [5+6]. A mutant form of GST-PIF or the 24 residue PIF peptide in which the amino acid equivalent to Asp978 in PRK2 is mutated to Ala (GST-D97δA- PIF), possesses markedly reduced affinity for PDKl [22]. Consistent with this, GST-D97δA-PIF or the mutant D97δA-PIF peptide poorly inhibited the phosphorylation of GST-p70 S6K-T2 by PDKl slightly (Fig 1). If Asp97δ in the PIF peptide is mutated to a phosphoserine residue instead of an Ala, to restore the negative charge, the resulting peptide interacted with PDKl with the same affinity as the wild type PIF peptide [22] and prevented PDKl from phosphorylating GST-p70 S6K-T2 (Fig 1). PDKl phosphorylates p70 S6 kinase at Thr412 in vitro. In order to determine whether PDKl could phosphorylate p70 S6K at Thr412, we raised phospho-specific antibodies that only recognise p70 S6K phosphorylated at Thr412 (termed T412-P antibody). This antibody did not recognise GST-p70 S6K-T2 that had been incubated with MgATP in the absence of PDKl. However, following the addition of PDKl which had either been expressed in 293 cells or bacteria, the GST-p70 S6K-T2 became recognised by the T412-P antibody (Fig2). Incubation of the T412-P antibody with the phosphorylated Thr412 peptide immunogen used to raise the antibody (but not with the dephosphorylated peptide) abolished its recognition of GST-p70 S6K-T2 (see Fig 3). The rate at which PDKl phosphorylated T412 (as well as Thr252) of GST-p-70 S6K-T2 was not increased in the presence of lipid vesicles containing phosphatidylinositol 3,4,5-trisphosphate (data not shown). PDKl phosphorylated the T252A mutant of GST-p-70 S6K-T2 at Thr412 to the same extent as the wild type GST-p70 S6K-T2. The T412A mutant of GST-p70 S6K-T2 was not recognised by the T412-P antibody after incubation with PDKl/MgATP. The 24 residue PIF peptide prevented PDKl from phosphorylating the p70 S6K at Thr412. A kinase-dead mutant of PDKl was unable to phosphorylate GST-p-70 S6K-T2 at Thr412 (Fig 2). The rate at which PDKl phosphorylates Thr412 is likely to be significantly lower than that at which it phosphorylates Thr252. 32 P- labelled GST-p70 S6K-T2 phosphorylated with PDKl was digested with either trypsin or Vδ protease and then subjected to peptide map analysis on HPLC as described previously [7]. This analysis revealed that the major 32 P-labelled peptide containing 20-30% of the total radioactivity applied to the HPLC column corresponding to the peptide phosphorylated at Thr252. Although several minor peptides were present in this analysis 7δ which each comprised <5% of the total phosphate, we were unable to attribute any of these to a peptide phosphorylated at Thr412. This analysis does not exclude the possibility that the recovery of the 32 P-labelled peptide phosphorylated at Thr-412 may be poor but suggests that the stoichiometry at which PDKl phosphorylated p70 SόK at Thr412 is much lower than that which it phosphorylates Thr252. PIF inhibits IGFl-induced activation of p70 S6K. In order to determine whether expression of PIF in cells could prevent the endogenous PDKl from phosphorylating and activating p70 S6K, HA-tagged full length p70 SόK (HA-p70 SόK) was transfected into 293 cells together with constructs encoding either GST-PIF, a mutant form of GST-PIF which interacts with PDKl weakly (GST-F977A-PIF) or GST itself. The wild type or mutant GST-PIF and GST itself were all expressed at a similar level, and were present at a much higher concentration than the endogenous PDKl or PRK2 (data not shown). The cells were subsequently stimulated with IGFl for 40 min (the time at which HA-p70 SόK is maximally activated, data not shown), the cells lysed and the HA-p70 SόK immunoprecipitated and assayed. Cells expressing HA-p70 S6K and GST exhibited a readily measurable basal p70 SόK activity in unstimulated cells, which was increased 10-fold in response to IGFl (Fig 3A). In contrast, cells expressing HA-p70 SόK and GST-PIF, possessed a basal HA-p70 SόK activity that was virtually undetectable, and IGF-stimulation caused only a very slight increase in the HA p70 S6K activity (Fig3A). In cells expressing HA-p70 SόK and GST-F977A-PIF, HA-p70 SόK was substantially activated by IGFl, although not to the same extent as in cells expressing HA-p70 SόK and GST (Fig 3A). This is probably explained by a weak interaction of GST-F977A-PIF with PDKl . PIF inhibits IGFl induced phosphorylation of p70 S6K at Thr412. As PIF inhibited P70 S6K activation in cells, we sought to determine the effect of PIF expression on the phosphorylation of p70 SόK at Thr412 and Thr252. We used the T412-P antibody to measure the phosphorylation of p70 S6K at Thr412. These experiments showed that IGFl triggered the phosphorylation of Thr412 (Fig 3A). This was abolished by incubation of the T412-P antibody with the phosphorylated Thr412 peptide immunogen used to raise the antibody (but not with the dephosphorylated peptide (Fig 3A) or a phosphopeptide corresponding to the sequence surrounding Thr252 (data not shown). Furthermore, a mutant form of HA-p70 S6K in which Thr412 was changed to an Ala was not recognised by the T412-P antibody following IGFl simulation (Fig 5C). When HA-p70 SόK was coexpressed in cells with GST-PIF, IGFl failed to induce the phosphorylation of HA-p70 SόK at Thr412 (Fig 3A). In contrast in cells expressing HA-p70 SόK and the mutant GST-F977A-PIF, the phosphorylation of HA-p70 SόK still occurred but a lower level than that observed in cells expressing HA-p70 S6K and GST. The decrease in Thr412 phosphorylation is consistent with the reduced activation of HA- p70 SόK in these cells compared to those expressing GST alone (Fig 3A). It should be noted however that contransfection of HAp70 SόK with the GST-F977A-PIF mutant induced a 50% maximal activation of P70 SόK, despite inducing a significantly greater reduction in the level of phosphorylation of T412 (Fig 3A). This finding demonstrates that the relationship between p70 phosphorylation at Thr412 and level of p70 SόK activity does not appear to be linear. One explanation for this observation is that the F977A-PIF mutant may inhibit more potently p70 SόK phosphorylation at Thr412 than Thr252; however, thus far we have not been able to raise phosphospecific antibodies recognising p70 SόK phosphorylated at Thr252 to explore this possibility. The overexpression of GST-PIF in cells also abolished the IGFl induced activation and phosphorlation at Thr412 of the p70 S6K-T2 mutant which lacks the C-terminal 104 residues (data not shown). PIF inhibits IGFl-induced phosphorylation of p70 S6K at Thr252. A mutant form of HA-p70 S6K in which Thr412 was altered to glutamic acid to mimic the presence of a phosphorylated residue at this position, possessed an elevated basal activity which was further activated by IGFl when co-expressed with GST or the mutant GST-F977A-PIF (Fig 3B). Previous work has established that the basal and IGFl-stimulated activity of this mutant, is mediated through phosphorylation of Thr252 [6]. In Fig 3B, we demonstrate that co-expression of HA-412E p70 S6K with PIF greatly reduced the basal activity of this mutant and largely prevented its activation by IGFl. This suggests that PIF also inhibits the phosphorylation of p70 SόK at Thr252. The overexpression of PIF cells also greatly reduced the basal and IGFl-stimulated activity of T412E p70 S6K-T2 mutant in cells (data not shown). PIF does not inhibit the activation of PKBα or its phosphorylation at Ser473. Previous work has shown that PIF does not prevent PDKl from phosphorylating PKB in the presence of 3-phosphoinositide lipids but instead enables PDKl to phosphorylate PKB at both Thr30δ and Ser473 (see introduction). Here we show that in marked contrast to the effect of GST-PIF on p70 S5K activation, expression of GST-PIF in 293 cells does not prevent IGFl from inducing a ~20-fold activation of HA-PKBα. This activation is similar to that observed when HA-PKBα is coexpressed with δi GST (Fig 4). Expression of GST-PIF did not inhibit or potentiate the IGFl -induced phosphorylation of HA-PKBα at Ser473 (the residue equivalent to Thr412 in p70 SόK) (Fig 4). GST-PIF is expressed at a similar level when contransfected with PKB and HA-p70 SόK (data not shown), indicating that the inability of PIF to affect the activation of PKB in cells is not due to it being expressed at a low level. A catalytically inactive mutant of PDKl prevents the activation and phosphorylation of p70 S6K. Consistent with earlier findings [7,δ], co-expression of HA-p70 S6K with wild type PDKl induced a large activation of HA p70 SόK which was not increased further by IGFl-stimulation (Fig 5A). We consistently observed a slight decrease in HA-p70 SόK activity in cells overexpressing PDKl following IGF-stimulation. The co-expression of wild type PDKl with HA-p70 SόK or T252A-p70 SόK also resulted in a large increase in Thr412 phosphorylation in unstimulated cells (Fig 5 A & 5B). In contrast, no immunoreative band was detected after immunoblotting with the T412-P antibody, when wild type PDKl and the HA-T412A p70 SόK mutant were co-expressed (Fig 5C). When a kinase-dead mutant of PDKl was co-expressed with HA-p70 SόK, the latter was no longer activated following IGFl-stimulation of cells, nor was it phosphorylated at Thr412 (Fig 5A). In Fig 5D we demonstrate that co-expression of HA-412E p70 SόK with a catalytically inactive PDKl reduced the basal level of HA-412E p70 SόK and largely prevented its activation by IGFl. This provides evidence that the overexpression of a kinase dead PDKl in cells also inhibits the phosphorylation of p70 SόK at Thr252. PIF prevents the interaction of PDKl with p70 S6 kinase. A recent study by Blenis and colleagues [29] reported that, when PDKl and p70 δ2 SόK were contransfected into cells, a small amount of PDKl was coimmunoprecipitated with p70 SόK, suggesting that these proteins may interact directly. Using Surface Plasmon Resonance measurements, we were able to detect a significant interaction (apparent K < , δ μM) between PDKl and p412E-p70 S6K-T2 (Fig 6). This interaction was abolished in the presence of the 24 residue wild type PIF peptide but not the Asp978Ala mutant of the PIF peptide (Fig 6), suggesting that both PIF and 412E-p70 S6K-T2 mutant compete for the same binding site on PDKl . In parallel experiments, a significantly weaker interaction between PDKl and wild type p70 S6K-T2 kinase was detected (Fig 6). Discussion Recent work has shown a high affinity-interaction between PIF and the kinase domain of PDKl, which enhances the rate at which PDKl phosphorylates PKBα and allows it to phosphorylate Ser473 as well as Thr30δ. In this study we have made the surprising observation that PIF prevents PDKl from phosphorylating p70 SόK (Figs 1 and 2) and expression of PIF in 293 cells prevents the IGFl -induced activation of p70 S6K (Fig 3) without affecting the activation of PKBα (Fig 4). Mutant forms of PIF which interact weakly with PDKl were much less effective at inhibiting the phosphorylation of p70 S6K by PDKl in vitro, or at inhibiting the IGFl -induced activation of the p70 S6K. These observations could be explained if p70 SόK, but not PKBα, needed to interact with PDKl at a site which overlaps with the PIF binding site, before p70 S6K can become phosphorylated by PDKl . This conclusion is supported by the findings in Fig 6 that p70 SόK does interact with PDKl , and this interaction is abolished in the presence of the PIF Peptide. The finding that the T412E mutant form of p70 SόK interacts with PDKl with higher affinity than the wild type enzyme, may also explain why the δ3 T412E mutant of p70 SόK was observed in previous studies to be a better substrate for PDKl than the wild type or T412A mutant of p70 S6K [7,δ]. Phosphorylation of PKBα by PDKl is not inhibited by the presence of PIF, and nor could we detect any significant interaction between PKBα and PDKl in vitro by surface plasmon resonance (data not shown). As PKBα and PDKl both interact with 3-phosphoinositides through their PH domains, it is possible that this is the primary determinant for co- localising these molecules at the plasma membrane and hence allowing PDKl to phosphorylate PKBα. In contrast, substrates for PDKl such as p70 SόK, which do not interact with 3-phosphoinositides may actually need to interact with PDKl with relatively high affinity, before they can become phosphorylated. Previous evidence that PDKl is an activator of p70 S6K rested largely on the demonstration that PDKl phosphorylates and activates p70 SόK in vitro and in cotransfection experiments. The finding in this smdy that expression of PIF can prevent the activation of p70 S6K in vivo, presumably by binding to PDKl, provides further evidence that PDKl is required for the activation of p70 SόK in cells. Interaction with PIF converts PDKl into a kinase that is capable of phosphorylating both Thr30δ and Ser473 sites of PKB. This demonstrates that PDKl has the intrinsic ability to phosphorylate the residue in the T- loop as well as the PDK2 motif of a least one AGC kinase family member. As the residues surrounding Thr412 of p70 SόK are highly homologous to those surrounding Ser473 of PKB, it was possible that PDKl, probably in complex with another protein(s), would also possess the intrinsic ability to phosphorylate p70 SόK at Thr252 and Thr412. The present study supports this hypothesis because firstly, overexpression of wild type PDKl triggers the phosphorylation of p70 SόK at Thr412 (Fig 5A). Secondly, the PDKl-catalysed phosphorylation of p70 S6K at Thr412 in vitro was δ4 prevented by PIF (Fig 2). Thirdly, expression with PIF prevents the IGF- induced phosphorylation of p70 SόK at Thr412 in cells (Fig 3A). Finally, the overexpression of a kinase-dead mutant of PDKl in cells not only prevented the activation of p70 S6, as reported by others [δ], but also prevented the phosphorylation of p70 SόK at Thr412 (Fig 5). Taken together, the data suggests that PDKl could phosphorylate p70 SόK at Thr412 in vivo. As PDKl phosphorylation of Thr412 of p70 SόK in vitro is not dependent upon 3-phosphoinositde lipids, it is possible that the sensitivity of PDKl to these lipids in cells is conferred by the interaction of PDKl with another protein. In this respect it should be recalled that the interaction of PDKl with PIF enables PDKl to be directly activated by 3-phosphoinositides [22]. It is also possible that a PDKl -interacting protein (s) could increase the rate at which PDKl phosphorylates both Thr252 and Thr412 of p70 SόK. It has been recently reported that catalytically inactive mutants of PKCλ[30] and PKCξ[29] antagonise the ability of agonists to activate p70 SόK in cells. These observations were interpreted as indicating that PKCλ/PKCξ may have a role in activating p70 SόK in cells. However, PKCλ and PKCζ are both AGC kinase family members which are likely to be activated by PDKl in vivo, and possess an acidic residue rather than Ser/Thr in their PKD2 consensus motif. Furthermore, PKCζ, like PIF has been shown to interact directly with the kinase domain of PDKl [16,18]. It is therefore possible that both PKCλ and PKCζ interact with PDKl in the same way as PIF, and so prevent PDKl from inducing the activation of p70 S6K. Recent work also implicated PKCζ in mediating a rapamycin-sensitive phosphorylation of the novel PKC isoform (PKCδ) at the residue equivalent to Thr412 of p70 SόK [31]. This study did not, however rule out the possibility that PDKl complexed to PKCζ acquires the ability to phosphorylate PKCδ at this residue, rather than PKCζ itself directly phosphorylating this residue. To complicate matters further, it has also recently been shown that conventional PKCα is capable of autophosphorylating itself at the residue equivalent to Thr412 of p70 S6K [32, reviewed 33]. Sabatini and colleagues have reported that mTOR phosphorylates p70 S6K directly at Thr412 [34]. However, much recent evidence suggests that the ability of rapamycin, an inhibitor of the mTor kinase, to suppress the activity of p70 S6K is mediated primarily through the rapamycin-induced activation of an mTor-regulated protein phosphatase which dephosphorylates p70 SόK [35-37]. It will be important to establish whether mTor or any other insulin-stimulated kinase, which can phosphorylate p70 S6K at Thr412 is inhibited by PIF. References 1. Proud, CG. (1995). Trends in Bioch. Sci 21, 181-185. 1. Lane et al (1993). Nature 363,170-172. 2. Jefferies et al (1997) EMBO J. 16, 3693-3704. 3. Pearson et al (1995) EMBO J. 14, 5278-52δ7. 4. Pullen N & Thomas G. (199δ) FEBS LETT.410, 78-δ2. 5. Weng et al (199δ) J. Biol. Chem.273, 16621-16629. 6. Alessi et al (1998) Curr. Biol. 8, 69-81. 7. Pullen et al (1998). Science, 279:707-710. 8. Alessi DR. & Cohen P. (1998). Curr. Opin. Genetics. Develop. 8:55- 62. 9. Mellor H & Parker P.J. (1998) Biochem. J., 332:281-292. 10. Webster et al (1993). Mol. Cell. Biol.13, 2031-2040. 11. Shepherd et al (1998). Biochem. J. 333: 471-479. 12. Belham et al (1999). Current Biol. 9, R93-R96. δ6 13. Alessi et al (1997). Curr. Biol. 7:261-269. 14. Stokoe et al (1997). Science 277:567-570. 15. LeGood et al (199δ). Science 281:2042-2045. lό. Chou et al (1998). Curr. Biol, 8: 1069-1077. 17. Dutil et al (1998). Curr.Biol.S: 1366-1375. 18. Kobayashi T & Cohen P. (1999) Biochem. J. 339, 319-328. 19. Park et al (1999). EMBO 7.18, 3024-3033. 20. Cheng et al (1998). Proc. Natl. Acad. Sci. USA 95:9849-9δ54. 21. Balendran et al (1999). Current Biology 9, 393-404. 22. Alessi et al (1996). FEBS Lett 399, 333-33δ. 23. Leighton et al (1995). FEBS Lett. 375, 2δ9-293. 24. Walker et al (199δ). Biochem. J. 331, 299-30δ. 25. Alessi et al (1997). Curr.Biol.7, 776-7δ9. 26. Casamayor et al (1999). Biochem. J. 342, 2δ7-292. 27. Alessi ef / (1996). £ £0 /.15:6541-6551. 2δ. Romanelli et al (1999) J. Mol. Cell. Biol 19, 2921-292δ. 29. Akimoto et al (199δ) Biochem. J. 273, 16621-16629. 30. Ziegler et al (1999). CHAT. Biol. 9, 522-529. 31. Dutil έtf al (1999) Cwrr. 5tø/. 8, 1366-1375. 32. Peterson et al (1999) Current Biol. 9, R521-524. 33. Burnett et al (199δ). Proc.Natl. Acad. Sci. 95, 1432-1437. 34.Hara et al (199δ) J. Brø/. Chem 273, 22160-2216δ. 35. Peterson <?f al (1999) Proc. Λfø/. ,4αzrf. Sci. 96, 443δ-4442. 37.Sabatini ef α/ (1999). Science 284, 1161-1164. Example 2: Identification of a hydrophobic pocket in the small lobe of the PDKl kinase domain which interacts with PIF and the C-terminal residues of PKA Abbreviations used (other than those defined in Example 1): PKA, cAMP dependent protein kinase; PKACT, C-terminal fragment of PKA composed of residues 129-350; PH, pleckstrin homology; PIF; PDKl interacting fragment. Materials and Methods Materials Complete protease inhibitor cocktail tablets and anti-Myc monoclonal antibodies were from Roche; tissue culture reagents were from Life Technologies: SensorChips SA were from BiaCore AB; biotinylated reagent and secondary anti-mouse IgG antibodies coupled to horse radish peroxidase were from Pierce. Glutathione-Sepharose and ECL reagent were from Amersham Pharmacia Biotech. Peptides: the 24 residue synthetic peptide whose sequence encompasses the PDKl binding site termed PIFtide (REPRILSEEEQEMFRDFDYIADWC), the mutant D978A-PIFtide (numbering based on the human PRK2 sequence REPRILSEEEQEMFRDFAYLADWC), unrelated peptides (YRRAAVPPSPSLSRHSSPHQAEDEEE, and KKVKPPFIPTIRGREDVSNFDDEFT used in control experiments for Fig 6), the PKB specific peptide substrate (RPRAATF), the PDKl peptide substrates T308tide (KTFCGTPEYLAPEVRR), and PDKtide (KTFCGTPEYLAPEVRREPRILSEEEQEMFRDFDYLADWC) were synthesised by Dr G Blomberg (University of Bristol, UK). General Methods Molecular biology techniques were performed using standard protocols. Site directed mutagenesis was performed using QuikChange kit (Stratagene) following instructions provided by the manufacturer. DNA 8δ constructs used for transfection were purified from bacteria using Qiagen plasmid Mega kit according to the manufacturer's protocol, and their sequence verified using an automated DNA sequencer (Model 373, Applied Biosy stems). Human kidney embryonic kidney 293 cells were cultured on 10cm dishes in Dulbecco's Modified Eagle's medium containing 10% foetal bovine serum. Phospholipid vesicles containing phosphatidylicholine, phosphatidylserine and jn-l-stearoyl-2-arachidonoyl- D-Ptdlns (3, 4, 5) P3 [26] were prepared as described previously [13]. PDKl constructs Full length PDKl (residues 1-556), PDKl (residues 52-556). PDKl (residues 52-404), PDKl (residues 1-360) and PDKl (1-426) constructs were expressed in 293 cells with an N-teπninal glutathione S-transferase (GST) tag from the pEBG2T vector [27] and affinity purified on glutathione-Sepharose [14]. The indicated Lysll5, Ilell9, Ginl50 and Leul55 mutants of PDKl used in this study were expressed and purified in a similar fashion. Between 0.5 and 1.0 mg of each GST-fusion protein was obtained by transfection of twenty 10cm diameter dishes of 293 cells and each protein was more than 90% homogeneous as judged by SDS poly aery lamide gel electrophoresis (data not shown). PDKl (residues 52- 556) was also expressed in Sf9 cells with a hexahistidine (His) tag at the N-terminus and purified as described previously [24]. Yeast two-hybrid screen Mye-tagged human PDKl was subcloned into the EcoKl/Sall site of pAS2-l (Clonetech) as a Gal4 DNA binding domain fusion. A yeast two- hybrid screen was carried out by co-transforming pAS2-l PDKl and a pACT2 human brain cDNA library (Clontech) into the yeast strain Y190. Transformed yeast cells were incubated for 10 days at 30°C on SD media supplemented with 25mm 3-aminotriazole and lacking histidine, leucine and tryptophan. Approximately 5 x 10 6 colonies were screened. Yeast two-hybrid analysis Site directed mutants of pAS2-l PDKl (L155D), (L155E) and (L155S) were constructed. Y190 strain yeasts were co-transformed with the indicated combinations of vectors and grown on SD media lacking histidine, uracil, tryptophan and leucine at 30°C until appearance of colonies. Yeast colonies were patched onto fresh media, incubated overnight at 30°C and filter lifts taken, β-glactosidase activity was tested by incubating filters in X-Gal at 30°C for 4h. Structural modelling The structure of the kinase domain of PDKl (residues 92-341) was modelled using the programme Swiss-Pdb Viewer [hhtp://www.expasy.ch/spdbv/main page.htm. [28] connecting to Swiss Model Automated Protein Modelling Server. Modelling was based on several structures of the PKA catalytic subunit available in the database (Protein Data Bank Identification: 1YDR, 1CTP, 1STC, 1ATP and ICDK). Sequence identity to PDKl within the catalytic region (residues 55-297 of mouse PKA) was 40% , with a similarity of 68 % . Binding of PIF to Myc-PDKl A pEBG2T plasmid encoding GST fused to the last 77 residues of PRK2 termed GST-PIF (lOμg) [24] and pCMV5 plasmid expressing Myc-PDKl wild type or the indicated mutants of PDKl (lOμg), were co-transfected into a 10cm diameter dish of 293 cells using a modified calcium phosphate method [29]. 48h post-transfection the cells were lysed in 0.6 ml of lysis buffer (50 mM Tris-HCl pH 7.5, 1 mM EGTA, ImM EDTA, 1 % (by mass) Triton-XlOO, ImM sodium orthovanadate, 50 mM sodium fluoride, 5mM sodium pyrophosphate, 0.27 M sucrose, ImM microcystin-LR, 0.1 % (by vol) β-mercaptoethanol and one tablet of protease inhibitor cocktail per 50 ml of buffer) cleared by centrifugation, and 0.5ml of supernatant was incubated for 2h at 4°C with 30μl of glutathione- Sepharose. The beads were washed twice in lysis buffer containing 0.5 M NaCl, followed by two further washes in lysis buffer. The beads were resuspended in 1 vol of Buffer containing 100 mM Tris/HCl pH 6.8, 4% (by mass) SDS, 20% (by vol) glycerol and 200 mM DTT and subjected to SDS polyacrylamide gel electrophoresis. The gels were either stained with Coomassie blue, or analysed by immunoblotting with anti Myc antibodies. Analysis of PIF-binding to Myc-PDKl Binding was analysed directly by surface plasmon resonance in an upgraded Bia-Lite™ system. PIFtide (comprising the last 24 residues of PRK2) was biotinylated though its C-terminal Cys and bound to an streptavidin-coated Sensor/Chip SA, as described previously [24]. Wild type or mutant preparations of GST-PDKl (10-400 nM) were injected in an intracellular type buffer, over the immobilised biotinylated PIFtide at a flow rate of 30 μl per min as described previously (James et al (1996) Biochem J 315, 709-713). Alternatively, the wild type or mutant preparations of GST-PDKl (1 μM) were incubated with PIFtide or D978A-PIFtide (0.10 μM) and the mixture injected over the immobilised peptides. The decrease is steady state binding between wild type and mutant GST-PDKl and peptide was used to determine the K j of interaction between PDKl and the peptide. The decrease in the maximal response at different concentrations of peptide was used to evaluate the relative affinities of both peptides for PDKl . The sensor chip surface was regenerated by pulses of lOmM NaOH. Measurement of PDKl catalytic activity PDKl's ability to phosphorylate Thr30δ of PKBα was measured using a mutant of GST-PKBα in which Ser473 was mutated to Asp (GST-473D PKBα) in the presence of phospholipid vesicles containing *y«-l-stearoyl-2- arachidonoyl-D-Ptdlns (3, 4, 5)P 3 [13]. The ability of wild type and mutant PDKl to phosphorylate the synthetic peptides T30δtide or PDKtide was carried out in 20μl assays containing 50 mM Tris/HCl pH 7.5, 0.1 % 2-mercaptoethanol lOmM MgCl 2 , lOOμM [γ 32 P]ATP ( " 500 cpm/pmol) 0.5μM microcystin-LR.PDKl and the peptide concentrations indicated under Results. After incubation for 10 min at 30 °C the reaction was stopped by addition of 20μl of 150 mM phosphoric acid. 35μl of the resultant mixture was spotted into Pδl phosphocellulose paper (2 x 2 cm) and the papers washed and analysed as described previously for assays of MAP kinase [30]. Wild type PIFtide or the mutant D97δA PIFtide peptides were included in the reactions as indicated. Control assays were carried out in parallel in which either PDKl, or peptide substrate were omitted; these values were always less than 5% of the activity measured in the presence of these reagents. One Unit of PDKl activity was defined as that amount required to catalyse the phosphorylation of 1 nmol of the T30δtide in 1 min. The assays were linear with time up to a final PDKl concentration of 5 U/ml. Thermal denaturation Heat denaturation was performed by incubating the indicated forms of PDKl (0.4 mg/ml) for 2 min at temperatures ranging from 30 to 65 °C. The heat treatment was terminated by the addition of a 10-fold volume excess of ice cold buffer (50 mM Tris/HCl pH 7.5, 1 mM DTT and 0.1 mg/ml BSA), and the samples incubated for 2 min in an ice-water bath before a 4 μl aliquot was assayed for activity towards T30δtide. Results PDKl interacts with the C-terminal fragment of PKA. A yeast two-hybrid screen was carried out to identify proteins expressed in human brain that interact with PDKl. We identified a clone corresponding to the C- terminal 223 amino acids of PKA (termed PKA CT ) that yielded a positive interaction with full length PDKl (Fig 7A), but not with the PH domain of PDKl (data not shown). PKA CT includes part of the kinase domain as well as amino acids in the C-terminal non catalytic region of PKA that show high sequence homology between AGC subfamily kinases (Fig 7B). The C-teπninal 62 amino acids of PKA possesses significant homology with PIF and terminates in the sequence motif (347-Phe-Xaa-Xaa- PheCOOH). This sequence is similar to the PDKl interacting motif in PIF (974Phe-Xaa-Xaa-Phe-Asp-Tyr979, numbering based on the human PRK2 sequence [24]) except that the Asp residue is replaced by the C- terminal carboxylate group of PKA and the C-terminal Tyr is missing. This suggested that the interaction of PKA CT with PDKl might be mediated by the C-terminal sequence 347-Phe-Xaa-Xaa-PheCOOH. The mutation of either or both of the C-terminal Phe347 and Phe350 to Ala of PKA CT abolished its interaction with PDKl (Fig 7A), but the addition of four Gly residues to the C-terminus of PKA CT to move the free carboxylate group to another position had no effect on the ability of PKA CT to interact with PDKl (Fig 7A). These findings indicate that both Phe residues but not the carboxylate group in this motif are required for the interaction of PKA CT with PDKl. Identification of a putative hydrophobic pocket in the kinase domain of PDKl that interacts with PIF PKA was the first protein whose 3-dimensional structure was solved at high resolution [31] and has established a structural framework for the catalytic domain of most protein kinases [reviewed in 32]. Analysis of the structure of PKA revealed that the non catalytic C-terminus forms a loop that interacts with the kinase domain (Fig δA). Most interestingly, the C- terminal residues of PKA implicated above in binding to the kinase domain of PDKl, interact with a deep hydrophobic pocket in the small lobe of the PKA catalytic domain (Fig 8B). This site does not overlap with the ATP or peptide substrate binding sites on PKA. The residues that make obvious hydrophobic interactions with the two Phe residues in the terminal 347Phe-Xaa-Xaa-Phe motif of PKA are Lys76, Val80, Lysl 11 and Leul 16 of PKA (Fig 2B). A sequence alignment of the kinase domains of PDKl and PKA indicated that the residues equivalent to Lys76 (Lysl 15 on PDKl) and Leullό (Leul 55 on PDKl) of PKA are conserved in PDKl (Fig δD). Molecular modelling of the structure of the kinase domain of PDKl based on that of PKA confirmed that PDKl is likely to possess a hydrophobic pocket in the equivalent region of its kinase domain and that Lysl 15 and Leu 155 in PDKl, are likely to lie in positions equivalent to Lys76 and Leullό in PKA. The residues on PKA equivalent to ValδO and Lyslll which form part of the hydrophobic pocket lie in the same position as Ilell9 and Glnl50, respectively of the PDKl kinase domain. The model of the PDKl kinase domain indicates that these residues, as well as Lysl 15 and Leu 155 may form part of a hydrophobic binding site (Fig 8D). Effect of mutation of Lysll5 and Leul55 on PIF-binding to PDKl The model for the hydrophobic pocket in PDKl predicts that Lysl 15 and Leul55 should participate in a hydrophobic interaction with the residues equivalent to Phe974 and Phe977 of PIF. We therefore mutated Lysl 15 to Ala and Leul55 to Ser, Asp or Glu and compared the ability of these PDKl mutants and wild type PDKl to interact with GST-PIF (Fig 9). As reported previously, a complex was readily observed between GST-PIF and wild type PDKl. In contrast, the K115A interacted very poorly with PIF, whilst none of the L155 mutants interacted significantly with PIF, although these PDKl mutants were expressed to the same level as wild type PDKl (Fig 9C and 9G). Surface Plasmon Resonance (SPR) measurements confirmed a high affinity interaction between wild type GST-PDKl and immobilised, biotinylated synthetic peptide termed PIFtide, whose sequence encompasses the PDKl binding site, as reported previously [24]. However the L155S, L155D and L155E mutants of PDKl had no detectable affinity for PIFtide (Fig 9D), whilst the K115A interacted weakly with PIFtide (Fig 9D). A yeast 2 hybrid screen also confirmed that the L155S, L155D, and L155E mutants of PDKl, failed to interact with PIF (Fig 10). Furthermore, the interaction of PKA CT with the L155S, L155D or L155E mutants of PDKl was greatly reduced in a yeast 2 hybrid screen, further suggesting that the carboxyl terminus of PKA interacts with the PDKl catalytic domain at the same site as PIF (Fig 10). The K115A, L155S, L155D and L155E mutants of PDKl were 50-60% as efficient as wild type PDKl in activating GST-473D-PKBα in the presence of MgATP and Ptdlns (3, 4, 5)P 3 (Fig 11). This indicated that the conformation of the active site of PDKl was not significantly impaired by these mutations. Effect of mutation of Dell9 and Glnl50 on PIF-binding to PDKl Ilell9 and Gin 150, which are also predicted to form part of the PIF- binding pocket on the small lobe of the PDKl kinase domain were mutated to Ala. In both pull down (Fig 9E and 9F) and Surface Plasmon Resonance experiments (Fig 9H) the I119A and Q150A mutants of PDKl interacted very weakly with PIF compared to wild type PDKl. These mutants also activated a GST-473D-PKBα at 60-70% of the rate of wild type PDKl (data not shown). Effect of PIF on the catalytic activity of PDKl towards a peptide substrate A recent study by Dong and colleagues [27] demonstrated that PDKl phosphorylates a synthetic peptide KT*FCGTPEYLAPEV-RR, here termed T30δtide, whose sequence encompasses residues 307 to 320 of PKBα with 2 Arg residues added to the C-terminus to make the peptide bind to Pδl paper. As it is unlikely that T30δtide would interact with the PIF-binding pocket of PDKl, we decided to use this substrate to investigate the effect of PIF-binding on the catalytic activity of PDKl. We confirmed that T30δtide was phosphorylated in vitro by PDKl although the K m was very high (> 10 mM). We also established that T30δtide was phosphorylated at the residue equivalent to Thr30δ of PKBα (indicated by an asterisk), by solid phase sequencing of 32 P-labelled T30δtide phosphorylated by PDKl (data not shown). PDKl activity towards T30δtide was increased up to 4-fold in the presence of PIFtide. The concentration required for half-maximal activation was 0.14 μM (Fig 11 A) which correlates with the affinity of PDKl for PIFtide (Kj of " 0.3 μM [24]). This increase in PDKl activity was observed with either full length PDKl or forms lacking the N- terminal or C-terminal non-catalytic regions (data not shown). The effects of PIFtide on PDKl activity were unaffected by preincubating these components for up to 30 min on ice prior to initiating the assay. Similarly, a mutant D97δA-PIFtide, which exhibits a 10-fold reduced affinity for PDKl [24], was δ-fold less effective at inducing PDKl activation (Fig 12A). Several unrelated peptides of similar size were unable to induce any activation of PDKl (data not shown). This strongly indicates that PDKl is activated directly by PIF. Furthermore, PIF did not alter the K,,, of PDKl for ATP (data not shown). GST-PDKl activity was reduced by 50% if the enzyme was heated for 2 min at 50°C (TM 50 value, Fig 12B). However, PDKl was stabilised in the presence of wild type PIFtide, the TM50 being increased by δ-10°C. PIF also caused a 6-10°C increase in the TM 50 value for all GST-PDKl transcription mutants tested which either lack the PH domain, the N- terminal 51 residues or both non-catalytic domains (data not shown). The L155D mutant of GST-PDKl was more heat labile than wild type PDKl with a TM 50 value of 42°C. As expected, PIF did not significantly stabilise this mutant (Fig 12B). Activity of PDKl mutants towards T308tide We next tested the specific activities of the PIF-binding pocket mutants towards T308tide. The L115A, L155S, L155D, and L155E mutants of PDKl phosphorylated T30δtide 3 to 5-fold more rapidly than wild type PDKl, ie at a rate similar to that of wild type PDKl in the presence of PIF (Fig 7). PIFtide did not further activate these mutants consistent with their inability to bind PIF. In contrast, the L119A and Q150A mutants of PDKl possessed a specific activity similar to wild type PDKl and were stimulated -2-fold in the presence of PIF. However, 10-fold more PIF was required for maximal activation compared to wild type PDKl, consistent with the reduced affinity of these mutants for PIF (Fig 9). PDKtide is a vastly superior peptide substrate for PDKl The results presented above suggested that a peptide substrate for PDKl might be phosphorylated with a much lower K^ value if it also contained the PDKl interacting sequence of PIF. We therefore synthesised a 39 amino acid polypeptide composed of T30δtide fused to PIFtide, and termed it PDKtide. This peptide was a vastly superior substrate for PDKl than T30δtide; its K m was -δOμM (compared to > 10 mM for T30δtide) and when assayed at lOOμM, PDKtide was phosphorylated at a rate over 100-fold greater than that using T30δtide (Fig 14A). The activity of PDKl towards PDKtide was inhibited by inclusion of PIFtide in the assay, in contrast to T30δtide phosphorylation which was stimulated by PIFtide (Fig 14B). Discussion The C-terminal residues of PKA, Phe-Xaa-Xaa-PheCOOH, correspond to part of the PDKl binding motif of PIF. These residues are known to interact with the small lobe of the kinase domain of PKA at a location distinct from the ATP or peptide substrate binding sites (Fig 2). In this paper we demonstrate that PKA CT also interacts with the kinase domain of PDKl in a yeast 2 hybrid screen, and that mutation to Ala of the residues 9δ in PKA CT equivalent to Phe347 or Phe350 abolishes/significantly reduces its interaction with PDKl (Fig 7). As the mutation of the equivalent Phe residues to Ala on PIF also abolishes its interaction with PDKl [24], these findings suggested that the PKA CT and PIF might interact at the same site in the PDKl kinase domain. The residues in the kinase domain of PKA known to interact with the C- terminus of this protein are present in PDKl, and their mutation either abolished or significantly diminished the interaction of PDKl with both PIF and PKA CT . These observations strongly suggest that PDKl possesses an equivalent hydrophobic pocket in its kinase domain that interacts with PIF and PKA CT . PDKl is itself a member of the AGC subfamily of protein kinases but, in contrast to PKA, it does not possess a hydrophobic Phe-Xaa-Xaa-Phe- motif at the equivalent position. PDKl is therefore likely to possess an unoccupied PIF-binding pocket in its kinase domain which is available to interact with the C-terminal hydrophobic motifs of PKA and other AGC subfamily members. The interaction of PIF with PDKl converts it from an enzyme that only phosphorylates PKBα at Thr30δ to a form that phosphorylates both Thr30δ and Ser 473 in a Ptdlnds (3, 4, 5)P 3 or Ptdlns (3, 4) P 2 dependent manner [24]. The PDKl binding motif in PIF (Phe-Xaa-Xaa-Phe- Asp- Tyr) could therefore be required as a pseudosubstrate sequence raising the possibility that PIF interacts with the substrate binding site of PDKl. However, if this were the case PT would be expected to prevent PDKl from phosphorylating PKBα at Ser473 rather than promoting this reaction. The finding that PIF interacts with a site on the kinase domain of PDKl which is distinct from the substrate binding site explains why this is not the case, and suggests that PIF may be capable of inducing conformational changes in the PDKl catalytic core which alter its substrate specifically. In order to assess the effect of PIF on the intrinsic catalytic activity of PDKl we used the peptide substrate T30δtide rather than a protein substrate of PDKl such as p70 S6 kinase which may interact with PDKl at a site that overlaps the PIF-binding pocket (see Example 1). Using this assay, we demonstrated that the PIF-binding pocket was likely to be important in regulating the activity of PDKl . When unoccupied, the PIF- binding pocket appears to suppress the activity of PDKl, because the mutation of key residues that form it, Lysl 15 and Leul55 enhanced PDKl activity towards T30δtide to the level equivalent to that of wild type PDKl in the presence of PIF (Fig 6). It is therefore likely that the binding of PIF transduces an allosteric transition which stabilises a functionally active conformation of PDKl . The interaction of PIF with PDKl requires an Asp residue (Asp97δ) at the position equivalent to Ser473 of PKBα. An interesting possibility was that the C-terminal carboxylate group of the Phe-Xaa-Xaa-PheCOOH motif of PKA CT may have played an analogous function to Asp97δ of PIF to enable binding to PDKl. However, this does not seem to be the case as the addition of four glycines to the C-terminus of PKA CT did not affect its interaction with PDKl. The C-terminal carboxylate group of Phe350 of PKA does not form any interaction with the hydrophobic pocket on the kinase domain of PKA but instead faces outwards from this site and forms a hydrogen bond with Gln35 in the N-terminal non-catalytic region of PKA [33]. The importance of this interaction has not yet been investigated by mutating Gln35 of PKA. Similarly, it is possible that Asp978 of PIF may not interact with the PIF binding pocket, but to a distinct region of PDKl . Sequence alignment of PKB, SGK and p70 S6 kinase indicates that these members of the AGC subfamily of kinases are also likely to possess a PIF- binding pocket in their kinase domains. These kinases are all activated by phosphorylation of a Ser/Thr residue at the position equivalent to Asp978 of PIF. It is therefore possible that the introduction of a negative charge at this site by phosphorylation causes the residue of this motif to interact with their own PIF-like binding pockets thereby leading to increased activity and stability, in a similar manner to the way in which PIF activates and stabilises PDKl. The observation that phosphorylation of the same site increases the stability of conventional PKC isoforms is consistent with this consensus[8]. The PIF-binding pocket may be the site that enables PDKl to interact with its substrates. This interaction may also induce a conformational change which enhances the rate at which these substrates are phosphorylated by PDKl. For example, the interaction of PKA with PDKl via the C- terminal Phe-Xaa-Xaa-PheCOOH motif of PKA may facilitate the phosphorylation of PKA at Thrl97. However, we have recently shown that PDKl is unable to interact with or phosphorylate p70 S6 kinase in the presence of PIF [25] and this is also true for SGK, PRK2 and PKCζ (data not shown). This suggests that p70 S6 kinase and SGK may require to interact with PDKl at a site that overlaps with the PIF-binding pocket in order to become phosphorylated [25]. P70S6 kinase when phosphorylated at its hydrophobic motif interacted with PDKl with much higher affinity. PKCζ is another protein kinase that interacts with PDKl [17, 18], which, like PRKl, PRK2 and PKCα, possesses an acidic residue rather than a Ser/Thr in the C-terminal hydrophobic motif. It is therefore likely that this region of PKCζ interacts with the PIF-binding pocket of PDKl, and this interaction enables PDKl to phosphorylate and hence to activate PKCζ. This is shown by Balendran et al (2000) J Biol Chem 275(27), 20806-20δl3 and discussed in Biondi et al (2000) EMBO J 19(5), 979-9δδ (both specifically incorporated herein by reference). Thus, the C-termini of these kinases may be acting as PDKl "docking sites". Consistent with the Phe-Xaa-Xaa-Phe-Asp-Tyr motif of PIF being a docking site for PDKl the addition of this motif to T30δtide greatly increases the rate at which it is phosphorylated by PDKl (Fig 14) PRKl, PRK2, PKCζ, and PKCi may have another role, to allow PDKl to phosphorylate PKB and other members of the AGC subfamily at the site equivalent to Ser473 on PKB [12, 24]. In summary, PDKl appears to possess a hydrophobic binding site in the small lobe of the kinase catalytic domain which regulates its activity as well as its interaction with substrates. These findings raise the possibility of developing novel drugs that interact with the PIF-binding pocket on PDKl. Such drugs could either activate or inhibit PDKl by modulating its interaction with particular substrates, and thus could switch on or switch off signal transduction pathways that are regulated by PDKl . Thus T30δtide could be used as a substrate to identify compounds that activate PDKl by mimicking the effect of PIF, while PDKtide may be the peptide of choice to identify compounds that disrupt the interaction of PDKl with PIF. References 1. Leevers et al (1999) Curr Opin Biol 11, 219-225 2. Shepherd et al (199δ) Biochem J 333: 471-479 3. Alessi D R and Downes C P (199δ) Biochem Biophys Acta 1436, 151-164 4. Proud C G (1995) Trends in Biochem Sci 21, 181-185 5. Pullen N & Thomas G, (1998) FEBS LETT 410, 78-δ2 6. Kobayashi T & Cohen P (1999) Biochem J 339, 319-32δ 7. Park et al (1999) EMBO J18, 3024-3033 δ. Mellor H and Parker P J, (199δ), Biochem J, 332, 2δl-292 9. Alessi, D R and Cohen P (199δ) Curr Opin Genet Dev 8, 55-62 10. Pearson et al (1995) EMBO J 14, 5278-52δ7 11. Belham et al (1999) Current Biol 9, R93-R96. 12. Peterson, R T & Schreiber, S L (1999) Current Biol 9, R521-524 13. Alessi et al (1997) Curr Biol 7, 261-269 14. Alessi et al (1997) Curr Biol 7, 776-7δ9 15. Stokoe et al (1997) Science 277, 567-570 16. Stephens et al (199δ) Science 279, 710-714 17. LeGood et al (199δ) Science, 281, 2042-2045 18. Chou et al (1998) Curr Biol, 8, 1069-1077 19. Dutil et al (1998) Curr Biol, , 8, 1366-1375 20. Alessi et al (1998) Curr Biol 8, 69-81 21. Pullen et al (199δ) Science, 279, 707-710 22. Chen et al (199δ) Proc Natl Acad, USA 95, 9δ49-9δ54 23. Etchebehere et al (1997) Eur J Biochem, 248, δ20-δ26 24. Balendran et al (1999) Current Biology 9, 393-404 26. Gaffhey P R J & Reese C B (1997) Bioorg Med Chem Lett, 7, 3171-3176 27. Sanchez et al (1994) Nature, 372, 794-79δ 2δ. Guex N and Peitsch M C (1997) Electrophoresis, 18, 2714-2723 29. Alessi et al (1996) EMBO J 15, 6541-6551 30. Alessi et al (1994), Methods ofEnzymology 255, 279-290 31. Knighton et al (1991) Science 253, 407-414 32. Taylor, S S & Radzio-Andzelm E (1994) Structure 2, 345-355 33. Zheng et al (1993) Biochemistry 32, 2154-61 34. Holland P M and Cooper J A (1999) Curr Biol 9, R329-R331 35. Jacobs et al (1999) Genes and Development, 10, 163-175 Example 3: PDKl hydrophobic PIF pocket is essential for phosphorylation and activation of S6K and SGK but not PKB In this example we demonstrate that the PIF binding pocket of PDKl plays a key role in enabling PDKl to phosphorylate and activate p70 ribosomal S6 kinase (S6K)[6,7] S6K1, serum and glucocorticoid induced kinase-1 (SGK)[8-10] SGK1 and mutant of PKBα that lacks its PH domain (ΔPH- PKBα). We also demonstrate that the hydrophobic motif of S6K1, SGK1 and ΔPH-PKBα plays a key role in allowing the kinases to become phosphorylated by PDKl in vitro and in vivo. In contrast neither the PIF binding pocket of PDKl or the hydrophobic motif of PKBα are required for the phosphorylation of PKBα by PDKl, in the presence of phosphatidylinositol(3,4,5)P 3 . We also provide evidence that non- phosphorylated forms S6K1 and SGK1 which are poor substrates for PDKl do not interact with PDKl. Removal of the C-terminal autoinhibitory domain of S6K1 enables PDKl to interact and phosphorylate S6K1. A mutation of SGK1 that mimics phosphorylation at its hydrophobic motif, also enables PDKl to interact and phosphorylate it. We suggest a model by which phosphorylation of PDKl substrates thus far been identified other than PKB are regulated by the direct interaction of their hydrophobic motif with the PIF binding pocket of PDKl . PKB is activated usually within 2 minutes of a cell being stimulated with insulin and growth factors [11-13]. It possesses an N-terminal plekstrin homology (PH) domain that interacts with PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 resulting in the recruitment of PKB to the plasma membrane where it becomes activated by the phosphorylation of 2 residues. One lies in the T- loop of the kinase domain (Thr30δ in PKBα) and the other is located C- terminal to the catalytic domain, in a region termed the "hydrophobic motir (Ser473 in PKBα) [14]. S6K [6] and SGK [δ-10] also possess residues equivalent to Thr30δ (Thr252 in S6K1 and Thr256 in SGK1) and Ser473 (Thr412 in S6K1 and Thr422 in SGK1) whose phosphorylation is required for activation of these kinases in vivo. The phosphorylation SόK and SGK at both its T-loop and hydrophobic motif like that of PKB, is dependent upon PI 3-kinase activation. In contrast to PKB, S6K and SGK do not possess a PH domain and do not interact with PtdIns(3,4,5)P 3 / PtdIns(3,4)P 2 . SόK and SGK are also activated markedly slower than PKBD following cell stimulation, with maximal activation occurring after 10-40 minutes [9, 10, 12]. PKB, S6K1 and SGK are phosphorylated at their T-loop by the 3- phosphoinositide-dependent protein kinasel (PDKl) [14]. This enzyme is also an AGC family member, and possess a PtdIns(3,4,5)P 3 / PtdIns(3,4)P 2 binding PH domain C-terminal to the catalytic domain. Following, PI 3- kinase activation, PDKl and PKB are thought to co-localise at the plasma membrane through their interactions with PtdIns(3,4,5)P 3 / PtdIns(3,4)P 2 . In addition to recruiting PKB to membranes of cells the binding of PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 to the PH domain of PKB may induce a conformational change that enables PDKl to phosphorylate it [14]. As S6K and SGK do not interact with PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 , nor is the rate at which these are phosphorylated by PDKl in vitro enhanced in the presence of PtdIns(3,4,5)P 3 / PtdIns(3,4)P 2 [9, 15], the mechanism by which activation of PI 3-kinases induces activation of SόK and SGK must be distinct from PKB. The kinase domain of PDKl was found in a yeast 2 hybrid screen to interact with a region of the protein kinase C-related kinase-2 (PRK2), termed the PDKl Interacting Fragment (PIF) [16]. PIF is situated C- teπninal to the kinase domain of PRK2, and contains a hydrophobic motif (Phe-Xaa-Xaa-Phe-Asp-Tyr), similar to that found in PKBα(Phe-Xaa-Xaa- Phe-Ser-Tyr), except that the residue equivalent to Ser473 is Asp. Mutation of the conserved aromatic residues in the hydrophobic motif of PIF or mutation of the Asp residue to either Ala or Ser prevented the interaction of PIF with PDKl [16]. Subsequent work demonstrated that a 24 amino acid fragment of PIF (termed PIFtide), that encompasses the hydrophobic motif of PRK2 bound to a hydrophobic pocket on the small lobe of the PDKl kinase domain which was termed, the "PIF binding pocket" [17], Three lines of evidence indicate that this interaction could serve as a "docking site" , enabling the recruitment of PDKl to PRK2 which is essential for its phosphorylation by PDKl. Firstly, a PDKl mutant in which the central residue (Leul55) in the PIF-binding pocket is mutated so that PDKl can not interact with PIF [17] possessed greatly reduced affinity for PRK2 [18]. Secondly, overexpression of PIF in 293 cells prevented the phosphorylation of PRK2 at its T-loop residue. Thirdly, mutation of a conserved Phe to Ala on the hydrophobic motif of PRK2 greatly reduced the affinity of PRK2 for PDKl and furthermore, this mutant was not phosphorylated at its T-loop residue when expressed in cells [18]. Similar findings were made for another PDKl substrate namely, protein kinase Cζ (PKCζ), which is similar in structure to PRK2 and also possesses a acidic residue in its hydrophobic motif at the residue equivalent to Ser473 of PKBαflδ]. In addition it is likely that the interaction of PDKl with the hydrophobic motif of PRK2 and PKCζ will directly activate PDKl, as PIFtide increased 4-fold the rate at which PDKl phosphorylated a peptide substrate that is derived from the T-loop of PKBα (T308tide) [17]. Furthermore, T308tide is a very poor substrate for PDKl, but if it is fused to PIFtide (PDKtide) it becomes a vastly superior substrate [17]. Recently, Frodin and colleagues [19] have also demonstrated that PDKl interacts with another AGC kinase substrates termed p90RSK only when it is phosphorylated at its hydrophobic motif and present evidence that this interaction recruits PDKl to p90RSK and may also activate PDKl. Here we investigated the role of the hydrophobic PIF binding pocket on PDKl, in enabling PDKl to phosphorylate and activate 3 of its AGC kinase substrates that are activated in response to insulin, namely, S6K1, SGK1, PKB as well as a mutant of PKBα that lacks its PH domain (ΔPH- PKBα) which like S6K and SGK does not interact with PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 . Our data indicate that the PIF binding pocket of PDKl plays a critical role in enabling PDKl to phosphorylate and activate S6K1, SGK1 and ΔPH-PKBα but not wild type PKBα. Our results suggest model in the phosphorylation of PDKl substrates other than PKB, would be regulated by the ability of the hydrophobic motif of these substrates to interact with PDKl. Materials and Methods Materials. Complete protease inhibitor cocktail tablets and anti-Myc monoclonal antibodies were from Roche; tissue culture reagents and microcystin-LR were from Life Technologies; glutathione-Sepharose and ECL reagent were form Amersham Pharmacia Biotech. Precast gradient SDS polyacrylamide gels were from invitrogen. Antibodies. The characterisation of the phospho-specific antibodies recognising SGK phosphorylated at its T-loop (Thr256) termed T256-P has been described previously [J. The phospho-specific antibody recognising PKBα phosphorylated at Thr30δ (termed T30δ-P) was raised in sheep against the peptide KDGATMKTFCGTP (corresponding to residues 301 to 313 of the human PKBα), in which the underlined residue is phosphothreonine. The antibody recognising S6K1 phosphorylated at Thr229 was raised in sheep against the peptide HDGTVTHTFCGTIEY (corresponding to residues 245 to 259 of long splice variant of human S6K1) in which the underlined residue is phosphothreonine. The antibodies were affinity purified on CH-Sepharose covalently coupled to the phosphorylated peptide, then passed through a column of CH- Sepharose coupled to the non-phosphorylated peptide. Antibodies that did not bind to the latter column were selected. Monoclonal antibody recognising the Myc epitope was from Roche, the monoclonal antibodies recognising GST and the FLAG epitope were purchased from Sigma. Horse radish peroxidase conjugated secondary antibodies were from Pierce. General methods. Molecular biology techniques were performed using standard protocols. Site directed mutagenesis was performed using a QuickChange kit (Stratagene) following instructions provided by the manufacturer. DNA constructs used for transfection were purified from bacteria using Qiagen plasmid Mega kit according to the maufacturer's protocol, and their sequence verified. Human kidney embryonic 293 cells were cultured on 10 cm diameter dishes in Dulbecco's modified Eagle's medium containing 10% foetal bovine serum. Transfections were performed using a modified calcium phosphate method and 10 μg of each plasmid per dish. Phospholipid visicles containing phosphatidiylcholine, phosphatidylserine and sn-l-stearoyl-2-arachidonoyl-D-PtdIns(3,4,5)P 3 [20] were prepared as previously described [21]. Buffers. Buffer A : 50 mM Tris-HCl pH 7.5, 1 mM EGTA, 1 mM EDTA, 1 % (by mass) Triton-X 100, 1 mM sodium orthovanadate, 50 mM sodium fluoride, 5 mM sodium pyrophosphate, 0.27 M sucrose, 1 μM microcystin-LR, 0.1 % (by vol) β-mercaptoethanol and 'complete' proteinase inhibitor cocktail (one tablet per 25 ml, Roche). Buffer B: 50 mM Tris/HCl pH 7.5, 0.1 mM EGTA, 10 mM β-mercaptoethanol and 0.27M sucrose. Protein expression and purification. Wild type-PDKl [22], PDK1[L155E], PDK1[K115A], PDK1[I119A], PDK1[Q150A] [17], and PDK1[L155A] [18], in the pEBG2T vector was used to express the wild type and indicated mutants of PDKl fused through their N-teπninus to glutathione S-transferase (GST). Wild type PDKl [22] and mutant PDK1[L155E] [17] in the pCMV5 vector was used to express these proteins with an N-terminal Myc tag. All S6K1, SGK1 and PKBα substrates employed in this study are illustrated in Fig 17. All S6K1 mutants lacking the C-terminal 104 residues are termed S6K1-T2. N-terminal Flag epitope tagged S6K1, S6K1-T2, S6K1[T412E], S6K1-T2[T412E] pCMT2-T2-S6Kl[T412E] were expressed in the pCMT2 vector [23]. N-terminal GST tagged S6K1, S6K1-T2 [9], S6K1-T2[F411A] were expressed in the pEBG2T vector. All SGK1 mutants expressed in this study lack the N-terminal 60 amino acids. N-terminal HA epitope tagged SGK1, SGK1[T422E] [9], SGK1[F421A] were expressed in the pEBG2T vector. N-terminal GST-tagged PKBα [21], PKBα [S473D] [16], PKBα [F472A], ΔPH-PKBα [22], ΔPH-PKBα[S473D], ΔPH-PKBα[F472A] were expressed in the pEBG2T vector. The GST fusion proteins were expressed in human embryonic kidney 293 cells. For the expression of each construct, twenty 10 cm diameter dishes of 293 cells were cultured and each dish fransfected with 10 μg of the pEBG-2T construct, using a modified calcium phosphate method [24] . 24 h post-transfection, the cells were deprived of serum for 16h and then lysed in 0.6 ml of ice-cold Buffer A, the lysates pooled, centrifuged at 4°C for 10 min at 13, 000 x g and the GST-fusion proteins were purified by affinity chromatography on glutathione-Sepharose and eluted in 50 mM Tris pH 7.5, 0.1 mMEGTA, 0.27 M Sucrose, 0.1 % (by vol) 2- mercaptoethanol and 20 mM glutathione as described previously [21]. Typically between 0.3 and 1.0 mg of each GST-fusion protein was obtained and each protein was more than 75 % homogeneous as judged by SDS polyacrylamide gel electrophoresis (data not shown). SGK1[T422E] when expressed in 293 cells is phosphorylated at Thr256 [9] and the purified GST-SGK1[T412E] was dephosphorylated by incubation with PP2A 30 mU/ml at 30 °C for one hour and the reaction was teπninated by the addition of microcystin-LR (lμM) the samples were left at 30 °C for a further 5 min and frozen in liquid nitrogen and stored at -80°C until required. Although, GST-SGK1 is not phosphorylated at Thr256, this was also subjected to treatment with PP2A to enable comparsion of phosphorylation of SGK1 and SGK1[T422E]. S6K1-T2 and S6K1-T2[T412E] were also expressed as His-tag proteins in a bacuolovirus/insect cell expression system and purified by nickel agarose affinity chromatography as described previously [25]. S6K1-T2[T412E] expressed in this manner is not phosphorylated at Thr252. Phosphorylation of AGC kinase substrates by PDKl. The phosphorylation of PDKl substrates was performed in a final volume of 20 μl in a buffer containing 50 mM Tris-HCl pH 7.5, 0.1 % (by vol) 2- mercaptoethanol, 10 mM magnesium chloride, 100 μM [γ- 32 P]ATP ( " 1000 c.p.mJpmol), 0.5 μM microcystin-LR, 0.6 μM AGC kinase substrate and 0.6 to 30 nM wild type PDKl or the indicated mutant of PDKl. After 10 min the reactions were stopped by addition of Laemmli Sample Buffer (100 mM Tris-HCl pH 6.δ, 4% (by mass) SDS, 20% (by volume) glycerol and 200 mM dithiothreitol (DTT), boiled, and the samples subjected to separation by SDS-polyacrylamide gel electrophoresis. The gels were exposed and analysed with a Fuji Phosphoimager known amounts of [γ- 32 P]ATP spotted onto a blank gels to permit quantification of the data. The experiments were performed so that the amount of PDKl did not phosphorylate more than 20% of the substrate. Control in which PDKl was omitted from the reaction was taken as the blank value. Activation of AGC kinase PDKl substrates. Phosphorylation reactions were carried out as above except that non-radioactive ATP replaced [γ- 32 P]ATP. Following 10 min at 30 °C, cocktail (30 μl) containing 50 mM Tris-HCl pH 7.5, 0.1 % (by vol) 2-mercaptoethanol, 10 mM magnesium chloride, 100 μM [γ- 32 P]ATP ( " 1000 c.p.m. pmol), 0.5 μM microcystin- LR and 100 μM peptide substrate crosstide (GRPRTSSFAEG) and [γ- 32 P]ATP ("600 c.p.m./pmol). Reactions were stopped by the addition of 25 μl of 0.2 M EDTA pH δ.0, spotted onto Pδl phosphocellulose paper, washed and analysed as described for the assay of MAP kinase [26]. The amount of PDKl was in the assay was varied so that the assay was in the linear range. One unit of activity is deficed as phosphorylation 1 nmol of substrate in 1 min. Binding of PDKl to SGKl and S6K1. For the data presented in Figure 22, 293 cells were cotransfected with 10 μg of the wild type or mutant PDKl plasmid and 10 μg of either the wild type or mutant S6K1 or SGKl. 36 h post-transfection the cells were lysed in 0.6 ml of Buffer A and the lysates were cleared by centrifugation at 13 000 x g for 10 min at 2 °C, and 0.5 ml of supernatant was incubated for 2 h at 4°C with 30 μl of glutathione-Sepharose. The beads were washed twice in Buffer A containing 0.5 M NaCl, followed by two further washes in Buffer A. The beads were resuspended in 30 μl Laemmli Sample Buffer and subjected to SDS polyacrylamide gel electrophoresis. The gels were either stained with Coomassie blue, or analysed by immunoblotting with either anti-Flag or anti-Myc antibodies (described below). Immunoblotting. For the Myc and Flag blots of cell lysates 5 μg of protein was used. Immunoblotting with the phosphospecific antibodies (0. 5-2 μg/ml) in the presence of 10 μg/ml dephospho peptide corresponding to the antigen used to raise the antibody in 50 mM Tris/HCl pH 7.5, 0.15M NaCl, 0.5% (by vol) Tween (TBS-Tween) containing in 50 mM Tris/HCl pH 7.5, 0.15M NaCl, 0.5% (by vol) Tween (TBS-Tween) 5% (by mass) skimmed milk. Detection was performed using horse radish peroxidase conjugated secondary antibodies and the enhanced chemiluminescence reagent. (Amersham/Pharamcia). For the Tδl6-P blots 25 μg of cell lysate protein was used. For the T410- P blots, 150 μg of cell lysate protein was immunoprecipitated using 5 μl of Flag affinity gel and washed as described above. Cell lysates or immunoprecipitates were made 1 % in SDS, subjected to SDS/polyacrylamide gel electrophoresis, and transferred to nitrocellulose. The nitrocellulose membranes were immunoblotted using either the anti- Myc (0.4 μg/ml ), anti-Flag antibodies (0.4 μg/ml) and 10% (by mass) skimmed milk. Results All the wild type and mutant forms of AGC kinase substrates employed in this study are defined in Fig 17. Unless stated otherwise the form of SόK used in this study lacks the C-terminal 104 residues as full length S6K1 is not an efficient substrate for PDKl in vitro [15, 27]. The form of SGKl used in this study lacks the N-terminal 61 amino acids as full length SGKl protein is unstable and can not be expressed at significant levels [9] . Role of PDKl "PIF-Pocket" on phosphorylation and activation of S6K1, SGKl and PKB by PDKl. We first investigated the role of the hydrophobic PIF binding pocket on PDKl, in enabling PDKl to phosphorylate and activate 3 of its AGC kinase substrates that are activated in response to insulin, namely, S6K1, SGKl and PKB. We initially tested whether a PDKl mutant (PDK1[L155E]) in which the hydrophobic PIF binding pocket has been disrupted [17] could phosphorylate and activate these AGC kinase substrates. Strikingly, the phosphorylation of S6K1 and SGKl by PDK1[L155E] was drastically reduced compared to wild type PDKl (Fig 18 and Table 1). We also employed mutants of S6K1 (S6K1[T422E]) and SGKl (SGK1[T422D]) in which their hydrophobic motif phosphorylation site was changed to an acidic residue which significantly increases the rate at which these are phosphorylated and activated by PDKl (Table 3 and [9, 15, 27]). We found that both S6K1[T422E] and SGK1[T422D] were also very poorly phosphorylated and activated by PDK1[L155E] compared to wild type PDKl (Fig 18). In contrast, PKBα and its acidic hydrophobic motif mutant, PKBα[S473D] were equally good substrates for wild type PDKl and PDK1[L155E]. Interestingly however, mutant forms of PKBα and PKBα[S473D] that lack the PH domain (ΔPH-PKBα and ΔPH- PKBα[S473D]) and are thus more similar in structure to S6K1 and SGKl (Fig 17) are very poor substrates for PDK1[L155E] compared to wild type PDKl. ΔPH-PKBα is phosphorylated by PDKl at a 50-100 fold lower rate than full length PKBα (Table 3) and its phosphorylation, like that of S6K1 and SGKl by PDKl, is not influenced by PtdIns(3,4,5)P 3 [9, 15]. It should be noted however, that ΔPH-PKBα is phosphorylated by PDKl at a 10-fold lower initial rate than S6K1 and SGKl and ΔPH-PKBα[S473D] is phosphorylated at " 100-fold lower rate than S6K1[T412E] and SGK1[T422E] (Table 3). Table 3: Relative phosphorylation of PDKl substrates Relative phosphorylation of PDKl substrates. PDKl substrates were phosphorylated in vitro, subjected to SDS-PAGE, and radioactivity associated with the bands measured using a Phospho-Imager and known amounts of ATP as standard. The phosphorylation rate of PKB[S473D] in the presence of Ptdlns (3,4,5)P3 was 2.6 mol/mol PDKl/min and was taken as the relative value of 100. Average values from a representative experiment performed in duplicates are shown. We previously observed that in the presence of PIFtide, PDKl is no longer able to phosphorylate S6K1 but retains its ability to activate PKBα [25] (see also Fig 19A & 19C & Table 3). Here we demonstrate that in the presence of PIFtide, PDKl's ability to phosphorylate and activate SGKl, ΔPH-PKBα as well as the hydrophobic motif mutants, S6K1[T412E], SGK1[T422D] and ΔPH-PKBα[S473D] is also markedly inhibited. These results demonstrate that PDKl requires it hydrophobic PIF binding pocket in order to phosphorylate and activate S6K1, SGKl and ΔPH-PKBα. Role of the hydrophobic motif on phosphorylation of S6K1, SGKl and PKB by PDKl. We next investigated the role of the hydrophobic motif of S6K1, SGKl and PKBα in enabling PDKl to phosphorylate these kinases. To achieve this we mutated the conserved Phe to Ala that lies prior to the hydrophobic motif phosphorylation site on these AGC kinases (Phe411 in S6K1, Phe 421 in SGKl and Phe 472 in PKBα) and tested the effect that this had on the phosphorylation and activation of these kinases by PDKl . This mutation was selected as the equivalent mutation in PRK2 and PKCζ prevented PDKl from interacting and phosphorylating these mutants. The rate at which PDKl phosphorylated S6K1[F411A], SGK1[F421A] and ΔPH-PKBα[F472A] compared to the wild type enzymes, was markedly reduced. Interestingly the basal rate at which PDKl phosphorylated these enzymes was not inhibited by PIFtide and was comparable to the rate at which these were phosphorylated by PDK1[L155E]. In contrast, PKB[F472A] was phosphorylated at a similar rate to the wild type PKB. We also expressed the wild type and the hydrophobic motif mutants of S6K1, SGK, PKBα and ΔPH-PKBα in 293 cells and measured the phosphorylation of these enzymes at their T-loop site before and after stimulation of cells with IGFl using the appropriate phospho-specific antibodies. IGFl stimulation of cells failed to induce the phosphorylation of S6K1[F411A], SGK1[F421A] and ΔPH-PKBα[F472A] at their T-loop residue, whereas the wild type kinases became phosphorylated (Fig 21D and 21E). S6K1[F411A] and SGK1[F421A] were also not significantly activated following IGFl stimulation which is consistent with the lack of phosphorylation of these enzymes at their T-loop residue (data not shown). In contrast, PKBα[F472A] was phosphorylated to similar extent as wild type PKBα in response to IGFl (Fig 18F). PKBα[F472A] in unstimulated cells possessed " 50% of the basal activity of wild type PKBα, but its activity was not further increased following stimulation with IGFl, despite becoming phosphorylated at Thr30δ (data not shown). In parallel experiments, IGFl induced equivalent phosphorylation of wild type-PKBα at Thr30δ and increased its activity over 10-fold. These results demonstrate that the hydrophobic motifs of S6K1, SGKl and ΔPH-PKBα are required in order for these kinases to become phosphorylated by PDKl efficiently at their T-loop motif. Role of LysllS, Ilell9 and Glnl50 in the PDKl "PIF-Pocket" on activation of S6K1, SGKl. In addition to Leu 155, there are other residues that were predicted to form part of the PIF binding pocket on PDKl, namely Lysll5, Ilell9 and Glnl50 [17]. PDK1[K115A], PDK1[I119A], PDK1[Q150A] in addition to PDK1[L155A] (unpublished data) that possess " 10-fold decreased affinity for PIFtide, while retaining activity towards peptide substrates that do not interact with the PIF binding pocket indicating that they are not catalytically impaired. The rate at which PDK1[K115A], PDK1[I119A], PDK1[Q150A] and PDK1[L155A] activated S6K1[412E] was only marginally lower than wild type PDKl . In contrast the rate at which these mutants of PDKl activated SGK1[T422D] was markedly reduced, indicating that Lysl 15, Ilell9 and Glnl50 may play a more dominant role in permitting PDKl to phosphorylate SGK1[T422D] than S6K1[T412E]. Consistent with the reduced affinity of these mutants of PDKl for PIFtide, low concentrations of PIFtide (2 μM) only inhibited the activation of S6K1-T2[T412E] and SGK1[T422D] by " 50% and high concentrations of PIF tide (35 μM) were required to inhibit these enzymes over 95 % . Under identical conditions 2 μM PIFtide inhibited the activation of S6K1[T412E] by wild type PDKl over " 20-fold. Interaction of S6K1 and SGKl with PDKl. Full length S6K1 is a very poor substrate for PDKl compared to S6K1 lacking the C-terminal 104 amino acids in its regulatory domain [15, 27]. We therefore tested whether this could be explained by the inability of full length S6K1 to interact with PDKl. To test this hypothesis we coexpressed in 293 cells GST-PDKl together with full length S6K1, full length S6K1[T412E] and the C- terminal truncated forms of these mutants (S6K1-T2 and S6K1- T2[T412E]) which have Flag epitope tags. Glutathione-Sepharose "pull- downs" of GST-PDKl from these extracts were immunoblotted for the presence of Flag epitope tagged S6K1. Although GST-PDKl and wild type and mutant forms of S6K1 were expressed to a similar level, full length S6K1 and full length S6K1[T412E] failed to interact with GST- PDKl, whilst the S6K1-T2 and S6K1-T2[T412E] both interacted with GST-PDKl. S6K1-T2[T412E] interacted moderately better with GST- PDKl compared to S6K1-T2. Under similar conditions we were unable to detect an interaction between GST-PDKl [LI 55E] and S6K1-T2 and S6K1- T2[T412E], indicating that PDKl interacts with S6K1-T2 through the PIF binding pocket. Wild type SGKl is phosphorylated at a 10-fold lower rate than SGK1[T422D] (Table 3 and [9]). We therefore tested whether this could be accounted for by differences in affinity of wild type SGKl and SGK1[T422D] for PDKl. To investigate this we coexpressed GST-SGKl and GST-SGKl [T422D] with Myc-PDKl in 293 cells. Glutathione- Sepharose "pull-downs" of GST-SGKl were immunoblotted for the presence of Myc-PDKl . As shown in Fig 20B Myc-PDKl only interacted with SGK1[T422D] but did not interact with the wild type SGKl. As expected SGK1[T422D] failed to interact with Myc-PDKl [L155E]. Discussion The results presented in this Example indicate that the hydrophobic motif of SόKl and SGKl function as a PDKl docking site that binds to the PIF binding pocket of PDKl. This recruits PDKl to SόKl and SGKl enabling PDKl to phosphorylate these enzymes at their T-loop site. This conclusion is supported by the finding that a mutant of PDKl in which the PIF binding pocket has been disrupted (PDK1[L155E]) can not phosphorylate SόKl or SGKl (Fig 18) and mutants of S6K1 and SGKl in which their hydrophobic motif have been disrupted can not be phosphorylated by wild type PDKl (Fig 19). These obsevations explain why PIFtide inhibits the phosphorylation of S6K1 and SGKl (Fig 20) as it interacts with the PIF binding pocket of PDKl thus preventing it from binding to S6K1 and SGKl. Overall these results provide further evidence that AGC kinases (other than PKB) interact through their C-terminal non catalytic residues with the PIF binding pocket of PDKl. These interactions will play an important role in regulating the access to PDKl to its substrates. SόKl requires phosphorylation of both the T-loop and hydrophobic motif to be activated [6] thus phosphorylation of S6K1 at its T-loop site by PDKl alone does not significantly activate it. Full length SόKl is a very poor substrate for PDKl compared to a mutant of SόKl that lacks its C- teπninal 104 residues encompassing the five in vivo Ser-Pro/Thr-Pro phosphorylation sites [15, 27]. We were unable to detect an interaction between full length SόKl or S6K1[T412E] and PDKl whilst a full length S6K1 mutant in which the hydrophobic motif phosphorylation site (Thr412) and all five phosphorylated C-terminal residues were mutated to acidic residues, was able to bind to PDKl but not to PDK1[L155E]. Removal of the C-terminal 104 residues of SόKl, also enabled S6K1-T2 and S6K1-T2[T412E] to interact with PDKl but not PDK1[L155E]. S6K1-T2[T412E] is phosphorylated by PDKl at a 5-fold higher initial rate than S6K1-T2 and consistent with previous binding studies [25] we observed that S6K1-T2[T412E] interacted with higher affinity to PDKl than S6K1-T2. These findings suggest a model for the activation of SόKl in which the first step would involve the phosphorylation of the C-terminal Ser-Pro/Thr-Pro by a proline directed kinase. This would not directly activate SόKl but induce a conformational change exposing its hydrophobic motif so that it can interact with the PIF binding pocket of PDKl, enabling PDKl to phosphorylate the T-loop residue of SόKl. This is consistent with the finding that in response to insulin stimulation of cells the C-terminal Ser-Pro/Thr-Pro of SόKl become phosphorylated before phosphorylation of the T-loop and hydrophobic motif [6, 23]. The interaction of PDKl with S6K1 would be enhanced further if SόKl was phosphorylated at its hydrophobic motif. However, this may not be a prerequisite for T-loop phosphorylation as a mutant of SόKl in which Thr412 is mutated to an Ala is still phosphorylated at its T-loop residue albeit to a lower extent that wild type SόKl [23]. SGKl, like SόKl requires phosphorylation of both its T-loop and hydrophobic motif to be activated in cells, but does not possess a C- teπninal tail following its hydrophobic motif that becomes phosphorylated at Ser-Pro/Thr-Pro motifs. Wild type SGKl that has not been phosphorylated at its hydrophobic motif (Thr422) is a poor substrate for PDKl and mutation of Thr412 to an acidic residue increases over 10-fold the rate at which it becomes phosphorylated by PDKl (Table 3 and [9]). Furthermore, when SGK1[T422D] is expressed in unstimulated 293 cells it is significantly phosphorylated at its T-loop residue (Thr256) whilst wild type SGKl is not [8, 9]. The finding that SGK1[T422D] interacts with PDKl (but not with PDK1[L155E]), in contrast no detectable interaction between wild type PDKl and SGKl was observed (Fig 21), is consistent with the conclusion of Kobayashi & Cohen [9] that the phosphorylation of SGKl at its hydrophobic motif plays the major role in regulating phosphorylation of SGKl at its T-loop residue. Consistent with this, a mutant of SGKl in which the hydrophobic motif phosphorylation site (THr422) is changed to Ala does not become phosphorylated at its T-loop phosphorylation site (THr256) following IGFl stimulation, whilst changing T422 to Asp results in SGKl being phosphorylated at Thr256 in unstimulated cells. Although the activation of SόKl and SGKl is dependent upon PI 3-kinase in vivo, it is not clear how PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 regulates this process. The phosphorylation of SόKl and SGKl in vitro by PDKl is not enhanced by the presence of PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 which interact with the PH domain of PDKl. This indicates that the binding of PDKl to PtdIns(3,4,5)P 3 /PtάTns(3,4)P 2 may not directly activate these enzymes. Instead of regulating the activity of PDKl, PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 could instead induce activation of the kinase(s) that phosphorylate the hydrophobic motif of SόKl and SGKl and/or the proline directed kinase(s) that phosphorylate SόKl at its C-terminal tail. If this mechanism operated in vivo, PDKl activity in cells would not need to be activated by insulin or growth factors as it would not be able to phosphorylate S6K1 or SGKl until these enzymes were phosphorylated at their hydrophobic motif/C-terminal tail. The finding in this study that the PIF binding pocket of PDKl is not required to enable PDKl to activate PKB, nor is the hydrophobic motif of PKB required to allow it to become phosphorylated at its T loop supports the conclusion of previous studies indicating that binding of PDKl and PKB to PtdIns(3,4,5)P 3 /PtdTns(3,4)P 2 is likely to be the primary determinant for bringing these molecules together [14]. This is also supported by the finding that the activation of PKB, like the activation of PI 3-kinase occurs very rapidly in cells thus indicating that the activation of PKB occurs shortly after the formation of PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 . Instead, as the activation of SόKl (and SGKl) occurs much more slowly than PKB; this indicates that there is a substantial delay between activation of PI 3-kinase and activation of S6K1 and SGKl. This delay could be accounted for by the time it takes for PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 to activate the S6Kl-C-terminal Ser- Pro/Thr-Pro kinase(s) and the S6K1/SGK1 hydrophobic motif kinase(s) which are likely be the rate limiting step in the activation of these kinases in cells. There is no evidence that phosphorylation of PKBα at Ser473 promotes phosphorylation of Thr30δ, as mutation of Ser473 to either Ala or Asp had no effect on insulin/IGFl induced phosphorylation of PKBα at Thr30δ [24]. It can not be ruled out that PDKl could interact with PKBα through a domain other than the PIF binding pocket and the binding of PKB and/or PDKl to PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 could expose this binding pocket(s). Indeed, there is evidence that the binding of PKB to PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 could induce a conformational change leading to the exposure of the Thr30δ and perhaps Ser473 phosphorylation sites [14]. VanObberghen and colleagues have also concluded that the PH domain of PDKl may inhibit it from phosphorylating PKBα [2δ]. This is based on the finding that a PDKl mutant lacking its PH domain or possessing a PH domain that can not interact with PtdIns(3,4,5)P 3 was far more effective at activating ΔPH-PKBα in a cotransfection experiment than the wild type PDKl. Although it should be noted that this observed in vivo effect is likely to be more complicated, as the rate at which ΔPH- PDKl and wild type PDKl phosphorylate ΔPH-PKBα is similar in vitro [22,29]. ΔPH-PKBα when expressed in 293 cells is phosphorylated at Thr30δ and Ser473 in response to insulin and this is prevented by inhibitors of PI 3- kinase [30, 31]. This observation was originally interpreted as evidence that PDKl and the enzyme which phosphorylates Ser473 were activated in vivo by PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 . However, in this study, we demonstrate that ΔPH-PKBα is not phosphorylated by PDKl by the same mechanism as wild type PKBα, as ΔPH-PKBα is not phosphorylated by PDK1[L155E] and disruption of the hydrophobic motif of ΔPH-PKBα largely prevented its phosphorylation by PDKl (Fig lδ to 19). Thus the ' mechanism by which PDKl phosphorylates ΔPH-PKBα is more like S6K1 and SGKl than PKBα. As mutation of Ser473 in ΔPH-PKBα to Asp increases the rate at which it is phosphorylated by PDKl (Table 3), it is possible that when ΔPH-PKBα is expressed in cells PtdIns(3,4,5)P 3 /PtdIns(3,4)P 2 does not activate PDKl but instead induces phosphorylation of Ser473 through the same hydrophobic motif kinase(s) that phosphorylate SόKl and SGKl, which subsequently converts ΔPH- PKBα into a PDKl substrate. Although ΔPH-PKBα is phosphorylated by PDKl at the same rate in the presence or absence of PtdIns(3,4,5)P 3 it should be emphasised that ΔPH- PKBα is phosphorylated by PDKl at " 50-fold lower rate than wild type PKBα in the presence of PtdIns(3,4,5)P 3 . Furthermore, ΔPH-PKBα is phosphorylated in vitro at a 10-100-fold lower rate than SGKl and SόKl by PDKl (Table 3). This might be explained if the C-terminal region of ΔPH-PKBα surrounding the hydrophobic motif, interacted with significantly lower affinity with PDKl than the equivalent region of SόKl and SGKl. It is possible that this region of SόKl and SGKl, has evolved to enable these enzymes to bind PDKl . However, the equivalent residues in PKB may not have evolved not to interact with the PIF binding pocket of PDKl, enabling PKB to be regulated by a distinct mechanism. When PKCζ and PRK2 are overexpressed in unstimulated 293 cells unlike SόKl, SGKl and PKB they are phosphorylated at their T-loop residue to a high stoichiometry and when these kinases are isolated from cells can not be phosphorylated further by PDKl in vitro. We also demonstrated that PRK2 and PKCζ and could directly interact through their hydrophobic motif to the PIF binding pocket of PDKl [18]. These observations imply that in contrast to SόKl, SGKl and PKB when PKCζ or PRK2 is overexpressed in unstimulated cells, their hydrophobic motif is able to interact directly with endogenous PDKl resulting in their T-loop residue becoming phosphorylated. However, it is likely that phosphorylation of the T-loop residue of endogenously expressed PKCζ and PRK2 will be under some regulation in cells. Parker and colleagues have presented evidence that the interaction of PRK2 with the small GTP binding protein Rho complexed to GTP, may promote the interaction of PRK2 with PDKl and enhance the phosphorylation of its T-loop residue [32]. In the case of PKCζ it is not clear how phosphorylation of its T-loop is regulated. Although there are several reports indicating that PKCζ is activated by insulin and growth factors in cells and that these agonists induce phosphorylation of PKCζ at its T-loop residue, we have thus far not been able to demonstrate any further activation of either endogenous or fransfected PKCζ in 293 cells. In unstimulated mouse embryonic stem cells endogenous PKCζ is significantly phosphorylated at its T-loop residue and this is not further increased by stimulation with IGFl or any other agonist that we have tried [33]. In mouse embryonic stem cells lacking PDKl, PKCζ is not phosphorylated at its T-loop residue, providing genetic evidence that PDKl mediates phosphorylation of PKCζ in vivo [33]. Recent work demonstrates that PKCζ in cells is complexed to other proteins termed hPar3 and hParό and that hParό in this complex is capable of interacting with the small GTP binding proteins Rac and CDC42 (reviewed in [34]). The evidence indicates that these proteins will play key roles in regulating the activity of PKCζ. However, it has not yet been investigated whether hPar3/hParό/CDC42/Racl could also function by regulating the phosphorylation PKCζ by PDKl. This complex could operate by controlling the access of the hydrophobic motif of PKCζ to enable interaction of PDKl in response to a specific signal. For example the binding of Rac/CDC42 to this complex may enable PDKl to interact and phosphorylate PKCζ. The model we propose in Fig 22 demonstrates the key step in the phosphorylation of PDKl substrates thus far been identified other than PKB are regulated by the direct interaction of their hydrophobic motif with the PIF binding pocket of PDKl . Instead PKB and PDKl are brought together mainly by their mutual interaction with PtdIns(3,4,5)P 3 . Although PRK2 and PKCζ are can interact directly with PDKl when overexpressed in 293 cells, the interaction of S6K1 and SGKl is regulated by the phosphorylation of these enzymes at their C-terminal residue(s). This model could account for the differences in the time course of activation of S6K1, SGKl and PKB in IGFl/growth factor stimulated cells. These findings indicate that drugs directed towards a specific non catalytic site on a protein kinase could inhibit the phosphorylation of a group of substrates without affecting the phosphorylation of another. Thus compounds that interact with the PIF-binding pocket on PDKl could affect the activation of S6K1 and SGKl but not PKB and will be more specific than an ATP competitive PDKl inhibitor which will inhibit the phosphorylation of all downstream targets. Thus such drugs are likely to have less side effects. References 1 Leevers, S. J., Vanhaesebroeck, B. and Waterfield, M. D. (1999) Curr Opin Cell Biol 11 , 219-225 2 Peterson, R. T. and Schreiber, S. L. (1999) Curr Biol 9, R521- R524 3 Belham, C, Wu, S. and Avruch, J. (1999) Curr Biol 9, R93-96 4 Alessi, D. R. and Cohen, P. (1998) Curr Opin Genet Dev 8, 55-62 5 Coffer, P. J., Jin, J. and Woodgett, J. R. (1998) Biochem J 335, 1- 13 6 Dufner, A. and Thomas, G. (1999) Exp Cell Res 253, 100-109 7 Avruch, J. (199δ) Mol Cell Biochem 182, 31-48 δ Park, J., Leong, M. L., Buse, P., Maiyar, A. C, Firestone, G. L. and Hemmings, B. A. (1999) Embo J 18, 3024-3033 9 Kobayashi, T. and Cohen, P. (1999) Biochem J 339, 319-328 10 Kobayashi, T., Deak, M., Morrice, N. and Cohen, P. (1999) Biochem J 344, lδ9-197 11 Kohn, A. D., Kovacina, K. S. and Roth, R. A. (1995) Embo J 14, 42δδ-4295 12 Cross, D. A., Alessi, D. R., Cohen, P., Andjelkovich, M. and Hemmings, B. A. (1995) Nature 378, 785-7δ9 13 Burgering, B. M. and Coffer, P. J. (1995) Nature 376, 599-602 14 Vanhaesebroeck, B. and Alessi, D. R. (2000) Biochem J 346, 561- 576 15 Alessi, D. R., Kozlowski, M. T., Weng, Q. P., Morrice, N. and Avruch, J. (199δ) Curr Biol 8, 69-81 16 Balendran, A., Casamayor, A., Deak, M., Paterson, A., Gaffney, P., Currie, R., Downes, C. P. and Alessi, D. R. (1999) Curr Biol 9, 393- 404 17 Biondi, R. M., Cheung, P. C, Casamayor, A., Deak, M., Currie, R. A. and Alessi, D. R. (2000) Embo J 19, 979-9δδ lδ Balendran, A., Biondi, R. M., Cheung, P. C, Casamayor, A., Deak, M. and Alessi, D. R. (2000) J Biol Chem 275, 20δ06-20δl3 19 Frodin, M., Jensen, C. J., Merienne, K. and Gammeltoft, S. (2000) Embo J 19, 2924-2934 20 Gaffney, P. R. J. and Reese, C. B. (1997) Bioorg.Med.Chem.Lett. 7, 3171-3176 21 Alessi, D. R., James, S. R., Downes, C. P., Holmes, A. B., Gaffney, P. R., Reese, C. B. and Cohen, P. (1997) Curr Biol 7, 261-269 22 Alessi, D. R., Deak, M., Casamayor, A., Caudwell, F. B., Morrice, N., Norman, D. G., Gaffney, P., Reese, C. B., MacDougall, C. N., Harbison, D., Ashworth, A. and Bownes, M. (1997) Curr Biol 7, 776-7δ9 23 Weng, Q. P., Kozlowski, M., Belham, C, Zhang, A., Comb, M. J. and Avruch, J. (199δ) J Biol Chem 273, 16621-16629 24 Alessi, D. R., Andjelkovic, M., Caudwell, B., Cron, P., Morrice, N., Cohen, P. and Hemmings, B. A. (1996) Embo J 15, 6541-6551 25 Balendran, A., Currie, R. A., Armstrong, C. G., Avruch, J. and Alessi, D. R. (1999) J Biol Chem 274, 37400-37406 26 Alessi, D. R., Cohen, P., Ashworth, A., Cowley, S., Leevers, S. J. and Marshall, C. J. (1995) Methods Enzymol 255, 279-290 27 Pullen, N., Dennis, P. B., Andjelkovic, M., Dufner, A., Kozma, S. C, Hemmings, B. A. and Thomas, G. (199δ) Science 279, 707-710 2δ Filippa, N., Sable, C. L., Hemmings, B. A. and Van Obberghen, E. (2000) Mol Cell Biol 20, 5712-5721 29 Currie, R. A., Walker, K. S., Gray, A., Deak, M., Casamayor, A., Downes, C. P., Cohen, P., Alessi, D. R. and Lucocq, J. (1999) Biochem J 337, 575-5δ3 30 Andjelkovic, M., Alessi, D. R., Meier, R., Fernandez, A., Lamb, N. J., Freeh, M., Cron, P., Cohen, P., Lucocq, J. M. and Hemmings, B. A. (1997) J Biol Chem 272, 31515-31524 31 Kohn, A. D., Takeuchi, F. and Roth, R. A. (1996) J Biol Chem 271, 21920-21926 32 Flynn, P., Mellor, H., Casamassima, A. and Parker, P. J. (2000) J Biol Chem 275, 11064-11070 33 Balendran, A., Hare, G. R., Kieloch, A., Williams, M. R. and Alessi, D. R. (2000) FEBS Lett 484, 217-223 34 Brazil, D. P. and Hemmings, B. A. (2000) Curr Biol 10, R592-594

Claims: CLAIMS 1. A method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, wherein the ability of the compound to inhibit, promote or mimic the interaction of the said hydrophobic pocket- containing protein kinase with an interacting polypeptide is measured and a compound that inhibits, promotes or mimics the said interaction is selected, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr. 2. The method of claim 1 wherein the polypeptide comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr, wherein Zaa represents a negatively charged amino acid residue. 3. The method of claim 1 wherein the polypeptide comprises the amino acid sequence Phe-Xaa-Xaa-Phe. 4. The method of claim 1 , 2 or 3 wherein the protein kinase is PDKl . 5. The method of claim 1, 2 or 3 wherein the protein kinase is SGK, PKB, PKA, p70 S6 kinase, p90 RSK, PKCα, PKCδ, PKCζ or PRK2. 6. The method of any of the previous claims wherein the interaction is an interaction of the hydrophobic pocket of the said protein kinase with the polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa- Phe/Tyr. 7. The method of any of the previous claims wherein the interacting polypeptide is part of the same polypeptide chain as the protein kinase. δ. The method of claim 7 wherein the interaction is an intramolecular interaction. 9. The method of any of the preceding claims wherein the ability of the compound to inhibit, promote or mimic the interaction of the protein kinase with the interacting polypeptide is measured using surface plasmon resonance. 10. A method of identifying a compound that modulates the protein kinase activity of a hydrophobic pocket-containing protein kinase as defined in claim 1 , wherein the effect of the said compound on the rate or degree of phosphorylation of a substrate polypeptide of the said hydrophobic pocket-containing protein kinase by the said hydrophobic pocket-containing protein kinase in the presence of an interacting polypeptide is determined, and a compound that modulates the said rate or degree of phosphorylation is selected, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr and is comprised in a separate polypeptide chain to the hydrophobic pocket- containing protein kinase, and wherein the substrate polypeptide has fewer than 400 amino acids. 11. The method of claim 10 wherein the substrate polypeptide comprises a portion that is the interacting polypeptide. 12. The method of claim 11 wherein the protein kinase is PDKl and the substrate polypeptide comprises or consists of the sequence KTFCGTPEYLAPEVRREPPJLSEEEQEMFRDFD YIADWC . 13. The method of claim 10 wherein the substrate portion and the interacting portion are on separate polypeptide chains. 14. The method of claim 13 wherein the hydrophobic pocket-containing protein kinase is PDKl, the substrate polypeptide comprises or consists of the sequence KTFCGTPEYLAPEV, and the interacting polypeptide comprises or consists of the sequence EPRILSEEEQEMFRDFDYIADWC. 15. A method of identifying a compound that modulates the protein kinase activity of a hydrophobic pocket-containing protein kinase as defined in claim 1, wherein the effect of the said compound on the rate or degree of phosphorylation of a substrate polypeptide of the said hydrophobic pocket-containing protein kinase by the said hydrophobic pocket-containing protein kinase is determined, and a compound that modulates the said rate or degree of phosphorylation is selected, wherein the effect of the compound is determined in the absence of an interacting polypeptide, wherein an interacting polypeptide is one which interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, and wherein the substrate polypeptide has fewer than 400 amino acids. 16. The method of claim 15 wherein hydrophobic pocket-containing protein kinase is PDKl and the substrate polypeptide consists of or comprises the amino acid sequence KTFCGTPEYLAPEV or KTFCGTPEYLAPEVRR. 17. A method of selecting or designing a compound that modulates the activity of a hydrophobic pocket-containing protein kinase as defined in any of the preceding claims, the method comprising the step of using molecular modelling means to select or design a compound that is predicted to interact with the said hydrophobic pocket-containing protein kinase, wherein a three-dimensional structure of a compound is compared with a three-dimensional structure of the said hydrophobic pocket and/or with a three-dimensional structure of an interacting polypeptide as defined in any of the preceding claims, and a compound that is predicted to interact with the said hydrophobic pocket is selected. lδ. A compound capable of modulating the protein kinase activity of a hydrophobic pocket-containing protein kinase as defined in claim 1, wherein the compound inhibits the interaction of the said protein kinase with an interacting polypeptide, wherein the interacting polypeptide interacts with the hydrophobic pocket of the said protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, wherein the compound does not comprise a polypeptide having the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr and is not PKA. 19. A compound capable of modulating the protein kinase activity of a hydrophobic pocket-containing protein kinase as defined in claim 1, wherein the compound modulates the rate or degree of phosphorylation of a substrate polypeptide of the said hydrophobic pocket-containing protein kinase by the said hydrophobic pocket-containing protein kinase in the absence of an interacting polypeptide, wherein an interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, and wherein the substrate polypeptide has fewer than 400 amino acids. 20. A compound that modulates the protein kinase activity of a hydrophobic pocket-containing protein kinase as defined in claim 1, wherein the compound modulates the rate or degree of phosphorylation of a substrate polypeptide of the said hydrophobic pocket-containing protein kinase by the said hydrophobic pocket-containing protein kinase in the presence of an interacting polypeptide, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr and is comprised in a separate polypeptide chain to the hydrophobic pocket- containing protein kinase, and wherein the substrate polypeptide has fewer than 400 amino acids. 21. A compound identifiable by the method of any one of claims 1 to 17, provided that the compound is not a polypeptide having the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr and is not full length PKA. 22. A mutated protein kinase, wherein the protein kinase before mutation has a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA, and wherein one or more residues defining the hydrophobic pocket of the protein kinase is mutated. 23. The mutated protein kinase of claim 22 wherein the protein kinase before mutation is PDKl, SGK, p70 S6 kinase or PKB. 24. The mutated protein kinase of claim 23 wherein the mutated residue(s) are the residues equivalent to residue Lys76, ValδO, Lyslll and/or Leullό of full length mouse PKA. 25. The mutated protein kinase of claim 24 wherein the residue at the position equivalent to residue Lys76 of full length mouse PKA is mutated to an Ala and/or the residue at the position equivalent to Leullό of full length mouse PKA is mutated to a Ser, Asp or Glu. 26. A preparation comprising a hydrophobic pocket-containing protein kinase as defined in claim 1, and a second, interacting compound, wherein the interacting compound interacts with the hydrophobic pocket of the protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa- Xaa-Phe/Tyr, wherein the said preparation further comprises a substrate polypeptide as defined in any of claims 10 to 16 and does not comprise all of the components found in a cell in which said protein kinase or compound is naturally found. 27. A preparation comprising a hydrophobic pocket-containing protein kinase as defined in claim 1, and a second, interacting compound, wherein the interacting compound interacts with the hydrophobic pocket of the protein kinase, wherein the said preparation does not comprise all of the components found in a cell in which said protein kinase or compound is naturally found, and wherein when the protein kinase is PDKl, the interacting compound is not a polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr. 2δ. A method of phosphorylating a substrate polypeptide for a hydrophobic pocket-containing protein kinase as defined in claim 1, wherein a preparation according to claim 27 is used. 29. A method of phosphorylating p70 S6 kinase on the residue equivalent to Thr412 of full length human p70 S6 kinase wherein the said p70 S6 kinase is exposed to recombinant PDKl . 30. A method of identifying a compound that modulates the activation and/or phosphorylation of p70 S6 kinase on the residue equivalent to Thr412 of full length human p70 S6 kinase by PDKl wherein the activation and/or phosphorylation of p70 S6 kinase on the residue equivalent to Thr412 of full length human p70 S6 kinase by PDKl is measured in the presence of more than one concentration of the compound. 31. A compound identified or identifiable by the method of claim 30. 32. The use of an interacting polypeptide as defined in claim 1 or a compound as defined in any of claims lδ to 21 in a method of stabilising a hydrophobic pocket-containing protein kinase as defined in claim 1, wherein the protein kinase is exposed to the compound or polypeptide. 33. A method of modulating in a cell the protein kinase activity of a hydrophobic pocket-containing protein kinase as defined in claim 1, wherein a recombinant interacting polypeptide is expressed in the cell, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase and/or has the amino acid sequence Phe/Tyr-Xaa- Xaa-Phe/Tyr. 34. A polypeptide comprising the amino acid sequence Phe/Tyr-Xaa- Xaa-Phe/Tyr, wherein said polypeptide does not comprise the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr or Phe/Tyr-Xaa-Xaa- Phe/Tyr-Ser/Thr-Phe/Tyr and is not full-length PKA. 35. A polypeptide according to claim 34 comprising or consisting essentially of the C-terminal 223 amino acids of full length PKA. 36. A fusion polypeptide of a polypeptide according to claim 34 or 35, wherein the fusion polypeptide is not full length PKA. 37. A polynucleotide encoding a polypeptide according to any one of claims 34 to 36 or a mutated protein kinase according to any of claims 22 to 25. 3δ. A recombinant polynucleotide suitable for expressing a polypeptide according to any one of claims 34 to 36 or a mutated protein kinase according to any of claims 22 to 25. 39. A host cell comprising a polynucleotide according to claim 37 or 3δ. 40. A method of making a polypeptide according to any one of claims 34 to 36 or a mutated protein kinase according to any of claims 22 to 25, the method comprising culturing a host cell according to claim 39 which expresses said polypeptide or mutated protein kinase and isolating said polypeptide. 41. A polypeptide obtainable by the method of claim 40. 42. A cell containing a recombinant nucleic acid suitable for expressing a hydrophobic pocket-containing protein kinase as defined in claim 1 , and a recombinant nucleic acid suitable for expressing a second polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr, wherein when the said protein kinase is PDKl, the said second polypeptide is not EDVKKHPFFRLIDWSALMDKKVKPPFIPTΓRGREDVSNFDDEFTSEA PILTPPREPRILSEEEQEMFRDFDYIADWC (PIF), and does not comprise the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Ser/Thr- Phe/Tyr. 43. A method of making a preparation comprising a hydrophobic pocket- containing protein kinase as defined in claim 1, and a second, interacting polypeptide, wherein the interacting polypeptide interacts with the hydrophobic pocket of the protein kinase, wherein the said preparation does not comprise all of the components found in a cell in which said protein kinase or compound is naturally found, and wherein when the protein kinase is PDKl, the interacting polypeptide is not a polypeptide comprising the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr-Zaa- Phe/Tyr, wherein the said protein kinase and the said interacting polypeptide are co-expressed in a cell according to claim 42. 44. A kit of parts useful in carrying out a method according to any one of claims 1 to 16, comprising a hydrophobic pocket-containing protein kinase as defined in claim 1 and a separate interacting polypeptide wherein the interacting polypeptide interacts with the hydrophobic pocket of the said protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa- Xaa-Phe/Tyr and does not comprise the amino acid sequence Phe/Tyr- Xaa-Xaa-Phe/Tyr-Zaa-Phe/Tyr or Phe/Tyr-Xaa-Xaa-Phe/Tyr-Ser/Thr- Phe/Tyr. 45. A kit of parts useful in carrying out a method according to any one of claims 10 to 16, comprising a hydrophobic pocket-containing protein kinase as defined in claim 1 and a substrate polypeptide as defined in any one of claims 10 to 16 and optionally a separate interacting polypeptide wherein the interacting polypeptide interacts with the hydrophobic pocket of the said protein kinase and/or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr. 46. A compound according to claim lδ to 21 or 31, or polypeptide according to any of claims 34 to 36 or polynucleotide according to claim 37 or 3δ for use in medicine. 47. The use of a compound or polypeptide or polynucleotide as defined in claim 46 in the manufacture of a medicament for the treatment of a patient in need of modulation of signalling by a hydrophobic pocket-containing protein kinase as defined in claim 1. 4δ. The use of claim 47 wherein the patient has cancer or diabetes or is in need of inhibition of apoptosis, for example a patient suffering from tissue injury or ischaemic injury, including stroke. 49. A polypeptide comprising non-overlapping interacting and substrate portions, wherein the interacting portion comprises the amino acid 13δ sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr and the substrate portion comprises a consensus sequence for phosphorylation by a hydrophobic pocket- containing protein kinase as defined in claim 1, wherein the amino acid sequence Phe/Tyr-Xaa-Xaa-Phe/Tyr and the said consensus sequence for phosphorylation are separated by between about 5 and 100 amino acids. 50. An interacting polypeptide which interacts with the hydrophobic pocket of a hydrophobic pocket-containing protein kinase as defined in claim 1 and or comprises the amino acid sequence Phe/Tyr-Xaa-Xaa- Phe/Tyr, immobilised on a surface of an article suitable for use as a test surface in a surface plasmon resonance method, wherein the interacting polypeptide is not PIF or PIFtide. 51. A method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA comprising the steps of (1) deteπnining the effect of a test compound on the protein kinase activity of the said protein kinase, and/or a mutant thereof, and (2) selecting a compound capable of modulating the protein kinase activity of the said protein kinase to different extents towards (i) a substrate that binds to the said hydrophobic pocket of the said protein kinase (hydrophobic pocket- dependent substrate) and (ii) a substrate (such as PKB) that does not bind, or binds to a lesser extent than the first said substrate (hydrophobic pocket-independent substrate), to the said hydrophobic pocket of the said protein kinase. 52. The method of claim 51 wherein a compound that inhibits the protein kinase activity of the said protein kinase to a greater extent towards the hydrophobic pocket-dependent substrate than towards the hydrophobic pocket-independent substrate is selected. 53. The method of claim 51 or 52 wherein the protein kinase is PDKl . 54. The method of claim 53 wherein the hydrophobic pocket-dependent substrate is SGK, PRK2, S6K1 or PKCζ. 55. The method of claim 53 or 54 wherein the hydrophobic pocket- independent substrate is PKB. 56. A method of identifying a compound that modulates the protein kinase activity of a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, ValδO and/or Lyslll of full-length mouse PKA (for example PDKl), comprising the step of deteπnining the effect of the compound on the protein kinase activity of, or ability of the compound to bind to (1) the said protein kinase mutated at a residue defining at least part of the said hydrophobic pocket of the protein kinase, for example the residue equivalent to lysine 76 of full-length mouse PKA. 57. A method according to claim 56 further comprising determining the effect of the compound on the protein kinase activity of, or ability of the compound to bind to, the protein kinase which is not mutated at the said residue defining at least part of the said hydrophobic pocket of PDKl . 5δ. The method of any one of claims 51 to 57 wherein the effect of the compound on the rate or degree of phosphorylation of a hydrophobic pocket-dependent substrate is determined. 59. The method of any one of claims 51 to 57 wherein a compound is selected that decreases the protein kinase activity of the protein kinase towards a hydrophobic pocket-dependent substrate and does not affect or increases the protein kinase activity of the protein kinase towards a hydrophobic pocket-independent subsfrate. 60. A kit of parts useful in carrying out a method according to any one of claims 51 to 59, comprising (1) a mutated protein kinase as defined in claim 56 and/or the protein kinase which is not a mutated said protein kinase as defined in claim 56 and (2) a hydrophobic pocket-dependent substrate and a hydrophobic pocket-independent substrate of the said protein kinase. 61. The use of a compound capable of inhibiting to a different extents the rate or degree of phosphorylation by a protein kinase having a hydrophobic pocket in the position equivalent to the hydrophobic pocket of mouse Protein Kinase A (PKA) that is defined by residues including Lys76, Leullό, Val80 and/or Lyslll of full-length mouse PKA (for example PDKl), of (1) a hydrophobic pocket-dependent substrate and (2) a hydrophobic pocket-independent substrate of the protein kinase, in the manufacture of a medicament for the freatment of a patient in need of inhibition to different extents of (1) phosphorylation of a hydrophobic pocket-dependent substrate of the said protein kinase and (2) phosphorylation of a hydrophobic pocket-dependent substrate of the said protein kinase. 62. The use according to claim 61 wherein the compound or composition inhibits to a greater degree the rate or degree of phosphorylation by the protein kinase of (1) a hydrophobic pocket-dependent substrate of the protein kinase than (2) a hydrophobic pocket-independent substrate of the protein kinase. 63. The use of claim 61 or 62 wherein the patient has diabetes or cancer.

Inventors: ALESSI DARIO, BIONDI RICARDO, ALESSI, DARIO, BIONDI, RICARDO
Applicants: ALESSI DARIO, BIONDI RICARDO, UNIV DUNDEE, ALESSI, DARIO, BIONDI, RICARDO, UNIVERSITY OF DUNDEE
Application year: 2000
Main classification: C12Q1
IPC codes: A61K38, A61K45, A61K48, A61P3, A61P9, A61P35, C07K14, C07K19, C12N1, C12N5, C12N9, C12N15, C12Q1, G01N33