UBIQUITIN LIGASE ASSAY

Abstract: The invention relates to assays for measuring ubiquitin ligase activity and for identifying modulators of ubiquitin ligase enzymes.

Description: UBIQUITIN LIGASE ASSAY FIELD OF THE INVENTION This invention is directed to assays for measuring the activity of ubitquitination enzymes. The invention is also directed to assays for identifying modulators of ubiquitination. BACKGROUND OF THE INVENTION Ubiquitin is a highly conserved 76 amino acid protein expressed in all eukaryotic cells. The levels of many intracellular proteins are regulated by a ubiquitin-dependent proteolytic process. This process involves the covalent ligation of ubiquitin to a target protein, resulting in a poly-ubiquitinated target protein which is rapidly detected and degraded by the 26S proteasome. The ubiquitination of these proteins is mediated by a cascade of enzymatic activity. Ubiquitin is first activated in an ATP-dependent manner by a ubiquitin activating enzyme (El). The C-terminus of a ubiquitin forms a high energy thiolester bond with El. The ubiquitin is then passed to a ubiquitin conjugating enzyme (E2; also called ubiquitin carrier protein), also linked to this second enzyme via a thiolester bond. The ubiquitin is finally linked to its target protein to form a terminal isopeptide bond under the guidance of a ubiquitin ligase (E3). In this process, chains of ubiquitin are formed on the target protein, each covalently ligated to the next through the activity of E3. The components of the ubiquitin ligation cascade have received considerable attention (for a review, see Weissman, Nature Reviews 2:169-178 (2001)). El and E2 are structurally related and well characterized enzymes. There are several species of E2 (at least 25 in mammals), some of which act in preferred pairs with specific E3 enzymes to confer specificity for different target proteins. While the nomenclature for E2 is not standardized across species, investigators in the field have addressed this issue and the skilled artisan can readily identify various E2 proteins, as well as species homologues (See Haas and Siepmann, FASEB J. 11:1257-1268 (1997)). E3 enzymes contain two separate activities: a ubiquitin ligase activity to conjugate ubiquitin to substrates and form polyubiquitin chains via isopeptide bonds, and a targeting activity to physically bring the ligase and substrate together. Substrate specificity of different E3 enzymes is the major determinant in the selectivity of the ubiquitin-dependent protein degradation process. Some E3 ubiquitin ligases are known to have a single subunit responsible for the ligase activity. Such E3 ligases that have been characterized include the HECT (homologous to E6-AP carboxy terminus) domain proteins, represented by the mammalian E6AP-E6 complex which functions as a ubiquitin ligase for the tumor suppressor p53 and which is activated by papillomavirus in cervical cancer (Huang et al., Science 286: 1321-26 (1999)). Single subunit ubiquitin ligases having a RING domain include Mdm2, which has also been shown to act as a ubiquitin ligase for p53, as well as Mdm2 itself. Other RING domain, single subunit E3 ligases include: TRAF6, involved in IKK activation; Cbl, which targets insulin and EGF; Sina/Siah, which targets DCC; Itchy, which is involved in haematopoesis (B, T and mast cells); and IAP, involved with inhibitors of apoptosis. The best characterized E3 ligase is the APC (anaphase promoting complex), which is a multi-subunit complex that is required for both entry into anaphase as well as exit from mitosis (see King et al., Science 274:1652-59 (1996) for review). The APC plays a crucial role in regulating the passage of cells through anaphase by promoting ubiquitin-dependent proteolysis of many proteins. In addition to degrading the mitotic B-type cyclin for inactivation of CDC2 kinase activity, the APC is also required for degradation of other proteins for sister chromatid separation and spindle disassambly. Most proteins known to be degraded by the APC contain a conserved nine amino acid motif known as the "destruction box" that targets them for ubiquitination and subsequent degradation. However, proteins that are degraded during Gl, including Gl cyclins, CDK inhibitors, transcription factors and signaling intermediates, do not contain this conserved amino acid motif. Instead, substrate phosphorylation appears to play an important role in targeting their interaction with an E3 ligase for ubiquitination (see Hershko et al., Ann. Rev. Biochem. 67:429-75 (1998)). In eukaryotes, a family of complexes with E3 ligase activity play an important role in regulating Gl progression. These complexes, called SCF's, consist of at least three subunits, SKP1, Cullins (having at least seven family members) and an F-box protein (of which hundreds of species are known) which bind directly to and recruit the substrate to the E3 complex. The combinatorial interactions between the SCF's and a recently discovered family of RING finger proteins, the ROC/APCl 1 proteins, have been shown to be the key elements conferring ligase activity to E3 protein complexes. Particular ROC/Cullin combinations can regulate specific cellular pathways, as exemplified by the function of APCl 1-APC2, involved in the proteolytic control of sister chromatid separation and exit from telophase into Gl in mitosis (see King et al., supra; Koepp et al., Cell 97:431-34 (1999)), and ROCl-Cullin 1, involved in the proteolytic degradation of I κ B α in NF- K B/I K B mediated transcription regulation (Tan et al., Mol. Cell 3(4):527-533 (1999); Laney et al., Cell 97:427-30 (1999)). Because the E3 complex is the major determinant of selection for protein degradation by the ubiquitin-dependent proteolytic process, modulators of E3 ligase activity may be used to upregulate or downregulate specific molecules involved in cellular signal transduction. Disease processes can be treated by such up- or down regulation of signal transducers to enhance or dampen specific cellular responses. This principle has been used in the design of a number of therapeutics, including Phosphodiesterase inhibitors for airway disease and vascular insufficiency, Kinase inhibitors for malignant transformation and Proteasome inhibitors for inflammatory conditions such as arthritis. Due to the importance of ubiquitination in cellular regulation and the wide array of different possible components in ubiquitin-dependent proteolysis, there is a need for a fast and simple means for assaying E3 ligase activity. Furthermore, such an assay would be very useful for the identification of modulators of E3 ligase. Accordingly, it is an object of the present invention to provide methods of assaying ubiquitin ligase activity, which methods may further be used to identify modulators of ubiquitin ligase activity. DESCRIPTION OF THE RELATED ART Tan et al, supra, disclose that ROCl/Cul 1 catalyzes ubiquitin polymerization in the absence of target protein substrate. Ohta et al., Mol. Cell 3(4):535-541 (1999) disclose that APC11/APC2 also catalyze ubiquitin polymerization in the absence of target protein .substrate, and that this activity is dependent on the inclusion of the proper E2 species. Rolfe et al., U.S. Patent No. 5,968,761 disclose an assay for identifying inhibitors of ubiquitination of a target regulatory protein. SUMMARY OF THE INVENTION The present invention provides methods for assaying ubiquitin ligase activity and screening for agents which modulate ubiquitin ligase activity. In one aspect, a method of assaying ubiquitin ligase activity is provided involving the steps of combining ubiquitin, El, E2 and E3 and measuring the amount of ubiquitin bound to E3. This method may further involve combining a candidate ubiquitin ligase modulator in the combining step. This method does not require a specific target protein to be ubiquitinated. In a preferred embodiment, a substrate protein for ubiquitination other than ubiquitin itself is specifically excluded. In one embodiment of the assay described above, ubiquitin is in the form of tagl- ubiquitin. In another embodiment, E3 is in the form of tag2-E3. In these embodiments tagl may be a label or a partner of a binding pair. In one embodiment, tagl is a fluorescent label, in which case measuring the amount of ubiquitin bound to E3 may be by measuring luminescence. In another embodiment, tagl is a member of a binding pair chosen from the group antigen, biotin and CBP. In this latter embodiment, the partner of a binding pair may be labeled by indirect labeling, which may be by a fluorescent label or a label enzyme. The label enzyme may be horseradish peroxidase, alkaline phosphatase or glucose oxidase. When the indirect labeling is by a fluorescent label, measuring the amount of ubiquitin bound to E3 may be by measuring luminescence. In the case that the indirect labeling is by a label enzyme, said enzyme may be reacted with a substrate which produces a fluorescent product, in which case, measuring the amount of ubiquitin bound to E3 may be by measuring luminescence. In one embodiment of the method above, tagl is a FLAG antigen. In this embodiment, indirect labeling may be by anti-FLAG. In one aspect of the above method, tag2 is a surface binding molecule, which may be His-tag. In this latter case, the assaying may be performed in a multi-well plate comprising a surface substrate comprising nickel. In a different embodiment of the method above, when tagl is a fluorescent label, the combining step further includes combining tag3 -ubiquitin. Tag3 may be the second member of a FRET pair with tagl or it may be a quencher of tagl . In this embodiment, measuring the amount of ubiquitin bound to E3 may be by measuring fluorescent emission, which may involve measuring the fluorescent emission spectrum. In this last embodiment, the method may further comprise comparing the measured fluorescent emission spectrum with the fluorescent emission spectrum of unbound tagl- and tag3- ubiquitin. When measuring the amount of ubiquitin bound to E3 is by measuring the fluorescent emission spectrum, this measuring may be continuous or at specific time points following the original combining of materials. In another aspect of the invention, a method of identifying modulators of ubiquitination enzymes is provided. This method involves combining tagl -ubiquitin, a candidate modulator, El, E2 and tag2-E3 and measuring the amount of tagl - ubiquitin bound to tag2-E3. In another embodiment, this method further comprises combining tagl -ubiquitin, a candidate modulator, El and tag2-E2 and measuring the amount of tagl -ubiquitin bound to the tag2-E2. In a preferred embodiment, target protein (i.e., a substrate protein other than ubiquitin itself) is specifically excluded in the method. In the embodiments of the method of identifying modulators of ubiquitination enzymes , tagl may be a label or a partner of a binding pair. If tagl is a label, it may be a fluorescent label, in which case, measuring the amount of bound tagl -ubiquitin may be by measuring luminescence. If tagl is a partner of a binding pair, the potential binding pair partners, labeling options and subsequent measuring options are substantially as described for tagl above for the method of assaying ubiquitin ligase activity. In the above method of identifying modulators of ubiquitination enzymes, tag2 and tag3 may be surface substrate binding molecules. Options for such molecules and conditions for performing the method are as described for the method of assaying ubiquitin ligase activity. In another aspect of the invention, a method of assaying ubiquitination enzyme activity is provided. This method comprises combining tagl -ubiquitin and tag2- ubiquitin, El, E2 and E3 under conditions in which ubiquitination can take place and measuring the amount or rate of ubiquitination. In this embodiment, tagl and tag2 constitute a FRET pair or tagl is a fluorescent label and tag2 is a quencher of tagl . In one embodiment, the method includes combining a candidate ubiquitination modulator with the other components. In a preferred embodiment of this method, measuring is by measuring the fluorescent emission spectrum from the combination, preferably continuously or at specific time points following combining the components. These measurements may be compared to the fluorescent emission spectrum of unbound tagl and tag2 ubiquitin. Also provided herein is a method of identifying a ubiquitination modulator. This method involved combining a candidate ubiquitination modulator, tagl -ubiquitin and tag2-ubiquitin, El, E2 and E3 under conditions in which ubiquitination can take place and measuring the amount or rate of ubiquitination. In this embodiment, tagl and tag2 constitute a FRET pair or tagl is a fluorescent label and tag2 is a quencher of tagl. In one embodiment, the method includes combining a candidate ubiquitination modulator with the other components. In a preferred embodiment of this method, measuring is by measuring the fluorescent emission spectrum from the combination, preferably continuously or at specific time points following combining the components. These measurements may be compared to the fluorescent emission spectrum of unbound tagl and tag2 ubiquitin. In the latter two assays described, the ubiquitin may be in the form tagl, 3 -ubiquitin and tag2,3 -ubiquitin, wherein tag3 is a member of a binding pair, preferably FLAG. In another embodiment of these assays, E3 may be in the form of tag4-E3, wherein tag4 is a surface substrate bonding molecule. In still another aspect of the invention, compositions are provided for use in assaying ubiquitination. The composition comprises tagl -ubiquitin and tag2-ubiquitin, wherein tagl and tag2 constitute a FRET pair or tagl is a fluorescent label and tag2 is a quencher of tagl. In one embodiment, the composition further comprises El, E2 and E3. In a preferred embodiment, the composition further comprises a candidate ubiquitination modulator. In yet another embodiment, the composition comprises a target protein. In addition, provided herein are compositions for use in assaying for a ubiquitination modulator. The composition comprises a candidate ubiquitination modulator, tagl- ubiquitin and tag2-ubiquitin, wherein tagl and tag2 constitute a FRET pair or tagl is a fluorescent label and tag2 is a quencher of tagl. In one embodiment, the composition further comprises El, E2 and E3. In one embodiment, the composition comprises a target protein. In preferred embodiments of the assays and compositions described above, E2 is selected from the group consisting of Ubc5, Ubc3, Ubc4 and UbcX. In ap referred embodiment, E3 comprises a RING finger protein, preferably selected from the group consisting of ROC 1, ROC2 and APCl 1. In a preferred embodiment, E3 comprises a Cullin, preferably selected from the group consisting of CUL1, CUL2, CUL3, CUL4A, CUL4B, CUL5 and APC2. In a preferred embodiment, E3 comprises a RING finger protein/Cullin combination, preferably selected from the group consisting of APCl 1/APC2, ROC1/CUL1, ROC1/CUL2 and ROC2/CUL5. Other aspects of the invention will become apparent to the skilled artisan from the following description of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the relative amounts of fluorescently labeled ubiquitin in a ubiquitin activating and conjugating assay. In these experiments, E2 is His-Ubch5c. Figure 2 shows the relative amounts of ubiquitin ligase activity resulting from various combinations of ubiquitination enzymes. In these experiments, E3 comprises the RING finger protein ROC1 and the Cullin Cull. Figure 3 shows relative ubiquitin ligase activity in an assay combining ubiquitin, El, E2 and E3. Figure 3 A shows relative ubiquitin ligase activity using varying amounts of El in the presence and absence of DMSO. Figure 3B shows relative ubiquitin ligase activity using varying amounts of ubiquitin and E3. Figure 4 shows the signal to noise ratio of fluorescent label in a ubiquitin ligase activity assay utilizing Flag-ubiquitin and an anti-Flag/anti-mouse antibody conjugated to HRP and Luminol fluorescent HRP substrate. The signal was measured from a reaction composition comprising El, E2 and E3, which E3 specifically bound the reaction receptacle surface substrate. The background was measured as the amount of fluorescence present after performing the assay in the absence of E3. Figure 5 shows the concentration-dependent effect of two ubiquitin ligase activity modulators in assays measuring ubiquitin ligase activity with two different E3 enzymes. Figure 5 A shows a concentration-dependent reduction in ubiquitin ligase activity in assays comprising either ROCl/Cull or ROC2/Cul5 as the components of the E3 ubiquitin ligase. Figure 5B shows a slightly different pattern of concentration- dependent reduction of ubiquitin ligase activity for another modulator. Figure 6 shows the proportions of ubiquitin ligase activity and ubiquitin conjugating activity in the presence and absence of two candidate ubiquitin ligase enzyme modulators for combinations of El, E2 and E3 and combinations of enzymes El and E2. Figure 6A shows a candidate ubiquitination enzyme modulator that affects only E3. Figure 6B shows candidate ubiquitination enzyme modulator that affects enzymes other than E3. Figure 7 shows the concentration-dependent effects of two candidate ubiquitin ligase modulators on ubiquitin ligase activity and ubiquitin conjugating activity. Figure 7A shows the results for a candidate modulator having a concentration-dependent effect on ubiquitin ligase activity (E1+E2+E3), but not have on ubiquitin conjugating activity (E1+E2), thus affecting only the E3 ligase. Figure 7B shows the results for a candidate modulator having a concentration-dependent effect on both ubiquitin conjugating activity and ubiquitin ligase activity, thus affecting a component other than the E3 ligase. Figures 8 A and 8B show the nucleic acid sequence encoding rabbit El and the amino acid sequence of rabbit E 1 , respectively. Figures 9 A and 9B show the nucleic acid sequence encoding the E2 Ubc5c and the amino acid sequence of the E2 Ubc5c, respectively. Figure 10 shows the amino acid sequence of the RING finger protein APC 11. Figure 11 shows the amino acid sequence of the RING finger protein ROC1. Figures 12A and 12 B show the nucleic acid sequence encoding the RING finger protein ROC2 and the amino acid sequence of ROC2, respectively. Figures 13A and 13B show the nucleic acid sequence encoding the Cullin CUL5 and the amino acid sequence of CUL5, respectively. Figures 14A and 14B show the nucleic acid sequence encoding the Cullin APC2 and the amino acid sequence of APC2, respectively. Figures 15 A, 15B and 15C show the amino acid sequences of human ubiquitin, Flag- ubiquitin and Flag-Cys-ubiquitin, respectively. The Flag and Flag-Cys portions of the sequence are shown in bold. Figures 16A and 16B show the E3 ligase-dependent incorporation of Flag-Ala-Cys- ubiquitin labeled with FRET fluorophores into E3 -ubiquitin complex. Isolation by HPLC shows emissions from free ubiquitin and ubiquitin attached to the E3 ligase. The traces show fluorescent emission at the wavelength described below, under excitation at 336 nm, the optimal excitation wavelength for IAEDANS. Figure 16A shows the fluorescence signals of IAEDANS (490 nm; larger peak) and fluorescein (515 nm; smaller peak) labeled ubiquitin following combination with El and E2 only. The free ubiquitin was isolation using high performance liquid chromatography (HPLC). Figure 16B shows the fluorescence signals of IAEDANS (490 nm; larger peak at each elution volume) and fluorescein (515 nm; smaller peak at each elution volume) labeled ubiquitin following combination with El and E2 and E3 (Rocl/Cull). The dashed line shows optical density of the protein solution (scale on right), revealing the high sensitivity of the fluorophores despite a very low concentration of protein. Figure 17 shows the fluorescence emission spectra of free ubiquitin labeled with the FRET donor/acceptor pair EDANS and fluorescein under excitation at 336 nm. The dashed line shows the emission spectra of free labeled ubiquitin (reactants), while the solid line shows the emission spectra of labeled ubiquitin bound to E3 (products). The greatly increased 515 :490 nm emission ratio of the E3 -bound ubiquitin as compared with the free ubiquitin shows the energy transfer from the EDANS donor to the fluorescein acceptor of this FRET donor/acceptor pair. DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to methods for assaying ubiquitin ligase activity. In a broad embodiment, the method provides measuring ubiquitin ligase activity directly where the reaction has occurred, thus obviating the need for target proteins and subsequent analysis such as separating ligated from unligated material in an SDS PAGE procedure. This allows multi-well array analysis and high throughput screening techniques for modulators of ubiquitination activity. In addition, the present methods allow the analysis of many different combinations of E3 components and E2/E3 combinations, without requiring prior identification of specific target substrates. In general, the method involves combining ubiquitin and ubiquitin ligation enzymes and measuring the amount of ubiquitin ligated to a ubiquitination substrate protein. In a preferred embodiment, the ubiquitination substrate protein is ubiquitin itself, and what is measured is poly-ubiquitin chains produced in the ligase reaction. Therefore, as used herein, "ubiquitination substrate protein" means a protein to which ubiquitin is bound through the activity of ubiquitination enzymes and "ubiquitination" and grammatical equivalents thereof means the binding of ubiquitin to a substrate protein. In a preferred embodiment, no specific target protein is used to measure ubiquitin ligase activity. By "target protein" herein is meant a protein other than ubiquitin to which ubiquitin is ligated by ubiquitination enzymes. In this embodiment, preferably, the poly-ubiquitin chains measured are bound to E3. In another preferred embodiment, the poly-ubiquitin chains measured may be bound to E3 or not. In a preferred embodiment, E3 is attached to the surface of a reaction vessel, such as the well of a multi-well plate. This embodiment facilitates the separation of ligated ubiquitin from unligated ubiquitin. Means of attaching E3 to the surface of a reaction vessel are described below. This embodiment allows the ubiquitin ligase reaction and detection and measurement of ligated ubiquitin to occur in the same vessel, making the assay useful for high-throughput screening applications. In another preferred embodiment, E3 is free in solution. In this embodiment, ubiquitination activity is monitored using a system that produces a signal which varies with the extent of ubiquitination, such as the fluorescence resonance energy transfer (FRET) system described in detail below. In a preferred embodiment, the ubiquitin is labeled, either directly or indirectly, as further described below, and the amount of label is measured. This allows for easy and rapid detection and measurement of ligated ubiquitin, making the assay useful for high-throughput screening applications. In one preferred embodiment, the signal of the label varies with the extent of ubiquitination, such as in the FRET system described below. One of ordinary skill in the art will recognize the applicability of the present invention to screening for agents which modulate ubiquitination. Accordingly, the present invention provides methods and compositions for assaying ubiquitin ligase activity. By "ubiquitin" herein is meant a polypeptide which is ligated to another polypeptide by ubiquitin ligase enzymes. The ubiquitin can be from any species of organism, preferably a eukaryotic species. Preferably, the ubiquitin is mammalian. More preferably, the ubiquitin is human ubiquitin. In a most preferred embodiment, the ubiquitin has the amino acid sequence depicted in figure 15 A. In a preferred embodiment, when ubiquitin is ligated to a target protein, that protein is targeted for degradation by the 26S proteasome. Preferred embodiments of the invention utilize a 76 amino acid human ubiquitin. Other embodiments utilize variants of ubiquitin, as further described below. Also encompassed by "ubiquitin" is naturally occurring alleles and man-made variants of such a 76 amino acid polypeptide. In a preferred embodiment, the ubiquitin has the amino acid sequence of that depicted in ATCC accession number P02248, incorporated herein by reference. ATCC accession numbers are found in Genbank. Sequences of GenBank accession numbers are incorporated herein by reference. GenBank is known in the art, see, e.g., Benson, DA, et al., Nucleic Acids Research 26:1-7 (1998) and http://www.ncbi.nlm.nih.gov/. Preferably, the ubiquitin has the amino acid sequence depicted in figure 15 A. In a preferred embodiment, variants of ubiquitin have an overall amino acid sequence identity of preferably greater than about 75%, more preferably greater than about 80%, even more preferably greater than about 85% and most preferably greater than 90% of the amino acid sequence depicted in figure 15 A. In some embodiments the sequence identity will be as high as about 93 to 95 or 98%. In another preferred embodiment, a ubiquitin protein has an overall sequence similarity with the amino acid sequence depicted in figure 15 A of greater than about 80%, more preferably greater than about 85%, even more preferably greater than about 90% and most preferably greater than 93%. In some embodiments the sequence identity will be as high as about 95 to 98 or 99%. As is known in the art, a number of different programs can be used to identify whether a protein (or nucleic acid as discussed below) has sequence identity or similarity to a known sequence. Sequence identity and/or similarity is determined using standard techniques known in the art, including, but not limited to, the local sequence identity algorithm of Smith & Waterman, Adv. Appl. Math. 2:482 (1981), by the sequence identity alignment algorithm of Needleman & Wunsch, J. Mol. Biol. 48:443 (1970), by the search for similarity method of Pearson & Lipman, PNAS USA 85:2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Drive, Madison, WI), the Best Fit sequence program described by Devereux et al, Nucl. Acid Res. i2:387-395 (1984), preferably using the default settings, or by inspection. Preferably, percent identity is calculated by FastDB based upon the following parameters: mismatch penalty of 1; gap penalty of 1; gap size penalty of 0.33; and joining penalty of 30, "Current Methods in Sequence Comparison and Analysis," Macromolecule Sequencing and Synthesis, Selected Methods and Applications, pp 127-149 (1988), Alan R. Liss, Inc. An example of a useful algorithm is PILEUP. PILEUP creates a multiple sequence alignment from a group of related sequences using progressive, pairwise alignments. It can also plot a tree showing the clustering relationships used to create the alignment. PILEUP uses a simplification of the progressive alignment method of Feng & Doolirtle, J. Mol. Evol. 35:351-360 (1987); the method is similar to that described by Higgins & Sharp CABIOS 5:151-153 (1989). Useful PILEUP parameters including a default gap weight of 3.00, a default gap length weight of 0.10, and weighted end gaps. Another example of a useful algorithm is the BLAST algorithm, described in Altschul et al., J. Mol. Biol. 215, 403-410, (1990) and Karlin et al, PNAS USA 90:5873-5787 (1993). A particularly useful BLAST program is the WU-BLAST-2 program which was obtained from Altschul et al., Methods in Enzvmology. 266: 460-480 (1996); http://blast.wustl/edu/blast/ README.html]. WU-BLAST-2 uses several search parameters, most of which are set to the default values. The adjustable parameters are set with the following values: overlap span— 1, overlap fraction = 0.125, word threshold (T) = 11. The HSP S and HSP S2 parameters are dynamic values and are established by the program itself depending upon the composition of the particular sequence and composition of the particular database against which the sequence of interest is being searched; however, the values may be adjusted to increase sensitivity. An additional useful algorithm is gapped BLAST as reported by Altschul et al. Nucleic Acids Res. 25:3389-3402. Gapped BLAST uses BLOSUM-62 substitution scores; threshold J 7 parameter set to 9; the two-hit method to trigger ungapped extensions; charges gap lengths of & a cost of 10+k; X u set to 16, and g set to 40 for database search stage and to 67 for the output stage of the algorithms. Gapped alignments are triggered by a score corresponding to ~22 bits. A percent amino acid sequence identity value is determined by the number of matching identical residues divided by the total number of residues of the "longer" sequence in the aligned region. The "longer" sequence is the one having the most actual residues in the aligned region (gaps introduced by WU-Blast-2 to maximize the alignment score are ignored). The alignment may include the introduction of gaps in the sequences to be aligned. In addition, for sequences which contain either more or fewer amino acids than the amino acid sequence depictd in figure 15 A, it is understood that in one embodiment, the percentage of sequence identity will be determined based on the number of identical amino acids in relation to the total number of amino acids. Thus, for example, sequence identity of sequences shorter than that of the sequence depicted in figure 15 A, as discussed below, will be determined using the number of amino acids in the shorter sequence, in one embodiment. In percent identity calculations relative weight is not assigned to various manifestations of sequence variation, such as, insertions, deletions, substitutions, etc. In one embodiment, only identities are scored positively (+1) and all forms of sequence variation including gaps are assigned a value of "0", which obviates the need for a weighted scale or parameters as described below for sequence similarity calculations. Percent sequence identity can be calculated, for example, by dividing the number of matching identical residues by the total number of residues of the "shorter" sequence in the aligned region and multiplying by 100. The "longer" sequence is the one having the most actual residues in the aligned region. Ubiquitin proteins of the present invention may be shorter or longer than the amino acid sequence depicted in figure 15 A. Thus, in a preferred embodiment, included within the definition of ubiquitin are portions or fragments of the amino acid sequence depicted in figure 15 A. In one embodiment herein, fragments of ubiquitin are considered ubiquitin proteins if they are ligated to another polypeptide by ubiquitin ligase enzymes. In addition, as is more fully outlined below, ubiquitin can be made longer than the amino acid sequence depicted in figure 15 A; for example, by the addition of tags, the addition of other fusion sequences, or the elucidation of additional coding and non- coding sequences. As described below, the fusion of a ubiquitin peptide to a fluorescent peptide, such as Green Fluorescent Peptide (GFP), is particularly preferred. The ubiquitin protein, as well as other proteins of the present invention, are preferably recombinant. A "recombinant protein" is a protein made using recombinant techniques, i.e. through the expression of a recombinant nucleic acid as described below. In a preferred embodiment, the ubiquitin of the invention is made through the expression of a nucleic acid as depicted in ATCC accession number M26880 or AB003730, or a fragment thereof. In a most preferred embodiment, the nucleic acid encodes the amino acid sequence depicted in figure 15 A. A recombinant protein is distinguished from naturally occurring protein by at least one or more characteristics. For example, the protein may be isolated or purified away from some or all of the proteins and compounds with which it is normally associated in its wild type host, and thus may be substantially pure. For example, an isolated protein is unaccompanied by at least some of the material with which it is normally associated in its natural state, preferably constituting at least about 0.5%, more preferably at least about 5% by weight of the total protein in a given sample. A substantially pure protein comprises at least about 75% by weight of the total protein, with at least about 80% being preferred, and at least about 90% being particularly preferred. The definition includes the production of a protein from one organism in a different organism or host cell. Alternatively, the protein may be made at a significantly higher concentration than is normally seen, through the use of an inducible promoter or high expression promoter, such that the protein is made at increased concentration levels. Alternatively, the protein may be in a form not normally found in nature, as in the addition of an epitope tag or amino acid substitutions, insertions and deletions, as discussed below. As used herein and further defined below, "nucleic acid" may refer to either DNA or RNA, or molecules which contain both deoxy- and ribonucleotides. The nucleic acids include genomic DNA, cDNA and oligonucleotides including sense and anti-sense nucleic acids. Such nucleic acids may also contain modifications in the ribose- phosphate backbone to increase stability and half life of such molecules in physiological environments. The nucleic acid may be double stranded, single stranded, or contain portions of both double stranded or single stranded sequence. As will be appreciated by those in the art, the depiction of a single strand ("Watson") also defines the sequence of the other strand ("Crick"); thus the sequences depicted in Figures 1 and 3 also include the complement of the sequence. By the term "recombinant nucleic acid" herein is meant nucleic acid, originally formed in vitro, in general, by the manipulation of nucleic acid by endonucleases, in a form not normally found in nature. Thus an isolated nucleic acid, in a linear form, or an expression vector formed in vitro by ligating DNA molecules that are not normally joined, are both considered recombinant for the purposes of this invention. It is understood that once a recombinant nucleic acid is made and reintroduced into a host cell or organism, it will replicate non- recombinantly, i.e. using the in vivo cellular machinery of the host cell rather than in vitro manipulations; however, such nucleic acids, once produced recombinantly, although subsequently replicated non-recombinantly, are still considered recombinant for the purposes of the invention. The terms "polypeptide" and "protein" may be used interchangeably throughout this application and mean at least two covalently attached amino acids, which includes proteins, polypeptides, oligopeptides and peptides. The protein may be made up of naturally occurring amino acids and peptide bonds, or synthetic peptidomimetic structures. Thus "amino acid", or "peptide residue", as used herein means both naturally occurring and synthetic amino acids. For example, homo-phenylalanine, citrulline and noreleucine are considered amino acids for the purposes of the invention. "Amino acid" also includes imino acid residues such as proline and hydroxyproline. The side chains may be in either the (R) or the (S) configuration. In the preferred embodiment, the amino acids are in the (S) or L-configuration. If non- naturally occurring side chains are used, non-amino acid substituents may be used, for example to prevent or retard in vivo degradation. In one embodiment, the present invention provides compositions containing protein variants, for example ubiquitin, El, E2 and/or E3 variants. These variants fall into one or more of three classes: substitutional, insertional or deletional variants. These variants ordinarily are prepared by site specific mutagenesis of nucleotides in the DNA encoding a protein of the present compositions, using cassette or PCR mutagenesis or other techniques well known in the art, to produce DNA encoding the variant, and thereafter expressing the DNA in recombinant cell culture as outlined above. However, variant protein fragments having up to about 100-150 residues may be prepared by in vitro synthesis using established techniques. Amino acid sequence variants are characterized by the predetermined nature of the variation, a feature that sets them apart from naturally occurring allelic or interspecies variation of the protein amino acid sequence. The variants typically exhibit the same qualitative biological activity as the naturally occurring analogue, although variants can also be selected which have modified characteristics as will be more fully outlined below. ' While the site or region for introducing an amino acid sequence variation is predetermined, the mutation per se need not be predetermined. For example, in order to optimize the performance of a mutation at a given site, random mutagenesis may be conducted at the target codon or region and the expressed variants screened for the optimal desired activity. Techniques for making substitution mutations at predetermined sites in DNA having a known sequence are well known, for example, M 13 primer mutagenesis and PCR mutagenesis. Rapid production of many variants may be done using techniques such as the method of gene shuffling, whereby fragments of similar variants of a nucleotide sequence are allowed to recombine to produce new variant combinations. Examples of such techniques are found in U.S. Patent Nos. 5,605,703; 5,811,238; 5,873,458; 5,830,696; 5,939,250; 5,763,239; 5,965,408; and 5,945,325, each of which is incorporated by reference herein in its entirety. Screening of the mutants is done using ubiquitin ligase activity assays of the present invention. Amino acid substitutions are typically of single residues; insertions usually will be on the order of from about 1 to 20 amino acids, although considerably larger insertions may be tolerated. Deletions range from about 1 to about 20 residues, although in some cases deletions may be much larger. Substitutions, deletions, insertions or any combination thereof may be used to arrive at a final derivative. Generally these changes are done on a few amino acids to minimize the alteration of the molecule. However, larger changes may be tolerated in certain circumstances. When small alterations in the characteristics of the protein are desired, substitutions are generally made in accordance with the following chart: Chart I Original Residue Exemplary Substitutions Ala Ser Arg Lys Asn Gin, His Asp Glu Cys Ser, Ala Gin Asn Glu Asp Gly Pro His Asn, Gin He Leu, Val Leu He, Val Lys Arg, Gin, Glu Met Leu, He Phe Met, Leu, Tyr Ser Thr Thr Ser Trp Tyr Tyr Trp, Phe Val He, Leu Substantial changes in function or immunological identity are made by selecting substitutions that are less conservative than those shown in Chart I. For example, substitutions may be made which more significantly affect: the structure of the polypeptide backbone in the area of the alteration, for example the alpha-helical or beta-sheet structure; the charge or hydrophobicity of the molecule at the target site; or the bulk of the side chain. The substitutions which in general are expected to produce the greatest changes in the polypeptide's properties are those in which (a) a hydrophilic residue, e.g. seryl or threonyl, is substituted for (or by) a hydrophobic residue, e.g. leucyl, isoleucyl, phenylalanyl, valyl or alanyl; (b) a cysteine or proline is substituted for (or by) any other residue; (c) a residue having an electropositive side chain, e.g. lysyl, arginyl, or histidyl, is substituted for (or by) an electronegative residue, e.g. glutamyl or aspartyl; or (d) a residue having a bulky side chain, e.g. phenylalanine, is substituted for (or by) one not having a side chain, e.g. glycine. The variants typically exhibit the same qualitative biological activity and will elicit the same immune response as the naturally-occurring analogue, although variants also are selected to modify the characteristics of the proteins as needed. Alternatively, the variant may be designed such that the biological activity of the protein is altered. For example, glycosylation sites may be altered or removed. Covalent modifications of polypeptides are included within the scope of this invention. One type of covalent modification includes reacting targeted amino acid residues of a polypeptide with an organic derivatizing agent that is capable of reacting with selected side chains or the N-or C-terminal residues of a polypeptide. Derivatization with bifunctional agents is useful, for instance, for crosslinking a protein to a water-insoluble support matrix or surface for use in the method for screening assays, as is more fully described below. Commonly used crosslinking agents include, e.g., l,l-bis(diazoacetyl)-2-phenylethane, glutaraldehyde, -hydroxy- succinimide esters, for example, esters with 4-azidosalicylic acid, homobifunctional imidoesters, including disuccinimidyl esters such as 3,3'-dithiobis(succinimidyl- propionate), bifunctional maleimides such as bis-N-maleimido-l,8-octane and agents such as methyl-3-[(p-azidophenyl)dithio]propioimidate. Other modifications include deamidation of glutaminyl and asparaginyl residues to the corresponding glutamyl and aspartyl residues, respectively, hydroxylation of proline and lysine, phosphorylation of hydroxyl groups of seryl or threonyl residues, methylation of the "-amino groups of lysine, arginine, and histidine side chains [T.E. Creighton, Proteins: Structure and Molecular Properties. W.H. Freeman & Co., San Francisco, pp. 79-86 (1983)], acetylation of the N-terminal amine, and amidation of any C-terminal carboxyl group. Another type of covalent modification of a polypeptide included within the scope of this invention comprises altering the native glycosylation pattern of the polypeptide. "Altering the native glycosylation pattern" is intended for purposes herein to mean deleting one or more carbohydrate moieties found in native sequence polypeptide, and/or adding one or more glycosylation sites that are not present in the native sequence polypeptide. Addition of glycosylation sites to polypeptides may be accomplished by altering the amino acid sequence thereof. The alteration may be made, for example, by the addition of, or substitution by, one or more serine or threonine residues to the native sequence polypeptide (for O-linked glycosylation sites). The amino acid sequence may optionally be altered through changes at the DNA level, particularly by mutating the DNA encoding the polypeptide at preselected bases such that codons are generated that will translate into the desired amino acids. Another means of increasing the number of carbohydrate moieties on a polypeptide is by chemical or enzymatic coupling of glycosides to the polypeptide. Such methods are described in the art, e.g., in WO 87/05330 published 11 September 1987, and in Aplin and Wriston, CRC Grit. Rev. Biochem.. pp. 259-306 (1981). Removal of carbohydrate moieties present on the polypeptide may be accomplished chemically or enzymatically or by mutational substitution of codons encoding for amino acid residues that serve as targets for glycosylation. Chemical deglycosylation techniques are known in the art and described, for instance, by Hakimuddin, et al., Arch. Biochem. Biophvs.. 259:52 (1987) and by Edge et al, Anal. Biochem.. 118:131 (1981). Enzymatic cleavage of carbohydrate moieties on polypeptides can be achieved by the use of a variety of endo-and exo-glycosidases as described by Thotakura et al., Meth. Enzvmol.138:350 (1987). Another type of covalent modification of a protein comprises linking the polypeptide to one of a variety of nonproteinaceous polymers, e.g., polyethylene glycol, polypropylene glycol, or polyoxyalkylenes, in the manner set forth in U.S. Patent Nos. 4,640,835; 4,496,689; 4,301,144; 4,670,417; 4,791,192 or 4,179,337. Polypeptides of the present invention may also be modified in a way to form chimeric molecules comprising a first polypeptide fused to another, heterologous polypeptide or amino acid sequence. In one embodiment, such a chimeric molecule comprises a fusion of a ubiquitin polypeptide (or an E2 or an E3, as defined below) with a tag polypeptide which provides an epitope to which an anti-tag antibody can selectively bind. The epitope tag is generally placed at the amino-or carboxyl-terminus of the polypeptide. The presence of such epitope-tagged forms of a polypeptide can be detected using an antibody against the tag polypeptide. Also, providing an epitope tag enables the polypeptide to be readily purified by affinity purification using an anti-tag antibody or another type of affinity matrix that binds to the epitope tag. In an alternative embodiment, the chimeric molecule may comprise a fusion of a polypeptide disclosed herein with an immunoglobulin or a particular region of an immunoglobulin. For a bivalent form of the chimeric molecule, such a fusion could be to the Fc region of an IgG molecule. Tags for components of the invention are defined and described in detail below. The present invention provides methods for assaying ubiquitin ligase activity. By "ubiquitin ligase" is meant a ubiquitination enzyme capable of catalyzing the covalent binding of a ubiquitin to another protein. Preferred embodiments of the invention involve combining ubiquitin and ubiquitination enzymes, including ubiquitin ligase, under conditions in which ubiquitination may take place, and measuring the amount of ubiquitin (poly-ubiquitin) bound to the ubiquitin ligase. In a preferred embodiment, the ubiquitin ligase is an E3 ubiquitin ligase, defined below. Embodiments of the present invention involve binding ubiquitin to a ubiquitination substrate protein. By "ubiquitination substrate protein" is meant a protein to which the ubiquitin ligase can catalyze the covalent binding of ubiquitin and includes target proteins and ubiquitin. In a preferred embodiment, the ubiquitination substrate protein is ubiquitin and the ubiquitin ligase catalyzes the formation of polyubiquitin chains. In a preferred embodiment, the polyubiquitin chains are formed by the ubiquitin ligase in the absence of any target protein. In one aspect, the invention provides methods for assaying ubiquitination. In these assays, the interaction of the different ubiquitination enzymes, the interaction of different subunits of individual ubiquitination enzymes, and the influence of candidate ubiquitination modulators can be observed and measured. In a preferred embodiment, the invention is directed to a method of assaying ubiquitin ligase activity. By "ubiquitin ligase activity", "ubiquitin ligation" and grammatical equivalents thereof is meant the catalysis of the covalent binding of ubiquitin to a substrate protein. Preferably, each ubiquitin is bound such that a subsequent ubiquitm polypeptide may be bound to it, to form chains comprising a plurality of ubiquitin molecules. In a preferred embodiment, ubiquitin ligase activity occurs in the absence of target proteins, thus the substrate protein is ubiquitin. In a preferred embodiment, the invention is additionally directed to a method of assaying ubiquitin activating activity. By "ubiquitin activating activity', "ubiquitin activation" and grammatical equivalents thereof is meant the binding of ubiquitin and El. Preferably, El forms a high energy thiolester bond with the ubiquitin. In a preferred embodiment, the invention is also directed to a method of assaying ubiquitin conjugating activity. By "ubiquitm conjugating activity", "ubiquitm conjugation" and grammatical equivalents thereof is meant the binding of activated ubiquitin with an E2. As will be appreciated by those in the art, due to the presence of the high energy thiolester bond in the E2 -ubiquitin conjugate, conjugated ubiquitin may be joined to other ubiquitin at a low rate in the absence of the catalytic activity of E3. Therefore, some of the ubiquitin measured in a ubiquitin conjugating activity assay will be in the form of poly-ubiquitin. The present invention provides methods and compositions comprising combining ubiquitin with other components. By "combining" is meant the addition of the various components into a receptacle under conditions in which ubiquitin ligase activity or ubiquitination may take place. In a preferred embodiment, the receptacle is a well of a 96 well plate or other commercially available multiwell plate. In an alternate preferred embodiment, the receptacle is the reaction vessel of a FACS machine. Other receptacles useful in the present invention include, but are not limited to 384 well plates and 1536 well plates. Still other receptacles useful in the present invention will be apparent to the skilled artisan. The addition of the components may be sequential or in a predetermined order or grouping, as long as the conditions amenable to ubiquitin ligase activity are obtained. Such conditions are well known in the art, and further guidance is provided below. In a preferred embodiment, one or more components of the present invention comprise a tag. By "tag" is meant an attached molecule or molecules useful for the identification or isolation of the attached component. Components having a tag are referred to as "tag-X", wherein X is the component. For example, a ubiquitin comprising a tag is referred to herein as "tag-ubiquitin". Preferably, the tag is covalently bound to the attached component. When more than one component of a combination has a tag, the tags will be numbered for identification, for example "tagl- ubiquitin". Components may comprise more than one tag, in which case each tag will be numbered, for example "tag 1,2-ubiquitin". Preferred tags include, but are not limited to, a label, a partner of a binding pair, and a surface substrate binding molecule. As will be evident to the skilled artisan, many molecules may find use as more than one type of tag, depending upon how the tag is used. By "label" is meant a molecule that can be directly (i.e., a primary label) or indirectly (i.e., a secondary label) detected; for example a label can be visualized and/or measured or otherwise identified so that its presence or absence can be known. As will be appreciated by those in the art, the manner in which this is done will depend on the label. Preferred labels include, but are not limited to, fluorescent labels, label enzymes and radioisotopes. By "fluorescent label" is meant any molecule that may be detected via its inherent fluorescent properties. Suitable fluorescent labels include, but are not limited to, fluorescein, rhodamine, tetramethylrhodamine, eosin, erythrosin, coumarin, methyl- coumarins, pyrene, Malacite green, stilbene, Lucifer Yellow, Cascade Blue™, Texas Red, IAEDANS, EDANS, BODIPY FL, LC Red 640, Cy 5, Cy 5.5, LC Red 705 and Oregon green. Suitable optical dyes are described in the 1996 Molecular Probes Handbook by Richard P. Haugland, hereby expressly incorporated by reference. Suitable fluorescent labels also include, but are not limited to, green fluorescent protein (GFP; Chalfie, et al, Science 263(5148):802-805 (Feb 11, 1994); and EGFP; Clontech - Genbank Accession Number U55762 ), blue fluorescent protein (BFP; 1. Quantum Biotechnologies, Inc. 1801 de Maisonneuve Blvd. West, 8th Floor, Montreal (Quebec) Canada H3H 1J9; 2. Stauber, R. H. Biotechniques 24(3):462-471 (1998); 3. Heim, R. and Tsien, R. Y. Curr. Biol. 6: 178-182 (1996)), enhanced yellow fluorescent protein (EYFP; 1. Clontech Laboratories, Inc., 1020 East Meadow Circle, Palo Alto, CA 94303), luciferase (Ichiki, et al., J. Immunol. 150(12):5408-5417 (1993)), β-galactosidase (Nolan, et al., Proc Natl Acad Sci USA 85(8):2603-2607 (Apr 1988)) and Renilla WO 92/15673; WO 95/07463; WO 98/14605; WO 98/26277; WO 99/49019; U.S. patent 5,292,658; U.S. patent 5,418,155; U.S. patent 5,683,888; U.S. patent 5,741,668; U.S. patent 5,777,079; U.S. patent 5,804,387; U.S. patent 5,874,304; U.S. patent 5,876,995; and U.S. patent 5,925,558) All of the above- cited references are expressly incorporated herein by reference. In some instances, multiple fluorescent labels are employed. In a preferred embodiment, at least two fluorescent labels are used which are members of a fluorescence resonance energy transfer (FRET) pair. FRET is phenomenon known in the art wherein excitation of one fluorescent dye is transferred to another without emission of a photon. A FRET pair consists of a donor fluorophore and an acceptor fluorophore. The fluorescence emission spectrum of the donor and the fluorescence absorption spectrum of the acceptor must overlap, and the two molecules must be in close proximity. The distance between donor and acceptor at which 50% of donors are deactivated (transfer energy to the acceptor) is defined by the Forster radius (R 0 ), o which is typically 10-100 A. Changes in the fluorescence emission spectrum comprising FRET pairs can be detected, indicating changes in the number of that are o in close proximity (i.e., within 100 A of each other). This will typically result from the binding or dissociation of two molecules, one of which is labeled with a FRET donor and the other of which is labeled with a FRET acceptor, wherein such binding brings the FRET pair in close proximity. Binding of such molecules will result in an increased fluorescence emission of the acceptor and/or quenching of the fluorescence emission of the donor. FRET pairs (donor/acceptor) useful in the invention include, but are not limited to, EDANS/fluorescien, IAEDANS/fluorescein, fluorescein/tetramethylrhodamine, fluorescein LC Red 640, fluorescein/Cy 5, fluorescein/Cy 5.5 and fluorescein/LC Red 705. In another aspect of FRET, a fluorescent donor molecule and a nonfluorescent acceptor molecule ("quencher") may be employed. In this application, fluorescent emission of the donor will increase when quencher is displaced from close proximity to the donor and fluorescent emission will decrease when the quencher is brought into close proximity to the donor. Useful quenchers include, but are not limited to, DABCYL, QSY 7 and QSY 33. Useful fluorescent donor/quencher pairs include, but are not limited to EDANS/DABCYL, Texas Red/DABCYL, BODIPY/DABCYL, Lucifer yellow/DABCYL, coumarin/DABCYL and fluorescein/QSY 7 dye. The skilled artisan will appreciate that FRET and fluorescence quenching allow for monitoring of binding of labeled molecules over time, providing continuous information regarding the time course of binding reactions. It is important to remember that ubiquitin is ligated to substrate protein by its terminal carboxyl group to a lysine residue, including lysine residues on other ubiquitin. Therefore, attachment of labels or other tags should not interfere with either of these active groups on the ubiquitin. Amino acids may be added to the sequence of protein, through means well known in the art and described herein, for the express purpose of providing a point of attachment for a label. In a preferred embodiment, one or more amino acids are added to the sequence of a component for attaching a tag thereto, preferably a fluorescent label. In a preferred embodiment, the amino acid to which a fluorescent label is attached is Cysteine. By "label enzyme" is meant an enzyme which may be reacted in the presence of a label enzyme substrate which produces a detectable product. Suitable label enzymes for use in the present invention include but are not limited to, horseradish peroxidase, alkaline phosphatase and glucose oxidase. Methods for the use of such substrates are well known in the art. The presence of the label enzyme is generally revealed through the enzyme's catalysis of a reaction with a label enzyme substrate, producing an identifiable product. Such products may be opaque, such as the reaction of horseradish peroxidase with tetramethyl benzedine, and may have a variety of colors. Other label enzyme substrates, such as Luminol (available from Pierce Chemical Co.), have been developed that produce fluorescent reaction products. Methods for identifying label enzymes with label enzyme substrates are well known in the art and many commercial kits are available. Examples and methods for the use of various label enzymes are described in Savage et al, Previews 247:6-9 (1998), Young, J. Virol. Methods 24:227-236 (1989), which are each hereby incoφorated by reference in their entirety. By "radioisotope" is meant any radioactive molecule. Suitable radioisotopes for use in the invention include, but are not limited to 14 C, 3 H, 32 P, 33 P, 35 S, 125 I, and 131 I. The use of radioisotopes as labels is well known in the art. In addition, labels may be indirectly detected, that is, the tag is a partner of a binding pair. By "partner of a binding pair" is meant one of a first and a second moiety, wherein said first and said second moiety have a specific binding affinity for each other. Suitable binding pairs for use in the invention include, but are not limited to, antigens/antibodies (for example, digoxigenin/anti-digoxigenin, dinitrophenyl (DNP)/anti-DNP, dansyl-X-anti-dansyl, Fluorescein/anti-fluorescein, lucifer yellow/anti-lucifer yellow, and rhodamine anti-rhodamine), biotin avid (or biotin/streptavidin) and calmodulin binding protein (CBP)/calmodulin. Other suitable binding pairs include polypeptides such as the FLAG-peptide [Hopp et al., BioTechnology. 6:1204-1210 (1988)]; the KT3 epitope peptide [Martin et al., Science. 255: 192-194 (1992)]; tubulin epitope peptide [Skinner et al., J. Biol. Chem.. 266:15163-15166 (1991)]; and the T7 gene 10 protein peptide tag [Lutz-Freyermuth et al, Proc. Natl. Acad. Sci. USA. 87:6393-6397 (1990)] and the antibodies each thereto. Generally, in a preferred embodiment, the smaller of the binding pair partners serves as the tag, as steric considerations in ubiquitm ligation may be important. As will be appreciated by those in the art, binding pair partners may be used in applications other than for labeling, as is further described below. As will be appreciated by those in the art, a partner of one binding pair may also be a partner of another binding pair. For example, an antigen (first moiety) may bind to a first antibody (second moiety) which may, in turn, be an antigen for a second antibody (third moiety). It will be further appreciated that such a circumstance allows indirect binding of a first moiety and a third moiety via an intermediary second moiety that is a binding pair partner to each. As will be appreciated by those in the art, a partner of a binding pair may comprise a label, as described above. It will further be appreciated that this allows for a tag to be indirectly labeled upon the binding of a binding partner comprising a label. Attaching a label to a tag which is a partner of a binding pair, as just described, is referred to herein as "indirect labeling". By "surface substrate binding molecule" and grammatical equivalents thereof is meant a molecule have binding affinity for a specific surface substrate, which substrate is generally a member of a binding pair applied, incorporated or otherwise attached to a surface. Suitable surface substrate binding molecules and their surface substrates include, but are not limited to poly-histidine (poly-his) or poly-histidine-glycine (poly- his-gly) tags and Nickel substrate; the Glutathione-S Transferase tag and its antibody substrate (available from Pierce Chemical); the flu HA tag polypeptide and its antibody 12CA5 substrate [Field et al., Mol. Cell. Biol. 8:2159-2165 (1988)]; the c- myc tag and the 8F9, 3C7, 6E10, G4, B7 and 9E10 antibody substrates thereto [Evan et al., Molecular and Cellular Biology. 5:3610-3616 (1985)]; and the Herpes Simplex virus glycoprotein D (gD) tag and its antibody substrate [Paborsky et al., Protein Engineering. 3(6):547-553 (1990)]. In general, surface binding substrate molecules useful in the present invention include, but are not limited to, polyhistidine structures (His-tags) that bind nickel substrates, antigens that bind to surface substrates comprising antibody, haptens that bind to avidin substrate (e.g., biotin) and CBP that binds to surface substrate comprising calmodulin. Production of antibody-embedded substrates is well known; see Slinkin et al, Bioconi. Chem. 2:342-348 (1991); Torchilin et al., supra; Trubetskoy et al., Bioconi. Chem. 3:323-327 (1992); King et al., Cancer Res. 54:6176-6185 (1994); and Wilbur et al., Bioconjugate Chem. 5:220-235 (1994) (all of which are hereby expressly incorporated by reference), and attachment of or production of proteins with antigens is described above. Calmodulin-embedded substrates are commercially available , and production of proteins with CBP is described in Simcox et al., Strategies 8:40-43 (1995), which is hereby incoφorated by reference in its entirety. As will be appreciated by those in the art, tag-components of the invention can be made in various ways, depending largely upon the form of the tag. Components of the invention and tags are preferably attached by a covalent bond. The production of tag-polypeptides by recombinant means when the tag is also a polypeptide is described below. Production of FLAG-labeled proteins is well known in the art and kits for such production are commercially available (for example, from Kodak and Sigma). Methods for the production and use of FLAG-labeled proteins are found, for example, in Winston et al, Genes andDevel. 13:270-283 (1999), incoφorated herein in its entirety, as well as product handbooks provided with the above-mentioned kits. Biotinylation of target molecules and substrates is well known, for example, a large number of biotinylation agents are known, including amine-reactive and thiol-reactive agents, for the biotinylation of proteins, nucleic acids, carbohydrates, carboxylic acids; see chapter 4, Molecular Probes Catalog, Haugland, 6th Ed. 1996, hereby incoφorated by reference. A biotinylated substrate can be attached to a biotinylated component via avidin or streptavidin. Similarly, a large number of haptenylation reagents are also known (Id.). Methods for labeling of proteins with radioisotopes are known in the art. For example, such methods are found in Ohta et al, Molec. Cell 3:535-541 (1999), which is hereby incoφorated by reference in its entirety. Production of proteins having His-tags by recombinant means is well known, and kits for producing such proteins are commercially available. Such a kit and its use is described in the QIAexpress Handbook from Quiagen by Joanne Crowe et al., hereby expressly incoφorated by reference. The functionalization of labels with chemically reactive groups such as thiols, amines, carboxyls, etc. is generally known in the art. In a preferred embodiment, the tag is functionalized to facilitate covalent attachment. The covalent attachment of the tag may be either direct or via a linker. In one embodiment, the linker is a relatively short coupling moiety, that is used to attach the molecules. A coupling moiety may be synthesized directly onto a component of the invention, ubiquitin for example, and contains at least one functional group to facilitate attachment of the tag. Alternatively, the coupling moiety may have at least two functional groups, which are used to attach a functionalized component to a functionalized tag, for example. In an additional embodiment, the linker is a polymer. In this embodiment, covalent attachment is accomplished either directly, or through the use of coupling moieties from the component or tag to the polymer. In a preferred embodiment, the covalent attachment is direct, that is, no linker is used. In this embodiment, the component preferably contains a functional group such as a carboxylic acid which is used for direct attachment to the functionalized tag. It should be understood that the component and tag may be attached in a variety of ways, including those listed above. What is important is that manner of attachment does not significantly alter the functionality of the component. For example, in tag-ubiquitin, the tag should be attached in such a manner as to allow the ubiquitin to be covalently bound to other ubiquitin to form polyubiquitin chains. As will be appreciated by those in the art, the above description of covalent attachment of a label and ubiquitin applies equally to the attachment of virtually any two molecules of the present disclosure. In a preferred embodiment, the tag is functionalized to facilitate covalent attachment, as is generally outlined above. Thus, a wide variety of tags are commercially available which contain functional groups, including, but not limited to, isothiocyanate groups, amino groups, haloacetyl groups, maleimides, succinimidyl esters, and sulfonyl halides, all of which may be used to covalently attach the tag to a second molecule, as is described herein. The choice of the functional group of the tag will depend on the site of attachment to either a linker, as outlined above or a component of the invention. Thus, for example, for direct linkage to a carboxylic acid group of a ubiquitin, amino modified or hydrazine modified tags will be used for coupling via carbodiimide chemistry, for example using l-ethyl-3-(3- dimethylaminopropyl)-carbodiimide (ED AC) as is known in the art (see Set 9 and Set 11 of the Molecular Probes Catalog, supra; see also the Pierce 1994 Catalog and Handbook, pages T-155 to T-200, both of which are hereby incoφorated by reference). In one embodiment, the carbodiimide is first attached to the tag, such as is commercially available for many of the tags described herein. In a preferred embodiment, ubiquitin is in the form of tag-ubiquitin. In a preferred embodiment, ubiquitin is in the form of tag-ubiquitin, wherein, tag is a partner of a binding pair. Preferably in this embodiment the tag is FLAG and the binding partner is anti-FLAG. Preferably in this embodiment, a label is attached to the FLAG by indirect labeling. Preferably, the label is a label enzyme. Most preferably, the label enzyme is horseradish peroxidase, which is reacted with a fluorescent label enzyme substrate. Preferably, the label enzyme substrate is Luminol. Alternatively, the label is a fluorescent label. In another preferred embodiment, ubiquitin is in the form of tag-ubiquitin, wherein the tag is a fluorescent label. In a particularly preferred embodiment, ubiquitin is in the form of tagl -ubiquitin and tag2 -ubiquitin, wherein tagl and tag2 are the members of a FRET pair. In an alternate preferred embodiment, ubiquitin is in the form of tagl -ubiquitin and tag2-ubiquitin, wherein tagl is a fluorescent label and tag2 is a quencher of the fluorescent label. In either of these preferred embodiments, when tagl -ubiquitin and tag2-ubiquitin are bound through the activity of a ubiquitin ligase, o o preferably tagl and tag2 are within 100 A of each other, more preferable within 70 A, o o still more preferably within 50 A, even more preferably within 40 A, and in some o cases, preferably within 30 A or less. In yet another preferred embodiment, ubiquitin is in the form of tagl,2-ubiquitin and tag 1,3 -ubiquitin, wherein tagl is a member of a binding pair, preferably FLAG, tag2 is a fluorescent label and tag3 is either a fluorescent label such that tag2 and tag3 are members of a FRET pair or tag3 is a quencher of tag2. In a preferred embodiment, one or more amino acids are added to the ubiquitin sequence, using recombinant techniques as described herein, to provide an attachment point for a tag, preferably a fluorescent label or a quencher. In a preferred embodiment, the one or more amino acids are Cys or Ala-Cys. Preferably, the one or more amino acids are attached to the N-terminal of the ubiquitin. In a preferred embodiment, the one or more amino acids intervenes the sequence of a FLAG tag and the ubiquitin. In a preferred embodiment, the tag, preferably a fluorescent label or a quencher, is attached to the added Cysteine. The present invention provides methods and compositions comprising combining ubiquitin and El . By "El" is meant a ubiquitin activating enzyme. In a preferred embodiment, El is capable of transferring ubiquitin to an E2, defined below. In a preferred embodiment, El binds ubiquitin. In a preferred embodiment, El forms a high energy thiolester bond with ubiquitin, thereby "activating" the ubiquitin. In a preferred embodiment, El proteins useful in the invention include those having the amino acid sequence of the polypeptide having ATCC accession numbers A38564, S23770, AAA61246, P22314, CAA40296 and BAA33144, incoφorated herein by reference. In a preferred embodiment, El has the amino acid sequence shown in figure 6B or is encoded by a nucleic acid comprising the sequence shown in figure 6 A. Preferably El is human El. El is commercially available from Affmiti Research Products (Exeter, U.K.). In a preferred embodiment, nucleic acids which may be used for producing El proteins for the invention include, but are not limited to, those disclosed by ATCC accession numbers M58028, X56976 and AB012190, incoφorated herein by reference. In a preferred embodiment, El is encoded by a nucleic acid having a sequence consisting essentially of the sequence shown in figure 6 A. Variants of the cited El proteins, also included in the term "El", can be made as described herein. In a preferred embodiment, the compositions of the invention comprise E2. By "E2" is meant a ubiquitin carrier enzyme (also known as a ubiquitin conjugating enzyme). In a preferred embodiment, ubiquitin is transferred from El to E2. In a preferred embodiment, the transfer results in a thiolester bond formed between E2 and ubiquitin. In a preferred embodiment, E2 is capable of transferring ubiquitin to an E3, defined below. In a preferred embodiment, the ubiquitination substrate protein is ubiquitin. In a preferred embodiment, proteins which may be used in the present invention as E2 include, but are not limited to, those having the amino acid sequences disclosed in ATCC accession numbers AAC37534, P49427, CAA82525, AAA58466, AAC41750, P51669, AAA91460, AAA91461, CAA63538, AAC50633, P27924, AAB36017, Q16763, AAB86433, AAC26141, CAA04156, BAA11675, Q16781, NP_003333, BAB18652, AAH00468, CAC16955, CAB76865, CAB76864, NP_05536, O00762, XP_009804, XP_009488, XP_006823, XP_006343, XP_005934, XP_002869, XP_003400XP_009365, XP_010361, XP_004699, XP_004019, 014933, P27924, P50550, P52485, P51668, P51669, P49459, P37286, P23567, P56554, and CAB45853, each of which is incoφorated herein by reference. Particularly preferred are sequences disclosed in ATCC accession numbers NP003331, NP003330, NP003329, P49427, AAB53362, NP008950, XP009488and AAC41750, also incoφorated by reference. The skilled artisan will appreciate that many different E2 proteins and isozymes are known in the filed and may be used in the present invention, provided that the E2 has ubiquitin conjugating activity. Also specifically included within the term "E2" are variants of E2, which can be made as described herein. In a preferred embodiment, E2 is one of Ubc5 (Ubch5, preferably Ubch5c), Ubc3 (Ubch3), Ubc4 (Ubch4) and UbcX (UbclO, UbchlO). In a preferred embodiment, E2 is Ubc5c. In a preferred embodiment, E2 has the amino acid sequence shown in Figure 7B or is encoded by a nucleic acid consisting essentially of the sequence shown in Figure 7A. In a preferred embodiment, nucleic acids which may be used to make E2 include, but are not limited to, those nucleic acids having sequences disclosed in ATCC accession numbers L2205, Z29328, M92670, L40146, U39317, U39318, X92962, U58522, S81003, AF031141, AF075599, AJ000519, XM009488, NM007019, U73379, L40146 and D83004, each of which is incoφorated herein by reference. As described above, variants of these and other E2 encoding nucleic acids may also be used to make variant E2 proteins. In a preferred embodiment, the nucleic acid used to make E2 comprises the sequence shown in Figure 7A. In a preferred embodiment, E2 has a tag, as defined above, with the complex being referred to herein as "tag-E2". Preferred E2 tags include, but are not limited to, labels, partners of binding pairs and substrate binding elements. In a most preferred embodiment, the tag is a His-tag or GST-tag. The present invention provides methods and compositions comprising E3. By "E3" is meant a ubiquitin ligase, as defined above, comprising one or more components associated with ligation of ubiquitin to a ubiquitination substrate protein for ubiquitin- dependent proteolysis. In a preferred embodiment, E3 comprises a ring finger protein and a Cullin. In a preferred embodiment, RING finger proteins include, but are not limited to, ROC1, ROC2 and APCl 1. In a preferred embodiment, Cullins include, but are not limited to, CUL1, CUL2, CUL3, CUL4A, CUL4B, CUL5 and APC2. In a preferred embodiment, RING finger proteins include, but are not limited to, proteins having the amino acid sequence disclosed in ATCC accession numbers AAD30147 and AAD30146 and 6320196, incoφorated herein by reference. In a more preferred embodiment, the ring finger protein has a sequence selected from the group consisting of that shown in Figure 8, Figure 9 and figure 10B. In a preferred embodiment, Cullins include, but are not limited to, proteins having the amino acid sequences disclosed in ATCC accession numbers 4503161, AAC50544, AAC36681, 4503163, AAC51190, AAD23581, 4503165, AAC36304, AAC36682, AAD45191, AAC50548, Q13620, 4503167 and AAF05751, each of which is incoφorated herein by reference. In addition, in the context of the invention, each of the RING finger proteins and Cullins encompass variants of the known or listed sequences, as described herein. In a preferred embodiment, the Cullin has a sequence as shown in Figure 1 IB or 12B. These E3 proteins and variants may be made as described herein. In a preferred embodiment, nucleic acids used to make the RING finger proteins include, but are not limited to, those having the nucleic acid sequences disclosed in ATCC accession numbers AF142059, AF142060 and nucleic acids 433493 to 433990 of NC 001136. In a preferred embodiment, Cullins are made from nucleic acids including, but not limited to, those having nucleic acid sequences disclosed in ATCC accession numbers NM 003592, U58087, AF062536, AF126404, NM 003591, U83410, NM 003590, AB014517, AF062537, AF064087, AF077188, U58091, NM 003478, X81882 and AF191337, each of which is incoφorated herein by reference. As described above, variants of these sequences are also encompassed by the invention. In a preferred embodiment, nucleic acid used to produce ROC2 comprises the sequence depicted in figure 12 A. In a preferred embodiment, nucleic acid used to produce CUL5 comprises the sequence depicted in figure 13 A. In a preferred embodiment, nucleic acid used to produce APC2 comprises the sequence depicted in figure 14 A. In a preferred embodiment, E3 comprises the RING finger protein/Cullin combination APCl 1/APC2. In another preferred embodiment, E3 comprises the RING fmger protein/Cullin combination ROCl/CULl. In yet preferred embodiment, E3 comprises the RING finger protein/Cullin combination ROC1/CUL2. In still another preferred embodiment, E3 comprises the RING finger protein/Cullin combination ROC2/CUL5. However, the skilled artisan will appreciate that any combination of E3 components may be produced and used in the invention described herein. In an alternate embodiment, E3 comprises the ligase E3 -alpha, E3A (E6-AP), HERC2, SMURF1, TRAF6, MDM2, Cbl, Sina/Siah, Itchy, IAP or NEDD-4. In this embodiment, the ligase has the amino acid sequence of that disclosed in ATCC accession number AAC39845, Q05086, CAA66655, CAA66654, CAA66656, AAD08657, NP_002383, XP_006284, AAC51970, XP_013050, BAB39389, Q00987, AAF08298 or P46934, each of which is incoφorated herein by reference. As above, variants are also encompassed by the invention. Nucleic acids for making E3 for this embodiment include, but are not limited to, those having the sequences disclosed in ATCC accession numbers AF061556, XM006284, U76247, XM013050, X898032, X98031, X98033, AF071172, Z12020, AB056663, AF199364 and D42055 and variants thereof. E3 may also comprise other components, such as SKPl and F-box proteins. The amino acid and nucleic acid sequences for SKPl are found in ATCC accession numbers AAC50241 and U33760, respectively. Many F-box proteins are known in the art and their amino acid and nucleic acid sequences are readily obtained by the skilled artisan from various published sources. In a preferred embodiment, the E3 components are produced recombinantly, as described herein. In a preferred embodiment, the E3 components are co-expressed in the same host cell. Co-expression may be achieved by transforming the cell with a vector comprising nucleic acids encoding two or more of the E3 components, or by transforming the host cell with separate vectors, each comprising a single component of the desired E3 protein complex. In a preferred embodiment, the RING finger protein and Cullin are expressed in a single host transfected with two vectors, each comprising nucleic acid encoding one or the other polypeptide, as described in further detail in the Examples. In a preferred embodiment, E3 has a tag, which complex is referred to herein as "tag- E3". Preferably, the tag is attached to only one component of the E3. Preferred E3 tags include, but are not limited to, labels, partners of binding pairs and substrate binding elements. More preferably, the tag is a surface substrate binding molecule. Most preferably, the tag is a His-tag or GST-tag. In an embodiment herein, ubiquitin and ubiquitination enzymes and their components are cloned and expressed as outlined below. Thus, probe or degenerate polymerase chain reaction (PCR) primer sequences may be used to find other related ubiquitination proteins from humans or other organisms. As will be appreciated by those in the art, particularly useful probe and/or PCR primer sequences include the unique areas of a nucleic acid sequence. As is generally known in the art, preferred PCR primers are from about 15 to about 35 nucleotides in length, with from about 20 to about 30 being preferred, and may contain inosine as needed. The conditions for the PCR reaction are well known in the art. It is therefore also understood that provided along with the sequences in the sequences cited herein are portions of those sequences, wherein unique portions of 15 nucleotides or more are particularly preferred. The skilled artisan can routinely synthesize or cut a nucleotide sequence to the desired length. Once isolated from its natural source, e.g., contained within a plasmid or other vector or excised therefrom as a linear nucleic acid segment, the recombinant nucleic acid can be further-used as a probe to identify and isolate other nucleic acids. It can also be used as a "precursor" nucleic acid to make modified or variant nucleic acids and proteins. Using the nucleic acids of the present invention which encode a protein, a variety of expression vectors are made. The expression vectors may be either self-replicating extrachromosomal vectors or vectors which integrate into a host genome. Generally, these expression vectors include transcriptional and translational regulatory nucleic acid operably linked to the nucleic acid encoding the protein. The term "control sequences" refers to DNA sequences necessary for the expression of an operably linked coding sequence in a particular host organism. The control sequences that are suitable for prokaryotes, for example, include a promoter, optionally an operator sequence, and a ribosome binding site. Eukaryotic cells are known to utilize promoters, polyadenylation signals, and enhancers. Nucleic acid is "operably linked" when it is placed into a functional relationship with another nucleic acid sequence. For example, DNA for a presequence or secretory leader is operably linked to DNA for a polypeptide if it is expressed as a preprotein that participates in the secretion of the polypeptide; a promoter or enhancer is operably linked to a coding sequence if it affects the transcription of the sequence; or a ribosome binding site is operably linked to a coding sequence if it is positioned so as to facilitate translation. As another example, operably linked refers to DNA sequences linked so as to be contiguous, and, in the case of a secretory leader, contiguous and in reading fram. However, enhancers do not have to be contiguous. Linking is accomplished by ligation at convenient restriction sites. If such sites do not exist, the synthetic oligonucleotide adaptors or linkers are used in accordance with conventional practice. The transcriptional and translational regulatory nucleic acid will generally be appropriate to the host cell used to express the protein; for example, transcriptional and translational regulatory nucleic acid sequences from Bacillus are preferably used to express the protein in Bacillus. Numerous types of appropriate expression vectors, and suitable regulatory sequences are known in the art for a variety of host cells. In general, the transcriptional and translational regulatory sequences may include, but are not limited to, promoter sequences, ribosomal binding sites, transcriptional start and stop sequences, translational start and stop sequences, and enhancer or activator sequences. In a preferred embodiment, the regulatory sequences include a promoter and transcriptional start and stop sequences. Promoter sequences encode either constitutive or inducible promoters. The promoters may be either naturally occurring promoters or hybrid promoters. Hybrid promoters, which combine elements of more than one promoter, are also known in the art, and are useful in the present invention. In addition, the expression vector may comprise additional elements. For example, the expression vector may have two replication systems, thus allowing it to be maintained in two organisms, for example in mammalian or insect cells for expression and in a procaryotic host for cloning and amplification. Furthermore, for integrating expression vectors, the expression vector contains at least one sequence homologous to the host cell genome, and preferably two homologous sequences which flank the expression construct. The integrating vector may be directed to a specific locus in the host cell by selecting the appropriate homologous sequence for inclusion in the vector. Constructs for integrating vectors are well known in the art. In addition, in a preferred embodiment, the expression vector contains a selectable marker gene to allow the selection of transformed host cells. Selection genes are well known in the art and will vary with the host cell used. A preferred expression vector system is a retroviral vector system such as is generally described in PCT/US97/01019 and PCT/US97/01048, both of which are hereby expressly incoφorated by reference. Proteins of the present invention are produced by culturing a host cell transformed with an expression vector containing nucleic acid encoding the protein, under the appropriate conditions to induce or cause expression of the protein. The conditions appropriate for protein expression will vary with the choice of the expression vector and the host cell, and will be easily ascertained by one skilled in the art through routine experimentation. For example, the use of constitutive promoters in the expression vector will require optimizing the growth and proliferation of the host cell, while the use of an inducible promoter requires the appropriate growth conditions for induction. In addition, in some embodiments, the timing of the harvest is important. For example, the baculoviral systems used in insect cell expression are lytic viruses, and thus harvest time selection can be crucial for product yield. Appropriate host cells include yeast, bacteria, archaebacteria, fungi, and insect and animal cells, including mammalian cells. Of particular interest are Drosophila melanogaster cells, Pichia pastoris and P. methanolica, Saccharomyces cerevisiae and other yeasts, E. coli, Bacillus subtilis, SF9 cells, SF21 cells, C129 cells, Saos-2 cells, Hi-5 cells, 293 cells, Neurospora, BHK, CHO, COS, and HeLa cells. Of greatest interest are Pichia pastoris and P. methanolica, E. coli, SF9 cells, SF21 cells and Hi-5 cells. In a preferred embodiment, the proteins are expressed in mammalian cells. Mammalian expression systems are also known in the art, and include retroviral systems. A mammalian promoter is any DNA sequence capable of binding mammalian RNA polymerase and initiating the downstream (3') transcription of a coding sequence for a protein into mRNA. A promoter will have a transcription initiating region, which is usually placed proximal to the 5' end of the coding sequence, and a TATA box, using a located 25-30 base pairs upstream of the transcription initiation site. The TATA box is thought to direct RNA polymerase II to begin RNA synthesis at the correct site. A mammalian promoter will also contain an upstream promoter element (enhancer element), typically located within 100 to 200 base pairs upstream of the TATA box. An upstream promoter element determines the rate at which transcription is initiated and can act in either orientation. Of particular use as mammalian promoters are the promoters from mammalian viral genes, since the viral genes are often highly expressed and have a broad host range. Examples include the SV40 early promoter, mouse mammary tumor virus LTR promoter, adeno virus major late promoter, heφes simplex virus promoter, and the CMV promoter. Typically, transcription termination and polyadenylation sequences recognized by mammalian cells are regulatory regions located 3' to the translation stop codon and thus, together with the promoter elements, flank the coding sequence. The 3' terminus of the mature mRNA is formed by site-specific post-translational cleavage and polyadenylation. Examples of transcription terminator and polyadenylation signals include those derived form SV40. The methods of introducing exogenous nucleic acid into mammalian hosts, as well as other hosts, is well known in the art, and will vary with the host cell used. Techniques include dextran-mediated transfection, calcium phosphate precipitation, polybrene mediated transfection, protoplast fusion, electroporation, viral infection, encapsulation of the polynucleotide(s) in liposomes, and direct microinjection of the DNA into nuclei. In a preferred embodiment, proteins are expressed in bacterial systems. Bacterial expression systems are well known in the art. A suitable bacterial promoter is any nucleic acid sequence capable of binding bacterial RNA polymerase and initiating the downstream (3') transcription of the coding sequence of a protein into mRNA. A bacterial promoter has a transcription initiation region which is usually placed proximal to the 5' end of the coding sequence. This transcription initiation region typically includes an RNA polymerase binding site and a transcription initiation site. Sequences encoding metabolic pathway enzymes provide particularly useful promoter sequences. Examples include promoter sequences derived from sugar metabolizing enzymes, such as galactose, lactose and maltose, and sequences derived from biosynthetic enzymes such as tryptophan. Promoters from bacteriophage may also be used and are known in the art. In addition, synthetic promoters and hybrid promoters are also useful; for example, the tac promoter is a hybrid of the trp and lac promoter sequences. Furthermore, a bacterial promoter can include naturally occurring promoters of non-bacterial origin that have the ability to bind bacterial RNA polymerase and initiate transcription. In addition to a functioning promoter sequence, an efficient ribosome binding site is desirable. In E. coli, the ribosome binding site is called the Shine-Delgarno (SD) sequence and includes an initiation codon and a sequence 3-9 nucleotides in length located 3 - 11 nucleotides upstream of the initiation codon. The expression vector may also include a signal peptide sequence that provides for secretion of the protein in bacteria. The signal sequence typically encodes a signal peptide comprised of hydrophobic amino acids which direct the secretion of the protein from the cell, as is well known in the art. The protein is either secreted into the growth media (gram-positive bacteria) or into the periplasmic space, located between the inner and outer membrane of the cell (gram-negative bacteria). The bacterial expression vector may also include a selectable marker gene to allow for the selection of bacterial strains that have been transformed. Suitable selection genes include genes which render the bacteria resistant to drugs such as ampicillin, chloramphenicol, erythromycin, kanamycin, neomycin and tetracycline. Selectable markers also include biosynthetic genes, such as those in the histidine, tryptophan and leucine biosynthetic pathways. These components are assembled into expression vectors. Expression vectors for bacteria are well known in the art, and include vectors for Bacillus subtilis, E. coli, Streptococcus cremoris, and Streptococcus lividans, among others. The bacterial expression vectors are transformed into bacterial host cells using techniques well known in the art, such as calcium chloride treatment, electroporation, and others. In one embodiment, proteins are produced in insect cells. Expression vectors for the transformation of insect cells, and in particular, baculovirus-based expression vectors, are well known in the art. In a preferred embodiment, proteins are produced in yeast cells. Yeast expression systems are well known in the art, and include expression vectors for Saccharomyces cerevisiae, Candida albicans and C. maltosa, Hansenula polymorpha, Kluyveromyces fragilis and K. lactis, Pichia guillerimondii P. methanolica and P. pastoris, Schizosaccharomyces pombe, and Yarroma lipolytica. Preferred promoter sequences for expression in yeast include the inducible GAL 1,10 promoter, the promoters from alcohol dehydrogenase, enolase, glucokinase, glucose-6-phosphate isomerase, glyceraldehyde-3-phosphate-dehydrogenase, hexokinase, phosphofructokinase, 3- phosphoglycerate mutase, pyruvate kinase, and the acid phosphatase gene. Yeast selectable markers include ADE2, HIS4, LEU2, TRP1, and ALG7, which confers resistance to tunicamycin; the neomycin phosphotransferase gene, which confers resistance to G418; and the CUPl gene, which allows yeast to grow in the presence of copper ions. The protein may also be made as a fusion protein, using techniques well known in the art. Thus, for example, the protein may be made as a fusion protein to increase expression, or for other reasons. For example, when the protein is a peptide, the nucleic acid encoding the peptide may be linked to other nucleic acid for expression puφoses. Similarly, proteins of the invention can be linked to protein labels, such as green fluorescent protein (GFP), red fluorescent protein (RFP), blue fluorescent protein (BFP), yellow fluorescent protein (YFP), etc. In a preferred embodiment, the protein is purified or isolated after expression. Proteins may be isolated or purified in a variety of ways known to those skilled in the art depending on what other components are present in the sample. Standard purification methods include electrophoretic, molecular, immunological and chromatographic techniques, including ion exchange, hydrophobic, affinity, and reverse-phase HPLC chromatography, and chromatofocusing. For example, the ubiquitin protein may be purified using a standard anti-ubiquitin antibody column. Ultrafiltration and diafiltration techniques, in conjunction with protein concentration, are also useful. For general guidance in suitable purification techniques, see Scopes, R., Protein Purification, Springer- Verlag, NY (1982). The degree of purification necessary will vary depending on the use of the protein. In some instances no purification will be necessary. Once made, the compositions find use in a number of applications, including, but not limited to, screens for modulators of ubiquitination. By "modulator" is meant a compound which can increase or decrease ubiquitination. The skilled artisan will appreciate that modulators of ubiquitination may affect enzyme activity, enzyme interaction with the substrate, interaction between ubiquitin and the substrate, or a combination of these. A modulator that specifically affects ubiquitin ligase activity is a ubiquitin ligase modulator. By "candidate", "candidate agent", "candidate modulator", "candidate ubiquitination modulator" or grammatical equivalents herein is meant any molecule, e.g. proteins (which herein includes proteins, polypeptides, and peptides), small organic or inorganic molecules, polysaccharides, polynucleotides, etc. which are to be tested for ubiquitination modulator activity. Candidate agents encompass numerous chemical classes. In a preferred embodiment, the candidate agents are organic molecules, particularly small organic molecules, comprising functional groups necessary for structural interaction with proteins, particularly hydrogen bonding, and typically include at least an amine, carbonyl, hydroxyl or carboxyl group, preferably at least two of the functional chemical groups. The candidate agents often comprise cyclical carbon or heterocyclic structures and/or aromatic or polyaromatic structures substituted with one or more chemical functional groups. Candidate modulators are obtained from a wide variety of sources, as will be appreciated by those in the art, including libraries of synthetic or natural compounds. As will be appreciated by those in the art, the present invention provides a rapid and easy method for screening any library of candidate modulators, including the wide variety of known combinatorial chemistry-type libraries. In a preferred embodiment, candidate modulators are synthetic compounds. Any number of techniques are available for the random and directed synthesis of a wide variety of organic compounds and biomolecules, including expression of randomized oligonucleotides. See for example WO 94/24314, hereby expressly incoφorated by reference, which discusses methods for generating new compounds, including random chemistry methods as well as enzymatic methods. As described in WO 94/24314, one of the advantages of the present method is that it is not necessary to characterize the candidate modulator prior to the assay; only candidate modulators that increase or decease ubiquitin ligase activity need be identified. In addition, as is known in the art, coding tags using split synthesis reactions may be done, to essentially identify the chemical moieties tested. Alternatively, a preferred embodiment utilizes libraries of natural compounds in the form of bacterial, fungal, plant and animal extracts that are available or readily produced. Additionally, natural or synthetically produced libraries and compounds are readily modified through conventional chemical, physical and biochemical means. Known pharmacological agents may be subjected to directed or random chemical modifications, including enzymatic modifications, to produce structural analogs. In a preferred embodiment, candidate modulators include proteins, nucleic acids, and chemical moieties. In a preferred embodiment, the candidate modulator are proteins, as defined above. In a preferred embodiment, the candidate modulators are naturally occurring proteins or fragments of naturally occurring proteins. Thus, for example, cellular extracts containing proteins, or random or directed digests of proteinaceous cellular extracts, may be tested, as is more fully described below. In this way libraries of procaryotic and eucaryotic proteins may be made for screening against any number of ubiquitin ligase compositions. Particularly preferred in this embodiment are libraries of bacterial, fungal, viral, and mammalian proteins, with the latter being preferred, and human proteins being especially preferred. In a preferred embodiment, the candidate modulators are peptides of from about 2 to about 50 amino acids, with from about 5 to about 30 amino acids being preferred, and from about 8 to about 20 being particularly preferred. The peptides may be digests of naturally occurring proteins as is outlined above, random peptides, or "biased" random peptides. By "randomized" or grammatical equivalents herein is meant that each nucleic acid and peptide consists of essentially random nucleotides and amino acids, respectively. Since generally these random peptides (or nucleic acids, discussed below) are chemically synthesized, they may incoφorate any nucleotide or amino acid at any position. The synthetic process can be designed to generate randomized proteins or nucleic acids, to allow the formation of all or most of the possible combinations over the length of the sequence, thus forming a library of randomized candidate bioactive proteinaceous agents. The library should provide a sufficiently structurally diverse population of randomized agents to effect a probabilistically sufficient range of diversity to allow interaction with a particular ubiquitin ligase enzyme. Accordingly, an interaction library must be large enough so that at least one of its members will have a structure that interacts with a ubiquitin ligase enzyme. Although it is difficult to gauge the required absolute size of an interaction library, nature provides a hint with the immune response: a diversity of 10 7 -10 8 different antibodies provides at least one combination with sufficient affinity to interact with most potential antigens faced by an organism. Published in vitro selection techniques have also shown that a library size of 10 7 to 10 8 is sufficient to find structures with affinity for a target. A library of all combinations of a peptide 7 to 20 amino acids in length, such as generally proposed herein, has the potential to code for 20 7 (10 9 ) to 20 20 . Thus, with libraries of 10 7 to 10 8 different molecules the present methods allow a "working" subset of a theoretically complete interaction library for 7 amino acids, and a subset of shapes for the 20 20 library. Thus, in a preferred embodiment, at least 10 6 , preferably at least 10 7 , more preferably at least 10 8 and most preferably at least 10 9 different sequences are simultaneously analyzed in the subject methods. Preferred methods maximize library size and diversity. In one embodiment, the library is fully randomized, with no sequence preferences or constants at any position. In a preferred embodiment, the library is biased. That is, some positions within the sequence are either held constant, or are selected from a limited number of possibilities. For example, in a preferred embodiment, the nucleotides or amino acid residues are randomized within a defined class, for example, of hydrophobic amino acids, hydrophilic residues, sterically biased (either small or large) residues, towards the creation of cysteines, for cross-linking, prolines for SH-3 domains, serines, threonines, tyrosines or histidines for phosphorylation sites, etc., or to purines, etc. In a preferred embodiment, the bias is towards peptides or nucleic acids that interact with known classes of molecules. For example, when the candidate modulator is a peptide, it is known that much of intracellular signaling is carried out via short regions of polypeptides interacting with other polypeptides through small peptide domains. For instance, a short region from the HIV-1 envelope cytόplasmic domain has been previously shown to block the action of cellular calmodulin. Regions of the Fas cytoplasmic domain, which shows homology to the mastoparan toxin from Wasps, can be limited to a short peptide region with death-inducing apoptotic or G protein inducing functions. Magainin, a natural peptide derived from Xenopus, can have potent anti-tumor and anti-microbial activity. Short peptide fragments of a protein kinase C isozyme (BPKC), have been shown to block nuclear translocation of BPKC in Xenopus oocytes following stimulation. And, short SH-3 target peptides have been used as psuedosubstrates for specific binding to SH-3 proteins. This is of course a short list of available peptides with biological activity, as the literature is dense in this area. Thus, there is much precedent for the potential of small peptides to have activity on intracellular signaling cascades. In addition, agonists and antagonists of any number of molecules may be used as the basis of biased randomization of candidate modulators as well. Thus, a number of molecules or protein domains are suitable as starting points for the generation of biased randomized candidate modulators. A large number of small molecule domains are known, that confer a common function, structure or affinity. In addition, as is appreciated in the art, areas of weak amino acid homology may have strong structural homology. A number of these molecules, domains, and/or corresponding consensus sequences, are known, including, but are not limited to, SH- 2 domains, SH-3 domains, Pleckstrin, death domains, protease cleavage/recognition sites, enzyme inhibitors, enzyme substrates, Traf, etc. In a preferred embodiment, the candidate modulators are nucleic acids. With reference to candidate modulators, by "nucleic acid" or "oligonucleotide" or grammatical equivalents herein means at least two nucleotides covalently linked together. A nucleic acid of the present invention will generally contain phosphodiester bonds, although in some cases, as outlined below, nucleic acid analogs are included that may have alternate backbones, comprising, for example, phosphoramide (Beaucage et al., Tetrahedron 49(10): 1925 (1993) and references therein; Letsinger, J. Org. Chem. 35:3800 (1970); Sprinzl et al., Eur. J. Biochem. 81. -579 (1977); Letsinger et al., Nucl. Acids Res. 14:3487 (1986); Sawai et al, Chem. Lett. 805 (1984), Letsinger et al., J. Am. Chem. Soc. 110:4470 (1988); and Pauwels et al., Chemica Scripta 26:141 91986)), phosphorothioate (Mag et al., Nucleic Acids Res. 19:1437 (1991); and U.S. Patent No. 5,644,048), phosphorodithioate (Briu et al, J. Am. Chem. Soc. 111:2321 (1989), O-methylphophoroamidite linkages (see Eckstein, Oligonucleotides and Analogues: A Practical Approach, Oxford University Press), and peptide nucleic acid backbones and linkages (see Egholm, J. Am. Chem. Soc. 114:1895 (1992); Meier et al., Chem. Int. Ed. Engl. 31:1008 (1992); Nielsen, Nature, 365:566 (1993); Carlsson et al., Nature 380:207 (1996), all of which are incoφorated by reference). Other analog nucleic acids include those with positive backbones (Denpcy et al., Proc. Natl. Acad. Sci, USA 92:6097 (1995); non-ionic backbones (U.S. Patent Nos. 5,386,023, 5,637,684, 5,602,240, 5,216,141 and 4,469,863; Kiedrowshi et al., Angew. Chem. Intl. Ed. English 30:423 (1991); Letsinger et al., J. Am. Chem. Soc. 110:4470 (1988); Letsinger et al., Nucleoside & Nucleotide 13:1597 (1994); Chapters 2 and 3, ASC Symposium Series 580, "Carbohydrate Modifications in Antisense Research", Ed. Y.S. Sanghui and P. Dan Cook; Mesmaeker et al., Bioorganic & Medicinal Chem. Lett. 4:395 (1994); Jeffs et al., J. Biomolecular NMR 34:17 (1994); Tetrahedron Lett. 37:743 (1996)) and non- ribose backbones, including those described in U.S. Patent Nos. 5,235,033 and 5,034,506, and Chapters 6 and 7, ASC Symposium Series 580, "Carbohydrate Modifications in Antisense Research", Ed. Y.S. Sanghui and P. Dan Cook. Nucleic acids containing one or more carbocyclic sugars are also included within the definition of nucleic acids (see Jenkins et al., Chem. Soc. Rev. (1995) ppl69-176). Several nucleic acid analogs are described in Rawls, C & E News June 2, 1997 page 35. All of these references are hereby expressly incoφorated by reference. These modifications of the ribose-phosphate backbone may be done to facilitate the addition of additional moieties such as labels, or to increase the stability and half-life of such molecules in physiological environments. As will be appreciated by those in the art, all of these nucleic acid analogs may find use in the present invention. In addition, mixtures of naturally occurring nucleic acids and analogs can be made. Alternatively, mixtures of different nucleic acid analogs, and mixtures of naturally occurring nucleic acids and analogs may be made. Particularly preferred are peptide nucleic acids (PNA) which includes peptide nucleic acid analogs. These backbones are substantially non-ionic under neutral conditions, in contrast to the highly charged phosphodiester backbone of naturally occurring nucleic acids. The nucleic acids may be single stranded or double stranded, as specified, or contain portions of both double stranded or single stranded sequence. The nucleic acid may be DNA, both genomic and cDNA, RNA or a hybrid, where the nucleic acid contains any combination of deoxyribo- and ribo-nucleotides, and any combination of bases, including uracil, adenine, thymine, cytosine, guanine, inosine, xathanine hypoxathanine, isocytosine, isoguanine, etc. As used herein, the term "nucleoside" includes nucleotides and nucleoside and nucleotide analogs, and modified nucleosides such as amino modified nucleosides. In addition, "nucleoside" includes non-narurally occurring analog structures. Thus for example the individual units of a peptide nucleic acid, each containing a base, are referred to herein as a nucleoside. As described above generally for proteins, nucleic acid candidate modulator may be naturally occurring nucleic acids, random nucleic acids, or "biased" random nucleic acids. For example, digests of procaryotic or eucaryotic genomes may be used as is outlined above for proteins. Where the ultimate expression product is a nucleic acid, at least 10, preferably at least 12, more preferably at least 15, most preferably at least 21 nucleotide positions need to be randomized, with more preferable if the randomization is less than perfect. Similarly, at least 5, preferably at least 6, more preferably at least 7 amino acid positions need to be randomized; again, more are preferable if the randomization is less than perfect. In a preferred embodiment, the candidate modulators are organic moieties. In this embodiment, as is generally described in WO 94/24314, candidate agents are synthesized from a series of substrates that can be chemically modified. "Chemically modified" herein includes traditional chemical reactions as well as enzymatic reactions. These substrates generally include, but are not limited to, alkyl groups (including alkanes, alkenes, alkynes and heteroalkyl), aryl groups (including arenes and heteroaryl), alcohols, ethers, amines, aldehydes, ketones, acids, esters, amides, cyclic compounds, heterocyclic compounds (including purines, pyrimidines, benzodiazepins, beta-lactams, tetracylines, cephalosporins, and carbohydrates), steroids (including estrogens, androgens, cortisone, ecodysone, etc.), alkaloids (including ergots, vinca, curare, pyrollizdine, and mitomycines), organometallic compounds, hetero-atom bearing compounds, amino acids, and nucleosides. Chemical (including enzymatic) reactions may be done on the moieties to form new substrates or candidate agents which can then be tested using the present invention. As will be appreciated by those in the art, it is possible to screen more than one type of candidate modulator at a time. Thus, the library of candidate modulators used may include only one type of agent (i.e. peptides), or multiple types (peptides and organic agents). The assay of several candidates at one time is further discussed below. The present invention provides methods and compositions comprising combining several components. In a preferred embodiment, a preferred combination is tag- ubiquitin, El, E2, and E3. Preferably the tag is a label, a partner of a binding pair, or a substrate binding molecule. More preferably, the tag is a fluorescent label or a binding pair partner. In a preferred embodiment, the tag is a binding pair partner and the ubiquitin is labeled by indirect labeling. In the indirect labeling embodiment, preferably the label is a fluorescent label or a label enzyme. In an embodiment comprising a label enzyme, preferably the substrate for that enzyme produces a fluorescent product. In a preferred embodiment, the label enzyme substrate is luminol. In a preferred embodiment, combining specifically excludes combining the components with a target protein. In another preferred embodiment, a preferred combination is Tagl -ubiquitin, tag2- ubiquitin, El, E2 and E3. Preferably, tagl and tag2 are labels, preferably fluorescent labels, most preferably tagl and tag2 constitute a FRET pair. In a preferred embodiment, a preferred combination is tagl -ubiquitin, El, E2 and tag2-E3. Preferably, tagl is a label, a partner of a binding pair, or a substrate binding molecule and tag2 is a different label, partner of a binding pair, or substrate binding molecule. More preferably, tagl is a fluorescent label or a member of a binding pair. When tagl is a member of a binding pair, preferably tagl is indirectly labeled. Still more preferably, tag-1 is indirectly labeled with a label enzyme. Preferably the label enzyme substrate used to reveal the presence of the enzyme produces a fluorescent product, and more preferably is luminol. In the presently described combination, preferably tag2 is a surface substrate binding element, more preferably a His-tag. In a preferred embodiment, a preferred combination is tagl -ubiquitin, El and tag2-E2. In this embodiment, preferably, tagl is a label, a partner of a binding pair, or a substrate binding molecule and tag2 is a different label, partner of a binding pair, or substrate binding molecule. More preferably, tag-1 is a label or a member of a binding pair. When tagl is a member of a binding pair, preferably tagl is indirectly labeled. In a preferred embodiment, the tagl label (direct or indirect) is a fluorescent label or a label enzyme. When the tagl label (direct or indirect) is a label enzyme, preferably the reaction substrate used to reveal the presence of the enzyme produces a fluorescent product, and more preferably is luminol. In the presently described combination, preferably tag2 is a substrate binding element, more preferably a His- tag. In a preferred embodiment, the compositions of the invention do not comprise a target protein. In this embodiment, ubiquitin is the sole ubiquitination substrate, as discussed above. This embodiment stands in contrast to previous ubiquitination enzyme assays, which required addition of a target protein as part of the composition. Because the different combinations of E3 and E2 and combinations of E3 subunits are specific to particular target proteins, the present assays are much more versatile, allowing any variation of such combinations without first identifying the specific target protein to which the combination is directed. The components of the present compositions may be combined in varying amounts. In a preferred embodiment, ubiquitin is combined at a final concentration of from 20 to 200 ng per 100 μl reaction solution, most preferable at about 100 ng per 100 μl reaction solution. In a preferred embodiment, El is combined at a final concentration of from 1 to 50 ng per 100 μl reaction solution, more preferably from 1 ng to 20 ng per 100 μl reaction solution, most preferably from 5 ng to 10 ng per 100 μl reaction solution. In a preferred embodiment, E2 is combined at a final concentration of 10 to 100 ng per 100 μl reaction solution, more preferably 10-50 ng per 100 μl reaction solution. In a preferred embodiment, E3 is combined at a final concentration of from 1 ng to 500 ng per 100 μl reaction solution, more preferably from 50 to 400 ng per 100 μl reaction solution, still more preferably from 100 to 300 ng per 100 μl reaction solution, most preferably about 100 ng per 100 μl reaction solution. The components of the invention are combined under reaction conditions that favor ubiquitin ligase activity and/or ubiquitination acitivty. Generally, this will be physiological conditions. Incubations may be performed at any temperature which facilitates optimal activity, typically between 4 and 40°C. Incubation periods are selected for optimum activity, but may also be optimized to facilitate rapid high through put screening. Typically between 0.5 and 1.5 hours will be sufficient. A variety of other reagents may be included in the compositions. These include reagents like salts, solvents, buffers, neutral proteins, e.g. albumin, detergents, etc. which may be used to facilitate optimal ubiquitination enzyme activity and/or reduce non-specific or background interactions. Also reagents that otherwise improve the efficiency of the assay, such as protease inhibitors, nuclease inhibitors, anti-microbial agents, etc., may be used. The compositions will also preferably include adenosine tri- phosphate (ATP). The mixture of components may be added in any order that promotes ubiquitin ligase activity or optimizes identification of candidate modulator effects. In a preferred embodiment, ubiquitin is provided in a reaction buffer solution, followed by addition of the ubiquitination enzymes. In an alternate preferred embodiment, ubiquitin is provided in a reaction buffer solution, a candidate modulator is then added, followed by addition of the ubiquitination enzymes. Once combined, preferred methods of the invention comprise measuring the amount of ubiquitin bound to E3. In an alternate preferred embodiment in which the combination lacks E3, preferred methods of the invention comprise measuring the amount of ubiquitin bound to E2. As will be understood by one of ordinary skill in the art, the mode of measuring will depend on the specific tag attached to the ubiquitin. As will also be apparent to the skilled artisan, the amount of ubiquitin bound will encompass not only the particular ubiquitin protein bound directly to the ubiquitination enzyme, but also the ubiquitin proteins bound to that particular ubiquitin in a polyubiquitin chain. In a preferred embodiment, ubiquitination is measured In a preferred embodiment, the tag attached to the ubiquitin is a fluorescent label. In a preferred embodiment, the tag attached to ubiquitin is an enzyme label or a binding pair member which is indirectly labeled with an enzyme label. In this latter preferred embodiment, the enzyme label substrate produces a fluorescent reaction product. In these preferred embodiments, the amount of ubiquitin bound is measured by luminescence. As used herein, "luminescence" or "fluorescent emission" means photon emission from a fluorescent label. In an embodiment where FRET pairs are used, fluorescence measurements may be taken continuously or at time-points during the ligation reaction. Equipment for such measurement is commercially available and easily used by one of ordinary skill in the art to make such a measurement. Other modes of measuring bound ubiquitin are well known in the art and easily identified by the skilled artisan for each of the labels described herein. For instance, radioisotope labeling may be measured by scintillation counting, or by densitometry after exposure to a photographic emulsion, or by using a device such as a Phosphorlmager. Likewise, densitometry may be used to measure bound ubiquitin following a reaction with an enzyme label substrate that produces an opaque product when an enzyme label is used. In preferred methods of the present invention, E3 is bound to a surface substrate. This may be done directly , as described above for the binding of a label to ubiquitin. This may also be accomplished using tag-E3, wherein the tag is a surface substrate binding molecule. In another preferred embodiment of the invention, E2 is bound to a surface substrate in the absence of E3. This may be done directly , as described above for the binding of a label to ubiquitin. This may also be accomplished using tag-E2, wherein the tag is a surface substrate binding molecule. In the two preferred embodiments described immediately above, E3 and E2 are in the form of tag-E3 and tag-E2, respectively, and are bound to a surface substrate via a surface substrate binding molecule tag. In general, any substrate binding molecule can be used. In a preferred embodiment, the tag is a His-tag and the surface substrate is nickel. In a preferred embodiment, the nickel surface substrate is present on the surface of the wells of a multi-well plate, such as a 96 well plate. Such multi-well plates are commercially available. The binding of the enzyme to a surface substrate facilitates the separation of bound ubiquitin from unbound ubiquitin. In the present embodiment, the unbound ubiquitin is easily washed from the receptacle following the ligation reaction. As will be appreciated by those of skill in the art, the use of any surface substrate binding element and receptacle having the surface substrate to which it binds will be effective for facilitating the separation of bound and unbound ubiquitin. In an alternative embodiment, E3 or E2 is bound, directly or via a substrate binding element, to a bead. Following ligation, the beads may be separated from the unbound ubiquitin and the bound ubiquitin measured. In a preferred embodiment, E3 or E2 is bound to beads and the composition used includes tag-ubiquitin wherein tag is a fluorescent label. In this embodiment, the beads with bound ubiquitin may be separated using a fluorescence-activated cell sorting (FACS) machine. Methods for such use are described in U.S. Patent Application Serial No. 09/047,119, which is hereby incoφorated in its entirety. The amount of bound ubiquitin can then be measured. In another embodiment, none of the ubiquitination enzymes is bound to a substrate. Preferably in this embodiment, the composition comprises tagl -ubiquitin, tag2- ubiquitin, El, E2 and E3. Preferably, tagl and tag2 are labels, preferably fluorescent labels, most preferably tagl and tag2 constitute a FRET pair. In this embodiment, ubiquitination is measured by measuring the fluorescent emission spectrum. This measuring may be continuous or at one or more times following the combination of the components. Alteration in the fluorescent emission spectrum of the combination as compared with unligated ubiquitin indicates the amount of ubiquitination. The skilled artisan will appreciate that in this embodiment, alteration in the fluorescent emission spectrum results from ubiquitin bearing different members of the FRET pair being brought into close proximity, either through the formation of poly-ubiquitin and/or by binding nearby locations on a protein, preferably a target protein. In a preferred embodiment, the compositions of the invention are used to identify ubiquitination modulators. In this embodiment, the composition includes a candidate modulator. In a preferred embodiment, the measured amount and/or rate of tag- ubiquitin binding to E3 is compared with the that when the candidate modulator is absent from the composition, whereby the presence or absence of the modulators effect on ubiquitin ligase activity is determined. In this embodiment, whether the modulator enhances or inhibits ubiquitination is also determined. In a preferred embodiment, the composition of the invention containing a candidate modulator lacks E3 and the amount and/or rate of ubiquitin binding to E2 is measured. This embodiment may also comprise the step of comparing the amount and/or rate of ubiquitin binding to E2 in a composition lacking both E3 and the candidate modulator, whereby the modulatory activity of the candidate on ubiquitination enzymes other than E3 is determined. In a preferred embodiment, the percentage difference in the amount of ubiquitin bound to E2 in the presence and absence of the candidate modulator is compared with the percentage difference in the amount bound to E3 in the presence and absence of candidate modulator, whereby the point of effect of the candidate modulator in the enzyme cascade is determined. That is, it is determined whether the candidate modulator affects E3 ubiquitin ligase activity or it affects El ubiquitin activating activity and/or E2 ubiquitin conjugating activity. In another preferred embodiment, the compositions of the invention are used to identify ubiquitination modulators. In this embodiment, the composition includes a candidate modulator. In a preferred embodiment, where tagl and tag2 constitute a FRET pair, the measured amount and/or rate of tagl -ubiquitin and tag2-ubiquitin binding to a substrate protein (as poly-ubiquitin and/or ubiquitin bound to a target protein) is compared with the amount or rate of such binding in the absence of the candidate modulator, whereby the presence or absence of the modulator's effect on ubiquitin ligase activity is determined. In this embodiment, whether the modulator enhances or inhibits ubiquitination is also determined. In a preferred embodiment, multiple assays are performed simultaneously in a high throughput screening system. In this embodiment, multiple assays may be performed in multiple receptacles, such as the wells of a 96 well plate or other multi-well plate. As will be appreciated by one of skill in the art, such a system may be applied to the assay of multiple candidate modulators and/or multiple combination of E3 components and/or E2-E3 pairings. In a preferred embodiment, the present invention is used in a high-throughput screening system for determining the ubiquitin ligase activity of different E2-E3 pairings and/or different E3 component combinations. In an alternate preferred embodiment, the present invention is used in a high throughput screening system for simultaneously testing the effect of individual candidate modulators. In another aspect, the invention provides a method of assaying for ubiquitination activity or ubiquitination enzyme activity in a mixture. Ubiquitin is introduced into a cell or mixture of protein, preferably a cell lysate, under conditions in which ubiquitination can take place. In this embodiment, the ubiquitin is in the form of tagl -ubiquitin and tag2-2-ubiquitin, wherein tagl and tag2 constitute a FRET pair or tagl is a fluorescent label and tag2 is a quencher of tagl. Fluorescent emission spectrum is measured as an indication of whether ubiquitination activity is present in the mixture or cell. In a preferred embodiment, the ubiquitin also comprises a member of a binding pair, such as FLAG. In this latter embodiment, components involved in ubiquitination can be isolated from the mixture using any one of a number of affinity-based separation means such as fluorescent beads coated with anti-FLAG antibody or amino precipitation using anti-FLAG antibodies, or using anti-FLAG antibody attached to a solid support. Other means of separating ubiquitin bound components of the cell or mixture will be readily apparent to the skilled artisan. Ubiquitin bound components so separated in this method may include E3 and target protein. The skilled artisan will appreciate that separation of these components for individual identification or subsequent investigation may be obtained by several means well known in the art, such as by HPLC or electrophoresis. It is understood by the skilled artisan that the steps of the assays provided herein can vary in order. It is also understood, however, that while various options (of compounds, properties selected or order of steps) are provided herein, the options are also each provided individually, and can each be individually segregated from the other options provided herein. Moreover, steps which are obvious and known in the art that will increase the sensitivity of the assay are intended to be within the scope of this invention. For example, there may be additionally washing steps, blocking steps, etc. The following examples serve to more fully describe the manner of using the above- described invention, as well as to set forth the best modes contemplated for carrying out various aspects of the invention. It is understood that these examples in no way serve to limit the true scope of this invention, but rather are presented for illustrative puφoses. All references cited herein are expressly incoφorated by reference in their entirety. EXAMPLES Example 1 Production of E2, E3 and ubiquitin E2 Production The open reading frame of E2 (Ubc5C) was amplified by PCR and cloned into the pGex-6p-l E.Coli. expression vector (Amersham Pharmacia) as BgHI-EcoRI fragments, with N-terminus in frame fused to the GST-tag. Materials and methods Plasmid is transformed in BL21 DE3 competent E.coli (Stratagene, cat # 230132). Cells are grown at 37°C in TB + lOOug/ml ampicillin and 0.4% glucose to an OD600 of about 0.6, induced with addition of 320uM IPTG and allowed to grow for another 3h before harvest. The pellets are washed once with cold PBS, then resuspended in about 6 volumes of lysis buffer (20 mM Tris, 10% glycerol, 0.5 M Nacl, 2.5mM EDTA, lmM TCEP plus ' Complete -EDTA Free Protease inhibitor tablets, 1 tablet 25ml of resuspended cells, pH 8.0). The suspension is homogenized and sonicated 3 x 30 sec. NP40, then added to a final concentration of 0.5 % and the tubes are rocked for 30 min at 4°C. Following centrifugation at 11000 φm for 25 to 30 min, the supernatant is incubated with Glutathione Sepharose 4B (Amersham, cat # 17- 0756-01) at a ratio of 1ml of beads per 100ml of original culture volume for 1 to 2 hours at 4°C with gentle rocking. The beads are pelleted and washed once with 10 bed volumes of the lysis buffer, then twice with 10 bed volumes of Prescission Protease buffer (50mM Tris-HCL, 150mM NaCl, ImM EDTA, lmM DTT, 0.1% NP-40, pH 7.O.). Prescission Protease (Amersham, product # 27-0843) is added at a ratio of 80ul (160 Units) per ml of GST resin, and allowed to incubate for 4h at 4°C. The supernatant containing the cleaved E2 protein is collected, and the resin is washed twice with one bed volume of Prescission buffer. All three fractions are analyzed by SDS-PAGE and pooled when appropriate. Ubiquitin Production Ubiquitin was cloned into the pFlag-Mac Expression Vector (Sigma) as a Hindlll- EcoRI fragment by PCR. This results in expression of amino-terminal Flag fusion ubiquitin in E.Coli. Materials and methods The induction of protein expression and cell lysis is similar to the above GST-E2 preparation, except that the supernatant is loaded over a FLAG-affinity resin (VWR, cat # IB 13020) at a ratio of 15ml of beads per IL of original culture. The resin is then washed with 10 bed volumes of lysis buffer. The protein is eluted from the column with : lOOmM Acetic acid, 10% glycerol, 200mM NaCl, 2.5 mM EDTA, 0.1% NP-40, pH 3.5. The elutions are collected as 1 bed volume fractions into tubes that contain l/10 th volume of 2M Tris, 80mM B-ME, pH 9.0 to neutralize the pH. The elution fractions are analyzed by SDS-PAGE and the appropriate fractions are pooled and dialyzed against 400 volumes of 20 mM Tris, 10% glycerol, 200mM NaCl, 2.5 mMEDTA, pH 8.0. Production of E3 Coding sequences for E3 complex were also amplified by PCR and baculoviruses were generated using the Bac-to-Bac system (GibcoBRL). E3 contains two subunits, which are expressed by co-infection of the two baculovirus in the same Hi-5 insect cells. One of the subunit is His-tagged, with the other associating subunit untagged. The detail procedure was done following the Bac to Bac Baculovirus Expression system by GibcoBRL. For example, ROC1 was cloned into the pFastBacHtb vector with a N-terminal His6-tag, while CULl was insert into the pFastBacl vector without any fusing tag. After transposition and Bacmid DNA transfection into SF-9 cells, Baculoviruses were harvested, amplified, and used to co-infect Hi-5 cells for protein expression. Materials and methods Cells are harvested, washed once with cold PBS, and resuspended in about 6 volumes of lysis buffer (20 mM Tris, 20% glycerol, 0.5 M Nacl, 15mM imidazole, ImM TCEP plus Complete -EDTA Free Protease inhibitor tablets, 1 tablet/25ml of resuspended cells, pH 8.0.). The suspension is then sonicated 3 x 30 sec, followed by addition of NP40 to a final concentration of 0.5 % and incubation for 30 min at 4°C. The lysate is then centrifuged and the supernatant is incubated with pre-equilibrated (lysis buffer + NP40) Ni- NTA Agarose beads (Qiagen, cat # 1000632) for 1 to 2 hrs. The pelleted beads are washed 2 times with lysis buffer, resuspended in 1 to 2 volumes of lysis buffer and transferred to a disposable column for elution. Elution is accomplished using 5 X 1 -bed volume aliquots of Lysis buffer + 250mM imidazole. Elution fractions are analyzed by SDS-PAGE and appropriate fractions are pooled. The elution pool is then desalted using either a desalting column or a centrifugal concentration device (more often used for large volumes.) When using centrifugal devices, the eluted pool is diluted 1 : 1 with lysis buffer that has no imidazole and spun at the appropriate speed until the volume is reduced by half. At this point an equal volume of fresh buffer is added and the device is respun. This is done a total of four times resulting in a 32 fold exchange. Example 2 Ubiquitin Conjugation Assay Ubiquitin conjugating activity of E1+E2 was measured using the following protocol with Flag-ubiquin, purified from E. coli, and the E2 Ubch5c, purified as His-Ubch5c from E. coli. Materials and Methods The following procedures were used for assays measuring ubiquitin conjugation. The wells of Nickel-substrate 96-well plates (Pierce Chemical) are blocked with 100 μl of 1% casein phosphate buffered saline (PBS) for 1 hour at room temperature, then washed with 200 μl of PBST (0.1% Tween-20 in PBS) 3 times. To each well is added the following Flag-ubiquitin (see above) reaction solution: Final concentration 62.5mM Tris pH 7.5 6.25 m MgCl 2 0.75 mM DTT 2.5 mM ATP 2.5 mM NaF 12.5 nM Okadaic acid lOOng Flag-ubiquitin (made as described above). The buffer solution is brought to a final volume of 80 μl with milipore-filtered water, followed by the addition of 10 μl of DMSO. To the above solution is then added 10 μl of El,His-E2 in 20mM Tris buffer, pH 7.5, and 5% glycerol. His-E2 is made as described above. El is obtained commercially (Affiniti Research Products, Exeter, U.K.). The following amounts of each enzyme are used for these assays: 5 ng/well of El; 25 nl/well E2. The reaction is then allowed to proceed at room temperature for 1 hour. Following the ubiquitin conjugation reaction, the wells are washed with 200 μl of PBST 3 times. For measurement of the enzyme-bound ubiquitin, 100 μl of Mouse anti-Flag (1:10,000) and ant-Mouse Ig-HRP (1:15,000) in PBST are added to each well and allowed to incubate at room temperature for 1 hour. The wells are then washed with 200 μl of PBST 3 times, followed by the addition of 100 μl of luminol substrate (1/5 dilution). Luminescence for each well is then measured using a fluorimeter. Results Ubiquitin Activating and Conjugating Activity Figure 1 A shows the luminescence measured for El alone and for El + his-E2, as described above. Example 3 Ubiquitin Ligase Assay Ubiquitin ligase activity of E1+E2+E3 was measured using the following protocol with Flag-ubiquin, purified from E. coli, the E2 Ubch5c, purified as GST-Ubch5c from E. coli with the GST tag removed, and the E3 His-ROC/Cull complex purified from Hi-5 cells by Baculovirus co-infection. This assay was also used to show the effects of candidate modulators on ubiquitin ligase activity. Materials and Methods The wells of Nickel-substrate 96-well plates (Pierce Chemical) are blocked with 100 μl of 1 casein/phosphate buffered saline (PBS) for 1 hour at room temperature, then washed with 200 μl of PBST (0.1% Tween-20 in PBS) 3 times. To each well is added the following Flag-ubiquitin (see above) reaction solution: Final concentration 62.5mM Tris pH 7.5 6.25 m MgCl 2 0.75 mM DTT 2.5 mM ATP 2.5 mM NaF 12.5 nM Okadaic acid lOOng Flag-ubiquitin (made as described above). The buffer solution is brought to a final volume of 80 μl with milipore-filtered water. For assays directed to identifying modulators of ubiquitm ligase activity, 10 μl of a candidate modulator compound in DMSO is then added to the solution. If no candidate modulator is added, 10 μl of DMSO is added to the solution. To the above solution is then added 10 μl of ubiquitination enzymes in 20mM Tris buffer, pH 7.5, and 5% glycerol. E2-Ubch5c and E3-HisROCl/Cull are made as described above. El is obtained commercially (Affiniti Research Products, Exeter, U.K.). The following amounts of each enzyme are used for these assays: 5 ng/well of El ; 25 nl/well E2; and 100 ng/well His-E3. The reaction is then allowed to proceed at room temperature for 1 hour. Following the ubiquitination reaction, the wells are washed with 200 μl of PBST 3 times. For measurement of the enzyme-bound ubiquitin, 100 μl of Mouse anti-Flag (1:10,000) and ant-Mouse Ig-HRP (1:15,000) in PBST are added to each well and allowed to incubate at room temperature for 1 hour. The wells are then washed with 200 μl of PBST 3 times, followed by the addition of 100 μl of luminol substrate (1/5 dilution). Luminescence for each well is then measured using a fluorimeter. Results Ubiquitin Ligase Activity Figure 1 shows the luminescence measured for several different combinations of ubiquitination enzymes. In these experiments, only E3 was in the form His-E3. The luminescence measurements show that the assay specifically measures the activity of the entire ubiquitination enzyme cascade, which requires the presence of all three ubiquitination enzymes in the reaction. Variations of Composition Components Figure 2 A shows the relative effect of varying the amount of El on ubiquitin ligase activity in the above procedure, in presence and absence of DMSO. The addition of about 10 ng per 100 μl reaction solution provides maximum ubiquitin ligase activity with the other components of the composition kept as detailed above. The presence of DMSO does not significantly affect the activity of the ubiquitination enzymes. The relative effect of varying E3 and ubiquitin concentration of the reaction composition is shown in Figure 2B. Generally speaking, maximum ubiquitin ligase activity was obtained with 200 to 300 ng per 100 μl of E3 at each concentration of ubiquitin, while increasing ubiquitin concentration generally increased ubiquitin ligase activity at each concentration of E3. It was also found that blocking of the wells with 1% casein improved the signal to noise ratio over either no blocking or blocking with 5 % bovine serum albumen (BSA). Background was determined after combining all of the components as above except His-E3 and measuring the resulting fluorescence after pre-treating the wells with 5% BSA, 1% casein or nothing. Results are shown in Figure 3. Identification of Modulators of Ubiquitin Ligase Activity To show that the assay is useful for identifying modulators of ubiquitin ligase activity, several candidate modulators were combined at varying concentrations with the assay components as described above. Figure 4 shows the results from two identified modulators of ubiquitin ligase activity. The modulators decreased ubiquitin ligase activity in a dose-dependent fashion for ubiquitination enzyme compositions comprising either ROC 1 /Cull or ROC2/Cul5 as the E3 component. Comparison of the effect of ubiquitin ligase activity modulators on reaction compositions, as described above, either containing El, E2 and His-E3 or containing El, His-E2 and lacking E3 shows whether the modulator affects E3 or an enzyme other than E3. In Figure 5 A, the identified modulator decreases ubiquitin ligase activity in the presence of E3, but does not alter activity in the absence of E3, showing that the modulator has a specific effect on E3 ligase activity. In contrast, results shown in Figure 5B for another modulator reveals that this compound reduces activity whether or not E3 is present, showing that this modulator affects a member of the ubiquitination enzyme cascade other than E3. Example 4 FRET Analysis of Ligated Ubiquitin Ubiquitin was prepared, labeled with either EDANS or fluorescein, and the fluorescence of each of these labels and their interaction as a FRET pair was measured to show binding of the labeled ubiquitin to E3 and FRET activity in the bound ubiquitin. Materials and Methods Ubiquitin were produced incoφorating Cys residues into the FLAG-ubiquitin sequence by site-directed mutagenensis using either the primer 5 '-CCCCCCAAGCTTTGCATGCAGATTTTCGTGAAGACCCTGACC-3 ' to produce FLAG-Cys-ubiquitin, or the primer 5 '-CCCCCCAAGCTTGCGTGCATGCAGATTTTCGTGAAGACCCTGACC-3 ' to produce FLAG-Ala-Cys-ubiquitin. Protein was expressed and purified as described above. Either fluoresceine 5-maleimide (peak emission at 515 nm) or 1,5-iodacetamide EDANS (IAEDANS; peak emission at 490 nm) was reacted with the thiol group on the cysteine of the ubiquitin produced as above to form a thioether. The labeling was performed in PBS with 1 mM TCEP. Labeled protein was separated from free label by gel filtration. Ubiquitin ligase assay was performed substantially as described above, with a few modifications, no nickel substrate was used in the reaction wells, so all of the components were free in solution. Equal amounts of fluoresceine labeled ubiquitin and IAEDANS labeled ubiquitin were used. The reaction was performed at room temperature for 2 hours in a volume of 100-150 μl, then stopped with 50 μl of 0.5M EDTA, pH 8. Following the reaction, the products were separated in PBS with 1 mM TCEP by HPLC on a Superdex-75 HR 10/30 size-exclusion column using fluorescence emission detection. A larger molecular weight cutoff gel-filtration column (e.g., Superdex 200 HR 10/30) could be used to resolve individual ligation species. Results Fluoresceine labeled ubiquitin and IAEDANS labeled ubiquitin was bound to E3 in approximately equal amounts. A comparison of the spectral analysis of fluorescent emission from the free (unligated) ubiquitin labeled with both fluorophores and the E3 -bound ubiqutin shows a distinct increase in ratio of emission at 515 nm versus 490 nm (figure 17). This shows that in ligated ubiquitin, the fluoφhores on different ubiquitin molecules are sufficiently close for FRET to be measured.

Claims: We Claim: 1. A method of assaying ubiquitin ligase activity comprising: a) combining, under conditions that favor ubiquitin ligase activity: i) tagl -ubiquitin; ϋ) El; iii) E2; and iv) E3; b) measuring the amount of tagl -ubiquitin bound to said E3. 2. The method of Claim 1, further comprising: a) combining: v) a candidate ubiquitin ligase modulator. 3. The method of Claim 1, wherein E2 is selected from the group consisting of Ubc5, Ubc3, Ubc4 and UbcX. 4. The method of Claim 1 , wherein E3 comprises a RING finger protein. 5. The method of Claim 4, wherein said RING finger protein is selected from the group consisting of ROC 1, ROC2 and APCll. 6. The method of Claim 1, wherein E3 comprises a Cullin. 7. The method of Claim 6, wherein said Cullin is selected from the group consisting of CULl, CUL2, CUL3, CUL4A, CUL4B, CUL5 and APC2. 8. The method of Claim 1 , wherein E3 comprises a RING finger protein/Cullin combination. 9. The method of Claim 8, wherein said RING finger protein/Cullin combination is selected from the group consisting of APC2/APC11, ROCl/CULl, ROC1/CUL2 and ROC2/CUL5. 10. The method of Claim 1, wherein said E3 is in the form tag2-E3. 11. The method of any one of Claims 1-10 wherein tagl is a label or a partner of a binding pair. 12. The method of Claim 11, wherein said tagl label is a fluorescent label. 13. The method of Claim 12, wherein said measuring is by measuring luminescence. 14. The method of Claim 11 , wherein said tag 1 partner of a binding pair is selected from the group consisting of an antigen, biotin, and CBP. 15. The method of Claim 14, wherein said tagl partner of a binding pair is labeled by indirect labeling. 16. The method of Claim 15, wherein said indirect labeling is with a fluorescent label or a label enzyme. 17. The method of Claim 16, wherein said measuring is by measuring luminescence. 18. Them method of Claim 16, wherein said label enzyme is selected from the group consisting of horseradish peroxidase, alkaline phosphatase and glucose oxidase. 19. The method of Claim 18, wherein said label enzyme is reacted with a label enzyme substrate which produces a fluorescent product. 20. The method of Claim 19, wherein said measuring is by measuring luminescence. 21. The method of Claim 14, wherein said antigen is FLAG. 22. The method of Claim 15, wherein said partner of a binding pair is FLAG and said indirect labeling is via anti-FLAG. 23. The method of Claim 16, wherein said tagl is FLAG. 24. The method of Claim 17, wherein said tagl is FLAG. 25. The method of Claim 11 , wherein said tag2 is a surface substrate binding molecule. 26. The method of Claim 25, wherein said tag2 surface substrate binding molecule is His-tag. 27. The method of Claim 26, wherein said assaying is performed in a multi-well plate comprising a surface substrate comprising nickel. 28. The method of Claim 12, further comprising: a) combining: v) tag3 -ubiquitin. 29. The method of Claim 28, wherein tag3 is the second member of a FRET pair with tagl or is a quencher of tagl. 30. The method of Claim 29, wherein said measuring is by measuring fluorescent emission. 31. The method of Claim 30, wherein said measuring is by measuring the fluorescent emission spectrum. 32. The method of Claim 31 , further comprising comparing said measured fluorescent emission spectrum with the fluorescent emission spectra of unbound tag3- and tagl -ubiquitin. 33. The method of Claim 31 , wherein said measuring is continuous over time or at specific time points following said combining. 34. A method of identifying a ubiquitination modulator comprising: a) combining, under conditions that favor ubiquitination activity: I) tagl -ubiquitin; ii) a candidate modulator; iii) El; iv) E2; and V) tag2-E3; b) measuring the amount of tagl -ubiquitin bound to said tag2-E3. 35. The method of Claim 31 , wherein E2 is selected from the group consisting of Ubc5, Ubc3, Ubc4 and UbcX. 36. The method of Claim 34, wherein E3 comprises a RING finger protein. 37. The method of Claim 36, wherein said RING finger protein is selected from the group consisting of ROC 1, ROC2 and APCll. 38. The method of Claim 34, wherein E3 comprises a Cullin. 39. The method od Claim 38, wherein said Cullin is selected from the group consisting of CULl, CUL2, CUL3, CUL4A, CUL4B, CUL5 and APC2. 40. The method of Claim 34, wherein E3 comprises a RING finger protein Cullin combination. 41. The method of Claim 40, wherein said RING finger protein/Cullin combination is selected from the group consisting of APC2/APC11, ROCl/CULl, ROC1/CUL2 and ROC2/CUL5. 42. The method of Claim 34, further comprising: c) combining, under conditions that favor ubiquitination activity: i) tagl -ubiquitin; ii) a candidate modulator iii) El; and iv) tag3-E2; d) measuring the amount of tagl -ubiquitin bound to said tag3-E2. 43. The method of any one of Claims 34-42, wherein tagl is a label or a partner of a binding pair. 44. The method of Claim 43, wherein said tagl label is a fluorescent label. 45. The method of Claim 44, wherein said measuring is by measuring luminescence. 46. The method of Claim 43, wherein said tagl partner of a binding pair is selected from the group consisting of an antigen, biotin, and CBP. 47. The method of Claim 46, wherein said tagl partner of a binding is labeled by indirect labeling. 48. The method of Claim 47, wherein said indirect labeling is with a fluorescent label or a label enzyme. 49. The method of Claim 48, wherein said measuring is by measuring luminescence. 50. Them method of Claim 48, wherein said label enzyme is selected from the group consisting of horseradish peroxidase, alkaline phosphatase and glucose oxidase. 51. The method of Claim 50, wherein said label enzyme is reacted with a label enzyme substrate which produces a fluorescent product. 52. The method of Claim 51 , wherein said measuring is by measuring luminescence. 53. The method of Claim 46, wherein said antigen is FLAG. 54. The method of Claim 47, wherein said tagl partner of a binding pair is FLAG. 55. The method of Claim 48, wherein said tagl partner of a binding pair is FLAG and said indirect labeling is via anti-FLAG. 56. The method of Claim 50, wherein said tagl is FLAG. 57. The method of Claim 43, wherein said tag2 is a surface substrate binding molecule. 58. The method of Claim 57, wherein said surface substrate binding molecule is His-tag. 59. The method of Claim 58, wherein said combining and measuring is performed in a multi-well plate comprising a surface substrate comprising nickel. 60. The method of Claim 43, wherein said tag3 is a surface substrate binding molecule. 61. The method of Claim 60, wherein said surface substrate binding molecule is selected from the group consisting of His-tag and GST-tag. 62. A method of assaying ubiquitination enzyme activity comprising: a) combining, under conditions that favor ubiquitination activity: i) tagl -ubiquitin and tag2-ubiquitin, wherein tagl and tag2 constitute a FRET pair or tagl is a fluorescent label and tag2 is a quencher of tagl; ϋ) El; iii) E2; and iv) E3; b) measuring the amount or rate of ubiquitination. 63. The method of Claim 62, further comprising: a) combining, under conditions that favor ubiquitination activity: v) a candidate ubiquitination modulator. 64. The method of Claim 62, wherein E2 is selected from the group consisting of Ubc5, Ubc3, Ubc4 and UbcX. 65. The method of Claim 62, wherein E3 comprises a RING finger protein. 66. The method of Claim 65, wherein said RING finger protein is selected from the group consisting of ROC 1, ROC2 and APCl 1. 67. The method of Claim 62, wherein E3 comprises a Cullin. 68. The method of Claim 67, wherein said Cullin is selected from the group consisting of CULl, CUL2, CUL3, CUL4A, CUL4B, CUL5 and APC2. 69. The method of Claim 62, wherein E3 comprises a RING finger protein/Cullin combination. 70. The method of Claim 69, wherein said RING finger protein/Cullin combination is selected from the group consisting of APC2/APC11, ROCl/CULl, ROC1/CUL2 and ROC2/CUL5. 71. The method of any one of Claims 62-70, wherein said measuring is by measuring the fluorescent emission spectrum. 72. The method of Claim 71, wherein said measuring is continuous over time or at specific time points following said combining. 73. The method of Claim 71 , further comprising comparing said measured fluorescent emission spectrum with the fluorescent emission spectra of unbound tagl- ubiquitin and tag2 -ubiquitin. 74. The method of Claim 71 , wherein said ubiquitin is in the form tagl ,3- ubiquitin and tag2,3 -ubiquitin, wherein tag3 is a member of a binding pair. 75. The method of Claim 74, wherein tag3 is FLAG. 76. The method of Claim 71, wherein said E3 is in the form tag4-E3, wherein tag4 is a surface substrate binding molecule. 77. The method of Claim 71, further comprising a target protein. 78. A method of identifying a unbiquitination modulator comprising: a) combining, under conditions that favor ubiquitination activity: i) tagl -ubiquitin and tag2 -ubiquitin, wherein tagl and tag2 constitute a FRET pair or tagl is a fluorescent label and tag2 is a quencher of tagl; ii) El; iii) E2; iv) E3; and v) a candidate ubiquitination modulator; b) measuring the amount or rate of ubiquitination. 79. The method of Claim 78, wherein E2 is selected from the group consisting of Ubc5, Ubc3, Ubc4 and UbcX. 80. The method of Claim 78, wherein E3 comprises a RING finger protein. 81. The method of Claim 80, wherein said RING finger protein is selected from the group consisting of ROC 1, ROC2 and APCl 1. 82. The method of Claim 78, wherein E3 comprises a Cullin. 83. he method of Claim 82, wherein said Cullin is selected from the group consisting of CULl, CUL2, CUL3, CUL4A, CUL4B, CUL5 and APC2. 84. The method of Claim 78, wherein E3 comprises a RING finger protein/Cullin combination. 85. The method of Claim 84, wherein said RING finger protein/Cullin combination is selected from the group consisting of APC2/APC11, ROCl/CULl, ROC1/CUL2 and ROC2/CUL5. 86. The method of any one of Claims 78-85, wherein said measuring is by measuring the fluorescent emission spectrum. 87. The method of Claim 86, wherein said measuring is continuous over time or at specific time points following said combining. 88. The method of Claim 86, further comprising comparing said measured fluorescent emission spectrum with the fluorescent emission spectra of unbound tagl- ubiquitin and tag2 -ubiquitin. 89. The method of Claim 86, wherein said ubiquitin is in the form tagl, 3- ubiquitin and tag2,3-ubiquitin, wherein tag3 is a member of a binding pair. 90. The method of Claim 89, wherein tag3 is FLAG. 91. The method of Claim 86, wherein said E3 is in the form tag4-E3 , wherein tag4 is a surface substrate binding molecule. 92. The method of Claim 86, further comprising: a) combining, under conditions that favor ubiquitination activity: vi) a target protein. 93. A composition for use in assaying ubiquitination comprising: tagl -ubiquitin and tag2-ubiquitin, wherein tagl is a fluorescent label and tag2 is the second member of a FRET pair with tagl or tag2 is a quencher of tagl. 94. The composition of Claim 93, further comprising: El, E2 and E3. 95. The composition of Claim 94, wherein said E2 is selected from the group consisting of Ubc5, Ubc3, Ubc4 and UbcX. 96. The composition of Claim 94, wherein E3 comprises a RING finger protein. 97. The composition of Claim 96, wherein said RING finger protein is selected from the group consisting of ROC 1, ROC2 and APCl 1. 98. The composition of Claim 94, wherein E3 comprises a Cullin. 99. The composition of Claim 98, wherein said Cullin is selected from the group consisting of CULl, CUL2, CUL3, CUL4A, CUL4B, CUL5 and APC2. 100. The composition of Claim 94, wherein E3 comprises a RING finger protein/Cullin combination. 101. The composition of Claim 100, wherein said RING finger protein/Cullin combination is selected from the group consisting of APC2/APC11, ROCl/CULl, ROC1/CUL2 and ROC2/CUL5. 102. The composition of any one of Claims 94- 101, further comprising a candidate ubiquitination modulator. 103. The composition of any one of Claims 94- 101, further comprising a target protein. 104. A composition for use in assaying for a ubiquitination modulator comprising: a candidate ubiquitination modulator and tagl -ubiquitin and tag2- ubiquitin, wherein tagl is a fluorescent label and tag2 is the second member of a FRET pair with tagl or tag2 is a quencher of tagl. 105. The composition of Claim 104, further comprising: El, E2 and E3. 106. The composition of Claim 105, wherein said E2 is selected from the group consisting of Ubc5, Ubc3, Ubc4 and UbcX. 107. The composition of Claim 105, wherein E3 comprises a RING finger protein. 108. The composition of Claim 107, wherein said RING finger protein is selected from the group consisting of ROC 1 , ROC2 and APC 11. 109. The composition of Claim 105, wherein E3 comprises a Cullin. 110. The composition of Claim 109, wherem said Cullin is selected from the group consisting of CULl , CUL2, CUL3, CUL4A, CUL4B, CUL5 and APC2. 111. The composition of Claim 105, wherein E3 comprises a RING finger protein/Cullin combination. 112. The composition of Claim 111, wherein said RING finger protein/Cullin combination is selected from the group consisting of APC2/APC11, ROCl/CULl, ROC1/CUL2 and ROC2/CUL5. 113. The composition of any one of Claims 105-112, further comprising a target protein.

Inventors: HUANG JIANING, ISSAKANI SARKIZ D, PRAY TODD R, SHEUNG JULIE, HUANG, JIANING, ISSAKANI, SARKIZ, D., PRAY, TODD, R., SHEUNG, JULIE
Applicants: HUANG JIANING, ISSAKANI SARKIZ D, PRAY TODD R, RIGEL PHARMACEUTICALS INC, SHEUNG JULIE, HUANG, JIANING, ISSAKANI, SARKIZ, D., PRAY, TODD, R., RIGEL PHARMACEUTICALS, INC., SHEUNG, JULIE
Application year: 2001
Main classification: C12Q1
IPC codes: C12Q1, G01N33