bluuebunny/crossref_metadata_2025 huggingface dataset:
Functional rescue of a constitutively desensitized β2AR through receptor dimerization
Authors: HEBERT E. Terence , LOISEL P. Thomas , ADAM Lynda , ETHIER Nathalie , ONGE ST. Stephane , BOUVIER Michel
Published: August 2015

We have recently demonstrated that wild-type β2-adrenergic receptors (β2AR) form homodimers and that disruption of receptor dimerization inhibits signalling via Gs [Hebert, Moffett, Morello, Loisel, Bichet, Barret and Bouvier (1996) J. Biol. Chem. 271, 16384-16392]. Here taking advantage of the altered functional properties of a non-palmitoylated, constitutively desensitized mutant β2AR (C341Gβ2AR), we sought to study whether physical interactions between mutant and wild-type β2AR expressed in Sf9 cells could occur and have functional consequences. Using metabolic labelling with [3H]palmitate and co-immunoprecipitation we demonstrated the existence of heterodimerization between wild-type and C341Gβ2AR. Furthermore, we show that, in co-expression experiments, wild-type receptors have a dominant positive effect resulting in the functional complementation of C341Gβ2AR. Indeed, when expressed alone, the mutant C341G receptor displays altered functional characteristics in that (1) the response of the receptor to agonist is reduced as compared to the wild-type receptor and (2) the desensitization of the receptor in response to prolonged exposure to agonist is minimal. In contrast, when C341G and the wild-type β2AR were expressed together, both the response to agonist and subsequent desensitization (at a constant level of total receptor) were equivalent to the wild-type β2AR expressed alone. This dominant positive effect was also seen when C341G was co-expressed with a second receptor mutant in which the two protein kinase A phosphorylation sites (S261, 262, 345, 346A β2AR) were mutated. Taken together these data suggest that intermolecular interactions between receptors may have both functional and structural implications for G-protein-mediated signalling.
Activation of a phospholipase Cβ2 deletion mutant by limited proteolysis
Authors: SCHNABEL Petra , CAMPS Montserrat
Published: August 2015

All phosphoinositide-specific phospholipases C (PLC) identified until today exhibit a high degree of similarity within two regions of 170 and 260 residues, respectively, which are designated regions X and Y. The PLCβ family, including four members designated PLCβ1, PLCβ2, PLCβ3 and PLCβ4, is regulated by heterotrimeric G proteins. In order to investigate structure-function relationships of PLCβ2, we expressed PLCβ2Δ, a deletion mutant of PLCβ2 which lacks most of the sequence downstream of region Y, in the baculovirus/insect cell system. The mutant was present in both soluble and particulate fractions of Sf9 cells and was demonstrated to be catalytically active and sensitive to βγ-subunits. Sf9 cytosol containing this mutant was subjected to limited proteolysis by trypsin and S. aureus protease V8, respectively. Immunochemical analysis revealed that both proteases cleaved the enzyme between the regions X and Y. Most interestingly, proteolytic cleavage at this site by both proteases stimulated the catalytic activity of PLC2β2Δ. The proteolytically activated enzyme was still sensitive to βγ-subunits and showed an unchanged concentration dependence on Ca2+. Gel filtration chromatography indicated that the fragments generated by cleavage between the regions X and Y were still connected and formed a functional heterodimeric complex. In order to visualize all fragments generated by protease V8, PLCβ2Δ was purified to homogeneity from Sf9 cytosol. Limited proteolysis of the purified enzyme by S. aureus protease V8 and subsequent SDS/PAGE and silver staining revealed that several cuts take place between the regions X and Y and that the resulting fragments remain intact. We hypothesize that the activating proteolytic cut induces a conformational change of the enzyme which might facilitate hydrolysis of the phospholipid substrate.
Infectivity of Trypanosoma cruzi strains is associated with differential expression of surface glycoproteins with differential Ca2+ signalling activity
Authors: RUIZ C. Rita , FAVORETO Silvio , DORTA L. Miriam , OSHIRO E. M. Maria , FERREIRA T. Alice , MANQUE M. Patricio , YOSHIDA Nobuko
Published: August 2015

Mammalian cell invasion assays, using metacyclic trypomastigotes of Trypanosoma cruzi G and CL strains, showed that the CL strain enters target cells in several-fold higher numbers as compared with the G strain. Analysis of expression of surface glycoproteins in metacyclic forms of the two strains by iodination, immunoprecipitation and FACS, revealed that gp90, undetectable in the CL strain, is one of the major surface molecules in the G strain, that expression of gp82 is comparable in both strains and that gp35/50 is expressed at lower levels in the CL strain. Purified gp90 and gp35/50 bound more efficiently than gp82 to cultured HeLa cells. However, the intensity of the Ca2+ response triggered in HeLa cells by gp82 was significantly higher than that induced by gp35/50 or gp90. Most of the Ca2+ signalling activity of the metacyclic extract towards HeLa cells was due to gp82 and was inhibitable by gp82-specific monoclonal antibody 3F6. Ca2+ mobilization was also triggered in metacyclic trypomastigotes by host-cell components; it was mainly gp82-mediated and more intense in the CL than in the G strain. We propose that expression of gp90 and gp35/50 at high levels impairs binding of metacyclic forms to host cells through productive gp82-mediated interaction, which leads to the target-cell and parasite Ca2+ mobilization required for invasion. Analysis of metacyclic forms of eight additional T. cruzi strains corroborated the inverse correlation between infectivity and expression of gp90 and gp35/50.
Molecular mechanism of lysosomal sialidase deficiency in galactosialidosis involves its rapid degradation
Authors: VINOGRADOVA V. Maia , MICHAUD Lorraine , MEZENTSEV V. Alexander , LUKONG E. Kiven , EL-ALFY Mohamed , MORALES R. Carlos , POTIER Michel , PSHEZHETSKY V. Alexey
Published: August 2015

Galactosialidosis is an inherited lysosomal storage disease caused by the combined deficiency of lysosomal sialidase and β-galactosidase secondary to the deficiency of cathepsin A/protective protein, which is associated with sialidase and β-galactosidase in a high-molecular weight (1.27 MDa) complex. Clinical phenotypes of patients as well as the composition of compounds which are stored in patient's tissues implicate sialidase deficiency as the underlying pathogenic defect. The recent cloning and sequencing of lysosomal sialidase [Pshezhetsky, Richard, Michaud, Igdoura, Wang, Elsliger, Qu, Leclerc, Gravel, Dallaire and Potier (1997), Nature Genet. 15, 316-320] allowed us to study the molecular mechanism of sialidase deficiency in galactosialidosis. By Western blotting, using antibodies against the recombinant human enzyme, and by NH2-terminal sequencing, we showed that sialidase is synthesized as a 45.5 kDa precursor and after the cleavage of the 47-amino acid signal peptide and glycosylation becomes a 48.3 kDa mature active enzyme present in the 1.27 kDa complex. Transgenic expression of sialidase in cultured skin fibroblasts from normal controls and from galactosialidosis patients, followed by immunofluorescent and immunoelectron microscopy showed that in both normal and affected cells the expressed sialidase was localized on lysosomal and plasma membranes, but the amount of sialidase found in galactosialidosis cells was ~ 5-fold reduced. Metabolic labelling studies demonstrated that the 48.3 kDa mature active form of sialidase was stable in normal fibroblasts (half-life ~ 2.7 h), whereas in galactosialidosis fibroblasts the enzyme was rapidly converted (half-life ~ 30 min) into 38.7 and 24 kDa catalytically inactive forms. Altogether our data provide evidence that the molecular mechanism of sialidase deficiency in galactosialidosis is associated with abnormal proteolytic cleavage and fast degradation.
Intergenic suppression of the γM23K uncoupling mutation in F0F1 ATP synthase by βGlu-381 substitutions: the role of the β380DELSEED386 segment in energy coupling
Authors: KETCHUM J. Christian , AL-SHAWI Marwan K. , NAKAMOTO K. Robert
Published: August 2015

We previously demonstrated that the Escherichia coli F0F1-ATP synthase mutation, γM23K, caused increased energy of interaction between γ- and β-subunits which was correlated to inefficient coupling between catalysis and transport [Al-Shawi, Ketchum and Nakamoto (1997) J. Biol. Chem. 272, 2300-2306]. Based on these results and the X-ray crystallographic structure of bovine F1-ATPase [Abrahams, Leslie, Lutter and Walker (1994) Nature (London) 370, 621-628] γM23K is believed to form an ionized hydrogen bond with βGlu-381 in the conserved β380DELSEED386 segment. In this report, we further test the role of γ-β-subunit interactions by introducing a series of substitutions for βGlu-381 and γArg-242, the residue which forms a hydrogen bond with βGlu-381 in the wild-type enzyme. βE381A, D, and Q were able to restore efficient coupling when co-expressed with γM23K. All three mutations reversed the increased transition state thermodynamic parameters for steady state ATP hydrolysis caused by γM23K. βE381K by itself caused inefficient coupling, but opposite from the effect of γM23K, the transition state thermodynamic parameters were lower than wild-type. These results suggest that the βE381K mutation perturbs the γ-β-subunit interaction and the local conformation of the β380DELSEED386 segment in a specific way that disrupts the communication of coupling information between transport and catalysis. βE381A, L, K, and R, and γR242L and E mutations perturbed enzyme assembly and stability to varying degrees. These results provide functional evidence that the β380DELSEED386 segment and its interactions with the γ-subunit are involved in the mechanism of coupling.
Amino acid substitutions in the N-terminal segment of cystatin C create selective protein inhibitors of lysosomal cysteine proteinases
Authors: MASON W. Robert , SOL-CHURCH Katia , ABRAHAMSON Magnus
Published: August 2015

We used site-directed mutagenesis to alter the specificity of human cystatin C, an inhibitor with a broad reactivity against cysteine proteinases. Nine cystatin C variants containing amino acid substitutions in the N-terminal (L9W, V10W, V10F and V10R) and/or the C-terminal (W106G) enzyme-binding regions were designed and produced in Escherichia coli. It was discovered that the inhibition profile of the cystatin could be altered by changing residues 9 and 10, which are proposed to bind in the S3 and S2 substrate-binding pockets respectively of the enzymes. All of the variants with substitutions in the N-terminal segment displayed decreased binding to cathepsins B and H, indicating that the S3 and S2 pockets of these enzymes cannot easily accommodate large aromatic residues. The introduction of a charged residue into S2 (variant V10R) created a more specific inhibitor to distinguish cathepsin B from cathepsin H. Cathepsin L showed a preference for larger aromatic residues in S2. In contrast, cathepsin S preferred phenylalanine to valine in S2, but bound less tightly to the V10W cystatin variant. The latter variant proved to be valuable for discriminating between cathepsin L and cathepsin S (Ki 2.4 and 190 pM respectively). The equilibrium dissociation constant of the complex between cathepsin L and variant L9W/W106G showed little difference in affinity from that of the cathepsin L complex with the singly substituted W106G variant. In contrast, the L9W/W106G variant displayed increased specificity for cathepsin S with a Ki of 10 pM. Our results clearly indicate differences in the specificity of interaction between the N-terminal region of cystatin C and cathepsins B, H, L and S, and that, although cystatin C has evolved to be a good inhibitor of all of the mammalian cysteine proteinases, more specific inhibitors of the individual enzymes can be engineered.
An analysis of the role of active site protic residues of cytochrome P-450s: mechanistic and mutational studies on 17α-hydroxylase-17,20-lyase (P-45017α also CYP17)
Authors: LEE-ROBICHAUD Peter , AKHTAR E. Monika , AKHTAR Muhammad
Published: August 2015

Certain cytochrome P-450s involved in the transformation of steroids catalyse not only the hydroxylation process associated with the group of enzymes, but also an acyl-carbon cleavage reaction. The hydroxylation occurs using an iron-monooxygen species while the acyl-carbon cleavage has been suggested to be promoted by an iron peroxide. In this paper we have studied the role of active site protic residues, Glu305 and Thr306, in modulating the two activities. For this purpose, the kinetic parameters for the hydroxylation reaction (pregnenolone → 17α-hydroxypregnenolone) and two different versions of acyl-carbon cleavage (17α-hydroxypregnenolone → dehydroepiandrosterone and 3β-hydroxyandrost-5-ene-17β-carbaldehyde → 3β-hydroxyandrost-5,16-diene+androst-5-ene-3β,17α-diol) were determined using the wild-type human CYP17 and its eight different single and double mutants. In addition the propensity of the proteins to undergo a subtle rearrangement converting the 450 nm active-form into an inactive counterpart absorbing at 420 nm, was monitored by measuring the of the P-450 → P-420 conversion. The results are interpreted to draw the following conclusions. The functional groups of Glu305 and Thr306 do not directly participate in the two proton delivery steps required for hydroxylation but may be important participants for the provision of a net work of hydrogen bonds for ‘activating’ water that then acts as a proton donor. The loss of any one of these residues is, therefore, only partially debilitating. That the mutation of Thr306 impairs the hydroxylation reaction more than it does the acyl-carbon cleavage is consistent with the detailed mechanistic scheme considered in this paper. Furthermore attention is drawn to the fact that the mutation of Glu305 and Thr306 subtly perturbed the architecture of the active site, which affects the geometry of this region of the protein and therefore its catalytic properties.
Lysophosphatidic acid-mediated signal-transduction pathways involved in the induction of the early-response genes prostaglandin G/H synthase-2 and Egr-1: a critical role for the mitogen-activated protein kinase p38 and for Rho proteins
Authors: REISER O. A. Christian , LANZ Thomas , HOFMANN Fred , HOFER Gerhard , RUPPRECHT D. Harald , GOPPELT-STRUEBE Margarete
Published: August 2015

During inflammatory processes of the kidney, lesions of the glomerulus lead to aggregation of thrombocytes and infiltration of macrophages, which can release bioactive mediators. One of these important signalling molecules is lysophosphatidic acid (LPA). Incubation of rat mesangial cells with LPA induced mRNA and protein expression of the early-response genes pghs-2 (for prostaglandin G/H synthase-2/cyclo-oxygenase-2) and egr-1. As shown by antisense experiments, induction of egr-1 was related to the strong mitogenic effect of LPA. LPA-mediated gene expression was inhibited by pertussis toxin, indicating coupling to G-proteins of the Gi family. Specific inhibition of proteins of the small G-protein subfamily Rho with toxin B from Clostridium difficile led to changes in mesangial cell morphology without induction of apoptosis. LPA-mediated expression of pghs-2 and egr-1 was reduced to base-line levels by toxin B, indicating a role for Rho proteins in LPA-mediated gene induction. Of the two mitogen-activated protein kinase (MAPK) pathways investigated, the MAPK kinase-extracellular signal-regulated kinase pathway was involved in the induction of both pghs-2 and egr-1 mRNA expression, as shown by the inhibitory effect of PD98059. Activation of the MAPK p38, however, was only related to pghs-2 expression, whereas egr-1 expression was not affected by treatment of mesangial cells with the specific inhibitor SB203580. Taken together our data provide evidence that LPA-mediated activation of MAPK kinase and Rho proteins leads to the induction of the functionally distinct early-response genes pghs-2 and egr-1, whereas activation of MAPK p38 revealed considerable differences between the regulation of these two genes.
Efficient CD28 signalling leads to increases in the kinase activities of the TEC family tyrosine kinase EMT/ITK/TSK and the SRC family tyrosine kinase LCK
Authors: GIBSON Spencer , TRUITT Ken , LU Yiling , LAPUSHIN Ruth , KHAN Humera , IMBODEN B. John , MILLS B. Gordon
Published: August 2015

Optimal T cell activation requires crosslinking of the T cell receptor (TCR) concurrently with an accessory receptor, most efficiently CD28. Crosslinking of CD28 leads to increased interleukin 2 (IL2) production, inhibition of anergy and prevention of programmed cell death. Crosslinking of CD28 leads to rapid increases in tyrosine phosphorylation of specific intracellular substrates including CD28 itself. Since CD28 does not encode an intrinsic tyrosine kinase domain, CD28 must activate an intracellular tyrosine kinase(s). Indeed, crosslinking of CD28 increases the activity of the intracellular tyrosine kinases EMT/ITK and LCK. The phosphatidylinositol 3-kinase (PI3K) and GRB2 binding site in CD28 is dispensable for optimal IL2 production in Jurkat T cells. We demonstrate herein that murine Y170 (equivalent to human Y173) in CD28 is also dispensable for activation of the SRC family tyrosine kinase LCK and the TEC family tyrosine kinase EMT/ITK. In contrast, the distal three tyrosines in CD28 are required for optimal IL2 production as well as for optimal activation of the LCK and EMT/ITK tyrosine kinases. The distal three tyrosines of CD28, however, are not required for recruitment of PI3K to CD28. Furthermore, PI3K is recruited to CD28 in JCaM1 cells which lack LCK and in which EMT/ITK is not activated by ligation of CD28. Thus optimal activation of LCK or EMT/ITK is not obligatory for recruitment of PI3K to CD28 and thus is also not required for tyrosine phosphorylation of the YMNM motif in CD28. Taken together the data indicate that the distal three tyrosines in CD28 are integral to the activation of LCK and EMT/ITK and for subsequent IL2 production.
Regulation of ATP supply during muscle contraction: theoretical studies
Authors: KORZENIEWSKI Bernard
Published: August 2015

The dynamic computer model of oxidative phosphorylation developed previously and successfully tested for large-scale changes in fluxes and metabolite concentrations was used to study the question of how the rate of ATP production by oxidative phosphorylation is adjusted to meet the energy demand during muscle contraction, which causes a great increase in ATP consumption in relation to the resting state. The changes in the respiration rate and ATP/ADP ratio after the onset of maximal work measured experimentally were compared with simulated changes in the respiration rate and ATP/ADP in several different cases, assuming direct activation of different steps by an external effector. On the basis of the computer simulations performed, it was possible to conclude which enzymes/metabolic blocks should be directly activated to cause the experimentally observable changes in fluxes and metabolite concentrations. The theoretical results obtained suggest that the parallel direct activation of actinomyosin-ATP-ase and oxidative phosphorylation by an external effector (for example calcium ions) is the main mechanism responsible for fitting of ATP production to ATP consumption, while the negative feedback via an increase in ADP concentration (decrease in ATP/ADP), which indirectly activates the ATP supply, plays only a minor role. Additionally, the conclusion is drawn that most of the oxidative phosphorylation steps should be directly activated in order to explain the observed changes in the respiration rate and ATP/ADP ratio (and also in other parameters) during muscle contraction. It is suggested that there should exist a universal external activator/regulatory mechanism which causes a parallel stimulation of different enzymes/processes. A possible nature of such an activator is shortly discussed.
Expression, processing and secretion of a proteolytically-sensitive insect diuretic hormone by Saccharomyces cerevisiae requires the use of a yeast strain lacking genes encoding the Yap3 and Mkc7 endoproteases found in the secretory pathway
Authors: COPLEY S. Kathrin , ALM M. Sheri , SCHOOLEY A. David , COURCHESNE E. William
Published: August 2015

A system is described for the heterologous expression of peptides in Saccharomyces cerevisiae. A synthetic gene encoding a precursor of the 41 amino acid Manduca sexta diuretic hormone (Mas-DH) was expressed at 0.8 mg/l purified peptide. A precursor of a mutant peptide of Mas-DH, Mas-DH[K22Q] was also expressed. The peptides were purified, then treated with peptidylglycine α-amidating enzyme to generate the α-amidated, mature, form of Mas-DH or Mas-DH[K22Q], which were biologically active. Successful expression of full-length Mas-DH+Gly depended upon the use of a protease-deficient yeast strain. In wild-type strains, Mas-DH+Gly was recovered only as proteolytic fragments, even in the presence of various protease inhibitors. Expression of Mas-DH+Gly in strains deficient in either the Mkc7 or the Yap3 protease reduced proteolysis, while no proteolysis of Mas-DH+Gly was detectable in a strain lacking both proteases. This protease-deficient strain may prove of general utility for expression of peptides. Analysis of recovered proteolytic fragments revealed a complex pattern of cleavage sites. Both the Yap3 and Mkc7 proteases preferred to cleave at a single Glu-Lys↓-Glu-Arg site. Analysis of secondary cleavage sites showed that Yap3 preferred to cleave after either Lys or Arg and Mkc7 after Lys. This paper is the first report on the in vivo activity and specificity of Yap3 and Mkc7 expressed at physiological levels.
The mitogen-activated protein (MAP) kinase cascade can either stimulate or inhibit DNA synthesis in primary cultures of rat hepatocytes depending upon whether its activation is acute/phasic or chronic
Authors: TOMBES M. Robert , AUER L. Kelly , MIKKELSEN Ross , VALERIE Kristopher , WYMANN P. Mathias , MARSHALL J. Christopher , MCMAHON Martin , DENT Paul
Published: August 2015

Bailie et al. [In Vitro Cell Dev. Biol. (1992) 28A, 621-624] reported that primary cultures of rat hepatocytes possess low affinity binding sites for nerve growth factor (NGF). NGF treatment of primary cultures of rat hepatocytes with a maximally effective concentration of NGF (20 ng/ml, 0.8 nM) caused acute phasic activation of Raf-1 and p42MAPkinase, and a smaller sustained activation of B-Raf. The transient increase in Raf-1 and p42MAPkinase activity returned to baseline within ~ 30 min. NGF treatment of hepatocytes did not induce expression of cyclin dependent kinase (cdk) inhibitor proteins, but instead stimulated cdk2 activity and increased [3H]thymidine incorporation into DNA. In contrast to hepatocytes, NGF treatment of PC12 pheochromocytoma cells caused large sustained activations of B-Raf and p42MAPkinase, and a lower phasic activation of Raf-1. The sustained activations of B-Raf and p42MAPkinase were for more than 5 h. Treatment of PC12 cells with NGF increased p21Cip1/WAF-1 expression, reduced cdk2 activity and inhibited DNA synthesis, the opposite to the effects of NGF treatment of hepatocytes. However when p42MAPkinase was chronically activated in hepatocytes, via infection with an inducible oestrogen receptor-Raf-1 fusion protein, expression of p21Cip-1/WAF1 and p16INK4a cdk inhibitor proteins increased, cdk2 activity decreased, and DNA synthesis decreased. Equally, treatment of hepatocytes with 50 mM ethanol elevated the basal activity of p42MAPkinase and temporally extended the ability of NGF treatment to activate p42MAPkinase. Ethanol and NGF co-treatment increased expression of p21Cip-1/WAF1 and p16INK4a cdk inhibitor proteins and decreased hepatocyte DNA synthesis. These data demonstrate that NGF can cause either acute/phasic or sustained activation of the MAP kinase cascade in different cell types. Acute activation of the MAP kinase cascade correlated with increased DNA synthesis. In contrast, sustained activation of the MAP kinase cascade correlated with increased expression of cdk inhibitor proteins, a reduction in cdk activity, and an inhibition of DNA synthesis. These data suggest a general mechanism exists where acute activation of the MAP kinase cascade promotes G1 progression/S phase entry and that chronic activation of the MAP kinase cascade inhibits this process.
Bombesin stimulates cholecystokinin secretion through mitogen-activated protein-kinase-dependent and -independent mechanisms in the enteroendocrine STC-1 cell line
Authors: NÉMOZ-GAILLARD Eric , CORDIER-BUSSAT Martine , FILLOUX Chantal , CUBER Jean-Claude , VAN OBBERGHEN Emmanuel , CHAYVIALLE Jean-Alain , ABELLO Jacques
Published: August 2015

Bombesin has been reported to stimulate cholecystokinin (CCK) secretion from rat duodeno-jejunal I-cells. Bombesin was shown to activate mitogen-activated protein kinases (MAPKs) in cell types such as Swiss 3T3 fibroblasts and rat pancreatic acinar cells. No information is available on whether MAPK is activated in intestinal endocrine cells upon bombesin stimulation. This was studied by using the CCK-producing enteroendocrine cell line STC-1. Bombesin stimulated markedly and transiently both p42MAPK and p44MAPK, with a maximum at 2 min, and a decrease to basal levels within 10 min. As expected, bombesin stimulated MAPK kinase 1 (MEK-1) activity. Activation of protein kinase C (PKC) with PMA also stimulated p42MAPK, p44MAPK and MEK-1. Treatment of cells with PD 098059 (at 10 µM or 30 µM), which selectively inhibits MEK phosphorylation, blocked bombesin-induced p42MAPK and p44MAPK activation for at least 90 min. However, PD 098059 inhibited bombesin- and PMA-stimulated CCK secretion during the first 15 min, but failed to significantly reduce CCK release at later times. Inhibition of PKC with staurosporine, or PKC down-regulation by prolonged treatment with PMA, both drastically decreased MEK-1, p42MAPK and p44MAPK activation upon bombesin stimulation. Additionally, PKC activation appeared to be required for both MAPK-dependent (early) and -independent (late) CCK responses to bombesin. It is concluded that the early CCK secretory response of STC-1 cells to bombesin involves MAPK pathway activation through a PKC-dependent mechanism, whereas the late phase of bombesin-induced CCK secretion, that also requires PKC, appears to result from a MAPK-independent process.
A human homologue of Escherichia coli ClpP caseinolytic protease: recombinant expression, intracellular processing and subcellular localization
Authors: CORYDON Thomas J. , BROSS Peter , HOLST Henrik U. , NEVE Søren , KRISTIANSEN Karsten , GREGERSEN Niels , BOLUND Lars
Published: August 2015

We have recently cloned a human cDNA (hClpP) with significant sequence similarity to the ATP-dependent Escherichia coli ClpP protease [Bross, Andresen, Knudsen, Kruse and Gregersen (1995) FEBS Lett. 377, 249–252]. In the present study, synthesis, intracellular processing and subcellular localization of hClpP have been analysed in intact cells and in a cell-free system. Using pulse-labelling/immunoprecipitation of Chang cells transfected with the hClpP cDNA, we observed two major bands with apparent molecular masses of approx. 39 and 37 kDa. A pulse–chase experiment showed that these bands were converted into one mature-enzyme band with a molecular mass of approx. 32 kDa that was stable for at least 24 h. The 37 kDa band co-migrated with a band produced upon expression of full-length hClpP in E. coli, and the 32 kDa band co-migrated with the product of E. coli-expressed hClpP in which the 56 N-terminal residues had been deleted, indicating that the 37 kDa moiety represents the precursor and that approx. 56 residues are cleaved off during maturation. The processing of hClpP in intact cells was dependent on mitochondrial membrane potential. These results were confirmed in an import assay system using in vitro transcription and translation directed by the hClpP cDNA and isolated rat liver mitochondria. No protease activity towards a series of fluorogenic peptides could be observed in extracts of Chang cells overexpressing hClpP, indicating that the protease may not be active without co-factors. Immunofluorescence studies using confocal-laser-scanning microscopy showed co-localization of the hClpP and the mitochondrially located Hsp60 (heat-shock protein 60). Taken together, the results reported here show that hClpP is localized inside mitochondria and that the trafficking and processing of hClpP resembles the typical biogenesis pathway for nuclear-encoded mitochondrial proteins.
Susceptibility of the cysteine-rich N-terminal and C-terminal ends of rat intestinal mucin Muc 2 to proteolytic cleavage
Authors: KHATRI Ismat A. , FORSTNER Gordon G. , FORSTNER Janet F.
Published: August 2015

The present study reveals that partial proteolytic degradation of rat Muc 2 mucin can occur rapidly even in the presence of a battery of proteinase inhibitors. During the initial steps of purification from homogenates of intestinal scrapings, degradation was rapid, causing release of the entire 118 kDa C-terminal glycopeptide and, as shown by N-terminal sequencing, a large (200 kDa) N-terminal glycopeptide fragment. Degradation could be prevented by adding 6 M guanidinium chloride provided that its presence was maintained throughout every step of purification. Even after purification, however, the mucin was still vulnerable to partial proteolysis unless it was stored in guanidinium chloride at -20 °C. These findings imply that a potent proteinase contaminant remains tightly bound to the mucin through every step of purification, or else that the mucin has autocatalytic properties. Because the C- and N-terminal regions of secretory mucins are required for their assembly into linear mucin polymers that form functional gels, our findings emphasize that extreme care is required to purify structurally intact mucin molecules. They also imply that the specific degradation steps described here are likely to occur rapidly after mucins are secreted into the intestinal lumen and come into contact with the products of sloughed cells.
Cloning and characterization of a novel sequence-specific single-stranded-DNA-binding protein
Authors: BAYARSAIHAN Dashzeveg , SOTO Ricardo J. , LUKENS Lewis N.
Published: August 2015

The promoter region of the chicken α2(I) collagen gene contains a pyrimidine-rich element that is well conserved in different mammalian species. This sequence can also form an unusual DNA structure as shown by its sensitivity to SI nuclease in vitro and it lies in a region that is DNase I-hypersensitive only when this promoter is active. We have recently reported that fibroblast nuclear proteins, including chicken Y-box-binding protein 1, bind to this single-stranded pyrimidine-rich sequence. Here we report the isolation, from a chick embryo fibroblast cDNA expression library, of a partial cDNA clone encoding a previously unknown protein, designated SSDP (sequence-specific single-stranded DNA-binding protein), that binds this single-stranded sequence. This clone contains 1199 bp of chicken sequence and has a single long open reading frame that encodes 284 amino acid residues. The affinity-purified recombinant protein encoded by this cDNA binds sequence-specifically to the single-stranded pyrimidine sequence. This cDNA sequence lacks significant similarity to any known gene in the data banks, but it is highly conserved in expressed sequence tags derived from both mouse and human. The corresponding amino acid sequence is remarkably conserved, having 97% identity with mouse and human expressed sequences. The corresponding mRNA is approx. 1800 nt in length and is expressed in both fibroblasts and chondrocytes. The high affinity of this protein for this conserved pyrimidine-rich region suggests that it might be involved in the transcriptional regulation of the α2(I) collagen gene.
Regulation of integrin function: evidence that bivalent-cation-induced conformational changes lead to the unmasking of ligand-binding sites within integrin α5β1
Authors: MOULD A. Paul , GARRATT Alistair N. , PUZON-McLAUGHLIN Wilma , TAKADA Yoshikazu , HUMPHRIES Martin J.
Published: August 2015

The molecular mechanisms that regulate integrin–ligand binding are unknown; however, bivalent cations are essential for integrin activity. According to recent models of integrin tertiary structure, sites involved in ligand recognition are located on the upper face of the seven-bladed β-propeller formed by the N-terminal repeats of the α subunit and on the von Willebrand factor A-domain-like region of the β subunit. The epitopes of function-altering monoclonal antibodies (mAbs) cluster in these regions of the α and β subunits; hence these mAbs can be used as probes to detect changes in the exposure or shape of the ligand-binding sites. Bivalent cations were found to alter the apparent affinity of binding of the inhibitory anti-α5 mAbs JBS5 and 16, the inhibitory anti-β1 mAb 13, and the stimulatory anti-β1 mAb 12G10 to α5β1. Analysis of the binding of these mAbs to α5β1 over a range of Mn2+, Mg2+ or Ca2+ concentrations demonstrated that there was a concordance between the ability of cations to elicit conformational changes and the ligand-binding potential of α5β1. Competitive ELISA experiments provided evidence that the domains of the α5 and β1 subunits recognized by mAbs JBS5/16 and 13/12G10 are spatially close, and that the distance between these two domains is increased when α5β1 is occupied by bivalent cations. Taken together, our findings suggest that bivalent cations induce a conformational relaxation in the integrin that results in exposure of ligand-binding sites, and that these sites lie near an interface between the α subunit β-propeller and the β subunit putative A-domain.
Modulation of Ins(2,4,5)P3-stimulated Ca2+ mobilization by Ins(1,3,4,5)P4: enhancement by activated G-proteins, and evidence for the involvement of a GAP1 protein, a putative Ins(1,3,4,5)P4 receptor
Authors: LOOMIS-HUSSELBEE Jefferson W. , WALKER Christopher D. , BOTTOMLEY Joanna R. , CULLEN Peter J. , IRVINE Robin F. , DAWSON Alan P.
Published: August 2015

We have previously shown that addition of Ins(1,3,4,5)P4 to permeabilized L1210 cells increases the amount of Ca2+ mobilized by a submaximal concentration of Ins(2,4,5)P3, and we suggested that, in doing this, Ins(1,3,4,5)P4 is not working via an InsP3 receptor but indirectly via an InsP4 receptor [Loomis-Husselbee, Cullen, Dreikhausen, Irvine and Dawson (1996) Biochem. J. 314, 811–816]. Here we have investigated whether this effect might be mediated by GAP1IP4BP, recently identified as a putative receptor for Ins(1,3,4,5)P4. GAP1IP4BP is a protein that interacts with one or more monomeric G-proteins, so we sought evidence for involvement of monomeric G-proteins in the effects of Ins(1,3,4,5)P4 in permeabilized L1210 cells. Guanosine 5´-[γ-thio]triphosphate (GTP[S]) enhanced the effect of Ins(1,3,4,5)P4 on Ins(2,4,5)P3-stimulated Ca2+ mobilization, but had no effect on the action of Ins(2,4,5)P3 alone. A specific enhancement of only the action of Ins(1,3,4,5)P4 was also seen with GTP[S]-loaded R-Ras or Rap1a (two G-proteins known to interact with GAP1IP4BP), whereas H-Ras was inactive at similar concentrations. Guanosine 5´-[β-thio]diphosphate (GDP[S]) did not alter the action of either Ins(2,4,5)P3 or Ins(1,3,4,5)P4. Finally, the addition of exogenous GAP1IP4BP, purified from platelets, markedly enhanced the effect of Ins(1,3,4,5)P4, and again, the amount of Ca2+ mobilized by Ins(2,4,5)P3 alone was unaltered. We conclude that the increase in Ins(2,4,5)P3-stimulated Ca2+ mobilization by Ins(1,3,4,5)P4 may be mediated by GAP1IP4BP or a closely related protein (such as GAP1m), and if so, the action of the GAP1 is not solely to regulate GTP loading of a G-protein, but rather it acts with a G-protein to cause its effect.
Polyclonal antibody catalytic variability
Authors: STEPHENS David B. , THOMAS Richard E. , STANTON John F. , IVERSON Brent L.
Published: August 2015

We have performed a systematic variability study of polyclonal antibody catalysis by using five rabbits immunized with the same hapten. Important results from this work are the following. (1) Similarities were observed in the catalytic polyclonal antibodies derived from all five rabbits. Four of the five rabbits produced polyclonal samples that were nearly the same in terms of catalytic activity, whereas the fifth rabbit, designated as rabbit 2, displayed a somewhat higher level of catalytic activity. The catalytic activities (as kcat/kuncat) of these polyclonal samples were similar to that from the best murine monoclonal antibody that had been previously elicited by the same hapten. (2) Titre was not an accurate indicator of polyclonal antibody catalytic activity. (3) A mathematical analysis to describe a distribution of Michaelis–Menten catalysts was performed to help interpret our results. (4) Kinetic analysis indicated that the binding parameters of the different samples were remarkably homogeneous, because one or two components were all that were required to fit the on-rate and off-rate data satisfactorily. Interestingly, the most active catalytic polyclonal sample, that from rabbit 2, displayed the slowest off-rate (so slow it could not be measured) and thus the highest overall affinity. (5) Catalytic analysis of eluted fractions of antibody from a substrate column indicated that each polyclonal sample was also relatively homogeneous in terms of catalytic parameters. The main conclusion of our study is that for this hapten–animal system, the overall catalytic immune response is relatively consistent at two levels. Consistent catalytic activity was observed between the polyclonal samples elicited in the different animals, and the elicited hapten-specific polyclonal antibodies were relatively homogeneous in terms of binding and catalytic parameters within each immunized animal. The observed similarities of the catalytic activity in the different animals is surprising, because the immune response is based on specific binding of antibodies to hapten. There is no known selective pressure to maintain consistent levels of catalytic activity. Our results can therefore be interpreted as providing evidence that for this hapten there is a fixed relationship between hapten structure and catalytic activity and/or consistent genetic factors that dominate the catalytic immune response.
Agonist-specific alterations in receptor–phospholipase coupling following inactivation of Gi2α gene
Authors: MATTERA Rafael , HAYEK Salim , SUMMERS Beth A. , GROVE Danette L.
Published: August 2015

Different forms of phospholipase A2, together with pertussis toxin-sensitive G-proteins, [Ca2+]i (intracellular Ca2+ concentration), protein kinase C, calmodulin, protein tyrosine kinases, mitogen-activated protein kinases and Ca2+/calmodulin-dependent protein kinase appear to play a role in agonist-mediated release of arachidonic acid. Here we report that fibroblasts from 14-day-old mouse embryos with inactivated Gi2α (α-subunit of the heterotrimeric G-protein Gi2) gene display a marked decrease in the ability of lysophosphatidic acid, thrombin and Ca2+ ionophores to release arachidonic acid compared with their normal counterparts. The requirement for Gi2α in the release of arachidonic acid following increased [Ca2+]i may be explained by the incomplete translocation of cytosolic phospholipase A2 observed in Gi2α-deficient cells. Paradoxically, inactivation of the Gi2α gene resulted in up-regulation of bradykinin receptors and their coupling to increased arachidonic acid release, phospholipase C activity and [Ca2+]i. A concomitant increase in basal phospholipase C activity was also observed in the Gi2α-deficient cells. These observations establish a pleiotropic and essential role for Gi2α in receptor-phospholipase coupling that contrasts with its less obligatory participation in agonist-mediated inhibition of adenylate cyclase.
The extended protein kinase C superfamily
Authors: MELLOR Harry , PARKER Peter J.
Published: August 2015

Members of the mammalian protein kinase C (PKC) superfamily play key regulatory roles in a multitude of cellular processes, ranging from control of fundamental cell autonomous activities, such as proliferation, to more organismal functions, such as memory. However, understanding of mammalian PKC signalling systems is complicated by the large number of family members. Significant progress has been made through studies based on comparative analysis, which have defined a number of regulatory elements in PKCs which confer specific location and activation signals to each isotype. Further studies on simple organisms have shown that PKC signalling paradigms are conserved through evolution from yeast to humans, underscoring the importance of this family in cellular signalling and giving novel insights into PKC function in complex mammalian systems.
Purification and characterization of a novel chitinase–lysozyme, of another chitinase, both hydrolysing Rhizobium meliloti Nod factors, and of a pathogenesis-related protein from Medicago sativa roots
Authors: MINIC Zoran , BROWN Spencer , De KOUCHKOVSKY Yaroslav , SCHULTZE Michael , STAEHELIN Christian
Published: August 2015

The symbiosis between Rhizobium meliloti and Medicago sativa (Leguminosae) involves the interaction of lipochito-oligosaccharides (Nod factors) excreted by bacteria with specific proteins of the host plant. The cleavage of Nod factors can be used as an enzymic assay to identify novel hydrolytic enzymes. Here a soluble extract of 3-day-old roots was fractionated by anion exchange, affinity chromatography, gel filtration and native electrophoresis. Two acidic chitinases (pI 4.6–5.4), CHIT24 and CHIT36, designated in accordance with their molecular mass in kDa, were separated. CHIT24 cleaves all tested Nod factors to produce lipotrisaccharides with the preference NodRm-V(S) > NodRm-IV > NodRm-IV(S) ⩾ NodRm-IV(Ac,S); it also hydrolyses colloidal 3H-chitin and has lysozyme activity. The kinetics of Nod factor degradation by CHIT24 depends on substrate structural parameters, namely the length of the oligosaccharide chain and sulphation (S) at the reducing end, but not much on acetylation (Ac) at the non-reducing end. The 25-residue N-terminal sequence of CHIT24 has no similarity with known chitinases or lysozymes, indicating that it is a novel type of hydrolase. CHIT36 also hydrolyses NodRm-V(S) into NodRm-III, but it is inactive towards NodRm-IV(S) and NodRm-IV(Ac,S) formed by R. meliloti. Finally, a 17 kDa protein, P17, was co-purified with CHIT24. It neither degrades Nod factors nor exhibits lysozyme activity and shows complete identity, at the 15-residue N-terminal sequence, with a class 10 pathogenesis-related protein, PR-10.
Rabbit cardiac and skeletal myocytes differ in constitutive and inducible expression of the glucose-regulated protein GRP94
Authors: VITADELLO Maurizio , COLPO Pierangelo , GORZA Luisa
Published: August 2015

The glucose-regulated protein GRP94 is a stress-inducible glycoprotein that is known to be constitutively and ubiquitously expressed in the endoplasmic reticulum of mammalian cells. From a rabbit heart cDNA library we isolated four overlapping clones coding for the rabbit homologue of GRP94 mRNA. Northern blot analysis shows that a 3200 nt mRNA species corresponding to GRP94 mRNA is detectable in several tissues and it is 5-fold more abundant in the heart than in the skeletal muscle. Hybridization analysis in situ shows that GRP94 mRNA accumulates in cardiac myocytes, whereas in skeletal muscles it is not detectable in myofibres. A monoclonal antibody raised by using a 35 kDa recombinant GRP94 polypeptide as immunogen detects a single reactive polypeptide of 94 kDa in a Western blot of liver and heart homogenates and does not react with skeletal muscle homogenates. Conversely, GRP94 mRNA and protein are detectable in both cardiac and skeletal muscle myocytes of fetal and neonatal rabbits. After 24 h of endotoxin administration to adult rabbits, GRP94 mRNA accumulation increases 3-fold in both heart and skeletal muscle and it is followed by a comparable increase in protein accumulation. However, hybridization and immunohistochemistry in situ do not reveal any change in the expression of GRP94 mRNA and protein in skeletal muscle myocytes after endotoxin treatment. Thus skeletal muscle fibres display a unique regulation of the GRP94 gene, which is up-regulated during perinatal development, whereas in the adult animal it is apparently silent and not responsive to endotoxin treatment.
Effect of tri-iodothyronine on leptin release and leptin mRNA accumulation in rat adipose tissue
Authors: FAIN John N. , BAHOUTH Suleiman W.
Published: August 2015

Leptin, the product of the obese gene, is produced by white adipocytes. The release of leptin, as well as leptin mRNA content, was enhanced in adipocytes isolated from hypothyroid rats. The administration of tri-iodothyronine (T3) 8 h before death inhibited leptin release by adipocytes incubated for 6 or 24 h. Direct addition of T3 to pieces of adipose tissue enhanced the loss of leptin mRNA seen over 24 h in the presence of dexamethasone plus the β3-adrenergic agonist Cl 316,243. In contrast, if pieces of adipose tissue were incubated with dexamethasone plus insulin, enhanced the T3 accumulation of leptin mRNA. These results indicate that T3 enhances net adipocyte leptin mRNA accumulation in a condition that approximates the fed state (presence of insulin) but inhibits leptin mRNA accumulation in a condition that approximates the fasted state (absence of insulin).
Overlapping antioxidant response element and PMA responseelement sequences mediate basal and β-naphthoflavone-induced expression of the human γ-glutamylcysteine synthetase catalytic subunit gene
Authors: WILD Angela C. , GIPP Jerry J. , MULCAHY R. Timothy
Published: August 2015

γ-Glutamylcysteine synthetase (GCS), the rate-limiting enzyme in the de novo synthesis of GSH, is a heterodimer, consisting of a catalytic (GCSh) and a regulatory subunit (GCSl). We previously demonstrated that the constitutive and β-naphthoflavone (β-NF)-induced expression of the GCSh gene is mediated by a distal antioxidant response element (ARE), ARE4, located 3.1 kb upstream of the transcriptional start site [Mulcahy, Wartman, Bailey and Gipp (1997) J. Biol. Chem. 272, 7445–7454]. ARE4 consists of a consensus ARE sequence (5´-GTGACTCAGCG-3´) containing an embedded PMA-responsive element (TRE, underlined). The relative significance of the two overlapping response elements to constitutive and β-NF-induced expression of the GCSh gene was determined by mutational analyses. The internal activator protein-1 (AP-1)-binding sequence mediated constitutive expression of promoter/reporter transgenes, but was not required for β-NF responsiveness. In gel-shift experiments, the TRE was necessary for binding of proteins from nuclear extracts prepared from untreated HepG2 cells. In contrast, induction by β-NF was dependent on an intact ARE sequence, particularly the terminal GC box of ARE4. The GC box of ARE4 was shown to be essential for both basal and β-NF-induced expression of reporter constructs. This element also influenced binding of nuclear proteins to ARE4, specifically in extracts isolated from β-NF-treated HepG2 cells. Because previous studies indicated that ARE4 may co-operate with a separate putative ARE, the role of the neighbouring sequence (ARE3), located 34 bases downstream of ARE4, was also evaluated. Mutation of this element within a GCSh promoter/reporter did not modify the basal or β-NF-induced expression of the transgene, demonstrating that ARE3 does not influence the constitutive or β-NF-induced expression of the GCSh gene.