Characterization of purified pp60c-src protein tyrosine kinase from human platelets

Autophosphorylation activates c-Src kinase through global structural rearrangements

The prototypical kinase c-Src plays an important role in numerous signal transduction pathways, where its activity is tightly regulated by two phosphorylation events. Phosphorylation at a specific tyrosine by C-terminal Src kinase inactivates c-Src, whereas autophosphorylation is essential for the c-Src activation process. However, the structural consequences of the autophosphorylation process still remain elusive. Here we investigate how the structural landscape of c-Src is shaped by nucleotide binding and phosphorylation of Tyr⁴¹⁶ using biochemical experiments, hydrogen/deuterium exchange MS, and atomistic molecular simulations. We show that the initial steps of kinase activation involve large rearrangements in domain orientation. The kinase domain is highly dynamic and has strong cross-talk with the regulatory domains, which are displaced by autophosphorylation. Although the regulatory domains become more flexible and detach from the kinase domain because of autophosphorylation, the kinase domain gains rigidity, leading to stabilization of the ATP binding site and a 4-fold increase in enzymatic activity. Our combined results provide a molecular framework of the central steps in c-Src kinase regulation process with possible implications for understanding general kinase activation mechanisms.

Sonic hedgehog stimulates neurite outgrowth in a mechanical stretch model of reactive-astrogliosis

Although recovery following a stroke is limited, undamaged neurons under the right conditions can establish new connections and take on-board lost functions. Sonic hedgehog (Shh) signaling is integral for developmental axon growth, but its role after injury has not been fully examined. To investigate the effects of Shh on neuronal sprouting after injury, we used an in vitro model of glial scar, whereby cortical astrocytes were mechanically traumatized to mimic reactive astrogliosis observed after stroke. This mechanical trauma impaired neurite outgrowth from post-natal cortical neurons plated on top of reactive astrocytes. Addition of Shh to the media, however, resulted in a concentration-dependent increase in neurite outgrowth. This response was inhibited by cyclopamine and activated by oxysterol 20(S)-hydroxycholesterol, both of which modulate the activity of the Shh co-receptor Smoothened (Smo), demonstrating that Shh-mediated neurite outgrowth is Smo-dependent. In addition, neurite outgrowth was not associated with an increase in Gli-1 transcription, but could be inhibited by PP2, a selective inhibitor of Src family kinases. These results demonstrate that neurons exposed to the neurite growth inhibitory environment associated with a glial scar can be stimulated by Shh, with signaling occurring through a non-canonical pathway, to overcome this suppression and stimulate neurite outgrowth.

Regulation of Src trafficking and activation by the endocytic regulatory proteins MICAL-L1 and EHD1

Localization of the non-receptor tyrosine kinase Src to the cell periphery is required for its activation and to mediate focal adhesion turnover, cell spreading and migration. Inactive Src localizes to a perinuclear compartment and the movement of Src to the plasma membrane is mediated by endocytic transport. However, the precise pathways and regulatory proteins that are responsible for SRC transport are incompletely understood. Here, we demonstrate that Src partially colocalizes with the endocytic regulatory protein MICAL-L1 (molecule interacting with CasL-like protein 1) in mammalian cells. Furthermore, MICAL-L1 is required for growth-factor- and integrin-induced Src activation and transport to the cell periphery in HeLa cells and human fibroblasts. Accordingly, MICAL-L1 depletion impairs focal adhesion turnover, cell spreading and cell migration. Interestingly, we find that the MICAL-L1 interaction partner EHD1 (EH domain-containing protein 1) is also required for Src activation and transport. Moreover, the MICAL-L1-mediated recruitment of EHD1 to Src-containing recycling endosomes is required for the release of Src from the perinuclear endocytic recycling compartment in response to growth factor stimulation. Our study sheds new light on the mechanism by which Src is transported to the plasma membrane and activated, and provides a new function for MICAL-L1 and EHD1 in the regulation of intracellular non-receptor tyrosine kinases.

Tyrosine 416 is phosphorylated in the closed, repressed conformation of c-Src

c-Src kinase activity is regulated by phosphorylation of Y527 and Y416. Y527 phosphorylation stabilizes a closed conformation, which suppresses kinase activity towards substrates, whereas phosphorylation at Y416 promotes an elevated kinase activity by stabilizing the activation loop in a manner permissive for substrate binding. Here we investigated the correlation of Y416 phosphorylation with c-Src activity when c-Src was locked into the open and closed conformations (by mutations Y527F and Q528E, P529E, G530I respectively). Consistent with prior findings, we found Y416 to be more greatly phosphorylated when c-Src was in an open, active conformation. However, we also observed an appreciable amount of Y416 was phosphorylated when c-Src was in a closed, repressed conformation under conditions by which c-Src was unable to phosphorylate substrate STAT3. The phosphorylation of Y416 in the closed conformation arose by autophosphorylation, since abolishing kinase activity by mutating the ATP binding site (K295M) prevented phosphorylation. Basal Y416 phosphorylation correlated positively with cellular levels of c-Src suggesting autophosphorylation depended on self-association. Using sedimentation velocity analysis on cell lysate with fluorescence detection optics, we confirmed that c-Src forms monomers and dimers, with the open conformation also forming a minor population of larger mass complexes. Collectively, our studies suggest a model by which dimerization of c-Src primes c-Src via Y416 phosphorylation to enable rapid potentiation of activity when Src adopts an open conformation. Once in the open conformation, c-Src can amplify the response by recruiting and phosphorylating substrates such as STAT3 and increasing the extent of autophosphorylation.

Bacterial Expression and Characterization of Catalytic Loop Mutants of Src Protein Tyrosine Kinase†

Protein tyrosine kinase Src is a key enzyme in mammalian signal transduction and an important target for anticancer drug discovery. Although recombinant expression in bacterial cells offers a convenient and rapid way for producing several other protein tyrosine kinases, active Src is difficult to produce in bacterial systems. However, a kinase-defective Src mutant (due to a single point mutation, Lys295Met) is expressed strongly in bacteria. We hypothesize that the difficulty with expressing active Src in bacteria is due to toxicity caused by Src kinase activity. To test this hypothesis, we generated a series of Src mutants by altering certain residues, especially His384, in the catalytic loop and examined their expression in the bacteria and their kinase activity. The results demonstrate that Src mutants with kinase activity above a certain threshold could not be purified from a bacterial expression system, while a variety of mutants with a kinase activity below this threshold could indeed be expressed and purified. These observations support the conclusion that Src activity is toxic to the bacteria, which prevents high-level expression of fully active Src. We further demonstrated that His384, a universally conserved residue among protein tyrosine kinases, is not essential for Src catalysis or its inactivation by C-terminal tail Tyr phosphorylation. Interestingly, His384 mutants undergo autophosphorylation on Tyr416 like wild-type Src but are not activated by autophosphorylation. The potential role of His384 in Src activation by autophosphorylation is discussed in the context of Src structure.

Microarrays for the functional analysis of the chemical-kinase interactome

A central challenge in chemical biology is profiling the activity of a large number of chemical structures against hundreds of biological targets, such as kinases. Conventional 32P-incorporation or immunoassay of phosphorylated residues produces high-quality signals for monitoring kinase reactions but is difficult to use in high-throughput screening (HTS) because of cost and the need for well-plate washing. The authors report a method for densely archiving compounds in nanodroplets on peptide or protein substrate-coated microarrays for subsequent profiling by aerosol deposition of kinases. Each microarray contains over 6000 reaction centers (1.0 nL each) whose phosphorylation progress can be detected by immunofluorescence. For p60c-src, the microarray produced a signal-to-background ratio of 36.3 and Z' factor of 0.63 for HTS and accurate enzyme kinetic parameters (KmATP = 3.3 microM) and IC50 values for staurosporine (210 nM) and PP2 (326 nM) at 10 microM adenosine triphosphate (ATP). Similarly, B-Raf phosphorylation of MEK-coated microarrays was inhibited in the nanoliter reactions by GW5074 at the expected IC50 of 9 nM. Common kinase inhibitors were printed on microarrays, and their inhibitory activities were systematically profiled against B-Raf (V599E), KDR, Met, Flt-3 (D835Y), Lyn, EGFR, PDGFRbeta, and Tie2. All results indicate that this platform is well suited for kinetic analysis, HTS, large-scale IC50 determinations, and selectivity profiling.

A lipid raft environment enhances Lyn kinase activity by protecting the active site tyrosine from dephosphorylation

The plasma membrane contains ordered lipid domains, commonly called lipid rafts, enriched in cholesterol, sphingolipids, and certain signaling proteins. Lipid rafts play a structural role in signal initiation by the high affinity receptor for IgE. Cross-linking of IgE-receptor complexes by antigen causes their coalescence with lipid rafts, where they are phosphorylated by the Src family tyrosine kinase, Lyn. To understand how lipid rafts participate in functional coupling between Lyn and FcepsilonRI, we investigated whether the lipid raft environment influences the specific activity of Lyn. We used differential detergent solubility and sucrose gradient fractionation to isolate Lyn from raft and nonraft regions of the plasma membrane in the presence or absence of tyrosine phosphatase inhibitors. We show that Lyn recovered from lipid rafts has a substantially higher specific activity than Lyn from nonraft environments. Furthermore, this higher specific activity correlates with increased tyrosine phosphorylation at the active site loop of the kinase domain. Based on these results, we propose that lipid rafts exclude a phosphatase that negatively regulates Lyn kinase activity by constitutive dephosphorylation of the kinase domain tyrosine residue of Lyn. In this model, cross-linking of FcepsilonRI promotes its proximity to active Lyn in a lipid raft environment.

Effect of autophosphorylation on the catalytic and regulatory properties of protein tyrosine kinase Src

The function of autophosphorylation in Src family protein tyrosine kinases is not fully understood. In this paper we compared the catalytic and ligand-binding properties of autophosphorylated and nonautophosphorylated (control) Src. The following are the main differences we found. First, while both forms had the same K(m) for ATP-Mg, autophosphorylated Src had significantly higher K(m) values for the phosphate-accepting substrates, polyE(4)Y, and RCM-lysozyme. The autophosphorylated form also had significantly higher V(max) values than the control. The substrate specificity, as measured by V(max)/K(m) ratio, was altered by autophosphorylation and was dependent on the phosphate-accepting substrate. Second, while autophosphorylation did not affect Src activation by free Mg(2+), Zn(2+), which inhibited Src by competing against an essential Mg(2+) activator, inhibited the control threefold more potently than the autophosphorylated form. Third, autophosphorylation significantly reduced the ability of its SH2 domain to bind phosphotyrosine. Fourth, a Pro-rich Src SH3 domain binding peptide activated the control, but not the autophosphorylated Src even though the apparent binding affinity was not significantly affected by autophosphorylation. These differences indicated that autophosphorylation induced significant and widespread changes in the catalytic and regulatory properties of Src. The implications of these findings relative to Src biological regulation are discussed.

Expression of the small tyrosine phosphatase (Stp1) in Saccharomyces cerevisiae: a study on protein tyrosine phosphorylation

Small tyrosine phoshatase 1 (Stp1) is a Schizosaccharomyces pombe low-molecular-mass phosphotyrosine-phosphatase 50% identical to Saccharomyces cerevisiae Ltp1. In order to investigate the role of Stp1 in yeast, a mutant was generated having the characteristic of a dominant negative molecule. Changes in protein tyrosine phosphorylation in S. cerevisiae proteome in response to Stp1 or its dominant negative mutant expression were analyzed by high-resolution two-dimensional (2-D) electrophoresis. The most remarkable result is the modification by phosphorylation on tyrosine of several proteins involved in carbohydrate metabolism. Twelve proteins were identified on the basis of their positions in the anti-phosphotyrosine immunoblot of the 2-D electrophoresis. Ten of these present tyrosyl residues that are within the consensus sequence for protein kinase CK2 (casein kinase-2). These data open the possibility for the identification of Stp1 substrates in yeast and provide hints about the nature of tyrosine phosphorylating agents in yeast and in other organisms where bona fide tyrosine kinases are lacking.

Purification of bovine thymus cytosolic C-terminal Src kinase (CSK) and demonstration of differential efficiencies of phosphorylation and inactivation of p56lyn and pp60c-src by CSK

The C-terminal src kinase (CSK) is a ubiquitously expressed, cytosolic enzyme capable of phosphorylating and inactivating several plasma membrane-bound src-family protein tyrosine kinases in vitro [Nada, S., Okada, M., MacAuley, A., Cooper, J.A., & Nakagawa, H. (1990) Nature 351, 69-72; Bergman, M., Mustelin, T., Oetken, C., Partanen, J., Flint, N.A., Amrein, K.E., Autero, M., Burn, P., & Alitalo, K. (1992) EMBO J. 11, 2919-2924]. We purified CSK to apparent homogeneity from bovine thymus cytosol to study in vitro how the purified enzyme recognizes the various src-family kinases as its substrates. A novel assay method was developed for assaying the ability of CSK to inactivate src-family tyrosine kinases. With this assay method, we demonstrated that CSK inactivated p56lyn with a significantly higher efficiency than pp60c-src. Phosphopeptide mapping of CSK-phosphorylated p56lyn and pp60c-src shows that the consensus tyrosine residue (also termed tail tyrosine) in the C-terminal regulatory domain of p56lyn was phosphorylated by CSK with an efficiency much higher than that of pp60c-src. Thus, the higher efficiency of inactivation of p56lyn by CSK is a result of the ability of p56lyn to serve as a better substrate of CSK. The synthetic peptides derived from the C-terminal portion of p56lyn and pp60c-src were much poorer substrates than the intact src-family kinases for CSK, indicating that the local structure around the tail tyrosine is not sufficient to direct efficient phosphorylation of p56lyn by CSK. Nevertheless, the slightly higher efficiency displayed by CSK in phosphorylating the peptide derived from the C-terminal portion of p56lyn than that from pp60c-src suggests that the structural differences between the C-terminal portions of p56lyn and pp60c-src contribute to the differential efficiencies displayed by CSK in phosphorylating the two kinases. Determination of the CSK-phosphorylation site in the src-C-terminal peptide by phosphopeptide mapping reveals that the whole C-terminal regulatory domain and an adjacent part of the protein kinase domain contain some of the structural determinants directing CSK to phosphorylate the consensus tail tyrosine of the src-family kinases.

Purification and characterization of human ZAP-70 protein-tyrosine kinase from a baculovirus expression system

The ZAP-70 protein tyrosine kinase is essential for T cell antigen receptor (TCR)-mediated signaling. The absence of ZAP-70 results in impaired differentiation of T cells and a lack of responsiveness to antigenic stimulation. In order to study the characteristics of ZAP-70 in vitro, we overexpressed an epitopically tagged human ZAP-70 in a recombinant baculovirus expression system and purified it by column chromatography. The kinase activity of purified, recombinant ZAP-70 required cation and exhibited a strong preference for Mn2+ over Mg2+. The apparent Km of ZAP-70 for ATP was approximately 3.0 microM. The activity of the recombinant ZAP-70, unlike that of the homologous protein tyrosine kinase, Syk, was not affected by binding of TCR-derived tyrosine phosphorylated immunoreceptor tyrosine-based activation motif peptides. Several proteins were tested as potential in vitro substrates of ZAP-70. Only alpha-tubulin and the cytoplasmic fragment of human erythrocyte band 3 (cfb3), which have a region of sequence identity at the phosphorylation site, proved to be good substrates, exhibiting Kmvalues of approximately 3.3 and approximately 2.5 microM, respectively ([ATP] = 50 microM). alpha- and beta-Casein were poor substrates for ZAP-70, and no activity toward enolase, myelin basic protein, calmodulin, histone proteins, or angiotensin could be detected. In contrast to the T cell protein tyrosine kinase, Lck, ZAP-70 did not phosphorylate the cytoplasmic portion of the TCRzeta chain or short peptides corresponding to the CD3epsilon or the TCRzeta immunoreceptor tyrosine-based activation motifs. Our studies suggest that ZAP-70 exhibits a high degree of substrate specificity.

Autophosphorylation induces autoactivation and a decrease in the Src homology 2 domain accessibility of the Lyn protein kinase

Lyn is a member of the Src family of protein-tyrosine kinases that can readily undergo autophosphorylation in vitro. The site of autophosphorylation is Tyr397 which corresponds to the consensus autophosphorylation site of other Src family tyrosine kinases. The rate of autophosphorylation is concentration-dependent, indicating that the reaction follows an intermolecular mechanism. Autophosphorylation results in a 17-fold increase in protein-tyrosine kinase activity. Kinetic analysis demonstrates that phosphorylation of a substrate peptide by Lyn following autophosphorylation occurs with a 63-fold decrease in Km but no significant change in Vmax, suggesting that autophosphorylation relieves the conformational constraint that prevents binding of the substrate peptide to the active site of the kinase. Using a phosphotyrosine-containing peptide (pYEEI) that has previously been shown to bind to the Src homology 2 (SH2) domain of Src family tyrosine kinases with high affinity, we found that autophosphorylation results in a significant decrease in accessibility of the Lyn SH2 domain, indicating that conformational changes in the protein kinase domain induced by autophosphorylation can be propagated to the SH2 domain. Our study suggests that autophosphorylation plays an important role in regulating Lyn by modulating both its kinase activity and its interaction with other phosphotyrosine-containing molecules.

A 56,000 Mr phosphoseryl protein in PC12 cell lysates strongly associates with protein-A sepharose beads and was observed in immune complex kinase assays for PP60c-src

Using an immune complex kinase assay to measure pp60c-src kinase activity, we have identified a 56,000 Mr protein (p56) from PC12 cell lysates that co-purified with pp60c-src by strong association with protein-A sepharose beads. The p56 protein was strongly phosphorylated on serine but no tyrosine or threonine phosphorylation was evident. However, pp60c-src was strongly phosphorylated on tyrosine, weakly phosphorylated on serine with no observed threonine phosphorylation. P56 was not a proteolytic breakdown product of pp60c-src, since it was neither tyrosine phosphorylated nor was it recognized by anti-src antibody. P56 was also not recognised by other antibodies to 56kD signalling molecules such as p56lck. The identify of p56 awaits further investigation but its appearance in immunoprecipitates of pp60c-src using protein-A sepharose beads is of interest but complicates the the interpretation of results from immune complex kinase assays in PC12 cells.

Purification and characterization of recombinant human p50csk protein-tyrosine kinase from an Escherichia coli expression system overproducing the bacterial chaperones GroES and GroEL

An Escherichia coli expression system overproducing the bacterial chaperones GroES and GroEL was engineered and has been successfully used to produce large quantities of the recombinant human protein-tyrosine kinase p50csk. The co-overproduction of the two chaperones with p50csk results in increased solubility of the kinase and allows purification of milligram amounts of active enzyme. Analysis of the purified protein by SDS/polyacrylamide gel electrophoresis reveals a single band with an apparent molecular mass of 50 kDa, indicating that recombinant human p50csk has been purified to near homogeneity. The purified enzyme displays tyrosine kinase activity as measured by both autophosphorylation and phosphorylation of exogenous substrates. Biochemical properties, including in vitro substrate specificity and enzymatic characteristics of the enzyme, have been assessed and compared with those of members of the Src family of protein-tyrosine kinases. Results indicate that p50csk and p56lck have different substrate specificities and that p50csk and p60c-src have similar kinetic parameters. The successful production and purification of an enzymatically active form of p50csk will enable further characterization of this important kinase and allow clarification of its physiological role. In addition, the results suggest that the approach described may be generally applicable to improve the solubility of recombinant proteins which otherwise are produced in an insoluble form in E. coli.

Tyrosine protein kinase inhibition and cancer

The various aspects of the research on tyrosine protein kinase inhibition and its connections with cancer are presented. The emphasis was made on the theoretical low toxic side effects of specific tyrosine protein kinase inhibitors. Particularly, the strategy of finding peptidic substrate-derived inhibitors or modulators is discussed, with an almost complete compendium of the tyrosine protein kinase peptidic substrates published so far. A series of data has been gathered that may serve as a basis for the discovery of selective and specific tyrosine protein kinase inhibitors by screening on molecular and cellular models. The potential of SH2 domain-interfering agents are also presented as a promising route to new anticancer compounds.

Recombinant pp60c-src from baculovirus-infected insect cells: purification and characterization

A simple and effective method has been developed to purify the recombinant protein tyrosine kinase pp60c-src from a baculovirus-insect cell expression system. The procedure includes affinity chromatography and HPLC. Milligram quantities of protein have been isolated with an activity of 3.9 mumol/min/mg protein using the substrate poly E4Y. This specific activity is many times higher than any published protocol. The enzyme is stable for months when stored in buffered 10% glycerol at -70 degrees C. This purification technique is compared to the immuno-affinity technique which is widely used for this enzyme. Enzyme kinetics were characterized with respect to substrate specificity, the effect of temperature, ionic strength, pH, and Mg+2 versus Mn+2 ions. Similar to the enzyme expressed in human cells, the recombinant enzyme demonstrated a higher Vmax and substrate specificity for poly E4Y over 5V-Agt-II. An activation energy of 14.2 kcal/mol was determined. Inhibition by increasing ionic strength is mostly due to an increase in Km for the poly E4Y substrate and hence was substrate dependent. The Km(ATP) was pH dependent while the Km(poly E4Y) was pH independent. For the phosphorylation of poly E4Y, free Mg+2 was stimulatory while Mn+2 was inhibitory. In contrast, Mn+2 stimulated the phosphorylation of 5V-Agt-II.

Substrate affinity of the protein tyrosine kinase pp60c-src is increased on thrombin stimulation of human platelets

Human blood platelets contain high levels of non-receptor protein tyrosine kinases of the Src family, particularly pp60c-src, suggesting an important role for these enzymes in platelet physiology. Indeed, in response to various agonists of platelet function, a number of proteins become phosphorylated at tyrosine residues. However, no enzymic activation of an Src-related tyrosine kinase has yet been shown in platelets. In searching for the kinase(s) responsible, we found that all agonists tested that directly or indirectly activate protein kinase C in platelets (phorbol 12-myristate, 13-acetate, thrombin, vasopressin, collagen, calcium ionophore A23187) increased the overall activity of pp60c-src determined by IgG phosphorylation in an immunocomplex assay in the presence of low ATP concentrations. On the other hand, elevation of cyclic AMP directly by forskolin or indirectly by prostaglandin E1, or elevation of cyclic GMP by sodium nitroprusside did not significantly affect the activity of the enzyme. To substantiate the differences in enzyme activity, we determined Km and Vmax, values of pp60c-src from resting and thrombin-stimulated platelets. Thrombin treatment increased substrate affinity of pp60c-src as indicated by a 2- to 3-fold decrease in the Km values for ATP and the exogenous protein substrate casein. Vmax. values were only slightly altered under the assay conditions used. To further rule out modifications of pp60c-src in phosphorylation as a probable cause of the changed substrate affinity, we analysed tryptic phosphopeptides of immunoprecipitated, 32P-labelled pp60c-src of unstimulated and stimulated platelets. The platelet agonists listed above induced an increase in pp60c-src phosphorylation at Ser-12, which is the amino acid phosphorylated by protein kinase C. Surprisingly, we found that elevation of cyclic AMP did not affect 32P labelling of pp60c-src. On the basis of our data, we suggest that phosphorylation at Ser-12 might be one of the signal-triggering events that cause the increase in substrate affinity of pp60c-src.

Integrated system for the screening of the specificity of protein kinase inhibitors

Tyrosine protein kinases (TPKs) play a major role in the transformation of cells. They are currently used as molecular targets for new generations of anticancer compounds. Numerous TPKs have been described from various tissues using either classical molecular biochemical techniques or cloning strategies. As a natural extension of these discoveries, a large number of "specific" inhibitors have been described in the literature. The major problem with these inhibitors is that there is no simple way to compare their specificity and/or selectivity from one report to another. We have set up a simple, straightforward technique to compare the inhibitory potency of 14 classical inhibitors towards six well-described and at least partially purified protein kinases. This technique is based on a new assay, easy to carry out and non-restrictive in terms of the type of protein substrate used. It permits direct comparisons between the results obtained from various sources. Data obtained showed that, when assessed in this integrated system, specificity and selectivity of many "classical" inhibitors are often weak, thus demonstrating that a universal technique such as ours is essential for the molecular screening of new protein kinase inhibitors. Compounds showing specificity for this panel of protein kinases will be more easily targeted to some defined types of oncogene and of transformed cells.

Heparin-associated thrombocytopenia: immune complexes are attached to the platelet membrane by the negative charge of highly sulphated oligosaccharides

The interaction of sulphated oligosaccharides (SO) with platelets and the antibody of heparin-associated thrombocytopenia (HAT type II) was investigated. 3H-heparin binding to platelets was inhibited by different SO, depending on their grade of sulphation. Dextran sulphate, pentosan polysulphate, and heparin were more effective than were LMW heparins. De-N-sulphated heparin and a LMW heparinoid (Org 10172) had no effect. Platelets preincubated with high-grade SO and washed, released serotonin in the presence of HAT sera without additional heparin. Platelets preincubated with HAT sera and then washed were not activated when heparin was added. Only high-grade SO which inhibited heparin binding to platelets caused platelet activation with HAT sera. However, low- and high-grade SO in high concentrations (0.11 g/l) inhibited serotonin release induced by HAT sera and heparin. 32P-phosphorylation of platelet proteins was enhanced by HAT-IgG and heparin and by heat-aggregated IgG, and was inhibited by the moab IV.3. High SO concentrations inhibited only the effect of HAT-IgG and not that of aggregated IgG. We assume that the antigen in HAT involves a releasable platelet protein with a binding site for SO. This was corroborated by studies with an anti-platelet factor 4 antibody causing Fc-receptor dependent platelet activation inhibitable by high SO concentrations.

Increased tyrosine kinase activity in pp60c-src immunoprecipitate from platelet activating factor stimulated human platelets: in vitro phosphorylation of a synthetic peptide

The involvement of pp60c-src tyrosine kinase was studied in human platelets stimulated with platelet activating factor (PAF). Immunoprecipitation of pp60c-src from platelets followed by immunoblot with pp60v-src monoclonal antibody revealed four protein bands of 60, 56, 50 and 29 kDa as detected by enzymographic web. The phosphorylation of these bands was increased in the pp60c-src immunoprecipitate from PAF stimulated platelets. To assay the tyrosine kinase activity, we used a 13 amino acid synthetic peptide (Arg-Arg-Leu-Ile-Glu-Asp-Ala-Glu-Tyr-Ala-Ala-Arg- Gly) which contains sequences similar to the phosphorylation site on pp60c-src. Incubation of the pp60c-src immunoprecipitate with the peptide and [32P]ATP caused phosphorylation of this peptide in vitro. This peptide phosphorylation was not observed when normal mouse IgG-bound protein(s) was used instead of pp60c-src immunoprecipitate. The peptide phosphorylation was markedly increased by pp60c-src immunoprecipitate obtained from PAF treated platelets. Lyso-PAF had no effect on the phosphorylation. PAF antagonists CV-6209 and WEB-2086 blocked PAF stimulated phosphorylation. This indicated structurally specific and PAF receptor dependency of this response. These results provide direct evidence that PAF stimulation of human platelets increased tyrosine kinase activity in pp60c-src immunoprecipitate.

Redistribution of granulophysin and SRC protein in normal and gray platelets after activation

In this study, two mAbs that recognize specifically the src protein pp60(c-src) in a wide variety of cells (mAb 327 and GD11) have been used to vizualize the src protein expression on human platelets by immunogold electron microscopy. The mAb D545 directed against the dense granule membrane p40 protein granulophysin was used as a control. Almost no pp60(c-src) could be detected on the plasma membrane from resting platelets. However, it appeared on the platelet surface after thrombin stimulation and was found preferentially on the pseudopods. The distribution of the src protein on thrombin-activated platelets was similar to that of granulophysin, the dense granule protein, although in a much lesser extent. In platelets from patients with the gray-platelet syndrome, devoid of alpha-granules, pp60(c-src) was absent on resting platelet surface but after thrombin activation expressed at the membrane surface to a normal extent. The results suggest that the src protein is indeed located in an intraplatelet component which is liberated during platelet stimulation, this finding being compatible with its dense granule localization.

Tetanus toxin inhibits depolarization-stimulated protein phosphorylation in rat cortical synaptosomes: effect on synapsin I phosphorylation and translocation

Synapsin I, a prominent phosphoprotein in nerve terminals, is proposed to modulate exocytosis by interaction with the cytoplasmic surface of small synaptic vesicles and cytoskeletal elements in a phosphorylation-dependent manner. Tetanus toxin (TeTx), a potent inhibitor of neurotransmitter release, attenuated the depolarization-stimulated increase in synapsin I phosphorylation in rat cortical particles and in synaptosomes. TeTx also markedly decreased the translocation of synapsin I from the small synaptic vesicles and the cytoskeleton into the cytosol, on depolarization of synaptosomes. The effect of TeTx on synapsin I phosphorylation was both time and TeTx concentration dependent and required active toxin. One- and two-dimensional peptide maps of synapsin I with V8 proteinase and trypsin, respectively, showed no differences in the relative phosphorylation of peptides for the control and TeTx-treated synaptosomes, suggesting that both the calmodulin- and the cyclic AMP-dependent kinases that label this protein are equally affected. Phosphorylation of synapsin IIb and the B-50 protein (GAP43), a known substrate of protein kinase C, was also inhibited by TeTx. TeTx affected only a limited number of phosphoproteins and the calcium-dependent decrease in dephosphin phosphorylation remained unaffected. In vitro phosphorylation of proteins in lysed synaptosomes was not influenced by prior TeTx treatment of the intact synaptosomes or by the addition of TeTx to lysates, suggesting that the effect of TeTx on protein phosphorylation was indirect. Our data demonstrate that TeTx inhibits neurotransmitter release, the phosphorylation of a select group of phosphoproteins in nerve terminals, and the translocation of synapsin I. These findings contribute to our understanding of the basic mechanism of TeTx action.

The molecular mechanism by which insulin stimulates glycogen synthesis in mammalian skeletal muscle

The ability of insulin to promote the phosphorylation of some proteins and the dephosphorylation of others is paradoxical. An insulin-stimulated protein kinase is shown to activate the type-1 protein phosphatase that controls glycogen metabolism, by phosphorylating its regulatory subunit at a specific serine. Furthermore, the phosphorylation of this residue is stimulated by insulin in vivo. Increased and decreased phosphorylation of proteins by insulin can therefore be explained through the same basic underlying mechanism.