Three-dimensional structure of the tyrosine kinase c-Src

Reciprocal regulation of Hck activity by phosphorylation of Tyr(527) and Tyr(416). Effect of introducing a high affinity intramolecular SH2 ligand

The Src family tyrosine kinase Hck possesses two phosphorylation sites, Tyr(527) and Tyr(416), that affect the catalytic activity in opposite ways. When phosphorylated, Tyr(527) and residues C-terminal to it are involved in an inhibitory intramolecular interaction with the SH2 domain. However, this sequence does not conform to the sequence of the high affinity SH2 ligand, pYEEI. We mutated this sequence to YEEI and show that this mutant form of Hck cannot be activated by exogenous SH2 ligands. The SH3 domain of Hck is also involved in an inhibitory interaction with the catalytic domain. The SH3 ligand Nef binds to and activates YEEI-Hck mutant in a similar manner to wild-type Hck, indicating that disrupting the SH3 interaction overrides the strengthened SH2 interaction. The other phosphorylation site, Tyr(416), is the autophosphorylation site in the activation loop. Phosphorylation of Tyr(416) is required for Hck activation. We mutated this residue to alanine and characterized its catalytic activity. The Y416A mutant shows a higher K(m) value for peptide and a lower V(max) than autophosphorylated wild-type Hck. We also present evidence for cross-talk between the activation loop and the intramolecular binding of the SH2 and SH3 domains.

Clustered cysteine residues in the kinase domain of v-Src: critical role for protein stability, cell transformation and sensitivity to herbimycin A

We have previously reported the activation of Src by mercuric chloride based on the sulfhydryl modification. To evaluate the significance of cysteine residues in v-Src, we replaced each cysteine to alanine by oligonucleotide-directed mutagenesis and examined its effect on cell transformation. Of ten cysteine residues scattered over v-Src, four cysteines clustered in kinase domain, Cys483, Cys487, Cys496 and Cys498, were important for protein stability and cell transformation, whereas those in SH2 domain were dispensable. A single mutation in Cys498 yielded suppression of kinase activity and a temperature-sensitivity in anchorage independent growth. Double mutation either in Cys483/Cys487 or in Cys496/Cys498 yielded clear temperature-sensitivity in cell transformation and in stability of Src protein. Instability of Src protein was magnified by quadruple mutation in the cysteines, which decreased the half-life of Src to be less than one quarter of that of wild-type. In addition, both Cys483/Cyr487 and Cys496/Cys498 kinases became resistant to in vitro inactivation by herbimycin A, which directly inactivates v-Src in addition to its effect on HSP90. Taken together, our results strongly suggest that the cysteine clustered motif of v-Src are critical for protein stability, cell transformation and in vitro inactivation by herbimycin A.

Complex effects of naturally occurring mutations in the JAK3 pseudokinase domain: evidence for interactions between the kinase and pseudokinase domains

The structure of Janus kinases (JAKs) is unique among protein tyrosine kinases in having tandem, nonidentical kinase and pseudokinase domains. Despite its conservation in evolution, however, the function of the pseudokinase domain remains poorly understood. Lack of JAK3 expression results in severe combined immunodeficiency (SCID). In this study, we analyze two SCID patients with mutations in the JAK3 pseudokinase domain, which allows for protein expression but disrupts the regulation of the kinase activity. Specifically, these mutant forms of JAK3 had undetectable kinase activity in vitro but were hyperphosphorylated both in patients' Epstein-Barr virus-transformed B cells and when overexpressed in COS7 cells. Moreover, reconstitution of cells with these mutants demonstrated that, although they were constitutively phosphorylated basally, they were unable to transmit cytokine-dependent signals. Further analysis showed that the isolated catalytic domain of JAK3 was functional whereas either the addition of the pseudokinase domain or its deletion from the full-length molecule reduced catalytic activity. Through coimmunoprecipitation of the isolated pseudokinase domain with the isolated catalytic domain, we provide the first evidence that these two domains interact. Furthermore, whereas the wild-type pseudokinase domain modestly inhibited kinase domain-mediated STAT5 phosphorylation, the patient-derived mutants markedly inhibited this phosphorylation. We thus conclude that the JAK3 pseudokinase domain is essential for JAK3 function by regulating its catalytic activity and autophosphorylation. We propose a model in which this occurs via intramolecular interaction with the kinase domain and that increased inhibition of kinase activity by the pseudokinase domain likely contributes to the disease pathogenesis in these two patients.

Role of the Bsk/Iyk non-receptor tyrosine kinase for the control of growth and hormone production in RINm5F cells

Bsk/Iyk, a murine non-receptor-tyrosine kinase which is expressed in fetal and adult islet of Langerhans was previously found to decrease NIH3T3 cell proliferation when expressed as a Y497/504F-mutant. We presently wanted to determine the effects of Bsk/Iyk on the proliferation of insulin producing cells. Cells expressing Bsk/IykY497/504F and Bsk/IykY504F display a decreased proliferation rate and express higher levels of the cell cycle inhibitor p27/Kip1 compared to control cells. These mutants also conferred diminished cell viability in response to INF-gamma and IL-1beta and contain higher levels of glucagon mRNA. Wild-type Bsk/Iyk is mainly localized at the plasma membrane whereas mutant Bsk/Iyk can enter the nucleus. In vitro kinase reactions using an exogenous substrate indicate a complicated mode of regulation of kinase activity by Y497 and Y504 with the latter being homologous to Y527 in pp60c-Src. These findings suggest that Bsk/Iyk might play a role in inhibiting cell proliferation, transducing cytokine-induced cytotoxicity and regulating hormone production of endocrine pancreatic cells.

c-Abl: activation and nuclear targets

The c-Abl tyrosine kinase and its transforming variants have been implicated in tumorigenesis and in many important cellular processes. c-Abl is localized in the nucleus and the cytoplasm, where it plays distinct roles. The effects of c-Abl are mediated by multiple protein-protein and protein-DNA interactions and its tyrosine kinase domain. At the biochemical level, the mechanism of c-Abl kinase activation and the identification of its target proteins and cellular machineries have in part been solved. However, the phenotypic outcomes of these molecular events remained in large elusive. c-Abl has been shown to regulate the cell cycle and to induce under certain conditions cell growth arrest and apoptosis. In this respect the interaction of c-Abl with p53 and p73 has attracted particular attention. Recent findings have implicated c-Abl in an ionizing irradiation signaling pathway that elicits apoptosis. In this pathway p73 is an important immediate downstream effector. Here I review the current knowledge about these nuclear processes in which c-Abl is engaged and discuss some of their possible implications on cell physiology. Cell Death and Differentiation (2000) 7, 10 - 16.

Early events in M-CSF receptor signaling

The M-CSF receptor (M-CSFR) is expressed in monocytes-macrophages and their progenitors, and drives growth and development of this blood cell lineage. The M-CSFR is a member of a small family of growth factor receptors exhibiting related structures but distinct tissue-specific functions. This review discusses the early molecular events in the M-CSF signaling mechanisms, positive signals, negative signals, the possible organization of individual signaling pathways, and the problem of achieving specificity in the signal transduction mechanism.

Syntaphilin: a syntaxin-1 clamp that controls SNARE assembly

Syntaxin-1 is a key component of the synaptic vesicle docking/fusion machinery that forms the SNARE complex with VAMP/synaptobrevin and SNAP-25. Identifying proteins that modulate SNARE complex formation is critical for understanding the molecular mechanisms underlying neurotransmitter release and its modulation. We have cloned and characterized a protein called syntaphilin that is selectively expressed in brain. Syntaphilin competes with SNAP-25 for binding to syntaxin-1 and inhibits SNARE complex formation by absorbing free syntaxin-1. Transient overexpression of syntaphilin in cultured hippocampal neurons significantly reduces neurotransmitter release. Furthermore, introduction of syntaphilin into presynaptic superior cervical ganglion neurons in culture inhibits synaptic transmission. These findings suggest that syntaphilin may function as a molecular clamp that controls free syntaxin-1 availability for the assembly of the SNARE complex, and thereby regulates synaptic vesicle exocytosis.

Regulation of c-Fes tyrosine kinase and biological activities by N-terminal coiled-coil oligomerization domains

The cytoplasmic protein-tyrosine kinase Fes has been implicated in cytokine signal transduction, hematopoiesis, and embryonic development. Previous work from our laboratory has shown that active Fes exists as a large oligomeric complex in vitro. However, when Fes is expressed in mammalian cells, its kinase activity is tightly repressed. The Fes unique N-terminal sequence has two regions with strong homology to coiled-coil-forming domains often found in oligomeric proteins. Here we show that disruption or deletion of the first coiled-coil domain upregulates Fes tyrosine kinase and transforming activities in Rat-2 fibroblasts and enhances Fes differentiation-inducing activity in myeloid leukemia cells. Conversely, expression of a Fes truncation mutant consisting only of the unique N-terminal domain interfered with Rat-2 fibroblast transformation by an activated Fes mutant, suggesting that oligomerization is essential for Fes activation in vivo. Coexpression with the Fes N-terminal region did not affect the transforming activity of v-Src in Rat-2 cells, arguing against a nonspecific suppressive effect. Taken together, these findings suggest a model in which Fes activation may involve coiled-coil-mediated interconversion of monomeric and oligomeric forms of the kinase. Mutation of the first coiled-coil domain may activate Fes by disturbing intramolecular coiled-coil interaction, allowing for oligomerization via the second coiled-coil domain. Deletion of the second coiled-coil domain blocks fibroblast transformation by an activated form of c-Fes, consistent with this model. These results provide the first evidence for regulation of a nonreceptor protein-tyrosine kinase by coiled-coil domains.

Diversity in protein recognition by PTB domains

Phosphotyrosine-binding (PTB) domains were originally identified as modular domains that recognize phosphorylated Asn-Pro-Xxx-p Tyr-containing proteins. Recent binding and structural studies of PTB domain complexes with target peptides have revealed a number of deviations from the previously described mode of interaction, with respect to both the sequences of possible targets and their structures within the complexes. This diversity of recognition by PTB domains extends and strengthens our general understanding of modular binding domain recognition.

Active site-directed protein regulation

Regulation of protein function is vital for the control of cellular processes. Proteins are often regulated by allosteric mechanisms, in which effectors bind to regulatory sites distinct from the active sites and alter protein function. Intrasteric regulation, directed at the active site and thus the counterpart of allosteric control, is now emerging as an important regulatory mechanism.

Multiple roles for Src in a PDGF-stimulated cell

The observation that platelet-derived growth factor (PDGF) increases the catalytic activity of Src family members (Src) suggests that they contribute to PDGF-dependent responses. The role of Src in PDGF-dependent cell cycle progression, phosphorylation of proteins, and chemotaxis has been tested by investigators using a variety of cell types and approaches, and it appears that the contribution of Src is highly variable. This idea is perhaps best illustrated by the finding that Src plays radically different roles downstream of the PDGF alpha- and beta-receptor subunits. Hence, Src is a versatile signal relay enzyme, whose contribution to a signaling cascade depends on variables such as the nature of the receptor via which the cell is activated, as well as the cell type itself.

Ubiquitin-mediated degradation of active Src tyrosine kinase

Src family tyrosine kinases are involved in modulating various signal transduction pathways leading to the induction of DNA synthesis and cytoskeletal reorganization in response to cell-cell or cell-matrix adhesion. The critical role of these kinases in regulating cellular signaling pathways requires that their activity be tightly controlled. Src family proteins are regulated through reversible phosphorylation and dephosphorylation events that alter the conformation of the kinase. We have found evidence that Src also is regulated by ubiquitination. Activated forms of Src are less stable than either wild-type or kinase-inactive Src mutants and can be stabilized by proteasome inhibitors. In addition, poly-ubiquitinated forms of active Src have been detected in vivo. Taken together, our results establish ubiquitin-mediated proteolysis as a previously unidentified mechanism for irreversibly attenuating the effects of active Src kinase.

Mechanism for phosphorylation-induced activation of the phagocyte NADPH oxidase protein p47(phox). Triple replacement of serines 303, 304, and 328 with aspartates disrupts the SH3 domain-mediated intramolecular interaction in p47(phox), thereby activating the oxidase

Activation of the superoxide-producing phagocyte NADPH oxidase requires interaction between p47(phox) and p22(phox), which is mediated via the SH3 domains of the former protein. This interaction is considered to be induced by exposure of the domains that are normally masked by an intramolecular interaction with the C-terminal region of p47(phox). Here we locate the intramolecular SH3-binding site at the region of amino acid residues 286-340, where Ser-303, Ser-304, and Ser-328 that are among several serines known to become phosphorylated upon cell stimulation exist. Simultaneous replacement of the three serines in p47(phox) with aspartates or glutamates, each mimicking phosphorylated residues, is sufficient for disruption of the intramolecular interaction and resultant access to p22(phox). The triply mutated proteins are also capable of activating the NADPH oxidase without in vitro activators such as arachidonate under cell-free conditions. In a whole-cell system where expression of the wild-type p47(phox) reconstitutes the stimulus-dependent oxidase activity, substitution of the kinase-insensitive residue alanine for Ser-328 as well as for Ser-303/Ser-304 leads to a defective production of superoxide. These findings suggest that phosphorylation of the three serines in p47(phox) induces a conformational change to a state accessible to p22(phox), thereby activating the NADPH oxidase.

SH3 domains with high affinity and engineered ligand specificities targeted to HIV-1 Nef

The avid binding of HIV-1 Nef to the Src homology-3 (SH3) domain of Hck (KD 250 nM) has been shown to involve an interaction between the RT-loop of Hck-SH3 and residues in Nef outside of its prototypic polyproline type II (PPII) helix-containing SH3-ligand region. Such distinctive interactions are thought to provide specificity and affinity for other SH3/ligand protein complexes as well. Here, we have constructed and successfully displayed on the surface of M13 bacteriophage particles a complex library of SH3 domains, which are derived from Hck but carry a random hexapeptide substitution in their RT-loops (termed RRT-SH3). Using this strategy we have identified individual RRT-SH3 domains that can bind to Nef up to 40-fold more avidly than Hck-SH3. Some of these high-affinity RRT-SH3 domains resembled Hck-SH3 in that they bound much less well to a Nef variant containing an engineered F90R mutation that interferes with docking of the native Hck RT-loop. In addition, we could also select RRT-SH3 domains with an opposite specificity, which were dependent on the Arg90 residue for strong binding, and bound 100-fold less well to unmodified Nef. These results demonstrate the utility of phage-display in engineering of signaling protein interaction domains, and emphasize the importance of the RT-loop in SH3 ligand selection, thus suggesting a general strategy for creating SH3 domains with desired binding properties.

A catalytically active Jak2 is required for the angiotensin II-dependent activation of Fyn

Recent work with interleukins has shown a convergence of tyrosine phosphorylation signal transduction cascades at the level of the Janus and Src families of tyrosine kinases. Here we demonstrate that activation of the seven-transmembrane AT(1) receptor by angiotensin II induces a physical association between Jak2 and Fyn, in vivo. This association requires the catalytic activity of Jak2 but not Fyn. Deletion studies indicate that the region of Jak2 that binds Fyn is located between amino acids 1 and 240. Studies of the Fyn SH2 and SH3 domains demonstrate that the SH2 domain plays the primary role in Jak2/Fyn association. Not surprisingly, this domain shows a marked preference for tyrosine-phosphorylated Jak2. Surface plasmon resonance estimated the dissociation equilibrium constant (K(d)) of this association to be 2.36 nM. Last, in vivo studies in vascular smooth muscle cells show that, in response to angiotensin II, Jak2 activation is required for Fyn activation and induction of the c-fos gene. The significance of these data is that Jak2, in addition to serving as a critical angiotensin II activated signal transduction kinase, also functions as a docking protein and participates in the activation of Fyn by providing phosphotyrosine residues that bind the SH2 domain of Fyn.

Functional roles for fatty acylated amino-terminal domains in subcellular localization

Several membrane-associating signals, including covalently linked fatty acids, are found in various combinations at the N termini of signaling proteins. The function of these combinations was investigated by appending fatty acylated N-terminal sequences to green fluorescent protein (GFP). Myristoylated plus mono/dipalmitoylated GFP chimeras and a GFP chimera containing a myristoylated plus a polybasic domain were localized similarly to the plasma membrane and endosomal vesicles, but not to the nucleus. Myristoylated, nonpalmitoylated mutant chimeric GFPs were localized to intracellular membranes, including endosomes and the endoplasmic reticulum, and were absent from the plasma membrane, the Golgi, and the nucleus. Dually palmitoylated GFP was localized to the plasma membrane and the Golgi region, but it was not detected in endosomes. Nonacylated GFP chimeras, as well as GFP, showed cytosolic and nuclear distribution. Our results demonstrate that myristoylation is sufficient to exclude GFP from the nucleus and associate with intracellular membranes, but plasma membrane localization requires a second signal, namely palmitoylation or a polybasic domain. The similarity in localization conferred by the various myristoylated and palmitoylated/polybasic sequences suggests that biophysical properties of acylated sequences and biological membranes are key determinants in proper membrane selection. However, dual palmitoylation in the absence of myristoylation conferred significant differences in localization, suggesting that multiple palmitoylation sites and/or enzymes may exist.

Critical amino acid substitutions in the Src SH3 domain that convert c-Src to be oncogenic

The Src homology 3 (SH3) domain, originally identified in v-Crk, plays an important role in signal transduction. The comparative study with c-src has revealed that v-src oncogene of Schmidt-Ruppin strain of Rous sarcoma virus has three point mutations in its SH3 domain and one in the upstream of SH3. To assess the role of these mutations, each of the single mutations was introduced into c-Src by oligonucleotide-directed mutagenesis and its effect on cell transformation was examined. While variant Src proteins that carry each one of single mutations could not transform cells, double mutation at positions 95 and 117 converted c-Src to be oncogenic and active in kinase. An additional mutation at position 124 together with one at 95 and 117 further activated Src kinase. By use of GST-fusion forms of v-Src SH3 and c-Src SH3, we found that these mutations in SH3 suppressed the binding of SH3 with c-Src protein, possibly with a linker region, while v-SrcSH3 retained the ability to bind a subset of cellular protein to the level similar to those of c-SrcSH3. Taken together, our results suggest that point mutations accumulated in SH3 region can activate, in concert, Src kinase by relaxing the interaction between SH3 and the linker region and subsequently convert Src to be oncogenic.

Tyrosine phosphatase SHP-1 is involved in CD66-mediated phagocytosis of Opa52-expressing Neisseria gonorrhoeae

Opa proteins of Neisseria gonorrhoeae bind to CD66 receptors on human phagocytes, thereby inducing efficient uptake of the bacteria in the absence of opsonins. The interaction of Opa proteins and CD66 receptors leads to activation of Src family tyrosine kinases, a process that is of critical importance for the efficient, CD66-mediated internalization. Here we show that during Opa-mediated stimulation of CD66 the activity of the host cell tyrosine phosphatase SHP-1 is strongly downregulated, concomitant with increases in the tyrosine phosphorylation of several cellular proteins. Since the SHP-1 tyrosine phosphorylation level itself is influenced by Opa-induced events, this phosphatase comprises an important regulatory checkpoint of the pathogen-triggered signaling cascade in human phagocytes.

Implications for Src kinases in hematopoiesis: signal transduction therapeutics

Signal transduction therapeutics is now the dominant theme of drug discovery, and its most immediate impact will be in cancer therapeutics. Blood cell proliferation, differentiation, and activation are controlled by cytokines, whose receptors contain tyrosine kinase catalytic domains or recruit cytosolic tyrosine kinases. Among the most important cytosolic protein tyrosine kinases are the Src and Jak families. Receptor or cytosolic protein tyrosine kinases activate a similar set of intracellular signaling molecules. In blood cells, excessive tyrosine kinase activity is associated with either cancer or autoreactive diseases. Therefore, tyrosine kinases and their substrates serve as excellent candidates for drug intervention. Herceptin has been approved for use in breast cancer. Other agents, such as SU101 and CGP 57418B, are well into phase I-III trials. Newer, more selective tyrosine kinase inhibitors are being evaluated for future use in the treatment of hematologic and solid tumors as well as a wide range of inflammatory or autoimmune diseases.

Inhibition of src family kinases by a combinatorial action of 5'-AMP and small heat shock proteins, identified from the adult heart

Src family kinases are implicated in cellular proliferation and transformation. Terminally differentiated myocytes have lost the ability to proliferate, indicating the existence of a down-regulatory mechanism(s) for these mitogenic kinases. Here we show that feline cardiomyocyte lysate contains thermostable components that inhibit c-Src kinase in vitro. This inhibitory activity, present predominantly in heart tissue, involves two components acting combinatorially. After purification by sequential chromatography, one component was identified by mass and nuclear magnetic resonance spectroscopies as 5'-AMP, while the other was identified by peptide sequencing as a small heat shock protein (sHSP). 5'-AMP and to a lesser extent 5'-ADP inhibit c-Src when combined with either HSP-27 or HSP-32. Other HSPs, including alphaB-crystallin, HSP-70, and HSP-90, did not exhibit this effect. The inhibition, observed preferentially on Src family kinases and independent of the Src tyrosine phosphorylation state, occurs via a direct interaction of the c-Src catalytic domain with the inhibitory components. Our study indicates that sHSPs increase the affinity of 5'-AMP for the c-Src ATP binding site, thereby facilitating the inhibition. In vivo, elevation of ATP levels in the cardiomyocytes results in the tyrosine phosphorylation of cellular proteins including c-Src at the activatory site, and this effect is blocked when the 5'-AMP concentration is raised. Thus, this study reveals a novel role for sHSPs and 5'-AMP in the regulation of Src family kinases, presumably for the maintenance of the terminally differentiated state.

Complex formation with focal adhesion kinase: A mechanism to regulate activity and subcellular localization of Src kinases

Tyrosine phosphorylation of focal adhesion kinase (FAK) creates a high-affinity binding site for the src homology 2 domain of the Src family of tyrosine kinases. Assembly of a complex between FAK and Src kinases may serve to regulate the subcellular localization and the enzymatic activity of members of the Src family of kinases. We show that simultaneous overexpression of FAK and pp60(c-src) or p59(fyn) results in the enhancement of the tyrosine phosphorylation of a limited number of cellular substrates, including paxillin. Under these conditions, tyrosine phosphorylation of paxillin is largely cell adhesion dependent. FAK mutants defective for Src binding or focal adhesion targeting fail to cooperate with pp60(c-src) or p59(fyn) to induce paxillin phosphorylation, whereas catalytically defective FAK mutants can direct paxillin phosphorylation. The negative regulatory site of pp60(c-src) is hypophosphorylated when in complex with FAK, and coexpression with FAK leads to a redistribution of pp60(c-src) from a diffuse cellular location to focal adhesions. A FAK mutant defective for Src binding does not effectively induce the translocation of pp60(c-src) to focal adhesions. These results suggest that association with FAK can alter the localization of Src kinases and that FAK functions to direct phosphorylation of cellular substrates by recruitment of Src kinases.

Activation of the Abl tyrosine kinase in vivo by Src homology 3 domains from the Src homology 2/Src homology 3 adaptor Nck

The nonreceptor tyrosine kinase c-Abl is tightly regulated in vivo, but the mechanisms that normally repress its activity are not well understood. We find that a construct encoding the first two Src homology 3 (SH3) domains of the Src homology 2/SH3 adaptor protein Nck can activate c-Abl in human 293T cells. A myristoylated Nck SH3 domain construct, which is expected to localize to membranes, potently activated Abl when expressed at low levels. An unmyristoylated Nck SH3 domain construct, which localizes to the cytosol and nucleus, also activated Abl but only at high levels of expression. Activation by both myristoylated and unmyristoylated Nck constructs required the C terminus of Abl; a C-terminally truncated form of Abl was not activated, although this construct could still be activated by deletion of its SH3 domain. Activation did not require the major binding sites in the Abl C terminus for Nck SH3 domains, however, suggesting that the mechanism of activation does not require direct binding to the C terminus. Activation of c-Abl by Nck SH3 domains provides a robust experimental system for analyzing the mechanisms that normally repress Abl activity and how that normal regulation can be perturbed.

Ubiquitin-dependent degradation of active Src

Signaling by members of the Src family of protein tyrosine kinases, such as Src and Fyn, is important in many biological responses, including gene transcription, cell-cycle progression, and cell adhesion and spreading [1] [2]. Unregulated Src kinase activity has been implicated in the progression of colon cancer and transformation of cultured cells [3] [4] [5] [6]. Thus, precise regulation of Src activity is critical for normal cell growth. Src kinase activity is downregulated by the carboxy-terminal Src kinase (Csk), a tyrosine kinase that phosphorylates a conserved tyrosine residue in the carboxy-terminal tail of Src [7] [8]. When phosphorylated, this tyrosine residue mediates an intramolecular interaction that results in a 'closed' or inactive conformation [1] [2] [9] [10]. Here, we report that loss of csk resulted in a reduction in the abundance of the Src and Fyn proteins, which could be restored by reintroducing catalytically active Csk. The effect of Csk on Src expression was not due to an increase in Src message, but to stabilization of the Src protein. Inhibition of proteasome activity also increased the level of Src protein in csk-deficient cells. Src was found to be ubiquitinated, and activation of Src increased the extent of polyubiquitination. Thus, ubiquitin-proteasome-dependent degradation represents an additional mechanism by which active Src can be downregulated.

SH2-kinase linker mutations release Hck tyrosine kinase and transforming activities in Rat-2 fibroblasts

Biochemical and structural studies of Src and related kinases demonstrate that two intramolecular interactions suppress kinase activity. These interactions involve binding of the SH2 domain to a phosphotyrosine residue in the C-terminal tail and association of the SH3 domain with a polyproline type II helix formed by amino acids linking the SH2 and kinase domains. Recent studies have shown that high affinity interaction of the SH3 domain of Hck with the human immunodeficiency virus type I Nef protein activates Hck tyrosine kinase and biological activities, suggesting a mechanism that involves disruption of the SH3-linker interaction. To test the role of this interaction in the regulation of Hck kinase activity in living cells, we substituted alanines for prolines 225 and 228 in the linker region and observed that the resulting mutant (Hck-2PA) demonstrated strong transforming activity in a Rat-2 fibroblast focus-forming assay. Hck-2PA also exhibited elevated tyrosine kinase activity in terms of autophosphorylation, endogenous substrate phosphorylation, and in an in vitro kinase assay. The transforming and kinase activities of Hck-2PA were remarkably similar to those observed with a Hck mutant activated by Phe substitution of the conserved tail Tyr residue and with wild-type Hck following co-expression with human immunodeficiency virus Nef. Introduction of the 2PA and tail mutations into a single Hck expression construct did not increase kinase or transforming activity relative to the individual mutations. These data provide new evidence that SH3-linker interaction may represent the dominant mechanism controlling Hck tyrosine kinase activity in vivo.

Nitric oxide controls src kinase activity through a sulfhydryl group modification-mediated Tyr-527-independent and Tyr-416-linked mechanism

c-Src kinase was activated when either murine NIH3T3 fibroblast cells or immunoprecipitated c-Src proteins were treated with nitric oxide generator, S-nitroso-N-acetyl penicillamine (SNAP) or sodium nitroprusside. Nitric oxide (NO) scavenger hemoglobin and N(2)O(3) scavenger homocysteine abolished the SNAP-mediated c-Src kinase activation. Phosphoamino acid analysis and peptide mapping of in vitro labeled phospho-c-Src proteins revealed that SNAP promoted the autophosphorylation at tyrosine, which preferentially took place at Tyr-416. Peptide mapping of in vivo labeled c-Src kinase excluded the involvement of phospho-Tyr-527 dephosphorylation in the SNAP-mediated activation mechanism. Correspondingly, protein-tyrosine phosphatase inhibitor Na(3)VO(4) did not abolish the SNAP-mediated activation of Src kinase, and the constitutively activated v-Src kinase was also further up-regulated in activity by SNAP. SNAP, however, failed to up-regulate the kinase activity of Phe-416 mutant v-Src. 2-Mercaptoethanol or dithiothreitol, which should disrupt N(2)O(3)-mediated S-nitrosylation and subsequent formation of the S-S bond, abolished the up-regulated catalytic activity, and the activity was regained after re-exposing the enzyme to SNAP. Exposure of Src kinase to SNAP promoted both autophosphorylation and S-S bond-mediated aggregation of the kinase molecules, demonstrating a linkage between the two events. These results suggest that the NO/N(2)O(3)-provoked S-nitrosylation/S-S bond formation destabilizes the Src structure for Tyr-416 autophosphorylation-associated activation bypassing the Tyr-527-linked regulation.

Phosphorylation of the insulin receptor kinase by phosphocreatine in combination with hydrogen peroxide: the structural basis of redox priming

Signaling by insulin requires autophosphorylation of the insulin receptor kinase (IRK) at Tyr1158, Tyr1162, and Tyr1163. Earlier experiments with (32)P-gamma-ATP indicated that the nonphosphorylated IRK (IRK-0P) is relatively inactive, and crystallographic data indicated that the ATP binding site of IRK-0P is blocked by its activation loop. We now show that phosphocreatine (PCr) in combination with hydrogen peroxide serves as an alternative phosphate donor and that ATP and PCr use distinct binding sites. Whereas phosphorylation of the IRK by ATP is inhibited by the nonhydrolyzable competitor adenylyl-imidodiphosphate, phosphorylation by PCr is enhanced. The IRK mutant Tyr1158Phe showed no phosphorylation with PCr but almost normal phosphorylation with ATP, whereas Tyr1162Phe was phosphorylated well with PCr but less then normal with ATP. 3-Dimensional models of IRK-0P revealed that the conversion of any of the four cysteine residues 1056, 1138, 1234, and 1245 into sulfenic acid produces structural changes that bring Tyr1158 into close contact with Asp1083 and render the well-known catalytic site at Asp1132 and Tyr1162 accessible from a direction that differs from the known ATP binding site. The mutant Cys1138Ala, in contrast, showed relatively inaccessible catalytic sites and weak catalytic activity in functional experiments. Taken together, these findings indicate that 'redox priming' of the IRK facilitates its autophosphorylation by PCr in the activation loop.

The role of SRC-CAS interactions in cellular transformation: ectopic expression of the carboxy terminus of CAS inhibits SRC-CAS interaction but has no effect on cellular transformation

Several lines of evidence indicate that the adapter molecule p130CAS (crk-associated substrate (CAS)) is required for src-mediated cellular transformation. CAS has been shown to be heavily tyrosine-phosphorylated in src-transformed cells, and genetic variants of src that are deficient in CAS binding are also unable to mediate cellular transformation. In this report, we investigated whether CAS phosphorylation and/or its association with src are required elements of the transformation process. Expression of the carboxy-terminal src binding domain of CAS in Rat 1 fibroblasts expressing a temperature-sensitive allele of v-src inhibited the formation of src-CAS complexes and also inhibited tyrosine phosphorylation of CAS. However, expression of this protein had no effect on morphological transformation, src-mediated actin rearrangements, or anchorage-independent growth of these cells when grown at the src-permissive temperature. Thus, the ability of activated src to mediate cellular transformation is either largely independent of endogenous CAS phosphorylation and/or its association with CAS or, alternatively, the carboxy-terminus of CAS may substitute for endogenous CAS in the process of src-mediated transformation.

Evidence for SH3 domain directed binding and phosphorylation of Sam68 by Src

Sam68 is a 68 kDa protein that associates with and is phosphorylated by the c-Src kinase at mitosis. It contains a KH domain implicated in RNA binding and several proline-rich motifs that resemble known SH3 binding sites. The SH3 domains of c-Src, phosphatidylinositol 3-OH kinase, phospholipase C-gamma and Grb2 protein (containing two SH3 domains), but not other SH3 domains tested, were capable of binding Sam68 in vitro. Synthetic peptides corresponding to the proline motifs of Sam68 inhibited with different efficiencies the binding of SH3 domains to Sam68 suggesting that the proline motifs of Sam68 function as specific SH3 domain binding sites. Mutation of Sam68 SH3 binding sites further indicated that the SRC SH3 domain mediates binding of Src to unphosphorylated Sam68. Phosphorylation of Sam68 by Src kinase was inhibited when the Src SH3 binding site of Sam68 was mutated or when corresponding peptides were added to in vitro kinase reactions indicating that binding of the Src SH3 domain to a specific site near the amino-terminus of Sam68 (including residues 38 - 45: PPLPHRSR) facilitates phosphorylation of Sam68 by the Src kinase domain. Sam68-based proline peptides had no effect on the phosphorylation of another in vitro substrate of Src, enolase. These results suggest that Src effectively mounts Sam68 through its SH3 domain, possibly as a mechanism to position the kinase domain close to substrate tyrosine residues in the carboxyl-half of the protein.

Regulation of the Src family tyrosine kinase Blk through E6AP-mediated ubiquitination

The Src family of nonreceptor tyrosine kinases are important regulators of a variety of cellular processes, including cytoskeletal organization, cell-cell contact, and cell-matrix adhesion. Activation of Src family kinases also can induce DNA synthesis and cellular proliferation; therefore, tight regulation of their kinase activities is important for the cell to maintain proliferative control. Posttranslational phosphorylation and dephosphorylation are recognized as the principle modifications by which the activities of the Src family of tyrosine kinases are regulated. We have discovered that this family of kinases also is regulated by ubiquitin-mediated proteolysis. Studies aimed at the identification of cellular targets for E6AP, an E3 ubiquitin protein ligase involved in ubquitin-mediated degradation, led us to the identification of members of the Src family kinases as potential substrates for E6AP. We have found that E6AP can bind to several of the Src family tyrosine kinases. Here we show that activated Blk is preferentially degraded by the ubiquitin-proteasome pathway and that its ubiquitination is mediated by E6AP. Identification of members of the Src tyrosine kinase family as substrates of the E6AP ubiquitin-protein ligase implicates a role for the ubiquitin pathway in regulating the activities of individual members of this important family of signaling molecules.

Benzodiazepine compounds as inhibitors of the src protein tyrosine kinase: screening of a combinatorial library of 1,4-benzodiazepines

We screened 1680 spatially separated compounds of a diverse combinatorial library of 1,4-benzodiazepines for their ability to inhibit the kinase activity of protein tyrosine kinases Src, Yes, Abl, Lck, Csk, and fibroblast growth factor receptor. This screening yielded novel ligands for the protein tyrosine kinase Src. In the 1, 4-benzodiazepine-2-one scaffold, the preferred substituent at position R(1) was 4-hydroxyphenylmethyl or a 3-indolemethyl derived from a tyrosine or tyrptophan used in building the benzodiazepine, while the substituent at R(2), introduced by alkylating agents, was preferably aromatic in nature. The preferred ring structure introduced on the bicyclic ring of the scaffold by acid chlorides was a p-hydroxy phenyl group. The lead compound, designated as N-L-Yaa, has a L-4-hydroxyphenylmethyl ring at R(1) and a biphenylmethyl substituent at R(2). The compound has an IC(50) of 73 microM against Src, 2- to 6-fold lower than against other protein tyrosine kinases and >10-fold lower than against other nucleotide-utilizing enzymes. The mechanism of binding of N-L-Yaa to Src is mixed against the peptidic substrate with a K(i) of 35 microM and noncompetitive against ATP-Mg with a K(i) of 17 microM. Multiple inhibition analysis of the lead compound in the presence of other competitive inhibitors demonstrated that the binding of the lead compound is nonexclusive to the other competitive inhibitor. The inhibitor was found to be nontoxic to the AFB-13-human fibroblasts cells and inhibited the colony formation of HT-29 colon adenocarcinoma cells that are dependent on Src activity.

Identification of residues involved in v-Src substrate recognition by site-directed mutagenesis

To study the role of the catalytic domain in v-Src substrate specificity, we engineered three site-directed mutants (Leu-472 to Tyr or Trp and Thr-429 to Met). The mutant forms of Src were expressed in Sf9 cells and purified. We analyzed the substrate specificities of wild-type v-Src and the mutants using two series of peptides that varied at residues C-terminal to tyrosine. The peptides contained either the YMTM motif found in insulin receptor substrate-1 (IRS-1) or the YGEF motif identified from peptide library experiments to be the optimal sequence for Src. Mutations at positions Leu-472 or Thr-429 caused changes in substrate specificity at positions P+1 and P+3 (i.e. one or three residues C-terminal to tyrosine). This was particularly evident in the case of the L-472W mutant, which had pronounced alterations in its preferences at the P+1 position. The results suggest that residue Leu-472 plays a role in P+1 substrate recognition by Src. We discuss the results in the light of recent work on the roles of the SH2, SH3 and catalytic domains of Src in substrate specificity.

Hierarchy of structure loss in MD simulations of src SH3 domain unfolding

To complement experimental studies of the src SH3 domain folding, we studied 30 independent, high-temperature, molecular dynamics simulations of src SH3 domain unfolding. These trajectories were observed to differ widely from each other. Thus, rather than analyzing individual trajectories, we sought to identify the recurrent features of the high-temperature unfolding process. The conformations from all simulations were combined and then divided into groups based on the number of native contacts. Average occupancies of each side-chain hydrophobic contact and hydrogen bond in the protein were then determined. In the symmetric funnel limit, the occupancies of all contacts should decrease in concert with the loss in total number of native contacts. If there is a lack of symmetry or hierarchy to the unfolding process, the occupancies of some contacts should decrease more slowly, and others more rapidly. Despite the heterogeneity of the individual trajectories, the ensemble averaging revealed an order to the unfolding process: contacts between the N and C-terminal strands are the first to disappear, whereas contacts within the distal beta-hairpin and a hydrogen-bonding network involving the distal loop beta-turn and the diverging turn persist well after the majority of the native contacts are lost. This hierarchy of events resembles but is somewhat less pronounced than that observed in our experimental studies of the folding of src SH3 domain.

Proline residues in CD28 and the Src homology (SH)3 domain of Lck are required for T cell costimulation

The Src family tyrosine kinases Lck and Fyn are critical for signaling via the T cell receptor. However, the exact mechanism of their activation is unknown. Recent crystal structures of Src kinases suggest that an important mechanism of kinase activation is via engagement of the Src homology (SH)3 domain by proline-containing sequences. To test this hypothesis, we identified several T cell membrane proteins that contain potential SH3 ligands. Here we demonstrate that Lck and Fyn can be activated by proline motifs in the CD28 and CD2 proteins, respectively. Supporting a role for Lck in CD28 signaling, we demonstrate that CD28 signaling in both transformed and primary T cells requires Lck as well as proline residues in CD28. These data suggest that Lck plays an essential role in CD28 costimulation.

CD45 Negatively Regulates Lyn Activity by Dephosphorylating Both Positive and Negative Regulatory Tyrosine Residues in Immature B Cells

Using CD45-deficient clones from the immature B cell line, WEHI-231, we previously demonstrated that CD45 selectively dephosphorylates the Src-family protein tyrosine kinase Lyn and inhibits its kinase activity. To further define the mechanisms of CD45 action on Lyn, we metabolically labeled Lyn from CD45-positive and -negative WEHI-231 cells and analyzed cyanogen bromide fragments by SDS-PAGE analysis. Phosphoamino acid analysis confirmed that Lyn is tyrosine phosphorylated with little serine or threonine phosphorylation. In CD45-negative cells, two bands at 8.2 and 4.1 kDa were phosphorylated in the absence of B cell Ag receptor (BCR) ligation. The 8.2-kDa band corresponded to a fragment containing the positive regulatory site (Tyr397), as assessed by its size and its phosphorylation in an in vitro kinase assay. The 4.1-kDa band was phosphorylated by COOH-terminal Src kinase, suggesting that it contains the COOH-terminal negative regulatory site (Tyr508). CD45 was also shown to dephosphorylate autophosphorylated Lyn in vitro. Thus, CD45 dephosphorylates not only the negative but also the positive regulatory tyrosine residues of Lyn. Furthermore, coimmunoprecipitations using anti-Igα Ab demonstrated that Lyn associated with the resting BCR was constitutively phosphorylated and activated in CD45-negative cells. In the parental cells, both regulatory sites were phosphorylated on BCR ligation. Taken collectively, these results suggest that CD45 keeps both BCR-associated and total cytoplasmic pools of Lyn in an inactive state, and a mechanism by which Lyn is activated by relative reduction of CD45 effect may be operative on BCR ligation.

Interaction between Wiskott-Aldrich Syndrome protein (WASP) and the Fyn protein-tyrosine kinase

Wiskott-Aldrich Syndrome (WAS) is a severe X-linked disorder characterised by immune deficiency, thrombocytopenia and eczema, resulting from abnormalities in a range of haematopoietic cell types. The protein that is defective in WAS, named WASP, appears to be involved in regulating changes in the cytoskeletal organisation of haematopoietic cells in response to external stimuli. In support of this idea, WASP has been found to be physically associated in haematopoietic cells in vivo with a number of SH3 domain-containing proteins involved in signal transduction, including the cytoplasmic protein-tyrosine kinase Fyn. Here, we have used a baculovirus expression system to explore the biochemical consequences of the interaction between WASP and Fyn. We find that the kinase activity of Fyn is stimulated as a result of binding to WASP, and that a cellular protein, which may be WASP itself, becomes phosphorylated on tyrosine as a result of the binding of WASP to Fyn.

Detecting protein function and protein-protein interactions from genome sequences

A computational method is proposed for inferring protein interactions from genome sequences on the basis of the observation that some pairs of interacting proteins have homologs in another organism fused into a single protein chain. Searching sequences from many genomes revealed 6809 such putative protein-protein interactions in Escherichia coli and 45,502 in yeast. Many members of these pairs were confirmed as functionally related; computational filtering further enriches for interactions. Some proteins have links to several other proteins; these coupled links appear to represent functional interactions such as complexes or pathways. Experimentally confirmed interacting pairs are documented in a Database of Interacting Proteins.

Activation of the lck tyrosine-protein kinase by the binding of the tip protein of herpesvirus saimiri in the absence of regulatory tyrosine phosphorylation

The Tip protein of herpesvirus saimiri 484 binds to the Lck tyrosine-protein kinase at two sites and activates it dramatically. Lck has been shown previously to be activated by either phosphorylation of Tyr394 or dephosphorylation of Tyr505. We examined here whether a change in the phosphorylation of either site was required for the activation of Lck by Tip. Remarkably, mutation of both regulatory sites of tyrosine phosphorylation did not prevent activation of Lck by Tip either in vivo or in a cell free in vitro system. Tip therefore appears to be able to activate Lck through an induced conformational change that does not necessarily involve altered phosphorylation of the kinase. Tip may represent the prototype of a novel type of regulator of tyrosine-protein kinases.

Mutations in the N-terminal regulatory region reduce the catalytic activity of Csk, but do not affect its recognition of Src

In addition to the C-terminal catalytic domain, Csk is a protein tyrosine kinase that has an N-terminal regulatory region that contains SH3 and SH2 domains. The role this region plays relative to the function of the catalytic domain is not clear. To study its role, we introduced either deletion or site-specific mutations within this region and analyzed the effect of such mutations on the catalytic activity of Csk and its ability to phosphorylate/inactivate Src protein tyrosine kinase, its physiological substrate in the cell. Deletion of the SH3 domain and the SH2 domain resulted in reductions of kinase activity by 70 and 96%, respectively. Mutations within the SH2 domain that abolished its ability to bind phosphotyrosine did not result in a significant loss of kinase activity. Mutation of Ser78 to Asp, located between the SH3 and the SH2 domains, resulted in a reduction of over 90% of the catalytic activity. The reduction in specific activity is not the result of any apparent physical instability of the mutants. Kinetic analyses indicate that the mutations did not affect the Km values for ATP-Mg or the polypeptide substrate. The ability of the mutants to phosphorylate and inactivate Src is directly correlated to their kinase activity. These results indicate that the regulatory region is important in optimizing the kinase activity of the catalytic domain, but apparently plays no direct or specific role in substrate recognition.

Disruption of coiled-coil domains in Fer protein-tyrosine kinase abolishes trimerization but not kinase activation

The protein-tyrosine kinase Fer and the highly homologous proto-oncoprotein Fps/Fes are implicated in signaling from a variety of growth factor and cytokine receptors. Here we examine the molecular basis of Fer kinase activation with an emphasis on the role of oligomerization. We show that Fer forms trimers in vivo and that disruption of either the first or second coiled-coil domain abolishes oligomerization, suggesting a cooperative interaction between these two domains. Although Fps/Fes also forms homotypic oligomers, probably via homologous coiled-coil domains, no heterotypic interactions were observed between Fer and Fps/Fes. Incorporation of catalytically inactive Fer peptides into the oligomeric complex caused only mild reduction of wild type Fer kinase activity, suggesting that kinase-inactive Fer would not behave as a potent dominant negative. Although oligomerization of Fer can potentiate autophosphorylation in trans at three major phosphorylation sites, these residues can likely also be phosphorylated in cis. In contrast, the testis-specific FerT isomer does not oligomerize and is able to autophosphorylate in cis at two of the same three residues autophosphorylated in Fer. These results suggest that although oligomerization potentiates autophosphorylation in trans, this is apparently not necessary for Fer activation.

Cleavage and relocation of the tyrosine kinase P59FYN during Fas-mediated apoptosis in T lymphocytes

Ligation of Fas with its natural ligand or with anti-Fas antibodies induces an apoptotic program in Fas sensitive cells. We report here the identification of the tyrosine kinase p59Fyn as a substrate for CPP32-like proteinases and more particularly caspase 3 during Fas-mediated apoptosis in Jurkat T cells. Inhibition of CPP32-like proteinases by Ac-Asp-Glu-Val-Asp-aldehyde but not by Ac-Tyr-Val-Ala-Asp-aldehyde prevents CPP32, PARP and p59Fyn cleavage indicating that CPP32 or CPP32-like proteinases are responsible for the cleavage of p59Fyn. Cleavage occurs in the N-terminal domain of p59Fyn between Asp19 and Gly20 and is accompanied by relocation of an active p57Fyn kinase to cytoplasm of Fas-stimulated Jurkat cells as judged by both biochemical and confocal microscopy experiments. Thus, p59Fyn relocation and activity may play an important role during Fas-mediated cell death in human T lymphocytes.

Cyclic AMP-dependent activation of the proenkephalin gene requires phosphorylation of CREB at serine-133 and a Src-related kinase

The transcription factor CREB [cyclic AMP response element (CRE)-binding protein] is activated by several kinase pathways on phosphorylation of serine-133. Phosphorylation of CREB at serine-133 is required for the induction of target gene expression. The proenkephalin gene is a target of cyclic AMP-dependent agonists like forskolin, and its expression is driven by the enhancer element CRE-2. It has been shown that CREB binds CRE-2 in extracts from striatum and hypothalamus. However, these studies did not show a functional requirement for CREB serine-133 phosphorylation in CRE-2 function. We demonstrate that CREB binds CRE-2 in primary astrocyte cultures and that transcriptional activation of CRE-2 requires CREB phosphorylation at serine-133. In addition, it has recently been shown that, at least in some contexts, CREB phosphorylation is not sufficient to activate target gene expression and that another intracellular signal seems to be required. Therefore, we also sought to determine if another signaling event, in addition to CREB phosphorylation, might be involved in cyclic AMP-mediated induction of the proenkephalin gene. We have found that the inhibition of src-related nonreceptor tyrosine kinases blocks forskolin-induced proenkephalin gene expression without having any effect on serine-133-phosphorylated CREB levels and that constitutively activated src kinase can activate the proenkephalin promoter.

NMR structure of phospho-tyrosine signaling complexes

A structural basis for activation and substrate specificity of src tyrosine kinases, and regulation of protein-protein association by tyrosine phosphorylation is described. Lyn, a src-family tyrosine kinase, recognizes and phosphorylates the immunoreceptor tyrosine-based activation motif, ITAM, a critical component in transmembrane signal transduction in hemopoietic cells. The structure of an ITAM peptide substrate bound to an active form of Lyn tyrosine kinase was determined by high-resolution NMR, and a model of the complex was generated using the crystallographic structure of Lck, a closely related Src-family kinase. The results provide a rationale for the conserved ITAM residues and specificity of Lyn, and suggest that substrate plays a role in stabilizing the kinase conformation optimal for catalysis. It is our hope that the Lck-ITAM peptide model complex will be useful in aiding structure-based drug design efforts that target substrate binding determinants in the design. Concerning the regulation of protein-protein association, we report on a complex between erythrocyte band 3 and two glycolytic enzymes, aldolase and glyceraldehyde-3-phosphate dehydrogenase. The formation of this complex is negatively regulated by tyrosine phosphorylation of band 3 by p72syk tyrosine kinase. In red blood cells, this association results in a decrease in glycolysis due to competitive inhibition of the glycolytic enzymes. The structure of band 3 recognized by the glycolytic enzymes was determined by solution NMR, and found to be a loop structure with tyrosine centrally positioned and excluded from intermolecular contact. This phosphorylation sensitive interaction, or PSI, loop may be the basis of a general mechanism for negative regulation through tyrosine phosphorylation.

Structural analysis of the lymphocyte-specific kinase Lck in complex with non-selective and Src family selective kinase inhibitors

BACKGROUND

The lymphocyte-specific kinase Lck is a member of the Src family of non-receptor tyrosine kinases. Lck catalyzes the initial phosphorylation of T-cell receptor components that is necessary for signal transduction and T-cell activation. On the basis of both biochemical and genetic studies, Lck is considered an attractive cell-specific target for the design of novel T-cell immunosuppressants. To date, the lack of detailed structural information on the mode of inhibitor binding to Lck has limited the discovery of novel Lck inhibitors.

RESULTS

We report here the high-resolution crystal structures of an activated Lck kinase domain in complex with three structurally distinct ATP-competitive inhibitors: AMP-PNP (a non-selective, non-hydrolyzable ATP analog); staurosporine (a potent but non-selective protein kinase inhibitor); and PP2 (a potent Src family selective protein tyrosine kinase inhibitor). Comparison of these structures reveals subtle but important structural changes at the ATP-binding site. Furthermore, PP2 is found to access a deep, hydrophobic pocket near the ATP-binding cleft of the enzyme; this binding pocket is not occupied by either AMP-PNP or staurosporine.

CONCLUSIONS

The potency of staurosporine against Lck derives in part from an induced movement of the glycine-rich loop of the enzyme upon binding of this ligand, which maximizes the van der Waals interactions present in the complex. In contrast, PP2 binds tightly and selectively to Lck and other Src family kinases by making additional contacts in a deep, hydrophobic pocket adjacent to the ATP-binding site; the amino acid composition of this pocket is unique to Src family kinases. The structures of these Lck complexes offer useful structural insights as they demonstrate that kinase selectivity can be achieved with small-molecule inhibitors that exploit subtle topological differences among protein kinases.

Phosphorylation of MAP kinases by MAP/ERK involves multiple regions of MAP kinases

Mitogen-activated protein (MAP) kinases are activated with great specificity by MAP/ERK kinases (MEKs). The basis for the specific activation is not understood. In this study chimeras composed of two MAP kinases, extracellular signal-regulated protein kinase 2 and p38, were assayed in vitro for phosphorylation and activation by different MEK isoforms to probe the requirements for productive interaction of MAP kinases with MEKs. Experimental results and modeling support the conclusion that the specificity of MEK/MAP kinase phosphorylation results from multiple contacts, including surfaces in both the N- and C-terminal domains.

Genetic analysis of B cell antigen receptor signaling

In B lymphocytes, a signaling complex that contributes to cell fate decisions is the B cell antigen receptor (BCR). Data from knockout experiments in cell lines and mice have revealed distinct functions for the intracellular protein tyrosine kinases (Lyn, Syk, Btk) in BCR signaling and B cell development. Combinations of intracellular signaling pathways downstream of these PTKs determine the quality and quantity of BCR signaling. For example, concerted actions of the PLC-gamma 2 and PI3-K pathways are required for proper calcium responses. Similarly, the regulation of ERK and JNK responses involves both PLC-gamma 2 and GTPases pathways. Since the immune response in vivo is regulated by alteration of these signaling outcomes, achieving a precise understanding of intracellular molecular events leading to B lymphocyte proliferation, deletion, anergy, receptor editing, and survival still remains a challenge for the future.