Molecular determinants for Csk-catalyzed tyrosine phosphorylation of the Src tail

Dissection of the catalytic and regulatory structure-function relationships of Csk protein tyrosine kinase

Protein tyrosine kinases (PTKs) are a large enzyme family that regulates many cellular processes. The key to their broad role in signaling is their tunable substrate specificity and regulatory mechanisms that allow each to respond to appropriate regulatory signals and phosphorylate the correct physiological protein substrates. Thus, in addition to the general PTK catalytic platform, each PTK acquires unique structural motifs that confer a unique combination of catalytic and regulatory properties. Understanding the structural basis for these properties is essential for understanding and manipulating the PTK-based signaling networks in normal and cancer cells. C-terminal Src kinase (Csk) and its homolog, Csk-homologous kinase (Chk), phosphorylate Src family kinases on a C-terminal Tyr residue and negatively regulate their kinase activity. While this regulatory function is biologically essential, Csk and Chk have also been excellent model PTKs for dissecting the structural basis of PTK catalysis and regulation. In this article, we review the structure-function studies of Csk and Chk that shed light on the regulatory and catalytic mechanisms of protein tyrosine kinases in general.

Structural versatility that serves the function of the HRD motif in the catalytic loop of protein tyrosine kinase, Src

Site-directed mutagenesis is a traditional approach for structure-function analysis of protein tyrosine kinases, and it requires the generation, expression, purification, and analysis of each mutant enzyme. In this study, we report a versatile high throughput bacterial screening system that can identify functional kinase mutants by immunological detection of tyrosine phosphorylation. Two key features of this screening system are noteworthy. First, instead of blotting bacterial colonies directly from Agar plates to nitrocellulose membrane, the colonies were cultured in 96-well plates, and then spotted in duplicate onto the membrane with appropriate controls. This made the screening much more reliable compared with direct colony blotting transfer. A second feature is the parallel use of a protein tyrosine phosphatase (PTP)-expressing host and a non-PTP-expressing host. Because high activity Src mutants are toxic to the host, the PTP system allowed the identification of Src mutants with high activity, while the non-PTP system identified Src mutants with low activity. This approach was applied to Src mutant libraries randomized in the highly conserved HRD motif in the catalytic loop, and revealed that structurally diverse residues can replace the His and Arg residues, while the Asp residue is irreplaceable for catalytic activity.

Csk-homologous kinase (Chk) is an efficient inhibitor of Src-family kinases but a poor catalyst of phosphorylation of their C-terminal regulatory tyrosine

Background

C-terminal Src kinase (Csk) and Csk-homologous kinase (Chk) are the major endogenous inhibitors of Src-family kinases (SFKs). They employ two mechanisms to inhibit SFKs. First, they phosphorylate the C-terminal tail tyrosine which stabilizes SFKs in a closed inactive conformation by engaging the SH2 domain in cis. Second, they employ a non-catalytic inhibitory mechanism involving direct binding of Csk and Chk to the active forms of SFKs that is independent of phosphorylation of their C-terminal tail. Csk and Chk are co-expressed in many cell types. Contributions of the two mechanisms towards the inhibitory activity of Csk and Chk are not fully clear. Furthermore, the determinants in Csk and Chk governing their inhibition of SFKs by the non-catalytic inhibitory mechanism are yet to be defined.

Methods

We determined the contributions of the two mechanisms towards the inhibitory activity of Csk and Chk both in vitro and in transduced colorectal cancer cells. Specifically, we assayed the catalytic activities of Csk and Chk in phosphorylating a specific peptide substrate and a recombinant SFK member Src. We employed surface plasmon resonance spectroscopy to measure the kinetic parameters of binding of Csk, Chk and their mutants to a constitutively active mutant of the SFK member Hck. Finally, we determined the effects of expression of recombinant Chk on anchorage-independent growth and SFK catalytic activity in Chk-deficient colorectal cancer cells.

Results

Our results revealed Csk as a robust enzyme catalysing phosphorylation of the C-terminal tail tyrosine of SFKs but a weak non-catalytic inhibitor of SFKs. In contrast, Chk is a poor catalyst of SFK tail phosphorylation but binds SFKs with high affinity, enabling it to efficiently inhibit SFKs with the non-catalytic inhibitory mechanism both in vitro and in transduced colorectal cancer cells. Further analyses mapped some of the determinants governing this non-catalytic inhibitory mechanism of Chk to its kinase domain.

Conclusions

SFKs are activated by different upstream signals to adopt multiple active conformations in cells. SFKs adopting these conformations can effectively be constrained by the two complementary inhibitory mechanisms of Csk and Chk. Furthermore, the lack of this non-catalytic inhibitory mechanism accounts for SFK overactivation in the Chk-deficient colorectal cancer cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-017-0186-x) contains supplementary material, which is available to authorized users.

Distal loop flexibility of a regulatory domain modulates dynamics and activity of C-terminal SRC kinase (csk)

The Src family of tyrosine kinases (SFKs) regulate numerous aspects of cell growth and differentiation and are under the principal control of the C-terminal Src Kinase (Csk). Csk and SFKs share a modular design with the kinase domain downstream of the N-terminal SH2 and SH3 domains that regulate catalytic function and membrane localization. While the function of interfacial segments in these multidomain kinases are well-investigated, little is known about how surface sites and long-range, allosteric coupling control protein dynamics and catalytic function. The SH2 domain of Csk is an essential component for the down-regulation of all SFKs. A unique feature of the SH2 domain of Csk is the tight turn in place of the canonical CD loop in a surface site far removed from kinase domain interactions. In this study, we used a combination of experimental and computational methods to probe the importance of this difference by constructing a Csk variant with a longer SH2 CD loop to mimic the flexibility found in homologous kinase SH2 domains. Our results indicate that while the fold and function of the isolated domain and the full-length kinase are not affected by loop elongation, native protein dynamics that are essential for efficient catalysis are perturbed. We also identify key motifs and routes through which the distal SH2 site might influence catalysis at the active site. This study underscores the sensitivity of intramolecular signaling and catalysis to native protein dynamics that arise from modest changes in allosteric regions while providing a potential strategy to alter intrinsic activity and signaling modulation.

Profiling the substrate specificity of protein kinases by on-bead screening of peptide libraries

A robust, high-throughput method has been developed to screen one-bead-one-compound peptide libraries to systematically profile the sequence specificity of protein kinases. Its ability to provide individual sequences of the preferred substrates permits the identification of sequence contextual effects and nonpermissive residues. Application of the library method to kinases Pim1, MKK6, and Csk revealed that Pim1 and Csk are highly active toward peptide substrates and recognize specific sequence motifs, whereas MKK6 has little activity or sequence selectivity against peptide substrates. Pim1 recognizes peptide substrates of the consensus RXR(H/R)X(S/T); it accepts essentially any amino acid at the S/T-2 and S/T+1 positions, but strongly disfavors acidic residues (Asp or Glu) at the S/T-2 position and a proline residue at the S/T+1 position. The selected Csk substrates show strong sequence covariance and fall into two classes with the consensus sequences of (D/E)EPIYϕXϕ and (D/E)(E/D)S(E/D/I)YϕXϕ (where X is any amino acid and ϕ is a hydrophobic amino acid). Database searches and in vitro kinase assays identified phosphatase PTP-PEST as a Pim1 substrate and phosphatase SHP-1 as a potential Csk substrate. Our results demonstrate that the sequence specificity of protein kinases is defined not only by favorable interactions between permissive residue(s) on the substrate and their cognate binding site(s) on the kinase but also by repulsive interactions between the kinase and nonpermissive residue(s).

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-mediated inhibitory signaling is regulated by sequential phosphorylation mediated by distinct nonreceptor tyrosine kinases: a case study involving PECAM-1

The activation state of many blood and vascular cells is tightly controlled by a delicate balance between receptors that contain immunoreceptor tyrosine-based activation motifs (ITAMs) and those that contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Precisely how the timing of cellular activation by ITAM-coupled receptors is regulated by ITIM-containing receptors is, however, poorly understood. Using platelet endothelial cell adhesion molecule 1 (PECAM-1) as a prototypical ITIM-bearing receptor, we demonstrate that initiation of inhibitory signaling occurs via a novel, sequential process in which Src family kinases phosphorylate the C-terminal ITIM, thereby enabling phosphorylation of the N-terminal ITIM of PECAM-1 by other Src homology 2 domain-containing nonreceptor tyrosine kinases (NRTKs). NRTKs capable of mediating the second phosphorylation event include C-terminal Src kinase (Csk) and Bruton's tyrosine kinase (Btk). Btk and Csk function downstream of phosphatidylinositol 3-kinase (PI3K) activation during ITAM-dependent platelet activation. In ITAM-activated platelets that were treated with a PI3K inhibitor, PECAM-1 was phosphorylated but did not bind the tandem SH2 domain-containing tyrosine phosphatase SHP-2, indicating that it was not phosphorylated on its N-terminal ITIM. Csk bound to and phosphorylated PECAM-1 more efficiently than did Btk and required its SH2 domain to perform these functions. Additionally, the phosphorylation of the N-terminal ITIM of Siglec-9 by Csk is enhanced by the prior phosphorylation of its C-terminal ITIM, providing evidence that the ITIMs of other dual ITIM-containing receptors are also sequentially phosphorylated. On the basis of these findings, we propose that sequential ITIM phosphorylation provides a general mechanism for precise temporal control over the recruitment and activation of tandem SH2 domain-containing tyrosine phosphatases that dampen ITAM-dependent signals.

Expression and purification of Src-family kinases for solution NMR studies

NMR analyses of the structure, dynamics, and interactions of the Src family kinases (SFKs) have been hindered by the limited ability to obtain sufficient amounts of properly folded, soluble protein from bacterial expression systems, to allow these studies to be performed in an economically viable manner. In this chapter, we detail our attempts to overcome these difficulties using the catalytic domain (SrcCD) of c-Src, the prototypical SFK, as an illustrative example. We describe in detail two general methods to express and purify SrcCD from Escherichia coli expression systems in both fully active wild-type and kinase-deficient mutant forms, allowing the efficient and cost-effective labeling by NMR-active isotopes for solution NMR studies.

Mechanistic insights into the activation of oncogenic forms of EGF receptor

Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that is commonly activated by mutation in non-small cell lung cancer. The mechanism of this oncogenic activation is not completely understood, but in contrast to that of the wild-type EGFR, it is proposed to be independent of kinase domain dimerization. Mechanistic studies on EGFR have mainly relied on cell-based assays or isolated kinase domain measurements. Here we show, using purified, near full-length human EGFR proteins (tEGFRs), that two oncogenic mutants are fully active independently of EGF and highly resistant to the therapeutic and endogenous inhibitors cetuximab, lapatinib and MIG6. Based on the pattern of inhibition and the effects of additional asymmetric kinase dimer interface mutations, we propose that these oncogenic EGFR mutants drive and strongly depend on the formation of the asymmetric kinase dimer for activation, which has implications for drug design and cancer treatment strategies.

Bacterial expression and purification of active hematopoietic cell kinase

Src family kinases (SFKs) are traditionally purified from eukaryotic expression systems. These expression systems can be costly, yield heterogeneously phosphorylated protein samples and present difficulties when metabolic labeling is required for structural studies. Therefore, many attempts have been made to develop bacterial purification systems for SFKs. So far, high-yield bacterial expression systems have only been achieved for SFK kinase domains or for inactive mutants of constructs containing the regulatory SH3 and SH2 domains, but not for their active forms. Herein described is a bacterial expression system for the wild type, active SFK Hck containing SH3, SH2 and kinase domains. Hck plays an important role in phagocyte function as well as the etiology of chronic myeloid leukemia as Hck is an interaction partner of Bcr-Abl. Structural studies of Hck are essential to fully understand the signaling processes involved in host defense and leukemogenesis. Successful bacterial expression of Hck was possible by a dual strategy: (1) co-expression with YopH phosphatase in order to control host toxicity, and (2) expression in a bacterial strain that is RNase E deficient, which dramatically increased overall expression levels. The expressed Hck construct is unphosphorylated and appears to be in an open conformation. Bacterially expressed Hck is capable of autophosphorylation, phosphorylates substrate at rates comparable to insect cell expressed Hck, and can be inhibited by staurosporine and Csk.

Structural elements and allosteric mechanisms governing regulation and catalysis of CSK-family kinases and their inhibition of Src-family kinases

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) are endogenous inhibitors constraining the activity of the oncogenic Src-family kinases (SFKs) in cells. Both kinases suppress SFKs by selectively phosphorylating their consensus C-terminal regulatory tyrosine. In addition to phosphorylation, CHK can suppress SFKs by a unique non-catalytic inhibitory mechanism that involves tight binding of CHK to SFKs to form stable complexes. In this review, we discuss how allosteric regulators, phosphorylation, and inter-domain interactions interplay to govern the activity of CSK and CHK and their ability to inhibit SFKs. In particular, based upon the published results of structural and biochemical analysis of CSK and CHK, we attempt to chart the allosteric networks in CSK and CHK that govern their catalysis and ability to inhibit SFKs. We also discuss how the published three-dimensional structure of CSK complexed with an SFK member sheds light on the structural basis of substrate recognition by protein kinases.

Proteins at work: a combined small angle X-RAY scattering and theoretical determination of the multiple structures involved on the protein kinase functional landscape

C-terminal Src kinase (Csk) phosphorylates and down-regulates the Src family tyrosine kinases (SFKs). Crystallographic studies of Csk found an unusual arrangement of the SH2 and SH3 regulatory domains about the kinase core, forming a compact structure. However, recent structural studies of mutant Csk in the presence of an inhibitor indicate that the enzyme accesses an expanded structure. To investigate whether wt-Csk may also access open conformations we applied small angle x-ray scattering (SAXS). We find wt-Csk frequently occupies an extended conformation where the regulatory domains are removed from the kinase core. In addition, all-atom structure-based simulations indicate Csk occupies two free energy basins. These basins correspond to ensembles of distinct global conformations of Csk: a compact structure and an extended structure. The transitions between these structures are entropically driven and accessible via thermal fluctuations that break local interactions. We further characterized the ensemble by generating theoretical scattering curves for mixed populations of conformations from both basins and compared the predicted scattering curves to the experimental profile. This population-combination analysis is more consistent with the experimental data than any rigid model. It suggests that Csk adopts a broad ensemble of conformations in solution, populating extended conformations not observed in the crystal structure that may play an important role in the regulation of Csk. The methodology developed here is broadly applicable to biological macromolecules and will provide useful information about what ensembles of conformations are consistent with the experimental data as well as the ubiquitous dynamic reversible assembly processes inherent in biology.

The C-terminal Src inhibitory kinase (Csk)-mediated tyrosine phosphorylation is a novel molecular mechanism to limit P2X3 receptor function in mouse sensory neurons

On sensory neurons, sensitization of P2X(3) receptors gated by extracellular ATP contributes to chronic pain. We explored the possibility that receptor sensitization may arise from down-regulation of an intracellular signal negatively controlling receptor function. In view of the structural modeling between the Src region phosphorylated by the C-terminal Src inhibitory kinase (Csk) and the intracellular C terminus domain of the P2X(3) receptor, we investigated how Csk might regulate receptor activity. Using HEK cells and the in vitro kinase assay, we observed that Csk directly phosphorylated the tyrosine 393 residue of the P2X(3) receptor and strongly inhibited receptor currents. On mouse trigeminal sensory neurons, the role of Csk was tightly controlled by the extracellular level of nerve growth factor, a known algogen. Furthermore, silencing endogenous Csk in HEK or trigeminal cells potentiated P2X(3) receptor responses, confirming constitutive Csk-mediated inhibition. The present study provides the first demonstration of an original molecular mechanism responsible for negative control over P2X(3) receptor function and outlines a potential new target for trigeminal pain suppression.

Optimized bacterial expression and purification of the c-Src catalytic domain for solution NMR studies

Progression of a host of human cancers is associated with elevated levels of expression and catalytic activity of the Src family of tyrosine kinases (SFKs), making them key therapeutic targets. Even with the availability of multiple crystal structures of active and inactive forms of the SFK catalytic domain (CD), a complete understanding of its catalytic regulation is unavailable. Also unavailable are atomic or near-atomic resolution information about their interactions, often weak or transient, with regulating phosphatases and downstream targets. Solution NMR, the biophysical method best suited to tackle this problem, was previously hindered by difficulties in bacterial expression and purification of sufficient quantities of soluble, properly folded protein for economically viable labeling with NMR-active isotopes. Through a choice of optimal constructs, co-expression with chaperones and optimization of the purification protocol, we have achieved the ability to bacterially produce large quantities of the isotopically-labeled CD of c-Src, the prototypical SFK, and of its activating Tyr-phosphorylated form. All constructs produce excellent spectra allowing solution NMR studies of this family in an efficient manner.

Direct and specific inactivation of protein tyrosine kinases in the Src and FGFR families by reversible cysteine oxidation

Accumulating evidence suggests that protein tyrosine phosphorylation-based signaling pathways are under the regulation of reactive oxygen species. Although protein tyrosine phosphatases are directly regulated by reversible oxidation, it is not clear whether protein tyrosine kinases (PTKs) are also directly regulated by reduction/oxidation (redox). In this study we report a mechanism of direct oxidative inactivation specific for the PTKs in the Src and fibroblast growth factor receptor (FGFR) families, key enzymes in mammalian signal transduction. Src is fully active when reduced and retains 8-25% of the full activity toward various substrates when oxidized. This inactivation is caused by oxidation of a specific cysteine residue (Cys-277), which results in homodimerization of Src linked by a disulfide bridge. Cys-277 is located in the Gly loop in the catalytic domain. This cysteine residue is conserved only in 8 of the >90 PTKs in the human kinome, including 3 of the 10 Src family kinases and all 4 kinases of the FGFR family. FGFR1 is also reversibly regulated by redox because of this cysteine residue, whereas Csk, a PTK that lacks a cysteine residue at the corresponding position, is not similarly regulated. These results demonstrate a mechanism of direct redox regulation conserved in certain specific PTKs.

Identification of N-terminal lobe motifs that determine the kinase activity of the catalytic domains and regulatory strategies of Src and Csk protein tyrosine kinases

Csk and Src protein tyrosine kinases are structurally homologous but use opposite regulatory strategies. The isolated catalytic domain of Csk is intrinsically inactive and is activated by interactions with the regulatory Src homology 3 (SH3) and SH2 domains, while the isolated catalytic domain of Src is intrinsically active and is suppressed by interactions with the regulatory SH3 and SH2 domains. The structural basis for why one isolated catalytic domain is intrinsically active while the other is inactive is not clear. In this study, we identified structural elements in the N-terminal lobe of the catalytic domain that render the Src catalytic domain active. These structural elements include the alpha-helix C region, a beta turn between the beta4 and beta5 strands, and an Arg residue at the beginning of the catalytic domain. These three motifs interact with one another to activate the Src catalytic domain, but the equivalent motifs in Csk directly interact with the regulatory domains that are important for Csk activation. The Src motifs can be grafted to the Csk catalytic domain to obtain an active Csk catalytic domain. These results, together with available Src and Csk tertiary structures, reveal an important structural switch that determines the kinase activity of a catalytic domain and dictates the regulatory strategy of a kinase.

Microtubule-mediated Src tyrosine kinase trafficking in neuronal growth cones

Src family tyrosine kinases are important signaling enzymes in the neuronal growth cone, and they have been implicated in axon guidance; however, the detailed localization, trafficking, and cellular functions of Src kinases in live growth cones are unclear. Here, we cloned two novel Aplysia Src kinases, termed Src1 and Src2, and we show their association with both the plasma membrane and the microtubule cytoskeleton in the growth cone by live cell imaging, immunocytochemistry, and cell fractionation. Activated Src2 is enriched in filopodia tips. Interestingly, Src2-enhanced green fluorescent protein-positive endocytic vesicles and tubulovesicular structures undergo microtubule-mediated movements that are bidirectional in the central domain and mainly retrograde in the peripheral domain. To further test the role of microtubules in Src trafficking in the growth cone, microtubules were depleted with either nocodazole or vinblastine treatment, resulting in an increase in Src2 plasma membrane levels in all growth cone domains. Our data suggest that microtubules regulate the steady-state level of active Src at the plasma membrane by mediating retrograde recycling of endocytosed Src. Expression of constitutively active Src2 results in longer filopodia that protrude from smaller growth cones, implicating Src2 in controlling the size of filopodia and lamellipodia.

Structural basis for the recognition of c-Src by its inactivator Csk

The catalytic activity of the Src family of tyrosine kinases is suppressed by phosphorylation on a tyrosine residue located near the C terminus (Tyr 527 in c-Src), which is catalyzed by C-terminal Src Kinase (Csk). Given the promiscuity of most tyrosine kinases, it is remarkable that the C-terminal tails of the Src family kinases are the only known targets of Csk. We have determined the crystal structure of a complex between the kinase domains of Csk and c-Src at 2.9 A resolution, revealing that interactions between these kinases position the C-terminal tail of c-Src at the edge of the active site of Csk. Csk cannot phosphorylate substrates that lack this docking mechanism because the conventional substrate binding site used by most tyrosine kinases to recognize substrates is destabilized in Csk by a deletion in the activation loop.

c-Src-mediated epithelial cell migration and invasion regulated by PDZ binding site

c-Src tyrosine kinase controls proliferation, cell adhesion, and cell migration and is highly regulated. A novel regulatory mechanism to control c-Src function that has recently been identified involves the C-terminal amino acid sequence Gly-Glu-Asn-Leu (GENL) of c-Src as ligand for PDZ domains. Herein, we determined the biological relevance of this c-Src regulation in human breast epithelial cells. The intact GENL sequence maintained c-Src in an inactive state in starved cells and restricted c-Src functions that might lead to metastatic transformation under normal growth conditions. c-Src with a C-terminal Leu/Ala mutation in GENL (Src-A) promoted the activation and translocation of cortactin and focal adhesion kinase and increased the motility and persistence of cell migration on the basement membrane. Src-A promoted increased extracellular proteolytic activity, and in acinar cultures, it led to the escape of cells through the basement membrane into the surrounding matrix. We ascribe the regulatory function of C-terminal Leu to the role of GENL in modulating c-Src activity downstream of cell matrix adhesion. We propose that the C terminus of c-Src via its GENL sequence presents a mechanism that restricts c-Src in epithelia and prevents progression toward an invasive phenotype.

Variant estrogen receptor-c-Src molecular interdependence and c-Src structural requirements for endothelial NO synthase activation

Little is known about the tyrosine kinase c-Src's function in the systemic circulation, in particular its role in arterial responses to hormonal stimuli. In human aortic and venous endothelial cells, c-Src is indispensable for 17beta-estradiol (E2)-stimulated phosphatidylinositol 3-kinase/Akt/endothelial NO synthase (eNOS) pathway activation, a possible mechanism in E2-mediated vascular protection. Here we show that c-Src supports basal and E2-stimulated NO production and is required for E2-induced vasorelaxation in murine aortas. Only membrane c-Src is structurally and functionally involved in E2-induced eNOS activation. Independent of c-Src kinase activity, c-Src is associated with an N-terminally truncated estrogen receptor alpha variant (ER46) and eNOS in the plasma membrane through its "open" (substrate-accessible) conformation. In the presence of E2, c-Src kinase is activated by membrane ER46 and in turn phosphorylates ER46 for subsequent ER46 and c-Src membrane recruitment, the assembly of an eNOS-centered membrane macrocomplex, and membrane-initiated eNOS activation. Overall, these results provide insights into a critical role for the tyrosine kinase c-Src in estrogen-stimulated arterial responses, and in membrane-initiated rapid signal transduction, for which obligate complex assembly and localization require the c-Src substrate-accessible structure.

Synthesis and Evaluation of 3-Phenylpyrazolo[3,4-d]pyrimidine-Peptide Conjugates as Src Kinase Inhibitors

3-Phenylpyrazolo[3,4-d]pyrimidine (PhPP) derivatives substituted with an alkyl or aryl carboxylic acid at the N1-endocyclic amine, such as PhPP-CH(2)COOH (IC(50)=250 microM), and peptides Ac-CIYKYY (IC(50)=400 microM) and Ac-YIYGSFK (IC(50)=570 microM) were weak inhibitors of polyE(4)Y phosphorylation by active c-Src. A series of PhPP-peptide conjugates were synthesized using PhPP as an ATP mimic and CIYKYY or YIYGSFK as a peptide substrate to improve the inhibitory potency against active c-Src kinase. PhPP derivatives were attached to the N terminus or the side chain of amino acids in the peptide template. Two N-terminal substituted conjugates, PhPP-CH(2)CO-CIYKYY (IC(50)=0.38 microM) and PhPP-CH(2)CO-YIYGSFK (IC(50)=2.7 microM), inhibited the polyE(4)Y phosphorylation by active c-Src significantly higher than that of the parent compounds. The conjugation of PhPP with the peptides produced a synergistic inhibition effect possibly through creation of favorable interactions between the conjugate and the kinase domain as shown by molecular modeling studies.

Src family kinases: regulation of their activities, levels and identification of new pathways

While the Src family of protein tyrosine kinases (SFK), and the main ancillary molecules involved in their regulation, have been studied for many years, the details of their interplay are not fully understood and thus remain under active investigation. Additionally, new players that coordinate their regulation and direct their signalling cascades are also being uncovered, shedding new light on the complexity of these signalling networks. Through the utilization of novel interaction assays, several new interconnecting mediators that are helping to show the elegance of Src family kinase regulation have been discovered. This review outlines SFK regulation, the discovery of the Csk binding protein (Phosphoprotein Associated with Glycosphingolipid-enriched microdomains, Cbp/PAG), and its role in regulating SFK kinase activity status, as well as protein levels. Further, details of the methods used to identify this dual mode of regulation can be applied to delineate the full gamut of SH2/SH3-directed SFK pathways and, indeed, those of any tyrosine kinase. Using Lyn as a model SFK, we and others have shown that Cbp recruits negative regulators of COOH-terminal Src kinase (Csk)/Csk-like protein-tyrosine kinase (Ctk) after Lyn is activated and bound to Cbp. Lyn phosphorylates Cbp on multiple tyrosine residues, including two that can bind Lyn's SH2 domain with high affinity. Lyn also phosphorylates Y314, which recruits Csk/Ctk to phosphorylate Lyn at its Y508 negative site, allowing an inactive conformation to form. However, the pY508 site has a low affinity for Lyn's SH2 domain, while the Cbp sites have high affinity. Thus, until these Cbp sites are dephosphorylated, Lyn can remain active. Intriguingly, phosphorylated Y314 also binds the suppressor of cytokine signalling 1 (SOCS1), resulting in elevated ubiquitination and degradation of Lyn. Thus, a single phosphotyrosine residue within Cbp co-ordinates a two-phase process involving distinct negative regulatory pathways that allow inactivation, followed by degradation, of SFKs.

Regulation of c-Src activity by the expression of wild-type v-Src and its kinase-dead double Y416F-K295N mutant

Active, wild-type v-Src and its kinase-dead double Y416F-K295N mutant were expressed in hamster fibroblasts. Expression of the active v-Src induced activation of endogenous c-Src and increased general protein-tyrosine phosphorylation in the infected cells. Expression of the kinase-dead mutant induced hypophosphorylation of Tyr416 of the endogenous c-Src. The inactivation of c-Src was reversible, as confirmed by in vitro kinase activity of c-Src immunoprecipitated from the kinase-dead v-Src-expressing cells. Both activation and inactivation of c-Src may be explained by direct interaction of the v-Src and c-Src that may either facilitate transphosphorylation of the regulatory Tyr416 in the activation loop, or prevent it by formation of transient dead-end complexes of the Y416F-K295N mutant with c-Src. The interaction was also indicated by co-localization of v- and c-Src proteins in immunofluorescent images of the infected cells. These results suggest that dimerization of Src plays an important role in the regulation of Src tyrosine kinase activity.

SRC tail phosphorylation is limited by structural changes in the regulatory tyrosine kinase Csk

Src family tyrosine kinases are down-regulated through phosphorylation of a single C-terminal tyrosine by the nonreceptor tyrosine kinase Csk. Despite the fundamental role of Csk in controlling cell growth and differentiation, it is unclear what limits this key signaling reaction and controls the production of catalytically repressed Src. To investigate this issue, stopped-flow fluorescence experiments were performed to determine which steps modulate catalysis. Both Src binding and phosphorylation can be monitored by changes in intrinsic tryptophan fluorescence. Association kinetics are biphasic with the initial phase corresponding to the bimolecular interaction of both proteins and the second phase representing a slow conformational change that coincides with the rate of maximum turnover. The kinetic transients for the phosphorylation reaction are also biphasic with the initial phase corresponding to the rapid phosphorylation and the release of phospho-Src. These data, along with equilibrium sedimentation and product inhibition experiments, suggest that steps involving Src association, phosphorylation, and product release are fast and that a structural change in Csk participates in limiting the catalytic cycle.

A new strategy to produce active human Src from bacteria for biochemical study of its regulation

Enzymological studies of Src protein tyrosine kinase have been hindered by the lack of a suitable bacterial expression system. Poor expression of active Src appears to be due to toxicity associated with its kinase activity. To overcome this problem, we fused Src to a protein tyrosine phosphatase with an affinity tag and an appropriate thrombin cleavage site. Upon affinity purification of the fusion protein, Src was released by thrombin digestion and further purified by FPLC. This strategy has been used to produce several Src mutants that display catalytic and regulatory properties similar to those from eukaryotic expression systems. Characterization of the Src mutants confirmed that inactivation of Src by Csk through tail tyrosine phosphorylation required the Src SH3 domain.

Conformational basis for SH2-Tyr(P)527 binding in Src inactivation

Src protein-tyrosine kinase contains a myristoylation motif, a unique region, an Src homology (SH) 3 domain, an SH2 domain, a catalytic domain, and a C-terminal tail. The C-terminal tail contains a Tyr residue, Tyr527. Phosphorylation of Tyr527 triggers Src inactivation, caused by Tyr(P)527 binding to the SH2 domain. In this study, we demonstrated that a conformational contribution, not affinity, is the predominant force for the intramolecular SH2-Tyr(P)527 binding, and we characterized the structural basis for this conformational contribution. First, a phosphopeptide mimicking the C-terminal tail is an 80-fold weaker ligand than the optimal phosphopeptide, pYEEI, and similar to a phosphopeptide containing three Ala residues following Tyr(P) in binding to the Src SH2 domain. Second, the SH2-Tyr(P)527 binding is largely independent of the amino acid sequence surrounding Tyr(P)527, and only slightly decreased by an inactivating mutation in the SH2 domain. Furthermore, even the unphosphorylated C-terminal tail with the sequence of YEEI suppresses Src activity by binding to the SH2 domain. These experiments demonstrate that very weak affinity is sufficient for the SH2-Tyr(P)527 binding in Src inactivation. Third, the effective intramolecular SH2-Tyr(P)527 binding is attributed to a conformational contribution that requires residues Trp260 and Leu255. Although the SH3 domain is essential for Src inactivation by Tyr(P)527, it does not contribute to the SH2-Tyr(P)527 binding. These findings suggest a conformation-based Src inactivation model, which provides a unifying framework for understanding Src activation by a variety of mechanisms.

Csk mediates G-protein-coupled lysophosphatidic acid receptor-induced inhibition of membrane-bound guanylyl cyclase activity

Natriuretic peptides (NPs) are involved in many physiological processes, including the regulation of vascular tone, sodium excretion, pressure-volume homeostasis, inflammatory responses, and cellular growth. The two main receptors of NP, membrane-bound guanylyl cyclases A and B (GC-A and GC-B), mediate the effects of NPs via the generation of cGMP. NP-stimulated generation of cGMP can be modulated by intracellular processes, whose exact nature remains to be elucidated. Thus, serum and lysophosphatidic acid (LPA), by unknown pathways, have been shown to inhibit the NP-induced generation of cGMP. Here we report that the nonreceptor-tyrosine-kinase Csk is an essential component of the intracellular modulation of atrial natriuretic peptide (ANP)-stimulated activation of GC-A. The genetic deletion of Csk (Csk(-)(/)(-)) in mouse embryonic fibroblasts blocked the inhibitory effect of both serum and LPA on the ANP-stimulated generation of cGMP. Moreover, using a chemical rescue approach, we also demonstrate that the catalytic activity of Csk is required for its modulatory function. Our data demonstrate that Csk is involved in the control of cGMP levels and that membrane-bound guanylyl cyclases can be critically modulated by other receptor-initiated intracellular signaling pathways.

Docking-based Substrate Recognition by the Catalytic Domain of a Protein Tyrosine Kinase, C-terminal Src Kinase (Csk)

Protein tyrosine kinases are key enzymes of mammalian signal transduction. Substrate specificity is a fundamental property that determines the specificity and fidelity of signaling by protein tyrosine kinases. However, how protein tyrosine kinases recognize the protein substrates is not well understood. C-terminal Src kinase (Csk) specifically phosphorylates Src family kinases on a C-terminal Tyr residue, which down-regulates their activities. We have previously determined that Csk recognizes Src using a substrate-docking site away from the active site. In the current study, we identified the docking determinants in Src recognized by the Csk substrate-docking site and demonstrated an interaction between the docking determinants of Src and the Csk substrate-docking site for this recognition. A similar mechanism was confirmed for Csk recognition of another Src family kinase, Yes. Although both Csk and MAP kinases used docking sites for substrate recognition, their docking sites consisted of different substructures in the catalytic domain. These results helped establish a docking-based substrate recognition mechanism for Csk. This model may provide a framework for understanding substrate recognition and specificity of other protein tyrosine kinases.

Structural basis for domain-domain communication in a protein tyrosine kinase, the C-terminal Src kinase

The catalytic activity of protein tyrosine kinases is commonly regulated by domain-domain interactions. The C-terminal Src kinase (Csk) contains a catalytic domain and the regulatory SH3 and SH2 domains. Both the presence of the regulatory domains and binding of specific phosphotyrosine-containing proteins to the SH2 domain activate Csk. The structural basis for both modes of activation is investigated here. First, the SH3-SH2 linker is crucial for Csk activation. Mutagenic and kinetic studies demonstrate that this activation is mediated by a cation-pi interaction between Arg68 and Trp188. Second, Ala scanning and kinetic analyses on residues in the SH2-catalytic domain interface identify three functionally distinct types of residues in mediating the communication between the SH2 and the catalytic domains. Type I residues are important in mediating a ligand-triggered activation of Csk because their mutation severely reduces Csk activation by the SH2 domain ligand. Type II residues are involved in suppressing Csk activity, and their mutation activates Csk, but makes Csk less sensitive to activation by the SH2 ligand. Both type I and type II residues are likely involved in mediating SH2 ligand-triggered activation of Csk. Type III residues are those located in the SH2 domain whose mutation severely decreases Csk catalytic activity without affecting the SH2 ligand-triggered activation. These residues likely mediate SH2 activation of Csk regardless of SH2-ligand interaction. These studies lead us to propose a domain-domain communication model that provides functional insights into the topology of Csk family of protein tyrosine kinases.

Characterization of the interactions between the active site of a protein tyrosine kinase and a divalent metal activator

BACKGROUND

Protein tyrosine kinases are important enzymes for cell signalling and key targets for anticancer drug discovery. The catalytic mechanisms of protein tyrosine kinase-catalysed phosphorylation are not fully understood. Protein tyrosine kinase Csk requires two Mg2+ cations for activity: one (M1) binds to ATP, and the other (M2) acts as an essential activator.

RESULTS

Experiments in this communication characterize the interaction between M2 and Csk. Csk activity is sensitive to pH in the range of 6 to 7. Kinetic characterization indicates that the sensitivity is not due to altered substrate binding, but caused by the sensitivity of M2 binding to pH. Several residues in the active site with potential of binding M2 are mutated and the effect on metal activation studied. An active mutant of Asn319 is generated, and this mutation does not alter the metal binding characteristics. Mutations of Glu236 or Asp332 abolish the kinase activity, precluding a positive or negative conclusion on their role in M2 coordination. Finally, the ability of divalent metal cations to activate Csk correlates to a combination of ionic radius and the coordination number.

CONCLUSION

These studies demonstrate that M2 binding to Csk is sensitive to pH, which is mainly responsible for Csk activity change in the acidic arm of the pH response curve. They also demonstrate critical differences in the metal activator coordination sphere in protein tyrosine kinase Csk and a protein Ser/Thr kinase, the cAMP-dependent protein kinase. They shed light on the physical interactions between a protein tyrosine kinase and a divalent metal activator.

Protein kinase structure and function analysis with chemical tools

Protein kinases are the largest enzyme superfamily involved in cell signal transduction and represent therapeutic targets for a range of diseases. There have been intensive efforts from many labs to understand their catalytic mechanisms, discover inhibitors and discern their cellular functions. In this review, we will describe two approaches developed to analyze protein kinases: bisubstrate analog inhibition and phosphonate analog utilization. Both of these methods have been used in combination with the protein semisynthesis method expressed protein ligation to advance our understanding of kinase-substrate interactions and functional elucidation of phosphorylation. Previous work on the nature of the protein kinase mechanism suggests it follows a dissociative transition state. A bisubstrate analog was designed against the insulin receptor kinase to mimic the geometry of a dissociative transition state reaction coordinate distance. This bisubstrate compound proved to be a potent inhibitor against the insulin receptor kinase and occupied both peptide and nucleotide binding sites. Bisubstrate compounds with altered hydrogen bonding potential as well as varying spacers between the adenine and the peptide demonstrate the importance of the original design features. We have also shown that related bisubstrate analogs can be used to potently block serine/threonine kinases including protein kinase A. Since many protein kinases recognize folded protein substrates for efficient phosphorylation, it was advantageous to incorporate the peptide-ATP conjugates into protein structures. Using expressed protein ligation, a Src-ATP conjugate was produced and shown to be a high affinity ligand for the Csk tyrosine kinase. Nonhydrolyzable mimics of phosphoSer/phosphoTyr can be useful in examining the functionality of phosphorylation events. Using expressed protein ligation, we have employed phosphonomethylene phenylalanine and phosphonomethylene alanine to probe the phosphorylation of Tyr and Ser, respectively. These tools have permitted an analysis of the SH2-phosphatases (SHP1 and SHP2), revealing a novel intramolecular stimulation of catalytic activity mediated by the corresponding phosphorylation events. They have also been used to characterize the cellular regulation of the melatonin rhythm enzyme by phosphorylation.

Endogenous and synthetic inhibitors of the Src-family protein tyrosine kinases

Src-family kinases (SFKs) are protooncogenic enzymes controlling mammalian cell growth and proliferation. The activity of SFKs is primarily regulated by two tyrosine phosphorylation sites: autophosphorylation of a conserved tyrosine (Y(A)) in the kinase domain results in activation while phosphorylation of the regulatory tyrosine (Y(T)) near the C-terminus leads to inactivation. The phosphorylated Y(T) (pY(T)) engages in intramolecular interactions that stabilise the inactive conformation of SFKs. These inhibitory intramolecular interactions include the binding of pY(T) to the SH2 domain and the binding of the SH2-kinase linker to the SH3 domain. Thus, SFKs are active upon (i) disruption of the inhibitory intramolecular interactions, (ii) autophosphorylation of Y(A) and/or (iii) dephosphorylation of pY(T). Since aberrant activation of SFKs contributes to cancer, SFKs in normal cells are kept inactive by multiple endogenous inhibitors classified as catalytic and non-catalytic inhibitors. The catalytic inhibitors include C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) that phosphorylate Y(T) of SFKs, as well as the protein tyrosine phosphatases that dephosphorylate pY(A) of the activated SFKs. The non-catalytic inhibitors inactivate SFKs by direct binding. CHK is unique among these inhibitors because it employs both catalytic and non-catalytic mechanisms to inhibit SFKs. Other known non-catalytic inhibitors include WASP, caveolin and RACK1, which function to down-regulate SFKs in specific subcellular locations. This review discusses how the various endogenous SFK inhibitors cooperate to regulate SFKs in normal cells. As chemical compounds that can selectively inhibit SFKs in vivo are potential anti-cancer therapeutics, this review also discusses how investigation into the inhibitory mechanisms of the endogenous inhibitors will benefit the design and screening of these compounds.

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK)--endogenous negative regulators of Src-family protein kinases

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) are endogenous inhibitors of the Src-family protein tyrosine kinases (SFKs). Since constitutive activation of SFKs contributes to cancer formation and progression, to prevent excessive activation of SFKs, their activity in normal cells is kept at the basal level by CSK and CHK. CSK and CHK inactivate SFKs by specifically phosphorylating a consensus tyrosine (called Y(T)) near their C-termini. Upon phosphorylation, the phospho-Y(T) engages in intramolecular interactions that lock the SFK molecule in an inactive conformation. SFKs are anchored to the plasma membrane, while CSK and CHK are localized predominantly in the cytosol. To inhibit SFKs, CSK and CHK need to translocate to the plasma membrane. Recruitment of CSK and CHK to the plasma membrane is mediated by the binding of their SH2, SH3 and/or kinase domains to specific transmembrane proteins, G-proteins and adaptor proteins located near the plasma membrane. For CSK, membrane recruitment often accompanies activation. CSK and CHK employ two types of direct interactions with SFKs to achieve efficient Y(T) phosphorylation: (i) short-range interactions involving binding of the active sites of CSK and CHK to specific residues near Y(T), (ii) long-range non-catalytic interactions involving binding of SFKs to motifs located distally from the active sites of CSK and CHK. The interactions between CSK and SFKs are transient in nature. Unlike CSK, CHK binds tightly to SFKs to form stable protein complexes. The binding is non-catalytic as it is independent of Y(T). More importantly, the tight binding alone is sufficient to completely inhibit SFKs. This non-catalytic inhibitory binding represents a novel mechanism employed by CHK to inhibit SFKs. Given that SFKs are implicated in cancer development, compounds mimicking the non-catalytic inhibitory mechanism of CHK are potential anti-cancer therapeutics.

Lateral hypothalamic signaling mechanisms underlying feeding stimulation: differential contributions of Src family tyrosine kinases to feeding triggered either by NMDA injection or by food deprivation

In rats, feeding can be triggered experimentally using many approaches. Included among these are (1) food deprivation and (2) acute microinjection of the neurotransmitter l-glutamate (Glu) or its receptor agonist NMDA into the lateral hypothalamic area (LHA). Under both paradigms, the NMDA receptor (NMDA-R) within the LHA appears critically involved in transferring signals encoded by Glu to stimulate feeding. However, the intracellular mechanisms underlying this signal transfer are unknown. Because protein-tyrosine kinases (PTKs) participate in NMDA-R signaling mechanisms, we determined PTK involvement in LHA mechanisms underlying both types of feeding stimulation through food intake and biochemical measurements. LHA injections of PTK inhibitors significantly suppressed feeding elicited by LHA NMDA injection (up to 69%) but only mildly suppressed deprivation feeding (24%), suggesting that PTKs may be less critical for signals underlying this feeding behavior. Conversely, food deprivation but not NMDA injection produced marked increases in apparent activity for Src PTKs and in the expression of Pyk2, an Src-activating PTK. When considered together, the behavioral and biochemical results demonstrate that, although it is easier to suppress NMDA-elicited feeding by PTK inhibitors, food deprivation readily drives PTK activity in vivo. The latter result may reflect greater PTK recruitment by neurotransmitter receptors, distinct from the NMDA-R, that are activated during deprivation-elicited but not NMDA-elicited feeding. These results also demonstrate how the use of only one feeding stimulation paradigm may fail to reveal the true contributions of signaling molecules to pathways underlying feeding behavior in vivo.

Phosphoryl transfer step in the C-terminal Src kinase controls Src recognition

All members of the Src family of nonreceptor protein tyrosine kinases are phosphorylated and subsequently down-regulated by the C-terminal Src kinase, Csk. Although the recognition of Src protein substrates is essential for a diverse set of signaling events linked to cellular growth and differentiation, the factors controlling this critical protein-protein interaction are not well known. To understand how Csk recognizes Src, the chemical/physical events that modulate apparent substrate affinity and turnover were investigated. Src is phosphorylated in a biphasic manner in rapid quench flow experiments, suggesting that the phosphoryl transfer step is fast and highly favorable and does not limit overall turnover. As opposed to other kinase-substrate pairs, turnover is not limited by the physical release of ADP based on stopped-flow fluorescence and catalytic trapping experiments, suggesting that other steps control net phosphorylation. The K(d) for Src is considerably larger than the K(m) based on single turnover kinetic and equilibrium sedimentation experiments. Taken together, the data are consistent with a mechanism whereby Csk achieves a low K(m) for the substrate Src, not by stabilizing protein-protein interactions but rather by facilitating a fast phosphoryl transfer step. In this manner, the phosphoryl transfer step functions as a chemical clamp facilitating substrate recognition.

Src protein–tyrosine kinase structure and regulation

Src and Src-family protein kinases are proto-oncogenes that play key roles in cell morphology, motility, proliferation, and survival. v-Src (a viral protein) is encoded by the chicken oncogene of Rous sarcoma virus, and Src (the cellular homologue) is encoded by a physiological gene, the first of the proto-oncogenes. From the N- to C-terminus, Src contains an N-terminal 14-carbon myristoyl group, a unique segment, an SH3 domain, an SH2 domain, a protein-tyrosine kinase domain, and a C-terminal regulatory tail. The chief phosphorylation sites of Src include tyrosine 416 that results in activation from autophosphorylation and tyrosine 527 that results in inhibition from phosphorylation by C-terminal Src kinase. In the restrained state, the SH2 domain forms a salt bridge with phosphotyrosine 527, and the SH3 domain binds to the kinase domain via a polyproline type II left-handed helix. The SH2 and SH3 domains occur on the backside of the kinase domain away from the active site where they stabilize a dormant enzyme conformation. Protein-tyrosine phosphatases such as PTPalpha displace phosphotyrosine 527 from the Src SH2 domain and mediate its dephosphorylation leading to Src kinase activation. C-terminal Src kinase consists of an SH3, SH2, and kinase domain; it lacks an N-terminal myristoyl group and a C-terminal regulatory tail. Its X-ray structure has been determined, and the SH2 lobe occupies a position that is entirely different from that of Src. Unlike Src, the C-terminal Src kinase SH2 and SH3 domains stabilize an active enzyme conformation. Amino acid residues in the alphaD helix near the catalytic loop in the large lobe of C-terminal Src kinase serve as a docking site for the physiological substrate (Src) but not for an artificial substrate (polyGlu(4)Tyr).

Role of the Brk SH3 domain in substrate recognition

Breast tumor kinase (Brk) is a nonreceptor tyrosine kinase that is overexpressed in a high percentage of breast carcinomas. Brk contains SH3, SH2, and tyrosine kinase catalytic domains in a similar arrangement as Src family kinases. In this study, we explored the roles of the SH3 and SH2 domains in Brk regulation and substrate binding. We introduced a series of mutations into Brk that were predicted to disrupt the intramolecular interactions involving the SH3 and SH2 domains. These mutant forms of Brk displayed higher activity than wild-type Brk when expressed in human embryonic kidney HEK293 cells. These studies also allowed us to pinpoint the intramolecular binding site for the SH3 domain. To examine substrate binding, we compared binding and phosphorylation of Sam68, a physiological substrate of Brk. These experiments showed that the SH3 domain plays a particularly important role in substrate recognition. We confirmed this conclusion using a series of synthetic peptides in which a substrate sequence was coupled to an SH3 or SH2 ligand. The SH3-binding substrate had a significantly lower K(m) than a control, while no difference was observed between an SH2-binding substrate and a control. Taken together, our data suggest that SH3 interactions will govern phosphorylation of many substrates by Brk.

A novel non-catalytic mechanism employed by the C-terminal Src-homologous kinase to inhibit Src-family kinase activity

Although C-terminal Src kinase (CSK)-homologous kinase (CHK) is generally believed to inactivate Src-family tyrosine kinases (SFKs) by phosphorylating their consensus C-terminal regulatory tyrosine (Tyr(T)), exactly how CHK inactivates SFKs is not fully understood. Herein, we report that in addition to phosphorylating Tyr(T), CHK can inhibit SFKs by a novel non-catalytic mechanism. First, CHK directly binds to the SFK members Hck, Lyn, and Src to form stable protein complexes. The complex formation is mediated by a non-catalytic Tyr(T)-independent mechanism because it occurs even in the absence of ATP or when Tyr(T) of Hck is replaced by phenylalanine. Second, the non-catalytic CHK-SFK interaction alone is sufficient to inactivate SFKs by inhibiting the catalytic activity of autophosphorylated SFKs. Third, CHK and Src co-localize to specific plasma membrane microdomains of rat brain cells, suggesting that CHK is in close proximity to Src such that it can effectively inactivate Src in vivo. Fourth, native CHK.Src complex exists in rat brain, and recombinant CHK.Hck complex exists in transfected HEK293T cells, implying that CHK forms stable complexes with SFKs in vivo. Taken together, our findings suggest that CHK inactivates SFKs (i) by phosphorylating their Tyr(T) and (ii) by this novel Tyr(T)-independent mechanism involving direct binding of CHK to SFKs. It has been documented that autophosphorylated SFKs can still be active, in some cases even when their Tyr(T) is phosphorylated. Thus, the ability of the Tyr(T)-independent mechanism to suppress the activity of both non-phosphorylated and autophosphorylated SFKs represents a fail-safe measure employed by CHK to down-regulate SFK signaling under all circumstances.

Determination of the substrate-docking site of protein tyrosine kinase C-terminal Src kinase

Protein tyrosine kinases (PTK) are key enzymes of mammalian signal transduction. For the fidelity of signal transduction, each PTK phosphorylates only one or a few proteins on specific Tyr residues. Substrate specificity is thought to be mediated by PTK-substrate docking interactions and recognition of the phosphorylation site sequence by the kinase active site. However, a substrate-docking site has not been determined on any PTK. C-terminal Src kinase (Csk) is a PTK that specifically phosphorylates Src family kinases on a C-terminal Tyr. In this study, by sequence alignment and site-specific mutagenesis, we located a substrate-docking site on Csk. Mutations in the docking site disabled Csk to phosphorylate, regulate, and complex with Src but only moderately affected its general kinase activity. A peptide mimicking the docking site potently inhibited (IC50 = 21 microM) Csk phosphorylation of Src but only moderately inhibited (IC50 = 422 microM) its general kinase activity. Determination of the substrate-docking site provides the structural basis of substrate specificity in Csk and a model for understanding substrate specificity in other PTKs.

Functions of the activation loop in Csk protein-tyrosine kinase

Autophosphorylation in the activation loop is a common mechanism regulating the activities of protein-tyrosine kinases (PTKs). PTKs in the Csk family, Csk and Chk, are rare exceptions for lacking Tyr residues in this loop. We probed the function of this loop in Csk by extensive site-specific mutagenesis and kinetic studies using physiological and artificial substrates. These studies led to several surprising conclusions. First, specific residues in Csk activation loop had little discernable functions in phosphorylation of its physiological substrate Src, as Ala scanning and loop replacement mutations decreased Csk activity toward Src less than 40%. Second, some activation loop mutants, such as a single residue deletion or replacing all residues with Gly, exhibited 1-2% of wild type (wt) activity toward artificial substrates, but significantly higher activity toward Src. Third, introduction of a thrombin cleavage site to the activation loop also resulted in loss of 98% of wt activity for poly(E4Y) and loss of 95% of wt activity toward Src, but digestion with thrombin to cut the activation loop, resulted in full recovery of wt activity toward both substrates. This suggested that the catalytic machinery is fully functional without the activation loop, implying an inhibitory role by the activation loop as a regulatory structure. Fourth, Arg313, although universally conserved in protein kinases, and essential for the activity of other PTKs so far tested, is not important for Csk activity. These findings provide new perspectives for understanding autophosphorylation as a regulatory mechanism and imply key differences in Csk recognition of artificial and physiological substrates.

Expression, purification, and biochemical characterization of Chk, a soluble protein tyrosine kinase

CSK family contains two protein tyrosine kinases: Csk (C-terminal Src kinase) and Chk (Csk homologous kinase). They are responsible for phosphorylating Src family protein tyrosine kinases on a C-terminal Tyr (Tyr527) and negatively regulating their activities. However, Chk and Csk have different expression patterns, mechanisms of regulation, and different biological functions, and appear to play different roles in the development of breast cancer. To obtain pure human Chk for biochemical characterization, its coding region was amplified by polymerase chain reaction and expressed as a fusion protein with glutathione S-transferase in Escherichia coli. The enzyme was highly expressed but unusually prone to proteolytic degradation during purification. Expression of the enzyme as a dual fusion protein with glutathione S-transferase on N-terminus and streptag, a 10 amino acid peptide, on C-terminus allowed purification of the full-length fusion protein. The purified enzyme was able to phosphorylate and inactivate Src. Chk (no inhibition up to 18.5 microM) and Csk (IC(50)= 1 microM) were differentially inhibited by PP2, probably due to the size difference of one residue (Thr265 in Csk versus Met304 in Chk) in the ATP-binding domain. The expression, purification, and initial characterizations of Chk provided an important step toward full characterization of Chk and Csk, two important enzymes in cellular regulation.

C-terminal Src kinase (CSK) modulates insulin-like growth factor-I signaling through Src in 3T3-L1 differentiation

IGF-I stimulates both proliferation and differentiation of adipocyte-precursor cells, preadipocytes in vivo and in vitro. We have previously shown that IGF-I stimulates proliferation of 3T3-L1 preadipocytes through activation of MAPK and MAPK activation by IGF-I is mediated through the Src family of nonreceptor tyrosine kinases. In addition, we have shown that when 3T3-L1 cells reach growth arrest and are stimulated to differentiate, IGF-I can no longer activate the MAPK pathway. We hypothesized that the loss of IGF-I signaling to MAPK in differentiating 3T3-L1 cells is due to loss of IGF-I activation of Src family kinases. We measured c-Src kinase activity in cell lysates from proliferating, growth-arrested and differentiating 3T3-L1 cells. Src activity increased 2- to 4-fold in IGF-I-stimulated proliferating cells; however, IGF-I had a marginal affect on Src activity in growth-arrested cells and inhibited Src activity localized at the membrane in differentiating cells. C-terminal Src kinase (CSK), a ubiquitously expressed nonreceptor tyrosine kinase, negatively regulates the Src family kinases by phosphorylation of the Src C-terminal tyrosine. IGF-I decreased phosphorylation of the Src C-terminal tyrosine in proliferating cells and increased phosphorylation of this site in differentiating cells. IGF-I stimulated CSK kinase activity 2-fold in differentiating 3T3-L1 cells. An association between CSK and c-Src was detected by immunoprecipitation following IGF-I stimulation of differentiating but not proliferating 3T3-L1 cells. These results suggest that the loss of IGF-I downstream mitogenic signaling in differentiating 3T3-L1 cells is due to a change in IGF-I activation of c-Src and CSK may mediate the inactivation of c-Src by IGF-I in 3T3-L1 adipogenesis.

Increased activity of c-Src and Csk in fibroblasts transformed by v-src oncogene

When c-Src and v-Src were immunoprecipitated together from hamster fibroblasts transformed by Rous sarcoma virus containing v-src oncogene, the total Src activity was almost threefold higher compared to c-Src activity in the control cells. The activity of v-Src immunoprecipitated separately, however, accounting for only 40% of the total Src activity, indicating that c-Src is activated upon transformation. An increased activity of Csk was also found in RSV-transformed cells. It decreased upon serum stimulation in parallel with an increase in Src kinase activity. In nontransformed cells, serum stimulation induced an enhanced Csk activity, but no changes in c-Src activity were observed. This may suggest that Csk may have more functions in hamster fibroblasts, in addition to its inhibitory effect on c-Src.

Dynamic coupling between the SH2 and SH3 domains of c-Src and Hck underlies their inactivation by C-terminal tyrosine phosphorylation

The effect of C-terminal tyrosine phosphorylation on molecular motions in the Src kinases Hck and c-Src is investigated by molecular dynamics simulations. The SH2 and SH3 domains of the inactive kinases are seen to be tightly coupled by the connector between them, impeding activation. Dephosphorylation of the tail reduces the coupling between the SH2 and SH3 domains in the simulations, as does replacement of connector residues with glycine. A mutational analysis of c-Src expressed in Schizosaccharomyces pombe demonstrates that replacement of residues in the SH2-SH3 connector with glycine activates c-Src. The SH2-SH3 connector appears to be an inducible "snap lock" that clamps the SH2 and SH3 domains upon tail phosphorylation, but which allows flexibility when the tail is released.