Activation of the proto-oncogene p60c-src by point mutations in the SH2 domain.

To investigate the importance of a conserved region spanning residues 137 to 241 in the noncatalytic domain of p60c-src (SH2 region), we used oligonucleotide-directed mutagenesis to change residues that are highly conserved in this region. Chicken embryo fibroblasts infected with a p60c-src variant containing arginine instead of tryptophan at residue 148 (W148R) appeared more rounded than cells overexpressing a normal c-src gene, and they formed colonies in soft agar. p60c-src variants containing serine instead of arginine at residue 155 (R155S) or isoleucine instead of glycine at residue 170 (G170I) also appeared transformed and were anchorage independent, but to a lesser extent than W148R. Mutation of residue 201 from histidine to leucine (H201L) had no observable effect. The in vitro kinase activity of cells infected with W148R or G170I was elevated twofold. Expression of p60W148R (or, to a lesser extent, of p60G170I) increased the number of proteins phosphorylated on tyrosine in infected cells. All of the mutants were phosphorylated in vivo on Tyr-527, instead of Tyr-416 as observed for p60v-src. Immunoprecipitated p60W148R and p60G170I were found to be associated with a phosphatidylinositol kinase activity, a factor which appears to be necessary for transformation by tyrosine-specific protein kinases. These results show that a single point mutation in the SH2 region of the cellular src gene can activate its transforming potential. This type of activation is in a new category of alterations at the amino terminus that activate but do not cause a shift in phosphorylation at the carboxy terminus.     

The SH2 and SH3 domains of pp60src direct stable association with tyrosine phosphorylated proteins p130 and p110.

Transformation of chicken embryo cells with the tyrosine kinase oncogene src results in the tyrosine phosphorylation of numerous cellular proteins. We have recently generated monoclonal antibodies to individual tyrosine phosphorylated cellular src substrates, several of which are directed to the phosphotyrosine-containing proteins p130 and p110. These proteins form stable complexes with activated variants of pp60src. Mutagenesis of the src homology domains (SH2 and SH3) of activated pp60src resulted in src variants with altered association with p130 and p110. Analysis of these variants showed that the SH3 domain was required for association of p110, while the SH2 domain contained residues necessary for the formation of the ternary complex involving p130, p110 and pp60src. Both the tyrosine phosphorylation status and pp60src association of p130 and p110 appeared to correlate, in part, with the extent of cell transformation. Biochemical analysis demonstrated that p130 and p110 were substrates of both serine/threonine and tyrosine kinases. In addition, p130 was redistributed from the nucleus to cellular membranes upon src transformation, whereas p110, which normally colocalized with cytoskeletal elements, was observed in adhesion plaques (podosomes) in src transformed cells. These data indicate that tyrosine phosphorylation of two different phosphoproteins may play a role during src transformation either by directing their interaction with pp60src, by redirecting subcellular distribution or both.     

Site-directed mutagenesis, proteolytic cleavage, and activation of human proheparanase

Heparanase is an endo-beta-D-glucuronidase that degrades heparan sulfate in the extracellular matrix and cell surfaces. Human proheparanase is produced as a latent 65-kDa polypeptide undergoing processing at two potential proteolytic cleavage sites, located at Glu109-Ser110 (site 1) and Gln157-Lys158 (site 2). Cleavage of proheparanase yields 8- and 50-kDa subunits that heterodimerize to form the active enzyme. The fate of the linker segment (Ser110-Gln157) residing between the two subunits, the mode of processing, and the protease(s) engaged in proheparanase processing are currently unknown. We applied multiple site-directed mutagenesis and deletions to study the nature of the potential cleavage sites and amino acids essential for processing of proheparanase in transfected human choriocarcinoma cells devoid of endogenous heparanase but possessing the enzymatic machinery for proper processing and activation of the proenzyme. Although mutagenesis at site 1 and its flanking sequences failed to identify critical residues for proteolytic cleavage, processing at site 2 required a bulky hydrophobic amino acid at position 156 (i.e. P2 of the cleavage site). Substitution of Tyr156 by Ala or Glu, but not Val, resulted in cleavage at an upstream site in the linker segment, yielding an improperly processed inactive enzyme. Processing of the latent 65-kDa proheparanase in transfected Jar cells was inhibited by a cell-permeable inhibitor of cathepsin L. Moreover, recombinant 65-kDa proheparanase was processed and activated by cathepsin L in a cell-free system. Altogether, these results suggest that proheparanase processing at site 2 is brought about by cathepsin L-like proteases. The involvement of other members of the cathepsin family with specificity to bulky hydrophobic residues cannot be excluded. Our results and a three-dimensional model of the enzyme are expected to accelerate the design of inhibitory molecules capable of suppressing heparanase-mediated enhancement of tumor angiogenesis and metastasis.     

The kinase, SH3, and SH2 domains of Lck play critical roles in T-cell activation after ZAP-70 membrane localization.

Antigenic stimulation of the T-cell antigen receptor initiates signal transduction through the immunoreceptor tyrosine-based activation motifs (ITAMs). When its two tyrosines are phosphorylated, ITAM forms a binding site for ZAP-70, one of the cytoplasmic protein tyrosine kinases essential for T-cell activation. The signaling process that follows ZAP-70 binding to ITAM has been analyzed by the construction of fusion proteins that localize ZAP-70 to the plasma membrane. We found that membrane-localized forms of ZAP-70 induce late signaling events such as activation of nuclear factor of activated T cells without any stimulation. This activity was observed only when Lck was expressed and functional. In addition, each mutation that affects the function of Lck in the kinase, Src homology 2 (SH2), and SH3 domains greatly impaired the signaling ability of the chimeric protein. Therefore, Lck functions in multiple manners in T-cell activation for the steps following ZAP-70 binding to ITAM.     

Physical and functional interactions between SH2 and SH3 domains of the Src family protein tyrosine kinase p59fyn.

The Src family protein tyrosine kinases participate in signalling through cell surface receptors that lack intrinsic tyrosine kinase domains. All nine members of this family possess adjacent Src homology (SH2 and SH3) domains, both of which are essential for repression of the enzymatic activity. The repression is mediated by binding between the SH2 domain and a C-terminal phosphotyrosine, and the SH3 domain is required for this interaction. However, the biochemical basis of functional SH2-SH3 interaction is unclear. Here, we demonstrate that when the SH2 and SH3 domains of p59fyn (Fyn) were present as adjacent domains in a single protein, binding of phosphotyrosyl peptides and proteins to the SH2 domain was enhanced, whereas binding of a subset of cellular polypeptide ligands to the SH3 domain was decreased. An interdomain communication was further revealed by occupancy with domain-specific peptide ligands: occupancy of the SH3 domain with a proline-rich peptide enhanced phosphotyrosine binding to the linked SH2 domain, and occupancy of the SH2 domain with phosphotyrosyl peptides enhanced binding of certain SH3-specific cellular polypeptides. Second, we demonstrate a direct binding between purified SH2 and SH3 domains of Fyn and Lck Src family kinases. Heterologous binding between SH2 and SH3 domains of closely related members of the Src family, namely, Fyn, Lck, and Src, was also observed. In contrast, Grb2, Crk, Abl, p85 phosphatidylinositol 3-kinase, and GTPase-activating protein SH2 domains showed lower or no binding to Fyn or Lck SH3 domains. SH2-SH3 binding did not require an intact phosphotyrosine binding pocket on the SH2 domain; however, perturbations of the SH2 domain induced by specific high-affinity phosphotyrosyl peptide binding abrogated binding of the SH3 domain. SH3-SH2 binding was observed in the presence of proline-rich peptides or when a point mutation (W119K) was introduced in the putative ligand-binding pouch of the Fyn SH3 domain, although these treatments completely abolished the binding to p85 phosphatidylinositol 3-kinase and other SH3-specific polypeptides. These biochemical SH2-SH3 interactions suggest novel mechanisms of regulating the enzymatic activity of Src kinases and their interactions with other proteins.     

Requirement of the SH3 and SH2 domains for the inhibitory function of tyrosine protein kinase p50csk in T lymphocytes.

Previous studies from our laboratory have shown that the cytosolic tyrosine protein kinase p50csk is involved in the negative regulation of T-cell activation (L.M. L. Chow, M. Fournel, D. Davidson, and A. Veillette, Nature [London] 365:156-160, 1993). This function most probably reflects the ability of Csk to phosphorylate the inhibitory carboxy-terminal tyrosine of p56lck and p59fynT, two Src-related enzymes abundantly expressed in T lymphocytes. Herein, we have attempted to better understand the mechanisms by which Csk participates in the inhibitory phase of T-cell receptor signalling. Our results demonstrated that the Src homology 3 (SH3) and SH2 domains of p50csk are crucial for its negative impact on T-cell receptor-mediated signals. As these two sequences were not essential for phosphorylation of the carboxy-terminal tyrosine of a Src-like product in yeast cells, we postulated that they mediate protein-protein interactions allowing the recruitment of p50csk in the vicinity of activated Lck and/or FynT in T cells. In complementary studies, it was observed that linkage of a constitutive membrane targeting signal to the amino terminus of Csk rescued the deleterious impact of a point mutation in the SH2 domain of p50csk. This observation suggested that the SH2 sequence is in part necessary to translocate p50csk from the cytoplasm to the plasma membrane, where Src-related enzymes are located. Nevertheless, constitutive membrane localization was unable to correct the effect of complete deletion of the SH3 or SH2 sequence, implying that these domains provide additional functions necessary for the biological activity of p50csk.     

Vmax. activation of pp60c-src tyrosine kinase from neuroblastoma neuro-2A.

A kinetic analysis of the tyrosine-specific protein kinase of pp60c-src from the C1300 mouse neuroblastoma cell line Neuro-2A and pp60c-src expressed in fibroblasts was carried out to determine the nature of the increased specific activity of the neuroblastoma enzyme. In immune-complex kinase assays with ATP-Mn2+ and the tyrosine-containing peptide angiotensin I as phosphoacceptor substrate, pp60c-src from the neuroblastoma cell line was characterized by a maximum velocity (Vmax.) that was 7-15-fold greater than the Vmax. of pp60c-src from fibroblasts. The neuroblastoma enzyme exhibited Km values for ATP (16 +/- 3 microM) and angiotensin I (6.8 +/- 2.6 mM) that were similar to Km values for ATP (25 +/- 3 microM) and angiotensin I (6.5 +/- 1.7 mM) of pp60c-src from fibroblasts. pp60v-src expressed in Rous-sarcoma-virus-transformed cells exhibited an ATP Km value (25 +/- 4 microM) and an angiotensin I Km value (6.6 +/- 0.5 mM) that approximated the values determined for pp60c-src in neuroblastoma cells and fibroblasts. These results indicate that the pp60c-src kinase from neuroblastoma cells has a higher turnover number than pp60c-src kinase from fibroblasts, and that the neural form of the enzyme would be expected to exhibit increased catalytic activity at the saturating concentrations of ATP that are found intracellularly.     

Thrombin-dependent association of phosphatidylinositol-3 kinase with p60c-src and p59fyn in human platelets.

Recent studies have shown that ligand-activated growth factor receptors as well as transforming versions of nonreceptor protein-tyrosine kinases physically associate with phosphatidylinositol-3 kinase (PI-3 kinase). Reasoning that PI-3 kinase might also play a role in the normal functions of nonreceptor kinases, we sought to determine whether association with PI-3 kinase might serve as a measure of nonreceptor protein-tyrosine kinase activation under physiological conditions. We found that p60c-src as well as p59fyn, the product of another member of the src family of proto-oncogenes, physically associated with a PI kinase activity within 5 s after exposure to thrombin. Furthermore, PI kinase reaction products generated in p60v-src, p60c-src or p59fyn containing immunoprecipitates were indistinguishable, demonstrating the identity of the associated enzyme as PI-3 kinase. These findings demonstrate a thrombin-dependent interaction between p60c-src or p59fyn and PI-3 kinase and suggest a role for nonreceptor protein-tyrosine kinases in human platelet signal transduction.     

Csk inhibition of c-Src activity requires both the SH2 and SH3 domains of Src.

The protein tyrosine kinase c-Src is negatively regulated by phosphorylation of Tyr527 in its carboxy-terminal tail. A kinase that phosphorylates Tyr527, called Csk, has recently been identified. We expressed c-Src in yeast to test the role of the SH2 and SH3 domains of Src in the negative regulation exerted by Tyr527 phosphorylation. Inducible expression of c-Src in Schizosaccharomyces pombe caused cell death. Co-expression of Csk counteracted this effect. Src proteins mutated in either the SH2 or SH3 domain were as lethal as wild type c-Src, but were insensitive to Csk, even though they were substrates for Csk in vivo. Peptide binding experiments revealed that Src proteins with mutant SH3 domains adopted a conformation in which the SH2 domain was not interacting with the tail. These data support the model of an SH2 domain-phosphorylated tail interaction repressing c-Src activity, but expand it to include a role for the SH3 domain. We propose that the SH3 domain contributes to the maintenance of the folded, inactive configuration of the Src molecule by stabilizing the SH2 domain-phosphorylated tail interaction. Moreover, the system we describe here allows for further study of the regulation of tyrosine kinases in a neutral background and in an organism amenable to genetic analysis.     

Structural differences between repressed and derepressed forms of p60c-src.

The kinase activity of p60c-src is derepressed by removal of phosphate from Tyr-527, mutation of this residue to Phe, or binding of a carboxy-terminal antibody. We have compared the structures of repressed and active p60c-src, using proteases. All forms of p60c-src are susceptible to proteolysis at the boundary between the amino-terminal region and the kinase domain, but there are several sites elsewhere that are more sensitive to trypsin digestion in repressed than in derepressed forms of p60c-src. The carboxy-terminal tail (containing Tyr-527) is more sensitive to digestion by pronase E and thermolysin when Tyr-527 is not phosphorylated. The kinase domain fragment released with trypsin has kinase activity. Relative to intact p60c-src, the kinase domain fragment shows altered substrate specificity, diminished regulation by the phosphorylated carboxy terminus, and novel phosphorylation sites. The results identify parts of p60c-src that change conformation upon kinase activation and suggest functions for the amino-terminal region.     

Association of phosphorylated insulin-like growth factor-I receptor with the SH2 domains of phosphatidylinositol 3-kinase p85.

Insulin-like growth factor-I (IGF-I) stimulates the production of 3-inositides and markedly increases the phosphatidylinositol 3-kinase activity that is immunoprecipitated by anti-phosphotyrosine antibodies, a portion of which is also associated with the IGF-I receptor. In this study, recombinant p85, the regulatory subunit of phosphatidylinositol 3-kinase, and fusion proteins containing various subdomains were used to investigate the association of p85 with the IGF-I receptor and to demonstrate that p85 is a direct in vitro substrate of the IGF-I receptor kinase. Solubilized IGF-I receptor was immobilized on antireceptor antibody-agarose beads. Following in vitro receptor phosphorylation and incubation with cell lysate, immobilized receptor became associated with phosphatidylinositol 3-kinase activity and with protein bands with molecular masses of 85 and 110 kDa, which correspond to the known molecular masses of the subunits of phosphatidylinositol 3-kinase. These associations were inhibited by the addition of recombinant intact p85 or SH2-containing fusion proteins, but not by fusion proteins containing its SH3 domain or breakpoint cluster homology region. A fusion protein containing the SH2 domains of Ras GTPase-activating protein also inhibited the association of phosphatidylinositol 3-kinase activity with immobilized IGF-I receptor, although less effectively than p85, whereas a similar construct containing the SH2 domain of pp60src was without effect. When immobilized phosphorylated IGF-I receptor was incubated with intact p85 or the SH2-containing fusion proteins, it became associated with and phosphorylated these proteins. These results demonstrate that at least in vitro, a tight association occurs between phosphorylated IGF-I receptor and phosphatidylinositol 3-kinase, that the region of phosphatidylinositol 3-kinase that contains its SH2 domains is directly involved in this association, and that this region is a direct substrate for IGF-I receptor tyrosine kinase. Furthermore, these results suggest that Ras GTPase-activating protein can also interact with the IGF-I receptor and that different SH2 domain-containing proteins interact with the IGF-I receptor with widely differing affinities.     

Two phosphorylation-independent sites on the p85 SH2 domains bind A-Raf kinase.

Src homology 2 (SH2) domains mediate phosphotyrosine (pY)-dependent protein:protein interactions involved in signal transduction pathways. We have found that the SH2 domains of the 85-kDa alpha subunit (p85) of phosphatidylinositol 3-kinase (PI3 kinase) bind directly to the serine/threonine kinase A-Raf. In this report we show that the p85 SH2:A-Raf interaction is phosphorylation-independent. The affinity of the p85 C-SH2 domain for A-Raf and phosphopeptide pY751 was similar, raising the possibility that a p85:A-Raf complex may play a role in the coordinated regulation of the PI3 kinase and Raf-MAP kinase pathways. We further show that the p85 C-SH2 domain contains two distinct binding sites for A-Raf; one overlapping the phosphotyrosine-dependent binding site and the other a separate phosphorylation-independent site. This is the first evidence for a second binding site on an SH2 domain, distinct from the phosphotyrosine-binding pocket.     

p68 Sam is a substrate of the insulin receptor and associates with the SH2 domains of p85 PI3K.

The 68 kDa Src substrate associated during mitosis is an RNA binding protein with Src homology 2 and 3 domain binding sites. A role for Src associated in mitosis 68 as an adaptor protein in signaling transduction has been proposed in different systems such as T-cell receptors. In the present work, we have sought to assess the possible role of Src associated in mitosis 68 in insulin receptor signaling. We performed in vivo studies in HTC-IR cells and in vitro studies using recombinant Src associated in mitosis 68, purified insulin receptor and fusion proteins containing either the N-terminal or the C-terminal Src homology 2 domain of p85 phosphatidylinositol-3-kinase. We have found that Src associated in mitosis 68 is a substrate of the insulin receptor both in vivo and in vitro. Moreover, tyrosine-phosphorylated Src associated in mitosis 68 was found to associate with p85 phosphatidylinositol-3-kinase in response to insulin, as assessed by co-immunoprecipitation studies. Therefore, Src associated in mitosis 68 may be part of the signaling complexes of insulin receptor along with p85. In vitro studies demonstrate that Src associated in mitosis 68 associates with the Src homology 2 domains of p85 after tyrosine phosphorylation by the activated insulin receptor. Moreover, tyr-phosphorylated Src associated in mitosis 68 binds with a higher affinity to the N-terminal Src homology 2 domain of p85 compared to the C-terminal Src homology 2 domain of p85, suggesting a preferential association of Src associated in mitosis 68 with the N-terminal Src homology 2 domain of p85. This association may be important for the link of the signaling with RNA metabolism.     

Computational binding studies of human pp60c-src SH2 domain with a series of nonpeptide, phosphophenyl-containing ligands

Monte Carlo/free energy perturbation (MC/FEP) simulations were performed on a series of nonpeptide ligands of the human pp60c-src SH2 domain in order to calculate relative free energies of binding for each compound and to understand the structural requirements for high affinity binding. The amido compound, exhibiting the highest experimental affinity, takes advantage of an interaction with a previously unobserved structural water.     

Novel recognition mode between Vav and Grb2 SH3 domains.

Vav is a guanine nucleotide exchange factor for the Rho/Rac family that is expressed exclusively in hematopoietic cells. Growth factor receptor-bound protein 2 (Grb2) has been proposed to play important roles in the membrane localization and activation of Vav through dimerization of its C-terminal Src-homology 3 (SH3) domain (GrbS) and the N-terminal SH3 domain of Vav (VavS). The crystal structure of VavS complexed with GrbS has been solved. VavS is distinct from other SH3 domain proteins in that its binding site for proline-rich peptides is blocked by its own RT loop. One of the ends of the VavS beta-barrel forms a concave hydrophobic surface. The GrbS components make a contiguous complementary interface with the VavS surface. The binding site of GrbS for VavS partially overlaps with the canonical binding site for proline-rich peptides, but is definitely different. Mutations at the interface caused a decrease in the binding affinity of VavS for GrbS by 4- to 40-fold. The structure reveals how GrbS discriminates VavS specifically from other signaling molecules without binding to the proline-rich motif.