Multiple SH2-mediated interactions in v-src-transformed cells

Novel nonphosphorylated peptides with conserved sequences selectively bind to Grb7 SH2 domain with affinity comparable to its phosphorylated ligand

The Grb7 (growth factor receptor-bound 7) protein, a member of the Grb7 protein family, is found to be highly expressed in such metastatic tumors as breast cancer, esophageal cancer, liver cancer, etc. The src-homology 2 (SH2) domain in the C-terminus is reported to be mainly involved in Grb7 signaling pathways. Using the random peptide library, we identified a series of Grb7 SH2 domain-binding nonphosphorylated peptides in the yeast two-hybrid system. These peptides have a conserved GIPT/K/N sequence at the N-terminus and G/WD/IP at the C-terminus, and the region between the N-and C-terminus contains fifteen amino acids enriched with serines, threonines and prolines. The association between the nonphosphorylated peptides and the Grb7 SH2 domain occurred in vitro and ex vivo. When competing for binding to the Grb7 SH2 domain in a complex, one synthesized nonphosphorylated ligand, containing the twenty-two amino acid-motif sequence, showed at least comparable affinity to the phosphorylated ligand of ErbB3 in vitro, and its overexpression inhibited the proliferation of SK-BR-3 cells. Such nonphosphorylated peptides may be useful for rational design of drugs targeted against cancers that express high levels of Grb7 protein.

Src family kinases as mediators of endothelial permeability: effects on inflammation and metastasis

Src family kinases (SFKs) are signaling enzymes that have long been recognized to regulate critical cellular processes such as proliferation, survival, migration, and metastasis. Recently, considerable work has elucidated mechanisms by which SFKs regulate normal and pathologic processes in vascular biology, including endothelial cell proliferation and permeability. Further, when inappropriately activated, SFKs promote pathologic inflammatory processes and tumor metastasis, in part through their effects on the regulation of endothelial monolayer permeability. In this review, we discuss the roles of aberrantly activated SFKs in mediating endothelial permeability in the context of inflammatory states and tumor cell metastasis. We further summarize recent efforts to translate Src-specific inhibitors into therapy for systemic inflammatory conditions and numerous solid organ cancers.

Specificity in Signal Transduction

Over the last two decades, a new and unifying concept of cellular organization has emerged in which modular protein-protein interactions provide an underlying framework through which signaling pathways are assembled and controlled. In this scheme, posttranslational modifications such as phosphorylation commonly exert their biological effects by regulating molecular interactions, exemplified by the ability of phosphotyrosine sites to bind selectively to SH2 domains. Although these interactions are rather simple in isolation, they can nonetheless be exploited to generate complex cellular systems. Here, I discuss experiments that have led to this view of dynamic cellular behavior and identify some current and future areas of interest in cell signaling.

Domain-dependent function of the rasGAP-binding protein p62Dok in cell signaling

p62Dok, the rasGAP-binding protein, is a common target of protein-tyrosine kinases. It is one of the major tyrosine-phosphorylated molecules in v-Src-transformed cells. Dok consists of an amino-terminal Pleckstrin homology domain, a putative phosphotyrosine binding domain, and a carboxyl-terminal tail containing multiple tyrosine phosphorylation sites. The importance and function of these sequences in Dok signaling remain largely unknown. We have demonstrated here that the expression of Dok can inhibit cellular transformation by the Src tyrosine kinase. Both the phosphotyrosine binding domain and the carboxyl-terminal tail of Dok (in particular residues 336-363) are necessary for such activity. Using a combinatorial peptide library approach, we have shown that the Dok phosphotyrosine binding domain binds phosphopeptides with the consensus motif of Y/MXXNXL-phosphotyrosine. Furthermore, Dok can homodimerize through its phosphotyrosine binding domain and Tyr(146) at the amino-terminal region. Mutations of this domain or Tyr(146) that block homodimerization significantly reduce the ability of Dok to inhibit Src transformation. Our results suggest that Dok oligomerization through its multiple domains plays a critical role in Dok signaling in response to tyrosine kinase activation.

Role of p120 Ras-GAP in directed cell movement

We have used cell lines deficient in p120 Ras GTPase activating protein (Ras-GAP) to investigate the roles of Ras-GAP and the associated p190 Rho-GAP (p190) in cell polarity and cell migration. Cell wounding assays showed that Ras-GAP-deficient cells were incapable of establishing complete cell polarity and migration into the wound. Stimulation of mutant cells with growth factor rescued defects in cell spreading, Golgi apparatus fragmentation, and polarized vesicular transport and partially rescued migration in a Ras-dependent manner. However, for directional movement, the turnover of stress fibers and focal adhesions to produce an elongate morphology was dependent on the constitutive association between Ras-GAP and p190, independent of Ras regulation. Disruption of the phosphotyrosine-mediated Ras-GAP/p190 complex by microinjecting synthetic peptides derived from p190 sequences in wild-type cells caused a suppression of actin filament reorientation and migration. From these observations we suggest that although Ras-GAP is not directly required for motility per se, it is important for cell polarization by regulating actin stress fiber and focal adhesion reorientation when complexed with 190. This observation suggests a specific function for Ras-GAP separate from Ras regulation in cell motility.

Angiotensin II stimulates tyrosine phosphorylation of phospholipase C-gamma-associated proteins. Characterization of a c-Src-dependent 97-kD protein in vascular smooth muscle cells

Stimulation of phospholipase C-gamma (PLC-gamma) is a critical event in angiotensin II (Ang II) signal transduction. We have previously shown that in rat aortic smooth muscle (RASM) cells Ang II stimulates tyrosine phosphorylation of PLC-gamma via activation of c-Src. Because we failed to demonstrate a direct association between c-Src and PLC-gamma, we hypothesized that a linker protein mediates the interaction between these molecules. To identify PLC-gamma-associated proteins, RASM cells were labeled with [32P]orthophosphate and stimulated with 100 nmol/L Ang II for 5 minutes. PLC-gamma was immunoprecipitated, and associated proteins were characterized by autoradiography and Western blotting with anti-phosphotyrosine antibodies. Ang II stimulated the phosphorylation of 47-, 60-, 84-, and 97-kD PLC-gamma-associated proteins. Because Ang II increased tyrosine phosphorylation of only the 97-kD protein, we characterized p97 further. An important role for Src in tyrosine phosphorylation of p97 was suggested by findings that p97 phosphorylation was inhibited by the selective Src-family kinase inhibitor CP-118,556, diminished in mouse aortic smooth muscle (MASM) cells from c-Src knockout mice compared with wild-type MASM cells, and increased in v-Src-transformed NIH-3T3 cells compared with wild-type NIH-3T3 cells. These studies are the first to define a PLC-gamma-associated protein that may be required for Ang II-mediated signal transduction.

Tyrosine phosphorylation of connexin 43 by v-Src is mediated by SH2 and SH3 domain interactions

Reduction of gap junctional communication in v-src transformed cells is accompanied by tyrosine phosphorylation of the gap junction protein, connexin 43 (Cx43). Cx43 is phosphorylated on tyrosine by v-Src. The Src homology 3 (SH3) and Src homology 2 (SH2) domains of v-Src mediate interactions with substrate proteins. SH3 domains interact with proline-rich peptide motifs. SH2 domains associate with short amino acid sequences containing phosphotyrosine. We present evidence that the SH3 and SH2 domains of v-Src bind to proline-rich motifs and a phosphorylated tyrosine residue in the C-terminal tail of Cx43. Cx43 bound to the SH3 domain of v-Src, but not c-Src, in vitro. Tyrosine-phosphorylated Cx43 bound to the SH2 domain of v-Src in vitro. v-Src coprecipitated with Cx43 from v-src-transformed Rat-1 fibroblasts. Mutations in the SH3 and SH2 domains of v-Src, and in the proline-rich region or tyrosine 265 of Cx43, reduced interactions between v-Src and Cx43 in vivo. Tyrosine phosphorylation of Cx43 was dependent on the association of v-Src and Cx43. These results provide further evidence for the direct involvement of v-Src in tyrosine phosphorylation of Cx43 and inhibition of gap junctional communication in v-src-transformed cells.

Complexes of focal adhesion kinase (FAK) and Crk-associated substrate (p130(Cas)) are elevated in cytoskeleton-associated fractions following adhesion and Src transformation. Requirements for Src kinase activity and FAK proline-rich motifs

The focal adhesion kinase (FAK) and Crk-associated substrate, p130(Cas) (Cas), have been implicated in diverse signaling pathways including those mediated by integrins, G-protein-coupled receptors, tyrosine kinase receptors, and the v-src and v-crk oncogenes. The recent identification of a direct interaction between FAK and Cas prompted the examination of potential regulation of FAK.Cas complexes by factors that result in concomitant increase in their phosphotyrosine content, namely cell adhesion and transformation by Src. Both conditions resulted in elevated FAK.Cas complex levels in nonionic detergent-insoluble fractions, indicating increased association with the cytoskeleton. For activated Src, this effect requires an active Src catalytic domain but not its Src homology 2 (SH2) or Src homology 3 (SH3) domains. FAK kinase domain tyrosines 576 and 577 are also required, suggesting that direct phosphorylation of these sites by Src may influence the solubility and/or stability of the complex. FAK-Cas association was only observed in the context of Cas binding to at least one of two distinct proline-rich sites on FAK. These findings firmly establish a direct interaction between FAK and Cas and demonstrate that Src can influence the subcellular localization of the complex by a tyrosine phosphorylation-dependent mechanism.

Direct binding of C-terminal region of p130Cas to SH2 and SH3 domains of Src kinase

p130Cas is a major tyrosine-phosphorylated protein that tightly binds v-Crk in v-crk-transformed cells and v-Src in v-src-transformed cells. The "substrate domain" of p130Cas contains 15 possible Src homology (SH) 2-binding motifs, most of which conform to the binding motif for the Crk SH2 domain. Another region near its C terminus contains possible binding motifs for the Src SH2 domain and proline-rich sequences that are candidates for SH3-binding sites. Using GST fusion proteins, we revealed that both SH2 and SH3 domains of Src bind p130Cas, whereas v-Crk binds p130Cas through its SH2 domain. We located the binding site of p130Cas for the Src SH3 domain at the sequence RPLPSPP in the region near its C terminus. Mutations within this sequence or at Tyr762 of p130Cas caused a significant reduction in the association of p130Cas with Src, and no association was detected when both of them were deleted. The kinase activity in v-Crk-transformed cells was also associated with p130Cas through this region. On the other hand, the deletion of the substrate domain abolished the binding with v-Crk. The association through the C-terminal region of p130Cas with Src kinase may facilitate effective hyperphosphorylation of tyrosine residues in the substrate domain of p130Cas, resulting in the binding of SH2-containing molecules to p130Cas.

Two SH2 domains of p120 Ras GTPase-activating protein bind synergistically to tyrosine phosphorylated p190 Rho GTPase-activating protein

p120 GTPase-activating protein (GAP) is a negative regulator of Ras that functions at a key relay point in signal transduction pathways that control cell proliferation. Among other proteins, p120 GAP associates with p190, a GAP for the Ras-related protein, Rho. To characterize the p120.p190 interaction further, we used bacterially expressed glutathione S-transferase fusion polypeptides to map the regions of p120 necessary for its interactions with p190. Our results show that both the N-terminal and the C-terminal SH2 domains of p120 are individually capable of binding p190 expressed in a baculovirus/insect cell system. Moreover, the two SH2 domains together on one polypeptide bind synergistically to p190, and this interaction is dependent on tyrosine phosphorylation of p190. In addition, mutation of the highly conserved Arg residues in the critical FLVR sequences of both SH2 domains of full-length p120 reduces binding to tyrosine-phosphorylated p190. The dependence on p190 phosphorylation for complex formation with p120 SH2 domains observed in vitro is consistent with analysis of the native p120.p190 complexes formed in vivo. These findings suggest that SH2-phosphotyrosine interaction is one mechanism by which the cell regulates p120.p190 association and thus may be a means for coordinating the Ras- and Rho-mediated signaling pathways.

Construction of an SH2 domain-binding site with mixed specificity

SH2 domains bind to specific phosphotyrosine-containing sites in a fashion dictated by the amino acids flanking the phosphotyrosine. Attention has focused on the role of the three COOH-terminal positions (+1 to +3) in generating specificity. Autophosphorylation of Tyr1021 in the tail of the beta-receptor for platelet-derived growth factor creates a specific binding site for the COOH-terminal SH2 domain of phospholipase C (PLC)-gamma 1. We show that the residues 4 and 5 amino acids COOH-terminal to Tyr1021 (+4 Leu and +5 Pro) are required for efficient PLC-gamma 1 binding, and that their replacement with the corresponding residues from a phosphatidylinositol 3'-kinase binding site abrogates stable association with PLC-gamma 1. In contrast, replacement of the +3 Pro with Met produces a Tyr1021 site with mixed specificity that binds both PLC-gamma 1 and phosphatidylinositol 3'-kinase. This motif is rendered specific for phosphatidylinositol 3'-kinase by further substitution of the +4 Leu. These results indicate that the +4 and +5 residues are important for the selective binding of specific SH2 domains. This study suggests that phosphotyrosine sites can be tailored to bind one or more SH2 domains with high affinity, depending on the combination of residues in the +1 to +5 positions.

Frequency of metastasis in Syrian hamster tumor cells selected for low levels of "typical" multidrug resistance

The metastatic capability of cells at the initial stages of selection for "typical" (P-glycoprotein-mediated) multidrug resistance (MDR) was studied. Two independent sublines, 2SC/4-1 and 2SC/4-2 (11-12.4-fold resistant), and their 23- to 23.7-fold resistant 2SC/20-1 and 2SC/20-2 variants were isolated from highly tumorigenic (TrD50 = 10 cells) and highly metastatic Rous sarcoma, virus-transformed Syrian hamster fibroblast HET-SR-2SC-LNM line for resistance to colchicine. 2SC/4 cells were less tumorigenic (TrD50 = 70 cells) but as highly metastatic as parental counterparts. In contrast, both 2SC/20 variants showed a decrease in tumorigenicity (TrD50 = 320 cells) and in the capability to produce spontaneous distant metastases. 2SC/20 cells almost lost the ability to colonize lungs in experimental metastasis assay. The autophosphorylation of pp60src tyrosine kinase in 2SC/20 cells was unaltered. The results suggest that i) selection of tumor cells for low levels of "typical" MDR leads to a decrease in the frequency of spontaneous and experimental metastases, and ii) alterations of malignancy in these cells are not caused by an impairment of function of a transforming oncogene.

Specificity of protein interactions with highly related SRC homology (SH) domains of FGR and FYN protein-tyrosine kinases

As an approach toward identification and isolation of cellular proteins that may act as substrates or effectors of the SRC-family of protein-tyrosine kinases, fusion proteins containing noncatalytic elements of two highly related SRC-family members were tested for their ability to recognize distinct molecules present in lysates of cells known to normally express both enzymes. Our results demonstrate differences of protein binding between the SH2 elements of FYN and FGR kinases, but do not discriminate proteins binding to their SH3 domains.

Biochemistry of the Src protein-tyrosine kinase: regulation by SH2 and SH3 domains

pp60c-Srs (c-Src) is the prototype for a family of cytoplasmic protein-tyrosine kinases involved in the control of signal transduction. In addition to the enzymatic kinase domain, c-Src has several noncatalytic domains which regulate Src tyrosine kinase activity in both a positive and a negative fashion. Phosphorylation of c-Src at Tyr527 in the noncatalytic C-terminal tail is a key mechanism for repression of c-Src tyrosine kinase activity. This inhibitory phosphorylation is apparently catalyzed by another cytoplasmic tyrosine kinase (Csk). Recent evidence suggests that the c-Src SH2 domain participates in this phosphorylation-dependent repression of kinase activity through an intramolecular association with the phosphotyrosine-containing C-terminus. The SH3 domain of c-Src also negatively regulates c-Src tyrosin kinase activity, although the mechanism is as yet unknown. However, in the background of constitutively active transforming Src variants, such as a c-Src mutant with an amino acid substitution eliminating Tyr527 (527F c-Src) or the retroviral oncogene v-src product pp60v-src (v-Src), both the SH2 and SH3 domains contribute positively to the enzymatic and biological activities of the Src tyrosine kinase through interactions with Src substrates and/or cellular regulators.

The cell cycle and c-Src

The activity of the proto-oncogene encoded c-Src product is tightly regulated in vivo. In recent years, a model has emerged of how this regulation is achieved. In particular, protein kinases and phosphatases that are potential regulators of c-Src activity in the cell cycle have been identified and characterized.

SH3--an abundant protein domain in search of a function

Src-homology 3 is a small protein domain of about 60 amino acid residues. It is probably made of beta-sheets. SH3 is present in a large number of eukaryotic proteins which are involved in signal transduction, cell polarization and membrane-cytoskeleton interactions. Here we review its occurrence and discuss possible functions of this domain.

A novel transforming protein (SHC) with an SH2 domain is implicated in mitogenic signal transduction

A new SH2-containing sequence, SHC, was isolated by screening cDNA libraries with SH2 representative DNA probes. The SHC cDNA is predicted to encode overlapping proteins of 46.8 and 51.7 kd that contain a single C-terminal SH2 domain, and an adjacent glycine/proline-rich motif with regions of homology with the alpha 1 chain of collagen, but no identifiable catalytic domain. Anti-SHC antibodies recognized three proteins of 46, 52, and 66 kd in a wide range of mammalian cell lines. These SHC proteins complexed with and were phosphorylated by activated epidermal growth factor receptor. The physical association of SHC proteins with activated receptors was recreated in vitro by using a bacterially expressed SHC SH2 domain. NIH 3T3 mouse fibroblasts that constitutively overexpressed SHC acquired a transformed phenotype in culture and formed tumors in nude mice. These results suggest that the SHC gene products couple activated growth factor receptors to a signaling pathway that regulates the proliferation of mammalian cells.