A 42-kD tyrosine kinase substrate linked to chromaffin cell secretion exhibits an associated MAP kinase activity and is highly related to a 42-kD mitogen-stimulated protein in fibroblasts.

The localization of the protein tyrosine kinase pp60c-src to the plasma membrane and to the membrane of secretory vesicles in neurally derived bovine chromaffin cells has suggested that tyrosine phosphorylations may be associated with the process of secretion. In the present study we have identified two cytosolic proteins of approximately 42 and 45 kD that become phosphorylated on tyrosine in response to secretagogue treatment. Phosphorylation of these proteins reached a maximum (3 min after stimulation) before maximum catecholamine release was observed (5-10 min after stimulation). Both secretion and tyrosine phosphorylation of p42 and p45 required extracellular Ca2+. Tyrosine-phosphorylated proteins of similar Mr have previously been identified in 3T3-L1 adipocytes stimulated with insulin (MAP kinase; Ray, L. B., and T. W. Sturgill. 1987. Proc. Natl. Acad. Sci. USA. 84:1502-1506) and in avian and rodent fibroblasts stimulated with a variety of mitogenic agents (Cooper, J. A., D. F. Bowen-Pope, E. Raines, R. Ross, and T. Hunter. 1982. Cell. 31:263-273; Nakamura, K. D., R. Martinez, and M. J. Weber. 1983. Mol. Cell. Biol. 3:380-390). Comparisons of the secretion-associated 42-kD protein of chromaffin cells with the 42-kD protein of Swiss 3T3 fibroblasts and 3T3-L1 adipocytes provide evidence that these three proteins are highly related. This evidence includes comigration during one-dimensional SDS-PAGE, cochromatography using ion exchange and hydrophobic matrices, similar isoelectric points, identical cyanogen-bromide peptide maps, and cochromatography of MAP kinase activity with the tyrosine-phosphorylated form of pp42. This protein(s), which appears to be activated in a variety of cell types, may serve a common function, perhaps in signal transduction involving a cascade of kinases.

A role for CD5 in TCR-mediated signal transduction and thymocyte selection.

CD5 is a transmembrane protein that is expressed on the surface of T cells and a subset of B cells. The absence of CD5 rendered thymocytes hyperresponsive to stimulation through the T cell antigen receptor (TCR) in vitro. Selection of T cells expressing three distinct transgenic TCRs was also abnormal in CD5-deficient mice. These observations indicate that CD5 can influence the fate of developing thymocytes by acting as a negative regulator of TCR-mediated signal transduction.

CD4, CD8 and the role of CD45 in T-cell activation.

CD4, CD8 and CD45 regulate the coupling of the T-cell receptor complex (CD3-TCR) to tyrosine kinase activation and phosphorylation of key substrates such as phospholipase C gamma 1. CD4 and CD8 contribute to activation signals through their cytoplasmic association with p56lck. Expression of the zeta-chain is required for functional synergy of the T-cell receptor with CD4 in the activation of phospholipase C gamma 1, which probably reflects an interaction between p56lck and zeta-associated kinase ZAP-70. CD45 expression is required for CD3-TCR signaling. CD45 may positively regulate signaling by dephosphorylating the carboxyl-terminal tyrosine of p56lck and p59fyn, and negatively regulate signaling by dephosphorylation of other TCR-associated substrates directly. One ligand for CD45 receptor has been identified as the B cell CD22 molecule. The positive and negative effects of CD45 are sensitive to the composition of CD45 in receptor complexes, and may be regulated by specific associations of CD45 isoforms with other receptors such as CD3-TCR, CD2 and CD4.

Tyrosine phosphorylation of a 120-kilodalton pp60src substrate upon epidermal growth factor and platelet-derived growth factor receptor stimulation and in polyomavirus middle-T-antigen-transformed cells.

The monoclonal antibody 2B12 is directed toward p120, a 120-kDa cellular protein originally identified as a protein tyrosine kinase substrate in cells expressing membrane-associated oncogenic variants of pp60src. In this report, we show that p120 was tyrosine phosphorylated in avian cells expressing membrane-associated, enzymatically activated variants of c-src, including variants having structural alterations in the src homology regions 2 and 3. In contrast, p120 was not tyrosine phosphorylated in cells expressing enzymatically activated, nonmyristylated pp60src. Furthermore, p120 was tyrosine phosphorylated in avian cells expressing middle T antigen, the transforming protein of polyomavirus, as well as in rodent cells stimulated with either epidermal growth factor (EGF) or platelet-derived growth factor. Analysis of the time course of p120 tyrosine phosphorylation in EGF-stimulated cells revealed a rapid onset of tyrosine phosphorylation. In addition, both the extent and duration of p120 phosphorylation increased when cells overexpressing the EGF receptor were stimulated with EGF. Biochemical analysis showed that p120 (in both normal and src-transformed cells) was membrane associated, was myristylated, and was phosphorylated on serine and threonine residues. Hence, p120 appears to be a substrate of both nonreceptor- and ligand-activated transmembrane receptor tyrosine kinases and of serine/threonine kinases and is perhaps a component of both mitogen-stimulated and tyrosine kinase oncogene-induced signaling pathways.

Transformation-specific tyrosine phosphorylation of a novel cellular protein in chicken cells expressing oncogenic variants of the avian cellular src gene.

We used myristylated and nonmyristylated c-src-based variants and phosphotyrosine-specific antibodies to reevaluate the role of tyrosine phosphorylation in cellular transformation by pp60src. Prior methods used to detect tyrosine-phosphorylated proteins failed to discriminate predicted differences in tyrosine phosphorylation which are clearly observed with phosphotyrosine-specific antibodies and Western blotting (immunoblotting). Here we report the observation of a 120,000-Mr protein whose phosphorylation on tyrosine correlates with the induction of morphological transformation. p120 was not observed in cells overexpressing the regulated, nononcogenic pp60c-src, whereas phosphorylation of p120 was greatly enhanced in cells expressing activated, oncogenic pp60527F. Furthermore, phosphorylation of p120 was not induced by expression of the activated but nonmyristylated src variant pp602A/527F, which is transformation defective. p120 partitioned preferentially with cellular membranes, consistent with the observation that transforming src proteins are membrane associated. Although a number of additional putative substrates were identified and partially characterized with respect to intracellular localization, tyrosine phosphorylation of these proteins was not tightly linked to transformation.

Stable association of activated pp60src with two tyrosine-phosphorylated cellular proteins.

We have identified two phosphotyrosine-containing cellular proteins with relative molecular masses of 130,000 (pp130) and 110,000 (pp110) daltons in chicken embryo cells that coimmunoprecipitated with pp60v-src and activated forms of chicken pp60c-src (pp60(527)F). Most if not all of the tyrosine-phosphorylated forms of pp130 and pp110 could be immunoprecipitated from lysates with any of several src protein-specific monoclonal antibodies directed against at least three spatially distinct epitopes. Consequently, of the more than 15 prominent phosphoproteins detected on immunoblots with phosphotyrosine-specific antibodies, pp130 and pp110 were selectively removed by src protein-specific immunoprecipitation, and their presence in the immunoprecipitates appears to have been due to a direct interaction with activated src proteins. src protein variants that induce different morphological phenotypes were altered in their ability to form detergent-stable complexes with pp130 and pp110 or with pp110 alone. Mutant src proteins, defective for myristylation, showed increased tyrosine phosphorylation of and association with pp110. Expression of src variants with mutations in the A box (pp60dl92/527F) or B box (pp60dl155/527F) of the src homology region induced differences in phosphorylation of pp130 and pp110, as well as changes in their association with variant src proteins. Sequences within the B-box region appeared to be necessary for stable complex formation with pp130 and pp110 and may be involved in the interaction of activated src proteins with cellular substrates.

Transformation by pp60src or stimulation of cells with epidermal growth factor induces the stable association of tyrosine-phosphorylated cellular proteins with GTPase-activating protein.

GTPase-activating protein (GAP) is a cytosolic protein that stimulates the rate of hydrolysis of GTP (GTP to GDP) bound to normal p21ras, but does not catalyze the hydrolysis of GTP bound to oncogenic, activated forms of the ras protein. Transformation of cells with v-src or activated transforming variants of c-src or stimulation of cells with epidermal growth factor resulted in the stable association of GAP with two tyrosine-phosphorylated cellular proteins of 64 kDa (p64) and 190 kDa (p190). Analysis of GAP immune complexes isolated from extracts of metabolically labeled src-transformed cells and epidermal growth factor-stimulated cells indicated that tyrosine phosphorylation of p64 and p190 appeared to be coincident with the stable association of these proteins with GAP. Quantitation of the amount of p64 associated with GAP in v-src-transformed cells, however, indicated that only 15 to 25% of tyrosine-phosphorylated p64 was found in complex with GAP. Mutations within the SH2 region of pp60src that render activated pp60src defective for transformation inhibited the efficient formation of complexes between GAP and the tyrosine-phosphorylated forms of p64 and p190. From these data, we suggest that tyrosine phosphorylation and stable association of p64 with GAP is an important step in mediating cellular signaling through the p21ras-GAP pathway.

Identification and characterization of a novel cytoskeleton-associated pp60src substrate.

Transformation of cells by the src oncogene results in elevated tyrosine phosphorylation of two related proteins, p80 and p85 (p80/85). Immunostaining with specific monoclonal antibodies revealed a striking change of subcellular localization of p80/85 in src-transformed cells. p80/85 colocalizes with F-actin in peripheral extensions of normal cells and rosettes (podosomes) of src-transformed cells. Sequence analysis of cDNA clones encoding p80/85 revealed an amino-terminal domain composed of six copies of a direct tandem repeat, each repeat containing 37 amino acids, a carboxyl-terminal SH3 domain, and an interdomain region composed of a highly charged acidic region and a region rich in proline, serine, and threonine. The multidomain structure of p80/85 and its colocalization with F-actin in normal and src-transformed cells suggest that these proteins may associate with components of the cytoskeleton and contribute to organization of cell structure.

CD5 negatively regulates the T-cell antigen receptor signal transduction pathway: involvement of SH2-containing phosphotyrosine phosphatase SHP-1.

The negative regulation of T- or B-cell antigen receptor signaling by CD5 was proposed based on studies of thymocytes and peritoneal B-1a cells from CD5-deficient mice. Here, we show that CD5 is constitutively associated with phosphotyrosine phosphatase activity in Jurkat T cells. CD5 was found associated with the Src homology 2 (SH2) domain containing hematopoietic phosphotyrosine phosphatase SHP-1 in both Jurkat cells and normal phytohemagglutinin-expanded T lymphoblasts. This interaction was increased upon T-cell receptor (TCR)-CD3 cell stimulation. CD5 co-cross-linking with the TCR-CD3 complex down-regulated the TCR-CD3-increased Ca2+ mobilization in Jurkat cells. In addition, stimulation of Jurkat cells or normal phytohemagglutinin-expanded T lymphoblasts through TCR-CD3 induced rapid tyrosine phosphorylation of several protein substrates, which was substantially diminished after CD5 cross-linking. The CD5-regulated substrates included CD3zeta, ZAP-70, Syk, and phospholipase Cgammal but not the Src family tyrosine kinase p56(lck). By mutation of all four CD5 intracellular tyrosine residues to phenylalanine, we found the membrane-proximal tyrosine at position 378, which is located in an immunoreceptor tyrosine-based inhibitory (ITIM)-like motif, crucial for SHP-1 association. The F378 point mutation ablated both SHP-1 binding and the down-regulating activity of CD5 during TCR-CD3 stimulation. These results suggest a critical role of the CD5 ITIM-like motif, which by binding to SHP-1 mediates the down-regulatory activity of this receptor.

Mutational activation of pp60(c-src) leads to a tumorigenic phenotype in a preneoplastic Syrian hamster embryo cell line.

Previous studies indicated that overexpression of wild-type avian c-src cannot induce neoplastic transformation of NIH 3T3 cells. In this study, we isolated and characterized novel spontaneously derived transforming mutants of avian pp60(c-src) from a Syrian hamster embryo-derived cell line, 10W, transfected with the avian c-src gene. Seventeen independently derived transfected 10W cell clones were injected into athymic nude mice. After a latency period, tumors eventually arose and were established in culture. The tumorigenic phenotype was always accompanied by the presence of the avian c-src DNA and functional expression of pp60(c-src). However, most of the tumor-derived cell lines expressed an electrophoretically altered form of pp60(c-src), suggesting mutations in src. Consistent with this hypothesis, DNAs isolated from the tumor-derived lines, but not the parental 10W cell lines, morphologically transformed NIH 3T3 cells in a focus-forming assay. We characterized pp60(c-src) in detail from three of the tumor-derived lines: 4AT, 4BT, and E2T. Two of these lines contained mutations within the exogenous c-src coding region. Line 4AT has an internal repeat of 29 amino acids immediately following Gln-513, which disrupts the spacing between the end of the kinase domain and Tyr-527, the negative regulatory site in pp60(c-src). Line 4BT has a 5-bp deletion following Phe-520, which results in loss of Tyr-527. However, the DNA sequence of the coding region of pp60(c-src) from a third line, E2T, was completely wild type. Cyanogen bromide cleavage analyses of the altered pp60(c-src) from lines 4AT and 4BT showed that Tyr-527, the site of negative regulation of c-src, is not phosphorylated, but Tyr-416, the site of in vitro autophosphorylation, is phosphorylated. However, in line E2T, Tyr-527 was phosphorylated, and Tyr-416 was phosphorylated to a lesser extent. Additionally, two proteins that indicate activation of src, p85 cortactin and p120(cas), are phosphorylated in at least six of the tumor-derived cell lines, although to a lesser extent in line E2T. These results suggest that dephosphorylation of Tyr-527 and phosphorylation of Tyr-416 correlate with activation of pp60(c-src) in the tumor-derived lines 4AT and 4BT, respectively. However, in line E2T, the high levels of pp60(c-src), in combination with a partial activation of the pp60(c-src) protein as indicated by phosphorylation of Tyr-416, appear to be involved in the neoplastic process, rather than mutation.

Novel tyrosine phosphorylations accompany the activation of pp60c-src during chemical carcinogenesis.

Treatment of normal Syrian hamster embryo (SHE) cells in vitro with various chemical carcinogens results in transformed preneoplastic cell lines. Continued passage of these preneoplastic cells gives rise to rare variant cells with enhanced capacity for tumorigenic growth. We have previously shown that tumor-derived SHE cell lines contain an activated proto-oncogene product, pp60c-src. Here we demonstrate that tumor-derived SHE cell lines contain several novel tyrosine phosphoproteins in addition to those found in preneoplastic parent cell lines. A correlation was observed between the activation of endogenous pp60c-src tyrosine kinase specific activity and the presence of new phosphotyrosine-containing proteins. Tyrosine phosphoproteins of approximate Mr 81 kilodaltons (kDa), 55 kDa, and 39 kDa were noted in different tumor-derived cell lines. The 81 kDa and 55 kDa proteins were membrane-associated phosphoproteins, whereas the 39 kDa protein was predominantly cytosolic. Additional signature tyrosine phosphoproteins in individual tumor-derived cell lines apparently were unique to the particular inducing carcinogen or target cell. These studies indicate that during chemical carcinogenesis, activation of the tyrosine kinase proto-oncogene protein pp60c-src coincides with the appearance of novel tyrosine phosphorylations.

Activation of pp60c-src tyrosine kinase specific activity in tumor-derived Syrian hamster embryo cells.

Tumor-derived Syrian hamster embryo (SHE) cell lines, induced in vitro by treatment with chemical carcinogens, contained increased levels of pp60c-src kinase activity compared to preneoplastic parental cell lines and normal SHE cells. The increased kinase activity did not result from an increase in the pp60c-src content of the SHE cell lines, but represented a 4-11 fold increase in pp60c-src kinase specific activity. Both the extent of phosphorylation and the velocity of pp60c-src phosphotransferase activity were increased in the tumor-derived cell lines. SHE cell lines producing chicken pp60c-src were isolated following co-transfection with plasmids bearing the chicken c-src and neoR genes. Chicken pp60c-src expressed in an asbestos-transformed, tumor-derived cell line showed an approximate 3-fold activation of tyrosine kinase activity compared to chicken pp60c-src expressed in the preneoplastic cell line. We suggest that these results indicate that activation of pp60c-src is mediated by trans-acting cellular factors present in the tumor-derived cells. Analysis of pp60c-src in normal SHE cells, preneoplastic cell lines and tumor-derived cell lines showed no alteration in the phosphorylation of tyr-527 or tyr-416, two tyrosine residues whose phosphorylation states have been associated with modulation of kinase activity. These studies indicate that the neoplastic progression of cells may be accompanied by the activation of proto-oncogene products, such as the pp60c-src tyrosine kinase, by mechanisms that may not directly involve genetic alteration of the proto-oncogene DNA sequence.

Evidence for LFA-1/ICAM-1 dependent stimulation of protein tyrosine phosphorylation in human B lymphoid cell lines during homotypic adhesion.

JK32.1 and SKW6.4 are Epstein-Barr virus (EBV)-positive human B cell lines that undergo spontaneous, lymphocyte function-associated antigen 1 (LFA-1) dependent homotypic adhesion in culture. This process is associated with induction of tyrosine phosphoproteins of molecular mass 90, 106, and 120 kDa and could be reproduced when these cells were centrifugationally aggregated. Antibodies to the beta 2 (CD18) chain of LFA-1 interfered with induction of p120, p106, and p90 during cellular aggregation. Response induction was abrogated when cells were incubated with protein tyrosine kinase (PTK) inhibitors (erbstatin, genistein, and geldanomycin) or cytochalasin B prior to aggregation. An in vitro kinase assay did not reveal activation of focal adhesion kinase. Although the role of LFA-1-dependent tyrosine phosphorylation in B cells is uncertain, patients with the leukocyte adhesion defect (LAD) exhibit humoral abnormalities. Moreover, aggregation did not induce specific tyrosine phosphoproteins in an EBV-transformed B cell line from a LAD patient. These results suggest that an LFA-1-dependent PTK pathway may play an important role in human B cell function.

Coengagement of CD2 with LFA-1 or VLA-4 by bispecific ligand fusion proteins primes T cells to respond more effectively to T cell receptor-dependent signals.

To examine the effects of ligand engagement and accessory molecule juxtaposition on T cell receptor (TCR) signaling, we prepared LFA-3/ICAM-1 Rg and LFA-3/VCAM-1 Rg bispecific immunoglobulin fusion proteins (Rg, recombinant globulin). These novel fusion proteins allowed us to examine the effects of ligand driven co-engagement of T cell proteins CD2 and LFA-1 or CD2 and VLA-4 on TCR-dependent mobilization of intracellular Ca2+. We observed that preincubation of resting T cells with LFA-3/ICAM-1 Rg or LFA-3/VCAM-1 Rg fusion proteins resulted in significantly enhanced mobilization of intracellular Ca2+ following TCR-accessory molecule cross-linking relative to T cells preincubated with each of the monospecific Rgs alone or with combinations of the monospecific Rg fusion proteins. In addition, such coengagement stimulated TCR-dependent activation and tyrosine phosphorylation of phospholipase C gamma 1 (PLC gamma 1). These results suggest that when T cells interact with antigen presenting cells the engagement of multiple cell adhesion molecules such as CD2, LFA-1, and VLA-4 primes the T cell to respond more effectively to signals delivered through the TCR.

Immunoaffinity purification of tyrosine-phosphorylated cellular proteins.

Transformation of cells by viral oncogene-encoded tyrosine kinases coincides with the phosphorylation of many cellular proteins on tyrosine. In order to study the potential cellular targets of oncogenic tyrosine kinases, tyrosine phosphoproteins were purified from cells by immunoaffinity chromatography with antibodies to phosphotyrosine. Tyrosine phosphoproteins were purified from both rat-1 cells and primary chicken embryo cells expressing transforming or non-transforming variants of the src oncogene. These proteins were released from anti-phosphotyrosine resins with hapten, and the protein mixtures contained 6-10 highly pure phosphoproteins including the src protein pp60src. The recovered proteins represented approximately 0.03% of total cellular proteins. All of the proteins were shown to contain phosphotyrosine; in addition, virtually all of these proteins were also phosphorylated on serine and threonine. This method thus provides a large-scale, single-step immunoaffinity purification of phosphotyrosine-containing proteins to a purity amenable for immunization protocols and characterization of individual polypeptides.

Developmental regulation of pp44/46, tyrosine-phosphorylated proteins associated with tyrosine/serine kinase activity in Trypanosoma brucei.

The pattern of tyrosine-phosphorylated proteins is developmentally regulated in Trypanosoma brucei. To examine the function and regulation of these tyrosine-phosphorylated molecules, monoclonal antibodies were generated using purified tyrosine-phosphorylated proteins as immunogens. Two monoclonal antibodies were obtained. Both react with a set of proteins at 44-46 kDa, collectively referred to as pp44/46, that are phosphorylated on serine and tyrosine. Differentiation of the parasite from slender bloodforms to procyclic forms was accompanied by increased abundance and tyrosine-phosphorylation of pp44/46. The monoclonal antibodies immunoprecipitated protein kinase activity capable of phosphorylating pp44/46 on serine and tyrosine, and myelin basic protein on serine. The data indicate that the prominent tyrosine-phosphorylated proteins induced upon differentiation are either themselves protein kinases or that they are associated with protein kinases.

The human p167 gene encodes a unique structural protein that contains centrosomin A homology and associates with a multicomponent complex.

The characterization of novel cytoplasmic, structural, and enzymatic proteins has been enhanced by a panel of monoclonal antibodies specific for protein substrates of transforming and nontransforming c-Src mutants. These protein substrates have included the focal adhesion kinase (FAK), cortactin, AFAP-110, p120CAS, and p130CAS. The monoclonal antibody 4G8 was generated as part of this panel of antibodies and was used to isolate the human gene for a 167-kD polypeptide. The cDNA sequence is 5,238 nucleotides in length with a predicted open reading frame consisting of 1,382 amino acids. The polypeptide is largely hydrophilic and highly charged. The central region of p167 has 88% identity with the entire 278-amino-acid encoded sequence of the murine centrosomin A gene. The carboxyl third of p167 contains a unique cluster of 10 amino acid repeats with the consensus sequence (A/M)DDDRGPRRG. The p167 protein was found primarily in the cytoplasm of lymphocytes and is part of a multicomponent protein complex with prominent members of 167, 120, 64, 45, 40, 38, and 25 kD. Finally, we illustrate the conservation of p167 and its associated complex, and demonstrate its expression in different human tissues and cell types. The data suggest that p167 is novel and has an important cellular function as a cytoplasmic structural protein.

Human T and B lymphocytes express a structurally conserved focal adhesion kinase, pp125FAK.

Clustering of beta 1-integrins on adherent cells with antibodies or ligands results in increased tyrosine phosphorylation and activation of a novel focal adhesion tyrosine kinase, pp125FAK. The genes encoding pp125FAK have been cloned previously from both chicken and mouse cDNA libraries, and the deduced amino acid sequences are nearly identical (94%). Two synthetic peptides derived from sequences at the carboxyl terminus of chicken pp125FAK were conjugated to ovalbumin to generate rabbit heteroantisera. Human pp125FAK was immunodetected in both T and B lymphocytes with these antisera. A basal state of pp125FAK tyrosine phosphorylation was observed in T and B lymphocytes, and its expression level was in general augmented among human T- and B-cell leukemia/lymphoma lines. Additionally, the full-length sequence of human T-cell pp125FAK (huT-FAK) was derived from a Jurkat T-cell cDNA library. huT-FAK is structurally identical with both mouse and chicken FAK, and shares 95% amino acid identity with chicken pp125FAK and has 97% homology with the mouse sequence. This high degree of evolutionary conservation between species suggests that pp125FAK is likely to have a crucial function in the cell. Expression of the full-length huT-FAK gene in COS cells showed an immunologically indistinct human pp125FAK protein compared with the endogenous primate pp125FAK. Taken together, the data indicate that this structurally conserved human T-cell pp125FAK likely functions in T- and B-cell lineages, and its altered expression in human lymphocyte tumor cell lines may contribute to their transformed phenotype.

Activation of murine T-cells via phospholipase-C gamma 1-associated protein tyrosine phosphorylation is reduced with aging.

Cross-linking of the T-cell receptor (CD3) induces activation of tyrosine kinases and the subsequent phosphorylation of intracellular protein substrates. We examined whether early events in signal transduction through CD3 or CD3 x CD4 receptor ligation were altered in aged murine T-lymphocytes. Both calcium mobilization and tyrosine phosphorylation of phospholipase C gamma 1 (PLC gamma 1) were decreased in T-lymphocytes from old mice. In addition, there was less tyrosine phosphorylation of a 35/36 kDa protein both in whole cell lysates and in PLC gamma 1 immunoprecipitates from old mice. This 35/36 kDa phosphoprotein binds specifically to the SH2 domains of PLC gamma 1. Using a fusion protein containing the SH2 domains of PLC gamma 1 and human IgG1 heavy chain, we identified three additional proteins that bind to the SH2 domains which were tyrosine phosphorylated following CD3 x CD4 ligation to a lesser degree with age. The tyrosine phosphorylation of two phosphoproteins binding to a fusion protein consisting of the SH2 domains of GAP (ras GTPase-activating protein) and human IgG1 heavy chain was also reduced with aging. The observed binding to SH2 domains was thiol redox sensitive. Thus, decreases in antioxidants with age may be responsible for inhibitory effects on PLC gamma 1-phosphatidylinositol signaling through redox regulation of tyrosine phosphoproteins.

SRChing for the substrates of Src.

By the mid 1980's, it was clear that the transforming activity of oncogenic Src was linked to the activity of its tyrosine kinase domain and attention turned to identifying substrates, the putative next level of control in the pathway to transformation. Among the first to recognize the potential of phosphotyrosine-specific antibodies, Parsons and colleagues launched a risky shotgun-based approach that led ultimately to the cDNA cloning and functional characterization of many of today's best-known Src substrates (for example, p85-Cortactin, p110-AFAP1, p130Cas, p125FAK and p120-catenin). Two decades and over 6000 citations later, the original goals of the project may be seen as secondary to the enormous impact of these protein substrates in many areas of biology. At the request of the editors, this review is not restricted to the current status of the substrates, but reflects also on the anatomy of the project itself and some of the challenges and decisions encountered along the way.