Steven E. Shoelson

Much progress has been made in elucidating early events in signal transduction by growth factor receptors with intrinsic tyrosine kinase activity. Upon ligand addition, these receptors dimerize and activate, becoming phosphorylated at a number of tyrosyl residues. These phosphorylation sites serve as docking points for proteins containing src homology-2 (SH2) domains. However, little is known about how phosphotyrosine phosphatases (PTPs), participate in these events. Recently, we and others molecularly cloned a ubiquitously expressed SH2 domain-containing PTP, SH-PTP2 (Syp, PTP1D, PTP2C), and found that it interacts directly with several activated growth factor receptors via its SH2 domains. Using a peptide competition assay, we now demonstrate that the major binding site for SH-PTP2 on the platelet-derived growth factor receptor is phosphotyrosine 1009. Immunoprecipitation studies indicate that SH-PTP2 is the previously unidentified "64-kDa" protein known to bind at this site. Addition of a phosphotyrosyl peptide comprising the region around Tyr-1009 stimulates SH-PTP2 activity 5-10-fold, whereas other phosphotyrosyl peptides from the platelet-derived growth factor receptor have no stimulatory effect. Our data suggest that binding of SH-PTP2 to the activated receptor in vivo should result in stimulation of SH-PTP2 activity.

The genome of avian sarcoma virus CT10 encodes a fusion protein in which viral Gag sequences are fused to cellular Crk sequences containing primarily Src homology 2 (SH2) and Src homology 3 (SH3) domains. Transformation of chicken embryo fibroblasts (CEF) with the Gag-Crk fusion protein results in the elevation of tyrosine phosphorylation on specific cellular proteins with molecular weights of 130,000, 110,000, and 70,000 (p130, p110, and p70, respectively), an event which has been correlated with cell transformation. In this study, we have identified the 70-kDa tyrosine-phosphorylated protein in CT10-transformed CEF (CT10-CEF) as paxillin, a cytoskeletal protein suggested to be important for organizing the focal adhesion. Tyrosine-phosphorylated paxillin was found to be complexed with v-Crk in vivo as evident from coimmunoprecipitation studies. Moreover, a bacterially expressed recombinant glutathione S-transferase (GST)-CrkSH2 fragment bound paxillin in vitro with a subnanomolar affinity, suggesting that the SH2 domain of v-Crk is sufficient for binding. Mapping of the sequence specificity of a GST-CrkSH2 fusion protein with a partially degenerate phosphopeptide library determined a motif consisting of pYDXP, and in competitive coprecipitation studies, an acetylated A(p)YDAPA hexapeptide was able to quantitatively inhibit the binding of GST-CrkSH2 to paxillin and p130, suggesting that it meets the minimal structural requirements necessary for the interaction of CrkSH2 with physiological targets. To investigate the mechanism by which v-Crk elevates the tyrosine phosphorylation of paxillin in vivo, we have treated normal CEF and CT10-CEF with sodium vanadate to inhibit protein tyrosine phosphatase activity. These data suggest that paxillin is involved in a highly dynamic kinase-phosphatase interplay in normal CEF and that v-Crk binding may interrupt this balance to increase the steady-state level of tyrosine phosphorylation. By contrast, the 130-kDa protein was not tyrosine phosphorylated upon vanadate treatment of normal CEF and only weakly affected in the CT10-CEF, suggesting that a different mechanism may be involved in its phosphorylation.

Of 34 tyrosine residues in insulin receptor substrate 1 (IRS-1), 14 are adjacent to acidic residues, suggesting that they might be phosphorylation sites. Synthetic peptides corresponding to sequences surrounding these tyrosines were used as substrates of the insulin receptor kinase. Surprisingly six of these, each within YMXM motifs, were phosphorylated with greatest efficiency (Km, 24-92 microM; kcat/Km, 0.6-2.1 x 10(4) M-1.sec-1). Substituted YMXM peptides revealed a strong preference of the insulin receptor kinase for methionine at Y + 1 and Y + 3 positions. When phosphorylated, related YMXM sequences are recognition motifs for binding to proteins with src-homology (SH2) domains. The combined hydrophobic and flexible nature of methionine side chains adjacent to the targeted tyrosines provides a versatile contact for recognition by diverse proteins involved in signal transduction.