TANDEM SH2 BINDING-SITES MEDIATE THE RASGAP-RHOGAP INTERACTION - A CONFORMATIONAL MECHANISM FOR SH3 DOMAIN REGULATION

Many cellular signaling proteins contain SH3 (Src homology 3) domains that mediate protein interactions via specific proline-containing pept ides. Unlike SH2 domains, whose interactions with tyrosine-containing peptides are promoted by phosphorylation of the SH2 binding site, the regulatory mechanism for SH3 interactions is unclear p120 RasGAP (GTPa se-activating protein), which contains an SH3 domain flanked by two SH 2 domains, forms an abundant SH2-mediated complex with p190 RhoGAP in cells expressing activated tyrosine kinases. We have identified two cl osely linked tyrosine-containing peptides in p190 that bind simultaneo usly to the RasGAP SH2 domains upon p190 phosphorylation. This interac tion is expected to bring the two SH2 domains into close proximity. Co nsequently, RasGAP undergoes a conformational change that results in a 100-fold increase in the accessibility of the target binding surface of its SH3 domain, These results indicate that the tandem arrangement of SH2 and SH3 domains found in a variety of cellular signaling protei ns can provide a conformational mechanism for regulating SH3-dependent interactions through tyrosine phosphorylation, In addition, it appear s that the role of p190 in the RasGAP signaling complex is to promote additional protein interactions with RasGAP via its SH3 domain.