c-Src signaling induced by the adapters Sin and Cas is mediated by Rap1 GTPas

Oncogenic Src proteins have been extensively studied to gain insight into t he signaling mechanisms of Src. To better understand signaling through wild -type Src, we used an approach that involves activation of Src signaling th rough the binding of physiologic ligands to the Src SH3 domain. To this end , we used full-length and truncated versions of the multiadapter molecules Cas and Sin to activate c-Src, and we examined the intracellular pathways t hat mediate Src signaling under these conditions. We show that although all proteins bind to and are phosphorylated by c-Src, quantitative differences exist in the ability of the different ligands to activate c-Src signaling. In addition, we show that Sin- and Cas-induced Src signaling, as assayed b y transcriptional activation, is exclusively mediated through a pathway tha t involves the adapter Crk and the GTP-binding protein Rap1. These data are in contrast to previous observations showing Ras to mediate signaling down stream of transforming Src alleles, In our system, we found that signaling through the oncogenic SrcY527 mutant is indeed mediated by Ras. In addition , we found that Rap1 also mediates oncogenic Src signaling. Our results sho w for the first time that Rap1 mediates c-Src kinase signaling and reveal m echanistic differences in the signaling properties of wild-type and transfo rming Src proteins.