Signaling of hepatocyte growth factor/scatter factor (HGF) to the small GTPase Rap1 via the large docking protein Gab1 and the adapter protein CRKL

Hepatocyte growth factor (HGF; scatter factor) is a multipotent protein wit h mitogenic, motogenic, and developmental functions. Upon activation, the H GF-receptor c-Met binds and phosphorylates the multisite docking protein Ga b1. Besides binding motifs for phosphatidylinositol 3-kinase and Grb2, Gab 1 contains multiple Tyr-X-X-Pro (YXXP) motifs which, when phosphorylated, a re potential binding sites for the adapter proteins c-Crk and Crk-like (CRK L). Stimulation of human embryonic kidney cells (HEK293) with HGF leads to Gab1 association with CRKL. The Gab1-CRKL interaction requires both, the SH 2 domain of CRKL and the region containing the YXXP motifs in Gab1, CRKL bi nds via its first SH3 domain to several downstream signal transducers, incl uding C3G an activator of the small GTPase Rap1. Indeed, Rap1 was rapidly a ctivated after HGF stimulation of HEK293 cells. Rap1 activation through HGF was suppressed through transfection of a truncated C3G protein which only contains the SH3-binding motifs of C3G. Transfection of nonmutated Gab1 led to a strong increase of Rap1.GTP in the absence of HGF. In contrast, trans fection of the Gab Delta YXXP mutant abolished the elevation of Rap1.GTP by HGF. A replating assay indicated that HGF decreases the adhesion of HEK293 cells. The results presented here delineate a novel signaling pathway from HGF to the GTPase Rap1 which depends on the interaction of the adapter pro tein CRKL with the exchange factor C3G and could be linked to cell migratio n.