Reconstitution of interactions between the Src tyrosine kinases and Ras GTPase-activating protein using a baculovirus expression system.

Ras GTPase-activating protein (GAP) has been implicated in mitogenic signal transduction downstream of oncogenic and receptor tyrosine kinases. Previous studies have suggested that GAP is phosphorylated by oncogenic viral Src (v-Src) and that GAP is associated with a complex containing normal cellular Src (c-Src) in vertebrate fibroblasts. To investigate molecular interactions between the Src kinases and GAP, we developed an in vitro system for reconstituting Src-GAP complexes. For this purpose, we constructed recombinant baculovirus vectors that direct expression of Rous sarcoma virus v-Src, chicken c-Src, and bovine GAP in infected Sf9 insect cells. In vitro reconstitution experiments using baculovirus-expressed proteins demonstrate that both v-Src and c-Src associate in complexes with GAP. In addition, in vitro and in vivo phosphorylation analyses indicate that GAP serves as a substrate for both the v-Src and c-Src tyrosine kinases. To determine which structural features of GAP are involved in interactions with the Src kinases, we constructed recombinant baculoviruses that encode deletion mutants of bovine GAP. Deletion of the GAP amino-terminal portion containing Src homology 2 regions, which are highly conserved structural motifs postulated to mediate interactions among proteins, diminishes GAP phosphorylation and association with Src. This reconstitution system should facilitate further studies of molecular interactions between the Src kinases and GAP.