The common src homology region 2 domain of cytoplasmic signaling proteins is a positive effector of v-fps tyrosine kinase function.

A conserved noncatalytic domain SH2 (for src homology region 2) is located immediately N terminal to the kinase domains of all cytoplasmic protein-tyrosine kinases. We found that the wild-type v-fps SH2 domain stimulated the enzymatic activity of the adjacent kinase domain 10-fold and functioned as a powerful positive effector of catalytic and transforming activities within the v-fps oncoprotein (P130gag-fps). Partial proteolysis of P130gag-fps and supporting genetic data indicated that the v-fps SH2 domain exerts its effect on catalytic activity through an intramolecular interaction with the kinase domain. Amino acid alterations in the SH2 domain that impaired kinase function interfered with association of the SH2 domain with the kinase domain. Deletion of a conserved octapeptide motif converted the v-fps SH2 domain from an activator to an inhibitor of tyrosine kinase activity. This latent inhibitory activity of v-fps SH2 has functional implications for phospholipase C-gamma and p21ras GTPase-activating protein, both of which have two distinct SH2 domains suggestive of complex regulation. In addition to regulating the specific activity of the kinase domain, the SH2 domain of P130gag-fps was also found to be required for the tyrosine phosphorylation of specific cellular proteins, notably polypeptides of 124 and 62 kilodaltons. The SH2 domain therefore appears to play a dual role in regulation of kinase activity and recognition of cellular substrates.