The endocytic protein intersectin is a major binding partner for the Ras exchange factor mSos1 in rat brain

We recently identified intersectin, a protein containing two EH and five SH 3 domains, as a component of the endocytic machinery. The N-terminal SH3 do main (SH3A), unlike other SH3 domains from intersectin or various endocytic proteins, specifically inhibits intermediate events leading to the formati on of clathrin-coated pits. We have now identified a brain-enriched, 170 kD a protein (p170) that interacts specifically with SH3A. Screening of combin atorial peptides reveals the optimal ligand for SH3A as Pp(V/I)PPR, and the 170 kDa mammalian son-of-sevenless (mSos1) protein, a guanine-nucleotide e xchange factor for Ras, contains two copies of the matching sequence, PPVPP R. Immunodepletion studies confirm that p170 is mSos1. Intersectin and mSos 1 are co-enriched in nerve terminals and are co-immunoprecipitated from bra in extracts. SH3A competes with the SH3 domains of Grb2 in binding to mSos1 , and the intersectin-mSos1 complex can be separated from Grb2 by sucrose g radient centrifugation, Overexpression of the SH3 domains of intersectin bl ocks epidermal growth factor-mediated Ras activation. These results suggest that intersectin functions in cell signaling in addition to its role in en docytosis and may link these cellular processes.