BIOCHEMICAL AND GENETIC-ANALYSIS OF THE DRK SH2 SH3 ADAPTER PROTEIN OF DROSOPHILA/

The Drk SH3-SH2-SH3 adaptor protein has been genetically identified in a screen for rate-limiting components acting downstream of the Sevenl ess (Sev) receptor tyrosine kinase in the developing eye of Drosophila , It provides a link between the activated Sev receptor and Sos, a gua nine nucleotide release factor that activates Ras1. We have used a com bined biochemical and genetic approach to study the interactions betwe en Sev, Drk and Sos. We show that Tyr2546 in the cytoplasmic tail of S ev is required for Drk binding, probably because it provides a recogni tion site for the Drk SH2 domain. Interestingly, a mutation at this si te does not completely block Sev function in vivo. This may suggest th at Sev can signal in a Drk-independent, parallel pathway or that Drk c an also bind to an intermediate docking protein. Analysis of the Drk-S os interaction has identified a high affinity binding site for Drk SH3 domains in the Sos tail. We show that the N-terminal Drk SH3 domain i s primarily responsible for binding to the tail of Sos in vitro, and f or signalling to Ras in vivo.