INTERACTION OF GRB2 VIA ITS SRC HOMOLOGY-3 DOMAINS WITH SYNAPTIC PROTEINS INCLUDING SYNAPSIN-I

Grb2 is a 25-kDa adaptor protein composed of a Src homology 2 (SH2) do main and two flanking Src homology 3 (SH3) domains. One function of Gr b2 is to couple tyrosine-phosphorylated proteins (through its SH2 doma in) to downstream effecters (through its SH3 domains). Using an overla y assay, we have identified four major Grb2-binding proteins in synapt ic fractions. These proteins interact with wild type Grb2 but not with Grb2 containing point mutations in each of its two SH3 domains corres ponding to the loss of function mutants in the Caenorhabditis elegans Grb2 homologue sem-5. Two of the proteins, mSos and dynamin, were prev iously shown to bind Grb2. The third protein of 145 kDa is brain speci fic and to our knowledge has not been previously described. The fourth protein is synapsin I. Dynamin is required for synaptic vesicle endoc ytosis and synapsin I is thought to mediate the interaction of synapti c vesicles with the presynaptic cytomatrix. These data suggest that Gr b2, or other proteins containing SH3 domains, may play a role in the r egulation of the exo/endocytotic cycle of synaptic vesicles and theref ore of neurotransmitter release.