SYNAPSIN-I INTERACTS WITH C-SRC AND STIMULATES ITS TYROSINE KINASE-ACTIVITY

Synapsin I is a synaptic vesicle-associated phosphoprotein that has be en implicated in the formation of presynaptic specializations and in t he regulation of neurotransmitter release, The nonreceptor tyrosine ki nase c-Src is enriched on synaptic vesicles, where it accounts for mos t of the vesicle-associated tyrosine kinase activity, Using overlay, a ffinity chromatography, and coprecipitation assays, we have nom shown that synapsin I is the major binding protein for the Src homology 3 (S H3) domain of c-Src in highly purified synaptic vesicle preparations, The interaction was mediated by the proline-rich domain D of sgnapsin I and was not significantly affected by stoichiometric phosphorylation of synapsin I at any of the known regulatory sites, The interaction o f purified c-Src and synapsin I resulted in a severalfold stimulation of tyrosine kinase activity and was antagonized by the purified c-Src- SH3 domain, Depletion of synapsin I from purified synaptic vesicles re sulted in a decrease of endogenous tyrosine kinase activity, Portions of the total cellular pools of synapsin I and Src were coprecipitated from detergent extracts of rat brain synaptosomal fractions using anti bodies to either protein species. The interaction between sgnapsin I a nd c-Src, as well as the synapsin I-induced stimulation of tyrosine ki nase activity, may be physiologically important in signal transduction and in the modulation of the function of axon terminals, both during synaptogenesis and at mature synapses.