Specificity of the binding of synapsin I to Src homology 3 domains

Synapsins are synaptic vesicle-associated phosphoproteins involved in synap se formation and regulation of neurotransmitter release. Recently, synapsin I has been found to bind the Src homology 3 (SH3) domains of Grb2 and c-Sr c. In this work we have analyzed the interactions between synapsins and an array of SH3 domains belonging to proteins involved in signal transduction, cytoskeleton assembly, or endocytosis. The binding of synapsin I was speci fic for a subset of SH3 domains. The highest binding was observed with SH3 domains of c-Src, phospholipase C-gamma, p85 subunit of phosphatidylinosito l 3-kinase, full-length and NH2-terminal Grb2, whereas binding was moderate with the SH3 domains of amphiphysins I/II, Crk, alpha-spectrin, and NADPH oxidase factor p47(phox) and negligible with the SH3 domains of p21(ras) GT Pase-activating protein and COOH-terminal Grb2. Distinct sites in the proli ne-rich COOH-terminal region of synapsin I were found to be involved in bin ding to the various SH3 domains. Synapsin II also interacted with SH3 domai ns with a partly distinct binding pattern. Phosphorylation of synapsin I in the COOH-terminal region by Ca2+/calmodulin-dependent protein kinase II or mitogen-activated protein kinase modulated the binding to the SH3 domains of amphiphysins I/II, Crk, and alpha-spectrin without affecting the high af finity interactions. The SH3-mediated interaction of synapsin I with amphip hysins affected the ability of synapsin I to interact with actin and synapt ic vesicles, and pools of synapsin I and amphiphysin I were shown to associ ate in isolated nerve terminals. The ability to bind multiple SH3 domains f urther implicates the synapsins in signal transduction and protein-protein interactions at the nerve terminal level.