Homo- and heterodimerization of synapsins

In vertebrates, synapsins constitute a family of synaptic vesicle proteins encoded by three genes. Synapsins contain a central ATP-binding domain, the C-domain, that is highly homologous between synapsins and evolutionarily c onserved in invertebrates. The crystal structure of the C-domain from synap sin I revealed that it constitutes a large (>300 amino acids), independentl y folded domain that forms a tight dimer with or without bound ATP, We now show that the C-domains of all synapsins form homodimers, and that in addit ion, C-domains from different synapsins associate into heterodimers, This c onclusion is based on four findings: 1) in yeast two-hybrid screens with fu ll-length synapsin IIa as a bait, the most frequently isolated prey cDNAs e ncoded the C-domain of synapsins; 2) quantitative yeast two-hybrid protein- protein binding assays demonstrated pairwise strong interactions between al l synapsins; 3) immunoprecipitations from transfected COS cells confirmed t hat synapsin II heteromultimerizes with synapsins I and III in intact cells , and similar results were obtained with bacterial expression systems; and 4) quantification of the synapsin III level in synapsin I/II double knockou t mice showed that the level of synapsin III is decreased by 50%, indicatin g that heteromultimerization of synapsin III with synapsins I or II occurs in vivo and is required for protein stabilization. These data suggest that synapsins coat the surface of synaptic vesicles as homo- and heterodimers i n which the C-domains of the various subunits have distinct regulatory prop erties and are flanked by variable C-terminal sequences. The data also impl y that synapsin III does not compensate for the loss of synapsins I and II in the double knockout mice.