title: Homo- and heterodimerization of synapsins.
abstract: 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 conserved in invertebrates. The crystal structure of the C-domain from synapsin I revealed that it constitutes a large (>300 amino acids), independently 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 addition, C-domains from different synapsins associate into heterodimers. This conclusion is based on four findings: 1) in yeast two-hybrid screens with full-length synapsin IIa as a bait, the most frequently isolated prey cDNAs encoded the C-domain of synapsins; 2) quantitative yeast two-hybrid protein-protein binding assays demonstrated pairwise strong interactions between all synapsins; 3) immunoprecipitations from transfected COS cells confirmed that 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 knockout mice showed that the level of synapsin III is decreased by 50%, indicating 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 in which the C-domains of the various subunits have distinct regulatory properties and are flanked by variable C-terminal sequences. The data also imply that synapsin III does not compensate for the loss of synapsins I and II in the double knockout mice.
authors.affiliation: Center for Basic Neuroscience and Department of Molecular Genetics, Howard Hughes Medical Institute, The University of Texas Southwestern Medical School, Dallas, Texas 75235, USA.
authors.forename: M
authors.initials: M
authors.lastname: Hosaka
authors.forename: T C
authors.initials: TC
authors.lastname: Südhof
mesh_terms: D000818:Animals; D000918:Antibody Specificity; D001923:Brain Chemistry; D003001:Cloning, Molecular; D019281:Dimerization; D051379:Mice; D018345:Mice, Knockout; D011485:Protein Binding; D020033:Protein Isoforms / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics; D016704:Synapsins / Q000235:genetics / Q000276:immunology / Q000378:metabolism*
chemical_list: D020033:Protein Isoforms; D011994:Recombinant Proteins; D016704:Synapsins