S100A1 codistributes with synapsin I in discrete brain areas and inhibits the F-actin-bundling activity of synapsin I

The Ca2+-sensor protein S100A1 was recently shown to bind in vitro to synapsins, a family of synaptic vesicle phosphoproteins involved in the regulation of neurotransmitter release. In this paper, we analyzed the distribution of S100A1 and synapsin I in the CNS and investigated the effects of the S100A1/synapsin binding on the synapsin functional properties. Subcellular fractionation of rat brain homogenate revealed that S100A1 is present in the soluble fraction of isolated nerve endings. Confocal laser scanning microscopy and immunogold immunocytochemistry demonstrated that S100A1 and synapsin codistribute in a subpopulation (5-20%) of nerve terminals in the mouse cerebral and cerebellar cortices. By forming heterocomplexes with either dephosphorylated or phosphorylated synapsin I, S100A1 caused a dose- and Ca2+-dependent inhibition of synapsin-induced F-actin bundling and abolished synapsin dimerization, without affecting the binding of synapsin to F-actin, G-actin or synaptic vesicles. These data indicate that: (i) synapsins and S100A1 can interact in the nerve terminals where they are coexpresssed; (ii) S100A1 is unable to bind to SV-associated synapsin I and may function as a cytoplasmic store of monomeric synapsin I; and (iii) synapsin dimerization and interaction with S100A1 are mutually exclusive, suggesting an involvement of S100A1 in the Ca2+-dependent regulation of synaptic vesicle trafficking.