The effect of synapsin I phosphorylation upon binding of synaptic vesicles to spectrin

We have previously demonstrated that brain spectrin is attached to small spherical synaptic vesicles via synapsin I. These studies utilized a novel microfiltration assay in which 125I-labelled synaptic vesicles were incubated with brain spectrin which was covalently attached to cellulosic membranes. In these studies purified dephosphosynapsin I was demonstrated to competitively inhibit the binding of the synaptic vesicles to the immobilized brain spectrin with a K I = 45 nM. In the current study we demonstrate that phosphorylation of synapsin I site 1 (0.74 mol P i/mol synapsin I) with cAMP-dependent protein kinase and sites 2 and 3 (2.0 mol P i/mol synapsin I) with Ca 2+-calmodulin kinase II had little effect upon its interaction with brain spectrin. cAMP-dependent protein kinase phosphorylated synapsin I and Ca 2+-calmodulin kinase II phosphorylated synapsin I both inhibited the binding of 125I-labelled synaptic vesicles to immobilized brain spectrin with a K I of 23 nM and 34 nM respectively. We conclude that phosphorylation of synapsin I does not down-regulate the interaction of synaptic vesicles with brain spectrin.