COORDINATED REGULATION OF SYNAPSIN-I INTERACTION WITH F-ACTIN BY CA2+ CALMODULIN AND PHOSPHORYLATION - INHIBITION OF ACTIN-BINDING AND BUNDLING

The synapsins are a family of synaptic vesicle phosphoproteins whose role seems to be to limit the availability of small synaptic vesicles for exocytosis by Linking them to the cytoskeleton. One member of the family, synapsin I, has been shown to bind calmodulin in a Ca2+-dependent manner. In this study, we have examined whether or not calmodulin can regulate one of the activities of synapsin I, namely, its interaction with F-actin. Synapsin I is an actin bundling protein: this activity is controlled by phosphorylation. Here we show that calmodulin in the presence of Ca2+ is a competitive inhibitor of both actin binding and bundling by synapsin I. Under the conditions of our assay (0.45 mu M synapsin I, 4 mu M F-actin), half-maximal inhibiton of actin binding and bundling by unphosphorylated synapsin I was found with 4.3 and 3.7 mu M calmodulin, respectively. The actin binding activity of synapsin I phosphorylated by cAMP-dependent protein kinase or by calmodulin-dependent protein kinase II showed similar sensitivity to calmodulin inhibition to unphosphorylated synapsin I. However, inhibition of bundling was potentiated. Half-maximal inhibition of bundling by synapsin I phosphorylated by cAMP-dependent kinase was achieved at approximately 0.5 mu M calmodulin. Half-maximal inhibition of bundling by synapsin I phosphorylated by calmodulin-dependent protein kinase II was achieved at less than 0.2 mu M calmodulin, although the maximum binding under the conditions of the assay was lower. Synapsin I phosphorylated by both cAMP-dependent kinase and calmodulin-dependent protein kinase II was half-maximally inhibited from actin binding by less than 100 nM calmodulin, although the binding and bundling characteristics of this form of synapsin I were weak even in the absence of calmodulin. We conclude that covalent (phosphorylation) and noncovalent (calmodulin) forms of regulation act together in regulating one of the activities of synapsin I.