Selective Ca2(+)-dependent interaction of calmodulin with the head domain of synapsin 1

The calcium-dependent regulatory protein calmodulin is a critical element in the machinery regulating exocytosis at nerve terminals. Okabe & Sobue [(1987) FEBS Lett. 213, 184-188] showed that calmodulin interacts with one of the proteins intimately connected with the neuronal exocytotic process, i.e. synapsin 1. We have investigated the site at which calmodulin interacts with synapsin 1. We find that it is possible to generate chemically cross-linked Ca2(+)-dependent complexes between synapsin 1 and calmodulin in vitro, and have used covalent cross-linking in conjunction with calmodulin affinity chromatography to identify fragments of synapsin 1 that interact with calmodulin. Ca2(+)-dependent calmodulin binding is restricted to the 'head' domain (residues 1-453 in bovine synapsin 1). Within this domain the binding site is located in a unique 11 kDa Staphylococcus aureus V8 proteinase generated fragment. This fragment does not contain the site for cyclic-AMP-dependent phosphorylation and therefore does not represent the N-terminus of the protein.