CA2-DEPENDENT INTERACTION WITH CALMODULIN IS CONSERVED IN THE SYNAPSIN FAMILY - IDENTIFICATION OF A HIGH-AFFINITY SITE()

The synapsins are a family of proteins associated with small synaptic vesicles that are implicated in synaptic maintenance and in the supply of vesicles for exocytosis. They are well characterized as substrates for protein kinases, and one class of synapsin, synapsin I, has been shown to bind, and be regulated by, calmodulin. A representative of th e synapsin II class is now shown to bind calmodulin. Optical biosensor assays of Ca2+-dependent calmodulin binding to recombinant rat synaps in IIb indicated an apparent K-D for calmodulin of 31 +/- 5 nM. Phosph orylation at Ser 10 increased the rates of calmodulin association (by a factor of 10) and dissociation (by a factor of 20). Fragment analysi s and predictions from the sequence indicated two potential calmodulin binding sequences in the conserved central (C) domain. Peptides repre senting these sequences (residues 122-143 and 313-334 in synapsin IIb) were synthesized. Peptide 122-143 was found to bind calmodulin (K-D 3 2 +/- 10 nM) and inhibit interaction of synapsin IIb with calmodulin. The interaction of peptide 313-334 was much weaker. Sequences similar to residues 122-143 are present in all published synapsin sequences. C almodulin binding by synapsins seems not to be confined to mammals: a recombinant Drosophila synapsin 1 fragment containing part of the C-do main showed Ca2+-dependent binding to mammalian calmodulin. We conclud e that calmodulin binding to synapsins is likely to be a general aspec t of regulation of synaptic function.