Interaction of calmodulin with synaptic plasma membranes isolated from sheep brain

Calmodulin binding to a membrane fraction enriched in synaptic plasma membranes of sheep brain cells was investigated with [(125)I]calmodulin. Calmodulin binding to these membranes is Ca(2+)-dependent with a half maximal saturation at the pCa value of about 5.5. The binding is reduced by replacing Ca(2+) with Mg(2+), but it is significantly enhanced when both cations are present in the medium. Cation-dependent binding is specific and saturable with an apparent K(D) of about 47-50 nM and a maximal capacity of about 4 pmol mg(?1) protein. The results indicate that synaptic plasma membranes isolated from sheep brain cells interact with calmodulin in a Ca(2+)-dependent, Mg(2+)-facilitated manner.