Ca2+-calmodulin binding to mouse alpha1 syntrophin: syntrophin is also a Ca2+-binding protein

Syntrophins are peripheral membrane proteins which have been found associated with dystrophin, the protein product of the Duchenne muscular dystrophy gene locus. Mouse alpha1 syntrophin binds the COOH-terminal domain of dystrophin, and calmodulin inhibits this interaction in a Ca2+-dependent fashion. Where calmodulin binds to syntrophin was investigated by constructing fusion proteins containing different regions of syntrophin's sequence. Syntrophin contains at least two regions which bind calmodulin in different ways. The COOH-terminal 24 residues contain a Ca2+-calmodulin binding site, named CBS-C, which binds calmodulin with an apparent affinity of 18 nM and which is highly conserved in all syntrophins. The amino-terminal 174 residue section of syntrophin contains other calmodulin binding, and binding occurs in either the presence or absence of Ca2+ with an apparent affinity of 100 nM. Syntrophin was shown to bind Ca2+ at two or more sites residing in the amino-terminal 274 residues, and Ca2+ binding to syntrophin affects calmodulin binding at high concentrations of syntrophin. Syntrophin A (residues 4-274) is predominantly a dimer in EGTA. A model of syntrophin's complex interactions with itself (i.e., oligomerization), calmodulin, and Ca2+ is presented.