A Major Calmodulin-Binding Protein Common to Various Vertebrate Tissues

A major calmodulin-binding protein (CaM-BP) of Mr240,000 was demonstrated in various rat tissues by using a125I-labeled CaM gel overlay technique. This protein (designated p240) was detected in the particulate fraction and to a lesser extent in the cytosol of all tissues studied. Binding of CaM to p240 was completely dependent on Ca2+. A second, exclusively soluble, CaM-BP (Mr115,000 common to several tissues and a number of other CaM-BPs with a more restricted tissue distribution were also observed by using this technique. CaM binding to p240 occurred in high amounts in plasma membranes from avian erythrocytes but was absent from mammalian erythrocyte membranes. Antibodies prepared against turkey erythrocyte p240 (anti-Tp240) crossreacted with p240 in other tissues. Identity between the proteins recognized by anti-Tp240 and CaM was confirmed by demonstrating that125I-labeled CaM could bind to p240 specifically immunoprecipitated from either rat brain or turkey erythrocytes by anti-Tp240. The p240 may be related to a previously described actin-binding protein and may represent a major site of action of CaM on the cytoskeleton.