Cadmium-113 nuclear magnetic resonance studies of proteolytic fragments of calmodulin: assignment of strong and weak cation binding sites.

Proteolytic fragments of bovine testis calmodulin were obtained by limited proteolysis with trypsin or thrombin. Cadmium-113 NMR studies showed that the tryptic fragment encompassing Ca2+ binding domains III and IV (TR2C) gave rise to a spectrum identical with that of the native protein. Two thrombic fragments containing either domains I, II, and III [TM1-(1-106)] or the single domain IV [TM2-(107-148)] both gave rise to one broad resonance only. These data indicate that domains III and IV comprise the two high-affinity Ca2+ binding sites in intact calmodulin and that disturbance of the structural relationship between domain III and domain IV markedly reduces the affinity of these two sites for Ca2+ ions. These observations are discussed with respect to other published accounts concerning the sequence in which the four calcium domains in calmodulin are filled.