Interaction of calmodulin and its fragments with Ca2+-ATPase and myosin light chain kinase

The binding of smooth muscle myosin light chain kinase (MLCK) and erythrocyte membrane Ca2+-ATPase to calmodulin (CM), or calmodulin fragments was investigated using CM-, or CM fragment-affinity column chromatography. Calmodulin fragments corresponding to amino acid residues 1-77 (TR1-C), 78-148 (TR2-C) and 107-148 (TR3-E) were used. The ability of calmodulin fragments to activate these enzymes was also studied. Fragments TR1-C and TR2-C were able to bind to Ca2+-ATPase but only TR2-C stimulated its activity. Only the TR2-C fragment bound MLCK but failed to activate this enzyme at the molar excess sufficient for activation of Ca2+-ATPase. These results suggest a different mode of calmodulin interaction with Ca2+-ATPase and MLCK.