Interaction of calmodulin with skeletal muscle myosin light chain kinase

Studies on myosin light chain kinase isolated from rabbit skeletal muscle show that the enzyme has a molecular weight of 80,000--84,000 with a sedimentation coefficient of 3.2 S and an apparent Stokes radius of 53 A. Gel filtration chromatography with a 3H-labeled calmodulin using a Hummel--Dryer technique shows that the enzyme will bind 1 mol of calmodulin per mol of enzyme, with an affinity of (1.9 +/- 0.5) x 10(7) M-1 in the absence of substrate. The calmodulin dependence of enzyme activation at limiting Mg2+ and light chain concentrations confirms this observation. The calcium dependence of activation of the enzyme--calmodulin complex is characterized by a Hill coefficient of 2.5, with half-activation occurring at 6.6 x 10(-7) M Ca2+. The amino acid composition shows a high percentage (9.1%) of proline, which may account for the large apparent Stokes radius and no clear resemblance to other skeletal muscle proteins. A comparison of the amino acid composition with that from turkey gizzard shows some resemblance.