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 Å. 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) × 107 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 × 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.
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