Isolation and properties of platelet myosin light chain kinase

A protein kinase which phosphorylates the 20 000-dalton light chain of platelet myosin has been isolated from human blood platelets and purified approximately 600-fold. Elution of a 7.5% polyacrylamide gel following electrophoresis of the partially purified enzyme yielded a single peak of kinase activity which could be aligned with a protein band on a stained gel. Assuming a globular shape, a native molecular weight of 83 000 (+/- 10%) was determined by gel filtration on Bio-Gel P-200. The kinase requires Mg2+ for activity and is not sensitive to the removal of trace Ca2+. The enzyme purified from human platelets phosphorylates the 20 000-dalton light chain of mouse fibroblast and chicken gizzard myosin, but does not phosphorylate human skeletal and cardiac myosin.