Isolation and characterization of myosin and two myosin fragments from human blood platelets.

Platelet myosin (thrombosthenin M) and two additional proteins corresponding to the head and rod portion of the myosin molecule have been prepared from human blood platelets. Characterization of these proteins by SDS-polyacrylamide gel electrophoresis, actin binding studies, assay of enzymic ATPase activity, and electron microscopy has shown that the platelet contractile proteins closely resemble the corresponding muscle proteins. Platelet myosin and platelet myosin-head bind to both muscle and platelet actin and have an EDTA + K-stimulated ATPase activity, which is suppressed by Mg(2+) in high salt concentration, whereas platelet rod does not possess either of these properties; platelet myosin and platelet myosin rod aggregate to form thick filaments at low ionic strength. Both intact platelet myosin and myosin head form typical arrowhead-shaped complexes with either platelet or muscle F-actin.