Platelet myosin. Localization of the rod myosin fragment and effect of its antibodies on platelet function.

Partial hydrolysis of human platelet or uterine smooth muscle myosin with insoluble papain yielded rod fragments, insoluble at low ionic strength and devoid of ATPase activity. This protein migrated on sodium dodecyl sulfate-8 M urea gel electrophoresis as a single band with a molecular weight of approximately 110,000 to 120,000 corresponding to the rod portion of myosin. Rabbit antibodies to platelet and uterine rod myosin were highly specific by the criteria of double-gel diffusion and immunoelectrophoresis; they reacted with a single precipitin line against actomyosin and rod myosin from their respective tissue sources, but failed to cross-react. Antibodies to platelet rod myosin inhibited superprecipitation of platelet actomyosin but did not inhibit its ATPase activity. The surface of blood platelets was stained by an indirect immunofluorescence technique with antiserum to human platelet rod myosin but not with antiserum to uterine rod myosin. Fluorescence was never intense, suggesting that there are not many myosin molecules in the platelet membrane or that few binding sites are available to the outer surface. Anti-platelet rod myosin inhibited clot retraction produced in titrated platelet-rich plasma by an enzyme from the venom of Bothrops atrax (reptilase) plus ADP or by thrombin. Anti-uterine rod myosin was not inhibitory. Anti-platelet rod myosin induced platelets to change their shape from disc to spiny spheres. It did not induce platelet aggregation but inhibited ADP-, collagen-, and epinephrine-induced aggregation as well as epinephrineand collagen-induced [‘%lserotonin release. These results suggest that platelet membrane actomyosin is implicated in platelet aggregation and that binding of the antibody may immobilize the platelet membrane myosin, thus preventing the triggering event at the membrane level which leads to aggregation, secretion, and contraction of platelets.