Ultrastructure of thrombosthenin, the contractile protein of human blood platelets.

Partially purified thrombosthenin with adenosine triphosphatase activity similar to that of actomyosin was subjected to electron microscopy. More than 50 percent of the material consisted of fibrils 80 to 100 angstroms in width. Occasional fibrils suggested a periodic structure. The morphology of isolated thrombosthenin resembled that of microfibrils in the cytoplasm and pseudopods of intact platelets.