Effect of calcium on protein composition of human platelet cytoskeletons

Triton X-100 residues (cytoskeletons) of human platelets were prepared in the presence of various concentrations of free calcium (Ca2+), and the polypeptide composition and ATPase activity were examined. Triton residues prepared in the presence of Ca2+ concentrations below 2 X 10(-7) M were composed primarily of polypeptides with an apparent molecular mass of 43 (actin), 105 (alpha-actinin-like protein) and 250 (actin-binding protein) kDa and showed low K+-EDTA-ATPase activity. When Triton residues were prepared at Ca2+ above 5 X 10(-7) M, a 200 kDa polypeptide (myosin heavy chain) and K+-EDTA-ATPase activity increased markedly, but actin-binding protein and alpha-actinin-like protein decreased. When N-(N-(L-3-trans-carboxyoxirane-2-carbonyl)-L-leucyl)agmatine, an inhibitor for Ca2+-dependent proteinase, was added to Triton lysis buffer containing high Ca2+, polypeptides of 250, 235 and 105 kDa remained associated with the residues. Under electron microscopic analysis, the treatment of platelets with Triton X-100 at low Ca2+ showed a network of microfilaments. When platelets were treated with high Ca2+, the microfilaments were disrupted and a few thick filaments and many granules appeared. However, when the inhibitor for Ca2+-proteinase was included in Triton lysis buffer, the microfilaments remained intact. These results suggested that an increase in Ca2+ concentration to more than 5 X 10(-7) M not only makes myosin associate with cytoskeletons but also regulates the organization of filamentous structures.