Structural properties of myosin from the particulate fraction of human blood platelets

Fractionation of human blood platelets has revealed that myosin, a contractile and mechanochemical protein, is present in both the soluble and particulate fraction. The aim of this study was to elucidate whether platelets contain more than one myosin isoform, especially in view of the fact that in other cellular systems (cardiac muscle, amoeba) several myosin isoenzymes were found. The particulate fraction was solubilized by Triton X-100, and the myosin was purified by the same procedure used for the cytoplasmic myosin. The final preparation contained, in addition to myosin, a 130-kDa polypeptide, which was observed also in myosin preparations obtained from the soluble fraction. The electrophoretic mobilities of the two myosins were identical under both dissociating and nondissociating conditions. Comparison of the molecular structure of the heavy chain of the two myosins by limited proteolysis with Staphylococcus aureus V8 protease showed that the proteolytic fragments of the two myosins were rather similar, with only minor alterations in the quantitative distribution of the products. Two-dimensional peptide mapping of the iodinated tryptic peptides of the myosin heavy chains indicated that at least one peptide is missing in the map of the particulate myosin, as compared to its soluble counterpart. According to the two-dimensional peptide map, the 130-kDa polypeptide seems to be a proteolytic fragment of the myosin heavy chain and most probably the rod portion of the molecule. The observed minor variations in the structure of myosins isolated from the soluble and the fractions of human platelets may reflect differences in their respective physiological functions.