1988 Volume 104 Issue 6 Pages 1027-1035
The physical properties of human platelet myosin in crude extracts were studied by means of Sepharose 4B gel filtration and sucrose density gradient centrifugation in the presence or absence of Mg-ATP. Platelet myosin extracted with a buffer containing 0-0.15 M KCl gave a Stokes radius of about 12.0-12.5 nm irrespective of the presence or absence of Mg-ATP. The sedimentation coefficients obtained in the presence of Mg-ATP were about 10-11 and 8.5S at 0.05-0.10 and 0.15 M KCl, respectively, whereas the values obtained in the absence of Mg-ATP were about 16, 9-12, and 8.5S at 0.05, 0.10, and 0.15 M KCl, respectively. The apparent molecular weight in the presence of Mg-ATP, therefore, was about 500, 000 and 420, 000 at 0.05-0.10 and 0.15 M KCl, respectively, while the molecular weight in the absence of Mg-ATP was about 790, 000, 460, 000-620, 000, and 440, 000 at 0.05, 0.10, and 0.15 M KCl, respectively. The purified monomeric platelet myosin that had been solubilized with Mg-ATP at 0.10 M KCl had a Stokes radius of about 12.5 nm, a sedimentation coefficient of about 9S, and an apparent molecular weight of 460, 000. On the other hand, while crude platelet myosin extracted at 0.6 M KCl with Mg-ATP gave a Stokes radius of about 20 nm, a sedimentation coefficient of about of 6S, and an apparent molecular weight of about 490, 000, each of these physical parameters obtained in the absence of Mg-ATP was much larger than that obtained in the presence of Mg-ATP because the myosin was associated with F-actin. In contrast, the Stokes radius, sedimentation coefficient, and molecular weight of purified monomeric platelet myosin at 0.6 M KCl were about 18.5 nm, 6S, and 460, 000, respectively, irrespective of the presence or absence of Mg-ATP. These results indicate that platelet myosin extracted at physiological ionic strength is mostly a partially folded monomeric molecule, that it dimerizes on decrease of ionic strength in the absence of Mg-ATP, and that it takes on an extended conformation on increase of ionic strength, and then associates with actin in the absence of Mg-ATP.
