MORPHOLOGICAL AND PHYSICOCHEMICAL CHANGES IN THE MYOSIN MOLECULES INDUCED BY HYDROSTATIC-PRESSURE

Hydrostatic pressure-induced morphological and physicochemical changes in monomeric myosin were investigated. The turbidity of a myosin solution increased after release of pressure, indicating aggregation of the molecules. Some molecules were single headed after exposure to 100-200 MPa, in contrast with an intact molecule having two heads. Small-sized oligomeric species, composed of several molecules, appeared after pressurization at 200 MPa. The oligomers were formed only through head-to-head association. Neither head-to-tail nor tail-to-tail interaction was observed. With increasing pressure to 300 MPa, monomeric myosin remarkably decreased, and most molecules formed oligomers, in which the myosin heads were tightly associated, forming a clump, the tails of the myosin molecules extending radially from the clump. Such an oligomer was shaped like a daisy wheel and its morphology was quite similar to that formed on heating reported previously [Yamamoto et al. (1990) J. Biochem. 108, 896-898]. The aggregation of myosin molecules upon pressurization was concomitant with an increase in hydrophobicity, which was measured spectrofluorometrically with 8-anilino-1-naphthalene sulfonate, a probe for apolar binding sites. Although the turbidity increased continuously with increasing pressure, the hydrophobicity remained at a constant level above 200 MPa at pH 6.0 and above 300 MPa at pH 7.0. The loss of myosin ATPase activity was accompanied by aggregation of the molecules. These results indicate that hydrophobic groups in the heads of myosin are exposed to the surface of the molecule on pressurization, so that hydrophobic interaction among the heads occurs, yielding aggregates. Beside the hydrophobic interaction, the contribution of other interaction(s) is also suggested.