A conformational transition in gizzard heavy meromyosin involving the head-tail junction, resulting in changes in sedimentation coefficient, ATPase activity, and orientation of heads

Gizzard heavy meromyosin (HMM) sediments in the ultracentrifuge as a single peak, whose sedimentation coefficient (S20,w) decreases from 9 to 7.5 S upon increasing the NaCl concentration from 0.02 to 0.3 M. This decrease is accompanied by a parallel increase in Mg2+-ATPase activity, suggesting that both changes have a common molecular basis. Phosphorylation decreases S20,w and increases ATPase activity, while ATP increases S20,w. Sedimentation equilibrium studies indicate that HMM undergoes no detectable aggregation at 0.02 or 0.4 M NaCl, remaining monomeric with a molecular weight of 3.4 X 10(5). In contrast, S20,w of subfragment 1 does not change with changes in ionic strength, and its ATPase activity does not decrease at low ionic strengths. Electron micrographs of samples of HMM prepared at low ionic strength show that up to half of the molecules are flexed, i.e. the heads are bent at the neck and project back toward the tail, while the remaining molecules have either one or both of the heads pointing away from the tail. In samples prepared at high ionic strength only about 10% of the molecules are flexed. There is a linear relationship between the fraction of flexed molecules and S20,w, with no significant bending or folding of the tail and no detectable change in the shape of the heads. This correlation suggests that the changes in ATPase activity and S20,w may be a result of the reorientation of the heads.