Heavy meromyosin (HMM) undergoes a conformational transition between a rapidly and a slowly sedimenting form, during which it sediments as a single peak in the ultracentrifuge with sedimentation coefficients between 7.5 and 9S. Changes in sedimentation velocity and ATPase activity produced by changes in ionic strength, phosphorylation of HMM or addition of MgATP are interpreted in terms of equilibria between the rapidly and slowly sedimenting forms, the observed values of activity and sedimentation velocity being determined by the fraction of HMM in each form. Phosphorylation of the 20 kDa light chain or raising the ionic strength decrease the sedimentation velocity, by decreasing the fraction of HMM in the rapidly sedimenting form, while addition of ATP increases sedimentation velocity upon forming a 9S HMM-ADP-Pi complex. Electron microscopic studies support this interpretation showing the presence of two distinct conformations of HMM--extended and flexed, which correspond to the 7.5S and 9S forms, respectively (Suzuki et al., 1985). In samples prepared at high ionic strengths, the heads extend away from the tail in a more or less random orientation, while at low ionic strength, the molecule is flexed at the head-tail junction assuming a more compact structure, that appears to account for its more rapid sedimentation rate. The degradation rates of the heavy chain and the 20 kDa light chain of HMM on digestion with papain indicate the presence of three forms of HMM differing in their susceptibility to papain. At 25 mM NaCl, HMM is rapidly digested in the absence of ATP, while addition of ATP decreases digestibility by a factor of ten, upon formation of a complex of HMM with the products of ATP hydrolysis. Above 0.4 M NaCl, HMM is degraded at an intermediate rate that is not affected by ATP. When the ionic strength is varied, the rate of disappearance of the heavy chain depends linearly on the sedimentation velocity in both the phosphorylated and dephosphorylated states, indicating that the rate of proteolysis is determined primarily by the fraction of HMM in the rapidly and slowly sedimenting forms. The same pattern is seen in the disappearance of the 20 kDa light chain of dephosphorylated HMM on cleavage at a site 4 kDa from the N-terminus, indicating that the cleavage of the light chain also depends on the fraction of HMM in the rapidly and slowly sedimenting forms.(ABSTRACT TRUNCATED AT 400 WORDS)