CROSS-BRIDGE MOVEMENT IN GLYCERINATED RABBIT PSOAS MUSCLE-FIBERS

The results of the present study demonstrate that the cross-linking properties of the S-1 and S-2 segments of myosin in glycerinated rabbit psoas muscle fibers are virtually identical with those observed in synthetic myosin filaments. Both systems show rapid cross-linking of the S-1 subunits to the thick filament surface at neutral pH and both exhibit a sharp decrease in the normalized rate constant (ks-1o) of cross-linking S-1 to the filament surface over the range pH 7.4-8.0. The most probable explanation of these results (Sutoh K., et al. (1978) Biochemistry 17, 1234-1239) is that the myosin heads in both preparations are in close contact with the filament backbone at neutral pH but are dissociated from the backbone at alkaline pH as a result of electrostatic repulsion between the proximal surfaces. The S-1 cross-linking rate also decreases in the glycerinated fiber system at neutral pH and at very low ionic strength (~0.015 M). When E. Rome's earlier ((1967) J. Mol. Biol. 27, 591-602) X-ray measurements of the filament lattice dimensions of glycerinated fibers in high pH and neutral pH-low ionic strength buffers are combined with our cross-linking results, we estimate a minimum length of ~18.5 nm for the S-1 subunit. Although the actin-attached S-1 subunit in the glycerinated fibers moves out from the thick filament surface at alkaline pH, a major fraction of the S-2 region of myosin appears to remain close to the thick filament surface based on the similar cross-linking rates of the heavy and light meromyosin segments. A significant tendency for heavy meromyosin segments to associate with the thick filament backbone was also observed in ultracentrifuge studies of tryptic digests of synthetic myosin filaments. © 1979, American Chemical Society. All rights reserved.