Rigor cross-bridges bind to two actin monomers in thin filaments of rabbit psoas muscle

The mode of binding of myosin subfragment-1 (S1) to actin is known to depend on their molar ratio: when actin is in excess, S1 binds to two actin monomers within the actin filament, and when S1 is in excess or is equimolar with actin, each S1 binds to one actin monomer. Since in vertebrate striated muscle actin is in molar excess over myosin, we expect that in fibers each myosin head binds to two actin monomers. To test this idea, we compared the conformation of the heads in native muscle with the conformation of S1 in fibers that were loaded with either high (S1 equimolar with actin) or low (excess of actin over S1) concentration of extrinsic S1. Conformation was assessed by the accessibility of heads to trypsin (measured by the rate of trypsinolysis) and by their orientation with respect to the muscle axis (measured by a combination of polarization of fluorescence and linear dichroism). In muscle fibers loaded with a high concentration of S1, the region of the heavy chain of the myosin head at the junction of 20 and 50 kDa proteolytic fragments was readily digested by trypsin and its orientation was approximately perpendicular to an axis of a thin filament. In contrast, when muscle fibers were loaded with a low concentration of S1, the 20/50 kDa junction of S1 was protected from trypsinolysis and its orientation was more parallel with a filament axis. Native muscle in rigor behaved like muscle irrigated with a low concentration of S1, i.e. the 20/50 kDa junction of the myosin head was protected from trypsinolysis and the orientation of the heads was parallel to the filament axis. We conclude that in rigor rabbit psoas muscle each myosin head binds to two actin monomers in a thin filament, and that this binding is different from the binding of S1 to actin in equimolar solutions.