Movement of scallop myosin on Nitella actin filaments: Regulation by calcium

In order to determine if Ca²⁺ regulates scallop myosin movement on actin, we have measured motility of scallop myosin along actin filaments using a direct visual assay. This procedure consists of covalently linking myosin to 1-μm beads and pipetting them onto a parallel array of actin filaments located on the cytoplasmic face of a Nitella internodal cell. In the absence of Ca²⁺, scallop myosin-coated beads exhibit no directed motion; however, in the presence of pCa²⁺ of greater than 5.84, these beads undergo linear translocations with average velocities of 2.0 μm/s. This Ca²⁺-sensitive motility requires the presence of regulatory light chains on the scallop myosin. Removal of regulatory light chains with 10 mM EDTA produces a 'desensitized' myosin, no longer sensitive to Ca²⁺, which moves at rates of 0.09-0.3 μs in the presence or absence of Ca²⁺. Readdition of regulatory light chains to preparations of desensitized myosin once again confers Ca²⁺-sensitive motility. The Ca²⁺ dependence of scallop-myosin motility shows a sharp transition, consistent with the Ca²⁺ activation sensitivity of the actin-activated ATPase. Furthermore, relative rates of movement of calcium-regulated myosins from various molluscan species are consistent with their respective rates of ATP hydrolysis. Thus, myosin motility along actin filaments provides a sensitive and direct assay of myosin activity and is suitable for studying myosin regulation.