Movement of scallop myosin on Nitella actin filaments: regulation by calcium.

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Abstract
To determine if Ca2+ regulates scallop [Aequipecten irradians, Placopecten magellanicus] myosin movement on actin, motility of scallop myosin along actin filaments was measured using a direct visual assay. This procedure consists of covalently linking myosin to 1 .mu.m 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; in the presence of pCa2+ of > 5.84, these beads undergo linear translocations with average velocities of 2.0 .mu.m/s. This Ca2+-sensitive motility requires the presense 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 .mu.ms 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. Relative rates of movement of Ca2+-regulated myosins from various molluscan species are consistent with their respective rates of ATP hydrolysis. Myosin motility along actin filaments provides a sensitive and direct assay of myosin activity and is suitable for studying myosin regulation.