Are actin filaments moving under unloaded conditions in the in vitro motility assay?

With sliding actin-filament motility assays, filament velocity should be independent of changes in the level of actomyosin activation under unloaded conditions. Using a simple modification of the motility assay to measure relative changes in isometric force (activation), we determined that isometric force increased 200-fold with thiophosphorylation of the myosin regulatory light chain, and that with thiophosphorylated myosin, isometric force was further increased by the addition of saturating smooth-muscle tropomyosin (100%) or tropomyosin plus calponin (500%), and decreased with the addition of saturating caldesmon (-100%). Under "reducing conditions," filament velocity (2.0 microns/s) was constant for mixtures of dephosphorylated and thiophosphorylated myosin containing > 5% thiophosphorylated myosin, and was unaffected by the addition of saturating concentrations of tropomyosin or caldesmon. In contrast, "standard assay conditions" were found to be nonreducing. With fully thiophosphorylated smooth-muscle myosin, saturating smooth-muscle tropomyosin increased velocity to 150% of control, and caldesmon halted all filament motion; with fully dephosphorylated myosin (< 0.002 mol/mol) filaments only moved when tropomyosin or tropomyosin plus calponin was added. Taken together, these observations suggest that under "standard conditions" a mechanical load is present that is eliminated by "reducing conditions." Filament velocity was insensitive to changes in cross-bridge density, under all conditions, suggesting that noncycling cross-bridges, generated by photochemical oxidation of myosin, is a likely source of mechanical loading.