Unitary distance of actin-myosin sliding studied using an in vitro force-movement assay system combined with ATP iontophoresis.

To obtain information about the mechanism of ATP-dependent actin-myosin sliding responsible for muscle contraction, we studied the "unitary" distance of sliding between a myosin-coated glass microneedle and actin filament arrays (actin cables) in a giant algal cell induced by iontophoretic application of ATP, attention being focused on the minimum distance of ATP-induced sliding when the amount of applied ATP was gradually decreased in the presence of hexokinase and D-glucose. The number of myosin heads interacting with actin cables was reduced to less than 100, as judged from the maximal force Po (approximately 100 pN) generated by myosin heads on the needle in the presence of 2 mM ATP. When the amount of iontophoretically applied ATP was decreased by reducing the amount of charge passed through the ATP electrode from 80 to 2 nC, the distance of ATP-induced actin-myosin sliding decreased almost linearly from approximately 100 to approximately 10 nm, no detectable actin-myosin sliding being observed with further reduction of the charge passed through the electrode. The amount of external load exerted by the bent microneedle was less than 1% of Po for the sliding distance < 50 nm. The actin-myosin sliding distances with a small amount of ATP slightly above the amount required to induce the minimum sliding distance were distributed around integral multiples of 10 nm, suggesting that the unitary distance of actin-myosin sliding coupled with ATP hydrolysis is of the order of 10 nm.