Unitary Distance of ATP-Induced Actin-Myosin Sliding Studied with an In Vitro Force-Movement Assay System

We studied the unitary distance of ATP-induced actin-myosin sliding using an in vitro force-movement assay system consisting of a myosin-coated glass microneedle and well organized actin filament arrays (actin cables) in the internodal cell of an alga Nitellopsis obtusa 1). The number of myosin heads interacting with actin cables was reduced to about 100, as judged from the isometric force of about 100 pN attained in the presence of 2 mM ATP. When the amount of iontophoretically applied ATP was reduced by decreasing the amount of charge passed through the ATP electrode from 80 to 2 nC, the distance of the ATP-induced actin-myosin sliding decreased almost linearly from about 100 to about 10 run, no detectable sliding being observed with further reduction of charge through the electrode. The sliding distances with small amounts of ATP (7–16 nC) distributed around integral multiples of 10 nm, suggesting the unitary distance of actin-myosin sliding of about 10 nm.