Velocities of unloaded muscle filaments are not limited by drag forces imposed by myosin cross-bridges

It is not known which kinetic step in the acto-myosin ATPase cycle limits contraction speed in unloaded muscles (V-0). Huxley's 1957 model [Huxley AF (1957) Prog Biophys Biophys Chem 7: 255-318] predicts that V-0 is limited by the rate that myosin detaches from actin. However, this does not explain why, as observed by Barany [Barany M (1967) J Gen Physiol 50(6, Suppl): 197-218], V-0 is linearly correlated with the maximal actin-activated ATPase rate (v(max)), which is limited by the rate that myosin attaches strongly to actin. We have observed smooth muscle myosin filaments of different length and head number (N) moving over surface-attached F-actin in vitro. Fitting filament velocities (V) vs. N to a detachment-limited model using the myosin step size d = 8 nm gave an ADP release rate 8.5-fold faster and t(on) (myosin's attached time) and r (duty ratio) similar to 10-fold lower than previously reported. In contrast, these data were accurately fit to an attachment-limited model, V = N.v.d, over the range of N found in all muscle types. At nonphysiologically high N, V = L/t(on) rather than d/t(on), where L is related to the length of myosin's subfragment 2. The attachment-limited model also fit well to the [ATP] dependence of V for myosin-rod cofilaments at three fixed N. Previously published V-0 vs. v(max) values for 24 different muscles were accurately fit to the attachment-limited model using widely accepted values for r and N, giving d = 11.1 nm. Therefore, in contrast with Huxley's model, we conclude that V-0 is limited by the actin-myosin attachment rate.