MOVEMENT AND FORCE PRODUCED BY A SINGLE MYOSIN HEAD

MUSCLE contraction is driven by the cyclical interaction of myosin wit h actin, coupled to the breakdown of ATP. Studies of the interaction o f filamentous myosin(1) and of a double-headed proteolytic fragment, h eavy meromyosin (HMM)(2,3), with actin have demonstrated discrete mech anical events, arising from stochastic interaction of single myosin mo lecules with actin. Here we show, using an optical-tweezers transducer (2,4), that a single myosin subfragment-1 (S1), which is a single myos in head, can act as an independent generator of force and movement. Ou r analysis accounts for the broad distribution of displacement amplitu des observed, and indicates that the underlying: movement (working str oke) produced by a single acto-S1 interaction is similar to 4 nm, cons iderably shorter than previous estimates(1-3,5) but consistent with st ructural data(6). We measure the average force generated by S1 or HMM to be at least 1.7 pN under isometric conditions.