Myosin motors: missing structures and hidden springs

High-resolution structures of the motor domain of myosin II and lower resol ution actin-myosin structures have led to the 'swinging lever arm' model fo r myosin force generation, The available kinetic data are not all easily re conciled with this model and understanding the final details of the myosin motor mechanism must await actin-myosin co-crystals. The observation that m yosin can populate multiple states in the absence of actin has nonetheless led to significant insights. The currently known myosin structures correspo nd to defined kinetic states that bind weakly (K-d > muM) to actin, It is p ossible that the myosin lever arm could complete its swing before strong bi nding to actin and force generation - a process that would correspond, in t he absence of load,to a Brownian ratchet. We further suggest that, under lo ad, internal springs within the myosin head could decouple force generation and lever arm movement.