Myosin cleft movement and its coupling to actomyosin dissociation

It has long been known that binding of actin and nucleotides to myosin are antagonistic, an observation that led to the biochemical basis for the crossbridge cycle of muscle contraction. Thus ATP binding to actomyosin causes actin dissociation, while actin binding to the myosin accelerates ADP and phosphate release. Structural studies have indicated that communication between the actin and nucleotide binding sites involves the opening and closing of the cleft between the upper and lower 50K domains of the myosin he ad. Here we test the proposal that the cleft responds to actin and nucleotide binding in a reciprocal manner and show that cleft movement is coupled to actin binding and dissociation. We monitored cleft movement using pyrene excimer fluorescence from probes engineered across the cleft.