The motor protein myosin-I produces its working stroke in two steps

Many types of cellular motility including muscle contraction, are driven by the cyclical interaction of the motor protein myosin with actin filaments, coupled to the breakdown of ATP. It is thought that myosin binds to actin and then produces force and movement as it 'tilts' or 'rocks' into one or m ore subsequent, stable conformations(1,2). Here we use an optical-tweezers transducer to measure the mechanical transitions made by a single myosin he ad while it is attached to actin We find that two members of the myosin-I f amily, rat liver myosin-I of relative molecular mass 130,000 (M-r 130K) and chick intestinal brush-border myosin-I, produce movement in two distinct s teps. The initial movement (of roughly 6 nanometres) is produced within 10 milliseconds of actomyosin binding, and the second step (of roughly 5.5 nan ometres) occurs after a variable time delay The duration of the period foll owing the second step is also variable and depends on the concentration of ATP. At the highest time resolution possible (about I millisecond), we cann ot detect this second step when studying the single-headed subfragment-1 of fast skeletal muscle myosin II. The slower kinetics of myosin-I have allow ed us to observe the separate mechanical states that contribute to its work ing stroke.