We have previously measured the process of displacementgeneration by a single head of muscle myosin (S1)using scanning probe nanometry. Given that the myosinhead was rigidly attached to a fairly large scanningprobe, it was assumed to stably interact with an underlyingactin filament without diffusing away as would bethe case in muscle. The myosin head has been shown tostep back and forth stochastically along an actin filamentwith actin monomer repeats of 5.5 nm and to produce anet movement in the forward direction. The myosin headunderwent 5 forward steps to produce a maximum displacementof 30 nm per ATP at low load (<1 pN). Here,we measured the steps over a wide range of forces up to4 pN. The size of the steps (~5.5 nm) did not change asthe load increased whereas the number of steps per displacementand the stepping rate both decreased. Therate of the 5.5-nm steps at various force levels produceda force-velocity curve of individual actomyosin motors.The force-velocity curve from the individual myosinheads was comparable to that reported in muscle, suggestingthat the fundamental mechanical properties inmuscle are basically due to the intrinsic stochasticnature of individual actomyosin motors. In order to explainmultiple stochastic steps, we propose a model arguingthat the thermally-driven step of a myosin head isbiased in the forward direction by a potential slope alongthe actin helical pitch resulting from steric compatibilitybetween the binding sites of actin and a myosin head.Furthermore, computer simulations show that multiplecooperating heads undergoing stochastic steps generatea long (>60 nm) sliding distance per ATP between actinand myosin filaments, i.e., the movement is looselycoupled to the ATPase cycle as observed in muscle.
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