Myosin-ATP chemomechanics.

The hydrodynamic size of rabbit skeletal muscle myosin subfragment 1 (S1) is decreased when S1 and MgATP form the steady-state intermediate S1-MgADP,P(i). The rotational decay time, tau, determined by transient electrical birefringence techniques was 259 ns for S1-MgADP,P(i) and 271 ns for S1-MgADP at 3 degrees C in low ionic strength solutions. The data were interpreted using a hydrodynamic model consisting of a rigid linear four-bead structure that had a point at the center of one of the inner beads about which the structure can bend. The structure of S1-MgADP was approximated by adjusting the bend angle to 20 degrees. The best fit to the S1-MgADP,P(i) decay time was then obtained when the angle was increased to 38 degrees. The results obtained using this simple model suggest that MgATP binding and hydrolysis changes the structure of S1 so that one end of it moves by at least 3.9 nm. The reverse of this process, during product release, would provide a displacement large enough to account for most of the ATP-driven filament sliding that occurs in muscle or in in vitro motility assays.