Evidence for cleft closure in actomyosin upon ADP release

Structural insights into the interaction of smooth muscle myosin with actin have been provided by computer-based fitting of crystal structures into th ree-dimensional reconstructions obtained by electron cryomicroscopy, and by mapping of structural and dynamic changes in the actomyosin complex. The a ctomyosin structures determined in the presence and absence of MgADP differ significantly from each other, and from all crystallographic structures of unbound myosin. Coupled to a complex movement (similar to 34 Angstrom) of the light chain binding domain upon MgADP release, we observed a similar to 9 degrees rotation of the myosin motor domain relative to the actin filamen t, and a closure of the cleft that divides the actin binding region of the myosin head. Cleft closure is achieved by a movement of the upper 50 kDa re gion, while parts of the lower 50 kDa region are stabilized through strong interactions with actin. This model supports a mechanism in which binding o f MgATP at the active site opens the cleft and disrupts the interface, ther eby releasing myosin from actin.