X-ray structures of the apo and MgATP-bound states of Dictyostelium discoideum myosin motor domain

Myosin is the most comprehensively studied molecular motor that converts en ergy from the hydrolysis of MgATP into directed movement. Its motile cycle consists of a sequential series of interactions between myosin, actin, MgAT P, and the products of hydrolysis, where the affinity of myosin for actin i s modulated by the nature of the nucleotide bound in the active site. The f irst step in the contractile cycle occurs when ATP binds to actomyosin and releases myosin from the complex. We report here the structure of the motor domain of Dictyostelium discoideum myosin II both in its nucleotide-free s tate and complexed with MgATP. The structure with MgATP was obtained by soa king the crystals in substrate. These structures reveal that both the apo f orm and the MgATP complex are very similar to those previously seen with Mg ATP gammaS and MgAMP-PNP. Moreover, these structures are similar to that of chicken skeletal myosin subfragment-l, The crystallized protein is enzymat ically active in solution, indicating that the conformation of myosin obser ved in chicken skeletal myosin subfragment-l is unable to hydrolyze ATP and most likely represents the pre-hydrolysis structure for the myosin head th at occurs after release from actin.