Molecular structural analysis of actomyosin to characterize the motor protein system

Muscle contraction results from the relative sliding motion between thick myosin and thin actin filaments. Actomyosin is a molecular machine that converts chemical energy produced by the hydrolysis of ATP into kinetic energy. The investigation of the contractile mechanism of muscles at the atomic/molecular level was motivated by the determination of the structures of actin and myosin head S1 monomers by X-ray diffraction analyses. In order to clarify microscopic kinetic function and material heterogeneity, the molecular structural analysis of actomyosin was carried out using the molecular mechanics simulation code "AMBER". The 3-D molecular structures of actomyosin employ three models which consist of three kinds of myosin head S1 (with ATP, with ADP, and without nucleotide) and F-actin itself to reveal the fundamental micromechanism of activation in the motility assay. The minimum-energy conformations of actomyosin in the three models were determined from molecular mechanics analyses. The differences in atomic coordinates and potential energy distributions show the existence of local packing and microstructural heterogeneity. Then, molecular fluctuations were studied by molecular dynamics analysis. The fluctuations reveal the dynamic properties at the atomic level and the possibility of change in the mesoscale structure as well as the emergence of the sliding motion of the entire molecule.