THE ACTOMYOSIN ENGINE

Two recent advances have extended our knowledge of the actomyosin interaction considerably. The first of these is the determination of the crystal structures of the actin monomer and of the myosin head. The structures of these proteins were fit into the quaternary protein complexes of the actin filament, and of the actin filament decorated by the myosin head, using data from fiber diffraction or electron microscopy as constraints. The atomic models of these protein complexes have led to new hypotheses concerning the large conformational changes that must occur in these proteins in order to generate force. The second advance has been the measurement of the forces and displacements of single motor proteins, in particular, of a single myosin molecule interacting with a single actin filament. These data lead to more unambiguous interpretations than did previous mechanical data, which were obtained using ensembles of motors that worked asynchronously on their filaments. This approach brings to the motor field the equivalent of what patch clamp techniques brought to the study of membrane channels: the ability to look at the function of a single molecule.