All conventional myosin IIs, whether isolated from skeletal, smooth, or invertebrate muscle sources, have two heads attached to an extended 16 nm alpha-helical coiled-coil tail. The head can be divided into a globular motor domain of approximate to 770 amino acids that contains the catalytic and actin binding sites, and a neck region of approximate to 70 amino acids which binds one essential and one regulatory light chain (ELC and RLC). The neck region with its associated LCs plays both structural and regulatory roles. While the mechanism and extent of regulation by the LCs varies for different myosins, the structural role may be a more fundamental feature of myosin II motors. Our understanding of the neck region has advanced rapidly in recent years primarily because of two types of information: (I) the high resolution structures of the LC binding domain from the thick-filament regulated scallop myosin (Xie et al., 1994) and of the head of unregulated skeletal myosin (Rayment et al., 1993), and (2) the ability to remove and/or mutate portions of both the heavy and light chains for analysis by in vitro motility assays.
