Myosin distribution and actin organization in different areas of antibody-labelled quick-frozen fibroblasts.

In cortical and subcortical areas of motile non-muscle cells myosin is found only on linear actin filament bundles that are aligned with the cell's long axis. Myosin is absent from actin filaments perpendicular to these bundles and from areas of cortical and subcortical actin, which has a complex geometrical array. These data suggest that in the non-muscle cell myosin exerts force in a unidirectional manner only, as it does in muscle. The presence of myosin up to the ends of cell processes suggests that, even in the cortex, this force transduction takes place over short-range distances. The absence of myosin rods in vivo but the presence of structures corresponding to single myosin molecules suggests that the force-generating unit for actomyosin-based movement in non-muscle cells is either a myosin dimer/small oligomer or single myosin molecule, attached to actin by their tail regions.