Differential effects of myosin-antibody complexes on contractile rings and circumferential belts in epitheloid cells

The role of myosin filaments during assembly and activity of microfilament rings was analyzed by microinjecting epitheloid cells (PtK2 and LLC-PK1 kidney cell lines) with specific anti-myosins. Six monoclonal antibodies directed against the light meromyosin (LMM) region of the myosin molecule were characterized with respect to epitope location, and their effects on actin-activated MgATPase as well as on assembly, structural integrity and stability of myosin filaments. All of these antibodies recognized LLC-PK1 myosin, but only three reacted with PtK2 myosin. The remaining three served as matching controls in experiments with this cell line. When injected in amounts sufficient to yield an excess of antibody over myosin, the reactive antibodies significantly delayed formation and constriction of the contractile ring in mitotic cells. These rings contained less myosin, but not less actin, than the controls. This indicates that the recruitment and alignment of actin in the cleavage furrow can occur independently of other components of the contractile ring. After completion of cytokinesis, the majority of the injected cells was unable to assemble a normal circumferential belt. This resulted in defective epitheloid sheets. Approximately one third of these cells showed grossly distorted cell shapes and an increase in locomotory activity. All these changes were fully reversible with time, suggesting that the effects of the antibodies were overcome by protein synthesis. The differential sensitivity seen between contractile rings and peripheral belts is discussed with respect to differences in their architecture, stability and proposed function.