MYOSIN-II TRANSPORT, ORGANIZATION, AND PHOSPHORYLATION - EVIDENCE FORCORTICAL FLOW SOLATION-CONTRACTION COUPLING DURING CYTOKINESIS AND CELL LOCOMOTION

The mechanism of cytokinesis has been difficult to define because of t he short duration and the temporal-spatial dynamics involved in the fo rmation, activation, force production, and disappearance of the cleava ge furrow. We have investigated the structural and chemical dynamics o f myosin II in living Swiss 3T3 cells from prometaphase through the se paration and migration of daughter cells. The structural and chemical dynamics of myosin II have been defined using the semiautomated, multi mode light microscope, together with a fluorescent analogue of myosin II and a fluorescent biosensor of myosin II regulatory light chain (RL C) phosphorylation at serine 19. The correlation of image data from li ve cells using different modes of light microscopy allowed interpretat ions not possible from single-mode investigations. Myosin II transport ed toward the equatorial plane from adjacent regions, forming three-di mensional fibers that spanned the volume of the equator during anaphas e and telophase. A global phosphorylation of myosin II at serine 19 of the RLC was initiated at anaphase when cortical myosin II transport s tarted. The phosphorylation of myosin II remained high near the equato rial plane through telophase and into cytokinesis, whereas the phospho rylation of myosin II at serine 19 of the RLC decreased at the poles. The timing and pattern of phosphorylation was the same as the shorteni ng of myosin II-based fibers in the cleavage furrow. Myosin II-based f ibers shortened and transported out of the cleavage furrow into the ta ils of the two daughter cells late in cytokinesis. The patterns of myo sin II transport, phosphorylation, and shortening of fibers in the mig rating daughter cells were similar to that previously defined for cell s migrating in a wound in vitro. The temporal-spatial patterns and dyn amics of myosin II transport, phosphorylation at serine 19 of the RLC, and the shortening and disappearance of myosin II-based fibers suppor t the proposal that a combination of the cortical flow hypothesis and the solation-contraction coupling hypothesis explain key aspects of cy tokinesis and polarized cell locomotion.