MYOSIN HEAVY-CHAIN PHOSPHORYLATION SITES REGULATE MYOSIN LOCALIZATIONDURING CYTOKINESIS IN LIVE CELLS

Conventional myosin II plays a fundamental role in the process of cyto kinesis where, in the form of bipolar thick filaments, it is thought t o be the molecular motor that generates the force necessary to divide the cell. In Dictyostelium, the formation of thick filaments is regula ted by the phosphorylation of three threonine residues in the tail reg ion of the myosin heavy chain. We report here on the effects of this r egulation on the localization of myosin in live cells undergoing cytok inesis. We imaged fusion proteins of the green-fluorescent protein wit h wild-type myosin and with myosins where the three critical threonine s had been changed to either alanine or aspartic acid. We provide evid ence that thick filament formation is required for the accumulation of myosin in the cleavage furrow and that if thick filaments are overpro duced, this accumulation is markedly enhanced. This suggests that myos in localization in dividing cells is regulated by myosin heavy chain p hosphorylation.