SINGLE-HEADED MYOSIN-II ACTS AS A DOMINANT-NEGATIVE MUTATION IN DICTYOSTELIUM

Conventional myosin II is an essential protein for cytokinesis, cappin g of cell surface receptors, and development of Dictyostelium cells, M yosin II also plays an important role in the polarization and movement of cells, All conventional myosins are double-headed molecules but th e significance of this structure is not understood since single-headed myosin II can produce movement and force in vitro, We found that expr ession of the tail portion of myosin II in Dictyostelium led to the fo rmation of single headed myosin II in vivo. The resultant cells contai n an approximately equal ratio of double- and single-headed myosin II molecules, Surprisingly, these cells were completely blocked in cytoki nesis and capping of concanavalin A receptors although development int o fruiting bodies was not impaired. We found that this phenotype is no t due to defects in myosin light chain phosphorylation. These results show that single-headed myosin II cannot function properly in vivo and that it acts as a dominant negative mutation for myosin II function, These results suggest the possibility that cooperativity of myosin II heads is critical for force production in vivo.