EXPRESSION OF LIGHT-MEROMYOSIN IN DICTYOSTELIUM BLOCKS NORMAL MYOSIN-II FUNCTION

The ability of myosin II to form filaments is essential for its functi on in vivo. This property of self association is localized in the ligh t meromyosin (LMM) region of the myosin II molecules. To explore this property in more detail within the context of living cells, we express ed the LMM portion of the Dictyostelium myosin II heavy chain gene in wild-type Dictyostelium cells. We found that the LMM protein was expre ssed at high levels and that it folded properly into alpha-helical coi led-coiled molecules. The expressed LMM formed large cytoplasmic inclu sions composed of entangled short filaments surrounded by networks of long tubular structures. Importantly, these abnormal structures seques tered the cell's native myosin II, completely removing it from its nor mal cytoplasmic distribution. As a result the cells expressing LMM dis played a myosin-null phenotype: they failed to undergo cytokinesis and became multinucleate, failed to form caps after treatment with Con A, and failed to complete their normal developmental cycle. Thus, expres sion of the LMM fragment in Dictyostelium completely abrogates myosin II function in vivo. The dominant-negative character of this phenotype holds promise as a general method to disrupt myosin II function in ma ny cell types without the necessity of gene targeting.