Opposite effects of overexpressed myosin Va or heavy meromyosin Va on vesicle distribution, cytoskeleton organization, and cell motility in nonmuscle cells.

Myosin Va, an actin-based motor protein that transports intracellular cargos, can bundle actin in vitro. Whether myosin Va regulates cellular actin dynamics or cell migration remains unclear. To address this, we compared Chinese Hamster Ovary (CHO) cells that stably express GFP fused to either full length mouse myosin Va (GFP-M5) or heavy meromyosin Va (GFP-M5Delta). GFP-M5 and GFP-M5Delta co-immunoprecipitate with CHO myosin Va and serve as overexpression of wild-type and dominant negative mutants of myosin Va. Compared to non-expressing control cells, GFP-M5-overexpressing cells have peripheral endocytic vesicles, spread slowly after plating, as well as produce robust interior actin stress fibers, myosin II bundles, and focal adhesions. However, these cells display normal cell migration and lamellipodial dynamics. In contrast, GFP-M5Delta-expressing cells have perinuclear endocytic vesicles, produce thin interior actin and myosin bundles and contain no interior focal adhesions. In addition, these cells spread rapidly, migrate slowly and display reduced lamellipodial dynamics. Similarly, neurite outgrowth is compromised in neurons cultured from transgenic Drosophila that express M5Delta-dsRed and in neurons cultured from Drosophila that produce a tailless version of endogenous myosin V. Together, these data suggest that myosin Va overexpression induces actin bundles in vivo whereas the tailless version fails to bundle actin and disrupts cell motility.