The tip of the coiled-coil rod determines the filament formation of smoothmuscle and nonmuscle myosin

Myosin II self-assembles to form thick filaments that are attributed to its long coiled-coil tail domain. The present study has determined a region cr itical for filament formation of vertebrate smooth muscle and nonmuscle myo sin IL A monoclonal antibody recognizing the 28 residues from the C-termina l end of the coiled-coil domain of smooth muscle myosin II completely inhib ited filament formation, whereas other antibodies recognizing other parts o f the coiled-coil did not. To determine the importance of this region in th e filament assembly in vivo, green fluorescent protein (GFP)-tagged smooth muscle myosin was expressed in COS-7 cells, and the filamentous localizatio n of the GFP signal was monitored by fluorescence microscopy. Wild type GFP -tagged smooth muscle myosin colocalized with F-actin during interphase and was also recruited into the contractile ring during cytokinesis. Myosin wi th the nonhelical tail piece deleted showed similar behavior, whereas delet ion of the 28 residues at the C-terminal end of the coiled-coil domain abol ished this localization. Deletion of the corresponding region of GFP-tagged nonmuscle myosin IIA also abolished this localization. We conclude that th e C-terminal end of the coiled-coil domain, but not the nonhelical tail pie ce, of myosin II is critical for myosin filament formation both in vitro an d in vivo.