A Molecular Pathway for Myosin II Recruitment to Stress Fibers

SummaryBackground: Cell migration and morphogenesis are driven byboth protrusive and contractile actin filament structures. Theassembly mechanisms of lamellipodial and filopodial actin filamentarrays, which provide the force for plasma membraneprotrusions through actin filament treadmilling, are relativelywell understood. In contrast, the mechanisms by whichcontractile actomyosin arrays such as stress fibers are generatedin cells, and how myosin II is specifically recruited tothese structures, are not known.Results: We demonstrate that four functionally distinct tropomyosinsare required for assembly of stress fibers in culturedosteosarcoma cells. Tm1, Tm2/3, and Tm5NM1/2 stabilizeactin filaments at distinct stress fiber regions. In contrast,Tm4 promotes stress fiber assembly by recruiting myosin IIto stress fiber precursors. Elimination of any one of the tropomyosinsfatally compromises stress fiber formation. Importantly,Dia2 formin is critical to stress fiber assembly bynucleating Tm4-decorated actin filaments at the cell cortex.Myosin II is specifically recruited through a Tm4-dependentmechanism to the Dia2-nucleated filaments, which subsequentlyassemble endwise with Arp2/3-nucleated actin filamentstructures to yield contractile stress fibers.Conclusions: These experiments identified a pathway,involving Dia2- and Arp2/3-promoted actin filament nucleationand several functionally distinct tropomyosins, that is requiredfor generation of contractile actomyosin arrays in cells.