A Molecular Pathway for Myosin II Recruitment to Stress Fibers

• DOI: 10.1016/j.cub.2011.03.007
• Publication Date: 2011-04-13T11:49:01Z
• Authors: Sari Tojkander; Gergana Gateva; Galina Schevzov; Pirta Hotulainen; Perttu Naumanen; Claire Martin; Peter W. Gunning; Pekka Lappalainen
• Keywords: Myosin Type II; 0301 basic medicine; Osteosarcoma; Agricultural and Biological Sciences(all); Biochemistry, Genetics and Molecular Biology(all); Formins; Membrane Proteins; Actomyosin; Tropomyosin; Actin-Related Protein 2-3 Complex; Actin Cytoskeleton; 03 medical and health sciences; Contractile Proteins; Cell Movement; Cell Line, Tumor; Stress Fibers; Morphogenesis; Humans; Protein Isoforms; RNA Interference; Carrier Proteins; Cytoskeleton; Adaptor Proteins, Signal Transducing

Cell migration and morphogenesis are driven by both protrusive and contractile actin filament structures. The assembly mechanisms of lamellipodial and filopodial actin filament arrays, which provide the force for plasma membrane protrusions through actin filament treadmilling, are relatively well understood. In contrast, the mechanisms by which contractile actomyosin arrays such as stress fibers are generated in cells, and how myosin II is specifically recruited to these structures, are not known.We demonstrate that four functionally distinct tropomyosins are required for assembly of stress fibers in cultured osteosarcoma cells. Tm1, Tm2/3, and Tm5NM1/2 stabilize actin filaments at distinct stress fiber regions. In contrast, Tm4 promotes stress fiber assembly by recruiting myosin II to stress fiber precursors. Elimination of any one of the tropomyosins fatally compromises stress fiber formation. Importantly, Dia2 formin is critical to stress fiber assembly by nucleating Tm4-decorated actin filaments at the cell cortex. Myosin II is specifically recruited through a Tm4-dependent mechanism to the Dia2-nucleated filaments, which subsequently assemble endwise with Arp2/3-nucleated actin filament structures to yield contractile stress fibers.These experiments identified a pathway, involving Dia2- and Arp2/3-promoted actin filament nucleation and several functionally distinct tropomyosins, that is required for generation of contractile actomyosin arrays in cells.