Periodicities designed in the tropomyosin sequence and structure define its functions

The protein actin forms filaments that provide cells with mechanical support and driving forces for movement. Actin contributes to biological processes such as sensing environmental forces, internalizing membrane vesicles, moving over surfaces, and dividing the cell in two. These cellular activities are complex; they depend on interactions of actin monomers and filaments with numerous other proteins. Here, we present a summary of the key questions in the field and suggest how those questions might be answered. Understanding actin-based biological phenomena will depend on identifying the participating molecules and defining their molecular mechanisms. Comparisons of quantitative measurements of reactions in live cells with computer simulations of mathematical models will also help generate meaningful insights.

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

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. Copyright © 2011 Elsevier Ltd. All rights reserved.