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Tropomodulins Control the Balance between Protrusive and Contractile Structures by Stabilizing Actin-Tropomyosin Filaments.

ABSTRACT: Eukaryotic cells have diverse protrusive and contractile actin filament structures, which compete with one another for a limited pool of actin monomers. Numerous actin-binding proteins regulate the dynamics of actin structures, including tropomodulins (Tmods), which cap the pointed end of actin filaments. In striated muscles, Tmods prevent actin filaments from overgrowing, whereas in non-muscle cells, their function has remained elusive. Here, we identify two Tmod isoforms, Tmod1 and Tmod3, as key components of contractile stress fibers in non-muscle cells. Individually, Tmod1 and Tmod3 can compensate for one another, but their simultaneous depletion results in disassembly of actin-tropomyosin filaments, loss of force-generating stress fibers, and severe defects in cell morphology. Knockout-rescue experiments reveal that Tmod's interaction with tropomyosin is essential for its role in the stabilization of actin-tropomyosin filaments in cells. Thus, in contrast to their role in muscle myofibrils, in non-muscle cells, Tmods bind actin-tropomyosin filaments to protect them from depolymerizing, not elongating. Furthermore, loss of Tmods shifts the balance from linear actin-tropomyosin filaments to Arp2/3 complex-nucleated branched networks, and this phenotype can be partially rescued by inhibiting the Arp2/3 complex. Collectively, the data reveal that Tmods are essential for the maintenance of contractile actomyosin bundles and that Tmod-dependent capping of actin-tropomyosin filaments is critical for the regulation of actin homeostasis in non-muscle cells.

SUBMITTER: Kumari R

PROVIDER: S-EPMC7065974 | biostudies-literature | 2020 Mar

REPOSITORIES: biostudies-literature

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Tropomodulins Control the Balance between Protrusive and Contractile Structures by Stabilizing Actin-Tropomyosin Filaments.

Kumari Reena R Jiu Yaming Y Carman Peter J PJ Tojkander Sari S Kogan Konstantin K Varjosalo Markku M Gunning Peter W PW Dominguez Roberto R Lappalainen Pekka P

Current biology : CB 20200206 5

Eukaryotic cells have diverse protrusive and contractile actin filament structures, which compete with one another for a limited pool of actin monomers. Numerous actin-binding proteins regulate the dynamics of actin structures, including tropomodulins (Tmods), which cap the pointed end of actin filaments. In striated muscles, Tmods prevent actin filaments from overgrowing, whereas in non-muscle cells, their function has remained elusive. Here, we identify two Tmod isoforms, Tmod1 and Tmod3, as k ...[more]

PMID: 32037094

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Project description:Experimental approaches such as fiber diffraction and cryo-electron microscopy reconstruction have defined regulatory positions of tropomyosin on actin but have not, as yet, succeeded at determining key atomic-level contacts between these proteins or fully substantiated the dynamics of their interactions at a structural level. To overcome this deficiency, we have previously employed computational approaches to deduce global dynamics of thin filament components by energy landscape determination and molecular dynamics simulations. Still, these approaches remain computationally challenging for any complex and large macromolecular assembly like the thin filament. For example, tropomyosin cable wrapping around actin of thin filaments features both head-to-tail polymeric interactions and local twisting, both of which depart from strict superhelical symmetry. This produces a complex energy surface that is difficult to model and thus to evaluate globally. Therefore, at this stage of our understanding, assessing global molecular dynamics can prove to be inherently impractical. As an alternative, we adopted a "divide and conquer" protocol to investigate actin-tropomyosin interactions at an atomistic level. Here, we first employed unbiased protein-protein docking tools to identify binding specificity of individual tropomyosin pseudorepeat segments over the actin surface. Accordingly, tropomyosin "ligand" segments were rotated and translated over potential "target" binding sites on F-actin where the corresponding interaction energetics of billions of conformational poses were ranked by the programs PIPER and ClusPro. These data were used to assess favorable interactions and then to rebuild models of seamless and continuous tropomyosin cables over the F-actin substrate, which were optimized further by flexible fitting routines and molecular dynamics. The models generated azimuthally distinct regulatory positions for tropomyosin cables along thin filaments on actin dominated by stereo-specific head-to-tail overlap linkage. The outcomes are in good agreement with current cryo-electron microscopy topology and consistent with long-thought residue-to-residue interactions between actin and tropomyosin.

Dataset Information

Tropomodulins Control the Balance between Protrusive and Contractile Structures by Stabilizing Actin-Tropomyosin Filaments.

Publications

Tropomodulins Control the Balance between Protrusive and Contractile Structures by Stabilizing Actin-Tropomyosin Filaments.

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