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The Structural Basis of Actin Organization by Vinculin and Metavinculin.

ABSTRACT: Vinculin is an essential adhesion protein that links membrane-bound integrin and cadherin receptors through their intracellular binding partners to filamentous actin, facilitating mechanotransduction. Here we present an 8.5-Å-resolution cryo-electron microscopy reconstruction and pseudo-atomic model of the vinculin tail (Vt) domain bound to F-actin. Upon actin engagement, the N-terminal "strap" and helix 1 are displaced from the Vt helical bundle to mediate actin bundling. We find that an analogous conformational change also occurs in the H1' helix of the tail domain of metavinculin (MVt) upon actin binding, a muscle-specific splice isoform that suppresses actin bundling by Vt. These data support a model in which metavinculin tunes the actin bundling activity of vinculin in a tissue-specific manner, providing a mechanistic framework for understanding metavinculin mutations associated with hereditary cardiomyopathies.

SUBMITTER: Kim LY

PROVIDER: S-EPMC4738167 | biostudies-literature | 2016 Jan

REPOSITORIES: biostudies-literature

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The Structural Basis of Actin Organization by Vinculin and Metavinculin.

Kim Laura Y LY Thompson Peter M PM Thompson Peter M PM Lee Hyunna T HT Pershad Mihir M Campbell Sharon L SL Alushin Gregory M GM

Journal of molecular biology 20151020 1

Vinculin is an essential adhesion protein that links membrane-bound integrin and cadherin receptors through their intracellular binding partners to filamentous actin, facilitating mechanotransduction. Here we present an 8.5-Å-resolution cryo-electron microscopy reconstruction and pseudo-atomic model of the vinculin tail (Vt) domain bound to F-actin. Upon actin engagement, the N-terminal "strap" and helix 1 are displaced from the Vt helical bundle to mediate actin bundling. We find that an analog ...[more]

PMID: 26493222

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Cardiomyopathy Mutations in Metavinculin Disrupt Regulation of Vinculin-Induced F-Actin Assemblies.

Project description:Debilitating heart conditions, notably dilated and hypertrophic cardiomyopathies (CMs), are associated with point mutations in metavinculin, a larger isoform of the essential cytoskeletal protein vinculin. Metavinculin is co-expressed with vinculin at sub-stoichiometric ratios in cardiac tissues. CM mutations in the metavinculin tail domain (MVt) occur within the extra 68-residue insert that differentiates it from the vinculin tail domain (Vt). Vt binds actin filaments (F-actin) and promotes vinculin dimerization to bundle F-actin into thick fibers. While MVt binds to F-actin in a similar manner to Vt, MVt is incapable of F-actin bundling and inhibits Vt-mediated F-actin bundling. We performed F-actin co-sedimentation and negative-stain EM experiments to dissect the coordinated roles of metavinculin and vinculin in actin fiber assembly and the effects of three known metavinculin CM mutations. These CM mutants were found to weakly induce the formation of disordered F-actin assemblies. Notably, they fail to inhibit Vt-mediated F-actin bundling and instead promote formation of large assemblies embedded with linear bundles. Computational models of MVt bound to F-actin suggest that MVt undergoes a conformational change licensing the formation of a protruding sub-domain incorporating the insert, which sterically prevents dimerization and bundling of F-actin by Vt. Sub-domain formation is destabilized by CM mutations, disrupting this inhibitory mechanism. These findings provide new mechanistic insights into the ability of metavinculin to tune actin organization by vinculin and suggest that dysregulation of this process by CM mutants could underlie their malfunction in disease.

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Metavinculin Tunes the Flexibility and the Architecture of Vinculin-Induced Bundles of Actin Filaments.

Project description:Vinculin is an abundant protein found at cell-cell and cell-extracellular matrix junctions. In muscles, a longer splice isoform of vinculin, metavinculin, is also expressed. The metavinculin-specific insert is part of the C-terminal tail domain, the actin-binding site of both isoforms. Mutations in the metavinculin-specific insert are linked to heart disease such as dilated cardiomyopathies. Vinculin tail domain (VT) both binds and bundles actin filaments. Metavinculin tail domain (MVT) binds actin filaments in a similar orientation but does not bundle filaments. Recently, MVT was reported to sever actin filaments. In this work, we asked how MVT influences F-actin alone or in combination with VT. Cosedimentation and limited proteolysis experiments indicated a similar actin binding affinity and mode for both VT and MVT. In real-time total internal reflection fluorescence microscopy experiments, MVT's severing activity was negligible. Instead, we found that MVT binding caused a 2-fold reduction in F-actin's bending persistence length and increased susceptibility to breakage. Using mutagenesis and site-directed labeling with fluorescence probes, we determined that MVT alters actin interprotomer contacts and dynamics, which presumably reflect the observed changes in bending persistence length. Finally, we found that MVT decreases the density and thickness of actin filament bundles generated by VT. Altogether, our data suggest that MVT alters actin filament flexibility and tunes filament organization in the presence of VT. Both of these activities are potentially important for muscle cell function. Perhaps MVT allows the load of muscle contraction to act as a signal to reorganize actin filaments.

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Project description:The ability of cells and tissues to respond differentially to mechanical forces applied in distinct directions is mediated by the ability of load-bearing proteins to preferentially maintain physical linkages in certain directions. However, the molecular basis and biological consequences of directional force-sensitive binding remain unclear. Vinculin (Vcn) is a load-bearing linker protein that exhibits directional catch bonding due to interactions between the Vcn tail domain (Vt) and filamentous (F)-actin. We developed a computational approach to predict Vcn residues involved in directional catch bonding and produced a set of associated Vcn variants with unaltered Vt structure, actin binding, or phospholipid interactions. Incorporation of the variants did not affect Vcn activation but reduced Vcn loading and altered exchange dynamics, consistent with the loss of directional catch bonding. Expression of Vcn variants perturbed the coordination of subcellular structures and cell migration, establishing key cellular functions for Vcn directional catch bonding.

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Metavinculin ABD-F-actin complex

Project description: Not available

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Vinexin: a novel vinculin-binding protein with multiple SH3 domains enhances actin cytoskeletal organization.

Project description:Using the yeast two-hybrid system and an in vitro binding assay, we have identified a novel protein termed vinexin as a vinculin-binding protein. By Northern blotting, we identified two types of vinexin mRNA that were 3 and 2 kb in length. Screening for full-length cDNA clones and sequencing indicated that the two mRNA encode 82- and 37-kD polypeptides termed vinexin alpha and beta, respectively. Both forms of vinexin share a common carboxyl-terminal sequence containing three SH3 domains. The larger vinexin alpha contains an additional amino-terminal sequence. The interaction between vinexin and vinculin was mediated by two SH3 domains of vinexin and the proline-rich region of vinculin. When expressed, vinexin alpha and beta localized to focal adhesions in NIH 3T3 fibroblasts, and to cell-cell junctions in epithelial LLC-PK1 cells. Furthermore, expression of vinexin increased focal adhesion size. Vinexin alpha also promoted upregulation of actin stress fiber formation. In addition, cell lines stably expressing vinexin beta showed enhanced cell spreading on fibronectin. These data identify vinexin as a novel focal adhesion and cell- cell adhesion protein that binds via SH3 domains to the hinge region of vinculin, which can enhance actin cytoskeletal organization and cell spreading.

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Vinculin and metavinculin exhibit distinct effects on focal adhesion properties, cell migration, and mechanotransduction.

Project description:Vinculin (Vcn) is a ubiquitously expressed cytoskeletal protein that links transmembrane receptors to actin filaments, and plays a key role in regulating cell adhesion, motility, and force transmission. Metavinculin (MVcn) is a Vcn splice isoform that contains an additional exon encoding a 68-residue insert within the actin binding tail domain. MVcn is selectively expressed at sub-stoichiometic amounts relative to Vcn in smooth and cardiac muscle cells. Mutations in the MVcn insert are linked to various cardiomyopathies. In vitro analysis has previously shown that while both proteins can engage filamentous (F)-actin, only Vcn can promote F-actin bundling. Moreover, we and others have shown that MVcn can negatively regulate Vcn-mediated F-actin bundling in vitro. To investigate functional differences between MVcn and Vcn, we stably expressed either Vcn or MVcn in Vcn-null mouse embryonic fibroblasts. While both MVcn and Vcn were observed at FAs, MVcn-expressing cells had larger but fewer focal adhesions per cell compared to Vcn-expressing cells. MVcn-expressing cells migrated faster and exhibited greater persistence compared to Vcn-expressing cells, even though Vcn-containing FAs assembled and disassembled faster. Magnetic tweezer measurements on Vcn-expressing cells show a typical cell stiffening phenotype in response to externally applied force; however, this was absent in Vcn-null and MVcn-expressing cells. Our findings that MVcn expression leads to larger but fewer FAs per cell, in conjunction with the inability of MVcn to bundle F-actin in vitro and rescue the cell stiffening response, are consistent with our previous findings of actin bundling deficient Vcn variants, suggesting that deficient actin-bundling may account for some of the differences between Vcn and MVcn.

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Elucidation of the Molecular Basis and Cellular Functions of Vinculin-Actin Directional Catch Bonding.

Project description:The ability of cells and tissues to differentially resist or adapt to mechanical forces applied in distinct directions is mediated by the ability of load-bearing proteins to preferentially maintain physical linkages in certain directions. However, the molecular basis and biological consequences of directional force-sensitive binding are unclear. Vinculin (Vcn) is a load-bearing linker protein that exhibits directional catch bonding due to interactions between the Vcn tail domain (Vt) and filamentous (F)-actin. We developed a computational approach to predict Vcn residues involved in directional catch bonding and produced a set of associated Vcn variants with unaltered Vt structure, actin binding, or phospholipid interactions. Incorporation of these variants into Vcn biosensors did not perturb Vcn conformation, but reduced Vcn loading consistent with loss of directional catch bonding. Expression of Vcn variants perturbed the coalignment of FAs and F-actin and directed cell migration, establishing key cellular functions for Vcn directional catch bonding.

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Vinculin nucleates actin polymerization and modifies actin filament structure.

Project description:Vinculin links integrins to the actin cytoskeleton by binding F-actin. Little is known with respect to how this interaction occurs or affects actin dynamics. Here we assess the consequence of the vinculin tail (VT) on actin dynamics by examining its binding to monomeric and filamentous yeast actins. VT causes pyrene-labeled G-actin to polymerize in low ionic strength buffer (G-buffer), conditions that normally do not promote actin polymerization. Analysis by electron microscopy shows that, under these conditions, the filaments form small bundles at low VT concentrations, which gradually increase in size until saturation occurs at a ratio of 2 VT:1 actin. Addition of VT to pyrene-labeled mutant yeast G-actin (S265C) produced a fluorescence excimer band, which requires a relatively normal filament geometry. In higher ionic strength polymerization-promoting F-buffer, substoichiometric amounts of VT accelerate the polymerization of pyrene-labeled WT actin. However, the amplitude of the pyrene fluorescence caused by actin polymerization is quenched as the VT concentration increases without an effect on net actin polymerization as determined by centrifugation assays. Finally, addition of VT to preformed pyrene-labeled S265C F-actin causes a concentration-dependent decrease in the maximum amplitude of the pyrene fluorescence band demonstrating the ability of VT to remodel the conformation of the actin filament. These observations support the idea that vinculin can link adhesion plaques to the cytoskeleton by initiating the formation of bundled actin filaments or by remodeling existing filaments.

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The interaction of vinculin with actin.

Project description:Vinculin can interact with F-actin both in recruitment of actin filaments to the growing focal adhesions and also in capping of actin filaments to regulate actin dynamics. Using molecular dynamics, both interactions are simulated using different vinculin conformations. Vinculin is simulated either with only its vinculin tail domain (Vt), with all residues in its closed conformation, with all residues in an open I conformation, and with all residues in an open II conformation. The open I conformation results from movement of domain 1 away from Vt; the open II conformation results from complete dissociation of Vt from the vinculin head domains. Simulation of vinculin binding along the actin filament showed that Vt alone can bind along the actin filaments, that vinculin in its closed conformation cannot bind along the actin filaments, and that vinculin in its open I conformation can bind along the actin filaments. The simulations confirm that movement of domain 1 away from Vt in formation of vinculin 1 is sufficient for allowing Vt to bind along the actin filament. Simulation of Vt capping actin filaments probe six possible bound structures and suggest that vinculin would cap actin filaments by interacting with both S1 and S3 of the barbed-end, using the surface of Vt normally occluded by D4 and nearby vinculin head domain residues. Simulation of D4 separation from Vt after D1 separation formed the open II conformation. Binding of open II vinculin to the barbed-end suggests this conformation allows for vinculin capping. Three binding sites on F-actin are suggested as regions that could link to vinculin. Vinculin is suggested to function as a variable switch at the focal adhesions. The conformation of vinculin and the precise F-actin binding conformation is dependent on the level of mechanical load on the focal adhesion.

| S-EPMC3635976 | biostudies-literature

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