Interaction of the N- and C-terminal domains of vinculin. Characterization and mapping studies

Molecular mechanisms of mechanotransduction in integrin-mediated cell-matrix adhesion

Cell-matrix adhesion complexes are multi-protein structures linking the extracellular matrix (ECM) to the cytoskeleton. They are essential to both cell motility and function by bidirectionally sensing and transmitting mechanical and biochemical stimulations. Several types of cell-matrix adhesions have been identified and they share many key molecular components, such as integrins and actin-integrin linkers. Mechanochemical coupling between ECM molecules and the actin cytoskeleton has been observed from the single cell to the single molecule level and from immune cells to neuronal cells. However, the mechanisms underlying force regulation of integrin-mediated mechanotransduction still need to be elucidated. In this review article, we focus on integrin-mediated adhesions and discuss force regulation of cell-matrix adhesions and key adaptor molecules, three different force-dependent behaviors, and molecular mechanisms for mechanochemical coupling in force regulation.

How far in-silico computing meets real experiments. A study on the structure and dynamics of spin labeled vinculin tail protein by molecular dynamics simulations and EPR spectroscopy

Background

Investigation of conformational changes in a protein is a prerequisite to understand its biological function. To explore these conformational changes in proteins we developed a strategy with the combination of molecular dynamics (MD) simulations and electron paramagnetic resonance (EPR) spectroscopy. The major goal of this work is to investigate how far computer simulations can meet the experiments.

Methods

Vinculin tail protein is chosen as a model system as conformational changes within the vinculin protein are believed to be important for its biological function at the sites of cell adhesion. MD simulations were performed on vinculin tail protein both in water and in vacuo environments. EPR experimental data is compared with those of the simulated data for corresponding spin label positions.

Results

The calculated EPR spectra from MD simulations trajectories of selected spin labelled positions are comparable to experimental EPR spectra. The results show that the information contained in the spin label mobility provides a powerful means of mapping protein folds and their conformational changes.

Conclusions

The results suggest the localization of dynamic and flexible regions of the vinculin tail protein. This study shows MD simulations can be used as a complementary tool to interpret experimental EPR data.

New insights into vinculin function and regulation

Vinculin is a cytoplasmic actin-binding protein enriched in focal adhesions and adherens junctions that is essential for embryonic development. Much is now known regarding the role of vinculin in governing cell-matrix adhesion. In the past decade that the crystal structure of vinculin and the molecular details for how vinculin regulates adhesion events have emerged. The recent data suggests a critical function for vinculin in regulating integrin clustering, force generation, and strength of adhesion. In addition to an important role in cell-matrix adhesion, vinculin is also emerging as a regulator of apoptosis, Shigella entry into host cells, and cadherin-based cell-cell adhesion. A close inspection of this work reveals that there are similarities between vinculin's role in focal adhesions and these processes and also some intriguing differences.

A helix replacement mechanism directs metavinculin functions

Cells require distinct adhesion complexes to form contacts with their neighbors or the extracellular matrix, and vinculin links these complexes to the actin cytoskeleton. Metavinculin, an isoform of vinculin that harbors a unique 68-residue insert in its tail domain, has distinct actin bundling and oligomerization properties and plays essential roles in muscle development and homeostasis. Moreover, patients with sporadic or familial mutations in the metavinculin-specific insert invariably develop fatal cardiomyopathies. Here we report the high resolution crystal structure of the metavinculin tail domain, as well as the crystal structures of full-length human native metavinculin (1,134 residues) and of the full-length cardiomyopathy-associated ΔLeu954 metavinculin deletion mutant. These structures reveal that an α-helix (H1′) and extended coil of the metavinculin insert replace α-helix H1 and its preceding extended coil found in the N-terminal region of the vinculin tail domain to form a new five-helix bundle tail domain. Further, biochemical analyses demonstrate that this helix replacement directs the distinct actin bundling and oligomerization properties of metavinculin. Finally, the cardiomyopathy associated ΔLeu954 and Arg975Trp metavinculin mutants reside on the replaced extended coil and the H1′ α-helix, respectively. Thus, a helix replacement mechanism directs metavinculin's unique functions.

Solution structure of the first SH3 domain of human vinexin and its interaction with vinculin peptides

Solution structure of the first Src homology (SH) 3 domain of human vinexin (V_SH3_1) was determined using nuclear magnetic resonance (NMR) method and revealed that it was a canonical SH3 domain, which has a typical beta-beta-beta-beta-alpha-beta fold. Using chemical shift perturbation and surface plasmon resonance experiments, we studied the binding properties of the SH3 domain with two different peptides from vinculin hinge regions: P856 and P868. The observations illustrated slightly different affinities of the two peptides binding to V_SH3_1. The interaction between P868 and V_SH3_1 belonged to intermediate exchange with a modest binding affinity, while the interaction between P856 and V_SH3_1 had a low binding affinity. The structure and ligand-binding interface of V_SH3_1 provide a structural basis for the further functional study of this important molecule.

Direct interaction of the C-terminal domain of alpha-catenin and F-actin is necessary for stabilized cell-cell adhesion

Alpha-catenin functions to anchor adherens junctions to the filamentous actin (F-actin) cytoskeleton, through direct and indirect binding mechanisms. When truncated at amino acid 865, alpha-catenin exhibited a markedly reduced F-actin binding affinity compared to wild-type. Expression of the truncated mutant in the alpha-catenin deficient colon carcinoma cell line, Clone A, could not restore an adhesive phenotype when compared. Furthermore, the truncated alpha-catenin fusion protein failed to concentrate at sites of cell-cell contact, to promote morphological changes associated with epithelial monolayers, and to stimulate resistance to shearing forces in a hanging drop aggregation assay. Subsequent attempts to isolate single residues governing the direct F-actin interaction, using neutralizing charge or reverse charge mutations of basic residues within a homology modeled alpha-catenin C-terminal 5-helix bundle, had no effect on F-actin cosedimentation. We conclude that direct attachment of alpha-catenin to F-actin is required to promote cadherin-mediated contact formation and strong cell-cell adhesive states.

The vinculin binding sites of talin and alpha-actinin are sufficient to activate vinculin

Vinculin regulates both cell-cell and cell-matrix junctions and anchors adhesion complexes to the actin cytoskeleton through its interactions with the vinculin binding sites of alpha-actinin or talin. Activation of vinculin requires a severing of the intramolecular interactions between its N- and C-terminal domains, which is necessary for vinculin to bind to F-actin; yet how this occurs in cells is not resolved. We tested the hypothesis that talin and alpha-actinin activate vinculin through their vinculin binding sites. Indeed, we show that these vinculin binding sites have a high affinity for full-length vinculin, are sufficient to sever the head-tail interactions of vinculin, and they induce conformational changes that allow vinculin to bind to F-actin. Finally, microinjection of these vinculin binding sites specifically targets vinculin in cells, disrupting its interactions with talin and alpha-actinin and disassembling focal adhesions. In their native (inactive) states the vinculin binding sites of talin and alpha-actinin are buried within helical bundles present in their central rod domains. Collectively, these results support a model where the engagement of adhesion receptors first activates talin or alpha-actinin, by provoking structural changes that allow their vinculin binding sites to swing out, which are then sufficient to bind to and activate vinculin.

Role of interaction with vinculin in recruitment of vinexins to focal adhesions

Although vinexin was originally identified as a protein binding to the proline-rich hinge region of vinculin, the functions and biochemical properties of the vinexin-vinculin interaction are not known. Here, we determined the affinity of the vinexin-vinculin interaction using surface plasmon resonance measurements and found that vinexin beta interacts with the C-terminal half of vinculin, which mimics an activated "open" form, with a threefold higher affinity than with the full-length "closed" vinculin. Coimmunoprecipitation experiments showed that cell adhesion on fibronectin enhances the vinexin-vinculin interaction. We also show that the interaction with vinculin is necessary for the efficient localization of vinexin alpha and beta at focal adhesions. These observations suggest a model that "activated" vinculin localized at focal adhesions recruits vinexins to focal adhesions.

Vinculin acts as a sensor in lipid regulation of adhesion-site turnover

The dynamics of cell adhesion sites control cell morphology and motility. Adhesion-site turnover is thought to depend on the local availability of the acidic phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2). PIP2 can bind to many cell adhesion proteins such as vinculin and talin, but the consequences of this interaction are poorly understood. To study the significance of phospholipid binding to vinculin for adhesion-site turnover and cell motility, we constructed a mutant, vinculin-LD, deficient in acidic phospholipid binding yet with functional actin-binding sites. When expressed in cells, vinculin-LD was readily recruited to adhesion sites, as judged by fluorescence recovery after photobleaching (FRAP) analysis, but cell spreading and migration were strongly impaired, and PIP2-dependent disassembly of adhesions was suppressed. Thus, PIP2 binding is not essential for vinculin activation and recruitment, as previously suggested. Instead, we propose that PIP2 levels can regulate the uncoupling of adhesion sites from the actin cytoskeleton, with vinculin functioning as a sensor.

Two distinct head-tail interfaces cooperate to suppress activation of vinculin by talin

Vinculin is autoinhibited by an intramolecular interaction that masks binding sites for talin and F-actin. Although a recent structural model explains autoinhibition solely in terms of the interaction between vinculin tail (Vt) and residues 1-258 (D1), we find an absolute requirement for an interface involving the D4 domain of head (Vh residues 710-836) and Vt. Charge-to-alanine mutations in Vt revealed a class of mutants, T12 and T19, distal to the V-(1-258) binding site, which showed increases in their Kd values for head binding of 100- and 42-fold, respectively. Reciprocal mutation of residues in the D4 domain that contact Vt yielded a head-tail interaction mutant of comparable magnitude to T19. These findings account for the approximately 120-fold difference in Kd values between Vt binding to V-(1-258), as opposed to full-length Vh-(1-851). The significance of a bipartite autoinhibitory site is evidenced by its effects on talin binding to Vh. Whereas Vt fails to compete with the talin rod domain for binding to V-(1-258), competition occurs readily with full-length Vh, and this requires the D4 interface. Moreover in intact vinculin, mutations in the D4-Vt interface stabilize association of vinculin and talin rod. In cells, these head-tail interaction mutants induce hypertrophy and elongation of focal adhesions. Definition of a second autoinhibitory site, the D4-Vt interface, supports the competing model of vinculin activation that invokes cooperative action of ligands at two sites. Together the D1-Vt and D4-Vt interfaces provide the high affinity (approximately 10(-9)) autoinhibition observed in full-length vinculin.

Structural basis for amplifying vinculin activation by talin

Talin interactions with vinculin are essential for focal adhesions. Curiously, talin contains three noncontiguous vinculin binding sites (VBS) that can bind individually to the vinculin head (Vh) domain. Here we report the crystal structure of the human Vh.VBS1 complex, a validated model of the Vh.VBS2 structure, and biochemical studies that demonstrate that all of talin VBSs activate vinculin by provoking helical bundle conversion of the Vh domain, which displaces the vinculin tail (Vt) domain. Thus, helical bundle conversion is a structurally conserved response in talin-vinculin interactions. Furthermore, talin VBSs bind to Vh in a mutually exclusive manner but do differ in their affinity for Vh and in their ability to displace Vt, suggesting that the strengths of these interactions could lead to differences in signaling outcome. These findings support a model in which talin binds to and activates multiple vinculin molecules to provoke rapid reorganization of the actin cytoskeleton.

Vinculin activation by talin through helical bundle conversion

Vinculin is a conserved component and an essential regulator of both cell-cell (cadherin-mediated) and cell-matrix (integrin-talin-mediated focal adhesions) junctions, and it anchors these adhesion complexes to the actin cytoskeleton by binding to talin in integrin complexes or to alpha-actinin in cadherin junctions. In its resting state, vinculin is held in a closed conformation through interactions between its head (Vh) and tail (Vt) domains. The binding of vinculin to focal adhesions requires its association with talin. Here we report the crystal structures of human vinculin in its inactive and talin-activated states. Talin binding induces marked conformational changes in Vh, creating a novel helical bundle structure, and this alteration actively displaces Vt from Vh. These results, as well as the ability of alpha-actinin to also bind to Vh and displace Vt from pre-existing Vh-Vt complexes, support a model whereby Vh functions as a domain that undergoes marked structural changes that allow vinculin to direct cytoskeletal assembly in focal adhesions and adherens junctions. Notably, talin's effects on Vh structure establish helical bundle conversion as a signalling mechanism by which proteins direct cellular responses.

Further characterization of the interaction between the cytoskeletal proteins talin and vinculin

The cytoskeletal protein talin, which is thought to couple integrins to F-actin, contains three binding sites (VBS1-VBS3) for vinculin, a protein implicated in the negative regulation of cell motility and whose activity is modulated by an intramolecular interaction between the vinculin head (Vh) and vinculin tail (Vt) domains. In the present study we show that recombinant talin polypeptides containing the three VBSs (VBS1, residues 498-636; VBS2, residues 727-965; and VBS3, residues 1943-2157) each bind tightly to the same or overlapping sites within vinculin(1-258). A short synthetic talin VBS3 peptide (residues 1944-1969) was sufficient to inhibit binding of a (125)I-labelled talin VBS3 polypeptide to vinculin(1-258), and NMR spectroscopy confirmed that this peptide forms a 1:1 complex in slow exchange with vinculin(1-258). Binding of the (125)I-labelled VBS3 polypeptide was markedly temperature dependent, but was not inhibited by 1 M salt or 10% (v/v) 2-methyl-2-propanol. Attempts to further define the talin-binding site within vinculin(1-258) using a gel-blot assay were unsuccessful, but near maximal talin-binding activity was retained by a construct spanning vinculin residues 1-131 in a yeast two-hybrid assay. Interestingly, the talin VBS3 polypeptide was a potent inhibitor of the Vh-Vt interaction, and the VBS3 synthetic peptide was able to expose the actin-binding site in intact vinculin, which is otherwise masked by the Vh-Vt interaction. The results suggest that under certain conditions, talin may be an effective activator of vinculin.

A lipid-regulated docking site on vinculin for protein kinase C

During cell spreading, binding of actin-organizing proteins to acidic phospholipids and phosphorylation are important for localization and activity of these proteins at nascent cell-matrix adhesion sites. Here, we report on a transient interaction between the lipid-dependent protein kinase Calpha and vinculin, an early component of these sites, during spreading of HeLa cells on collagen. In vitro binding of protein kinase Calpha to vinculin tail was found dependent on free calcium and acidic phospholipids but independent of a functional kinase domain. The interaction was enhanced by conditions that favor the oligomerization of vinculin. Phosphorylation by protein kinase Calpha reached 1.5 mol of phosphate/mol of vinculin tail and required the C-terminal hydrophobic hairpin, a putative phosphatidylinositol 4,5-bisphosphate-binding site. Mass spectroscopy of peptides derived from in vitro phosphorylated vinculin tail identified phosphorylation of serines 1033 and 1045. Inhibition of C-terminal phospholipid binding at the vinculin tail by mutagenesis or deletion reduced the rate of phosphorylation to < or =50%. We suggest a possible mechanism whereby phospholipid-regulated conformational changes in vinculin may lead to exposure of a docking site for protein kinase Calpha and subsequent phosphorylation of vinculin and/or vinculin interaction partners, thereby affecting the formation of cell adhesion complexes.

Molecular complexity and dynamics of cell-matrix adhesions

Currently &gt;50 proteins have been reported to be associated with focal contacts and related ECM adhesions. Most of these contain multiple domains through which they can interact with different molecular partners, potentially forming a dense and heterogeneous protein network at the cytoplasmic faces of the adhesion site. The molecular and structural diversity of this ‘submembrane plaque’ is regulated by a wide variety of mechanisms, including competition between different partner proteins for the same binding sites, interactions triggered or suppressed by tyrosine phosphorylation, and conformational changes in component proteins, which can affect their reactivity. Indeed, integrin-mediated adhesions can undergo dynamic changes in structure and molecular properties from dot-like focal complexes to stress-fiber-associated focal contacts, which can further ‘mature’ to form fibronectin-bound fibrillar adhesions. These changes are driven by mechanical force generated by the actin- and myosin-containing contractile machinery of the cells, or by external forces applied to the cells, and regulated by matrix rigidity.

Molecular architecture of adherens junctions

Adherens junctions are composed of a cadherin-catenin complex and its associated proteins. Recently, an increasing number of novel members of adherens junctions, including membrane and PDZ proteins, have been reported. Interactions among these components in adherens junctions seem to be dynamically regulated during the formation of adherens junction complexes in epithelial cells.

Conformational change in the vinculin C-terminal depends on a critical histidine residue (His-906)

A phospholipid-controlled interaction between the N-terminal and C-terminal domains of vinculin is thought to be a major mechanism that regulates binding activities of the protein. To probe the mechanisms underlying these interactions we used chemical modification and site-directed mutagenesis directed at histidine residues. Diethylpyrocarbonate (DEPC) modification of the C-terminal, but not the N-terminal, domain greatly decreased affinity of the N-terminal-C-terminal binding, implicating histidine residues in the C-terminal. Mutation of either or both C-terminal histidines (at positions 906 and 1026), however, did not affect N-C binding at neutral pH. The H906A mutation did prevent DEPC effects and also prevented the normal decrease in binding affinity for the N-terminal at lower pH. We found that the wild type C-terminal domain, but not the H906A mutant, underwent a conformational change at pH 6.5, reflected in an altered circular dichroism spectrum and apparent oligomerization. Phospholipid also induced conformational changes in the wild type C-terminal domain but not in the H906A mutant, even though the mutant protein did bind to the phospholipid. Finally, the sensitivity of the N-C interaction to phospholipid was much reduced by the H906A mutation. These results show that H906 plays a key role in the conformational dynamics of the C-terminal domain and thus the regulation of vinculin.