Polyphosphoinositides inhibit the interaction of vinculin with actin filaments.

Binding of vinculin to adhesion plaque proteins is restricted by an intramolecular association of vinculin's head and tail regions. Results of previous work suggest that polyphosphoinositides disrupt this interaction and thereby promote binding of vinculin to both talin and actin. However, data presented here show that phosphatidylinositol 4,5-bisphosphate (PI4,5P2) inhibits the interaction of purified tail domain with F-actin. Upon re-examining the effect of PI4,5P2 on the actin and talin-binding activities of intact vinculin, we find that when the experimental design controls for the effect of magnesium on aggregation of PI4,5P2 micelles, polyphosphoinositides promote interactions with the talin-binding domain, but block interactions of the actin-binding domain. In contrast, if vinculin is trapped in an open confirmation by a peptide specific for the talin-binding domain of vinculin, actin binding is allowed. These results demonstrate that activation of the actin-binding activity of vinculin requires steps other than or in addition to the binding of PI4,5P2.     

Activation of vinculin induced by cholinergic stimulation regulates contraction of tracheal smooth muscle tissue

Vinculin localizes to membrane adhesion junctions where it links actin filaments to the extracellular matrix by binding to the integrin-binding protein talin at its head domain (Vh) and to actin filaments at its tail domain (Vt). Vinculin can assume an inactive (closed) conformation in which Vh and Vt bind to each other, masking the binding sites for actin and talin, and an active (open) conformation in which the binding sites for talin and actin are exposed. We hypothesized that the contractile activation of smooth muscle tissues might regulate the activation of vinculin and thereby contribute to the regulation of contractile tension. Stimulation of tracheal smooth muscle tissues with acetylcholine (ACh) induced the recruitment of vinculin to cell membrane and its interaction with talin and increased the phosphorylation of membrane-localized vinculin at the C-terminal Tyr-1065. Expression of recombinant vinculin head domain peptide (Vh) in smooth muscle tissues, but not the talin-binding deficient mutant head domain, VhA50I, inhibited the ACh-induced recruitment of endogenous vinculin to the membrane and the interaction of vinculin with talin and also inhibited vinculin phosphorylation. Expression of Vh peptide also inhibited ACh-induced smooth muscle contraction and inhibited ACh-induced actin polymerization; however, it did not affect myosin light chain phosphorylation, which is necessary for cross-bridge cycling. Inactivation of RhoA inhibited vinculin activation in response to ACh. We conclude that ACh stimulation regulates vinculin activation in tracheal smooth muscle via RhoA and that vinculin activation contributes to the regulation of active tension by facilitating connections between actin filaments and talin-integrin adhesion complexes and by mediating the initiation of actin polymerization.     

A conformational switch in vinculin drives formation and dynamics of a talin-vinculin complex at focal adhesions

Dynamic interactions between the cytoskeleton and integrins control cell adhesion, but regulatory mechanisms remain largely undefined. Here, we tested the extent to which the autoinhibitory head-tail interaction (HTI) in vinculin regulates formation and lifetime of the talin-vinculin complex, a proposed mediator of integrin-cytoskeleton bonds. In an ectopic recruitment assay, mutational reduction of HTI drove assembly of talin-vinculin complexes, whereas ectopic complexes did not form between talin and wild-type vinculin. Moreover, reduction of HTI altered the dynamic assembly of vinculin and talin in focal adhesions. Using fluorescence recovery after photobleaching, we show that the focal adhesion residency time of vinculin was enhanced up to 3-fold by HTI mutations. The slow dynamics of vinculin correlated with exposure of its cryptic talin-binding site, and a talin-binding site mutation rescued the dynamics of activated vinculin. Significantly, HTI-deficient vinculin inhibited the focal adhesion dynamics of talin, but not paxillin or alpha-actinin. These data show that talin conformation in cells permits vinculin binding, whereas the autoinhibited conformation of vinculin constitutes the barrier to complex formation. Down-regulation of HTI in vinculin to Kd approximately 10(-7) is sufficient to induce talin binding, and HTI is essential to the dynamics of vinculin and talin at focal adhesions. We therefore conclude that vinculin conformation, as modulated by the strength of HTI, directly regulates the formation and lifetime of talin-vinculin complexes in cells.     

Detection of metavinculin in human platelets using a modified talin overlay assay

Talin was purified from human platelets and proteolytically cleaved by the calcium-dependent protease (CDP II) to two stable fragments of 200 and 47 kDa. The 200 kDa fragment was radiolabeled and used in Western blot overlay assays of fractionated platelet proteins. This procedure revealed vinculin to be the major talin binding protein. However, in addition, a less abundant protein of approximately 150 kDa also interacted strongly with the talin fragment. Using conventional immunoblot analysis we have confirmed that this protein is metavinculin, a protein previously believed to be confined to cardiac and smooth muscle tissue.     

Functional studies of the domains of talin

The protein talin has two domains of approximately 200 and 47 kD, which can be cleaved apart by a variety of proteases. To examine the function of these two structural domains of talin, we have digested purified talin with a calcium-dependent protease and separated the resulting fragments chromatographically. Both fragments were radioiodinated and used to probe Western blots of whole fibroblasts and chicken gizzard extracts. The large talin fragment bound to vinculin and metavinculin. The small fragment did not demonstrate any binding in this assay. The fragments were labeled fluorescently and microinjected into fibroblasts in tissue culture. The large talin fragment incorporated quickly into focal adhesions where it remained stable for at least 14 h. The small fragment associated with focal adhesions of fibroblasts but was also distributed diffusely in the cytoplasm and the nucleus. These experiments suggest that talin has at least two sites that contribute to its localization in focal adhesions. Intact talin microinjected into Madin-Darby bovine kidney epithelial cells localized to the focal adhesions but was excluded from the zonulae adherentes, despite the localization of vinculin to both of these sites. In contrast, the large talin fragment, when microinjected into these epithelial cells, incorporated into both focal adhesions and zonulae adherentes. The difference in localization between the large talin fragment and intact talin seems to be due to the removal of the small domain. This difference in localization suggests that talin binding sites in zonulae adherentes have limited accessibility.     

The adhesion plaque protein, talin, is phosphorylated in vivo in chicken embryo fibroblasts exposed to a tumor-promoting phorbol ester.

Talin is a high molecular weight phosphoprotein that is localized at adhesion plaques. We have found that talin phosphorylation increases 3.0-fold upon exposure of chicken embryo fibroblasts to the tumor-promoting phorbol ester, phorbol 12-myristate 13-acetate. Talin isolated from tumor promoter-treated cells is phosphorylated on serine and threonine residues. Vinculin, a 130 kDa talin-binding protein, also exhibits increased phosphorylation in vivo in response to tumor promoter, but to a lesser degree than does talin. Because tumor-promoting phorbol esters augment protein kinase C activity, we have compared the ability of purified protein kinase C to phosphorylate talin and vinculin in vitro. Both talin and vinculin were found to be substrates for protein kinase C; however, talin was phosphorylated to a greater extent than was vinculin. Cleavage of protein kinase C-phosphorylated talin by the calcium-dependent protease (Type II) revealed that while both the resulting 190-200 and 46 kDa proteolytic peptides were phosphorylated, the majority of label was contained within the 46-kDa fragment. Although incubation of chicken embryo fibroblasts with tumor-promoting phorbol ester induces a dramatic increase in talin phosphorylation, we detected no change in the organization of stress fibers and focal contacts in these cells. Exposure of the cells to tumor promoter did, however, result in a loss of actin and talin-rich cell surface elaborations that resemble focal contact precursor structures.     

Vinculin binding site mapped on talin with an anti-idiotypic antibody.

Vinculin and talin are major adhesion plaque components which interact in vitro and presumably in vivo. The amino acid sequence of talin is now known so details of its domain structure can be mapped. We localized vinculin binding sites in the talin sequence by overlaying peptide maps of talin with an anti-idiotypic vinculin antibody that recognizes talin and with 125I-vinculin. A rabbit injected only twice with vinculin and producing anti-vinculin antibodies spontaneously generated a second antibody that recognizes talin. Vinculin and anti-vinculin antibodies specifically compete with this second antibody for binding to talin as determined by solid-phase binding and overlay assays. The antibody is thus most likely an anti-idiotypic antibody which mimics a region of vinculin that interacts with talin. The binding site of the anti-idiotypic antibody on talin was mapped to the 196 amino acids spanning residues 1653 to 1848. A second vinculin binding site identified with an 125I-vinculin blot overlay technique was located between residues 483 and 1652. The observation that talin has two immunologically distinct vinculin binding sites suggests that vinculin may have two different talin binding sites or one "complex" site with two interacting regions.     

Intermolecular versus intramolecular interactions of the vinculin binding site 33 of talin

The cytoskeletal proteins talin and vinculin are localized at cell‐matrix junctions and are key regulators of cell signaling, adhesion, and migration. Talin couples integrins via its FERM domain to F‐actin and is an important regulator of integrin activation and clustering. The 220 kDa talin rod domain comprises several four‐ and five‐helix bundles that harbor amphipathic α‐helical vinculin binding sites (VBSs). In its inactive state, the hydrophobic VBS residues involved in binding to vinculin are buried within these helix bundles, and the mechanical force emanating from bound integrin receptors is thought necessary for their release and binding to vinculin. The crystal structure of a four‐helix bundle of talin that harbors one of these VBSs, coined VBS33, was recently determined. Here we report the crystal structure of VBS33 in complex with vinculin at 2 Å resolution. Notably, comparison of the apo and vinculin bound structures shows that intermolecular interactions of the VBS33 α‐helix with vinculin are more extensive than the intramolecular interactions of the VBS33 within the talin four‐helix bundle.     

Interaction of the 47-kDa talin fragment and the 32-kDa vinculin fragment with acidic phospholipids: a computer analysis.

In recent in vitro experiments, it has been demonstrated that the 47-kDa fragment of the talin molecule and the 32-kDa fragment of the vinculin molecule interact with acidic phospholipids. By using a computer analysis method, we determined the hydrophobic and amphipathic stretches of these fragments and, by applying a purpose-written matrix method, we ascertained the molecular amphipathic structure of alpha-helices. Calculations for the 47-kDa mouse talin fragment (residues 1-433; NH2-terminal region) suggest specific interactions of residues 21-39, 287-342, and 385-406 with acidic phospholipids and a general lipid-binding domain for mouse talin (primary amino acid sequence 385-401) and for Dictyostelium talin (primary amino acid sequence 348-364). Calculations for the 32-kDa chicken embryo vinculin fragment (residues 858-1066; COOH-terminal region) and from nematode vinculin alignment indicate for chicken embryo vinculin residues 935-978 and 1020-1040 interactions with acidic phospholipids. Experimental confirmation has been given for vinculin (residues 916-970), and future detailed experimental analyses are now needed to support the remaining computational data.     

Interaction of iodinated vinculin, metavinculin and α-actinin with cytoskeletal proteins

Iodinated vinculin, metavinculin and α-actinin were used to probe the interaction of these proteins with electrophoretically separated cytoskeletal proteins. Using the gel overlay technique, we detected strong binding of ¹²⁵I-vinculin and ¹²⁵I-metavinculin to α-actinin, 175 kDa polypeptide, talin, vinculin and metavinculin themselves, and moderate binding to actin.¹²⁵I-α-actinin was capable of interacting with vinculin and metavinculin. The specific binding of ¹²⁵-I-α-actinin to vinculin and metavinculin immobilized on a polysterene surface was also demonstrated. We suggest that the ability of vinculin and α-actinin to form a complex may be realized in microfilament-membrane linkages.     

Paxillin: a new vinculin-binding protein present in focal adhesions

The 68-kD protein (paxillin) is a cytoskeletal component that localizes to the focal adhesions at the ends of actin stress fibers in chicken embryo fibroblasts. It is also present in the focal adhesions of Madin-Darby bovine kidney (MDBK) epithelial cells but is absent, like talin, from the cell-cell adherens junctions of these cells. Paxillin purified from chicken gizzard smooth muscle migrates as a diffuse band on SDS-PAGE gels with a molecular mass of 65-70 kD. It is a protein of multiple isoforms with pIs ranging from 6.31 to 6.85. Using purified paxillin, we have demonstrated a specific interaction in vitro with another focal adhesion protein, vinculin. Cleavage of vinculin with Staphylococcus aureus V8 protease results in the generation of two fragments of approximately 85 and 27 kD. Unlike talin, which binds to the large vinculin fragment, paxillin was found to bind to the small vinculin fragment, which represents the rod domain of the molecule. Together with the previous observation that paxillin is a major substrate of pp60src in Rous sarcoma virus-transformed cells (Glenney, J. R., and L. Zokas. 1989. J. Cell Biol. 108:2401-2408), this interaction with vinculin suggests paxillin may be a key component in the control of focal adhesion organization.     

The cytoskeletal protein talin contains at least two distinct vinculin binding domains

We have mapped the vinculin-binding sites in the cytoskeletal protein talin as well as those sequences which target the talin molecule to focal contacts. Using a series of overlapping talin-fusion proteins expressed in E. coli and 125I-vinculin in both gel-overlay and microtitre well binding assays, we present evidence for three separable binding sites for vinculin. All three are in the tail segment of talin (residues 434-2541) and are recognized by the same fragment of vinculin (residues 1-258). Two sites are adjacent to each other and span residues 498-950, and the third site is more than 700 residues distant in the primary sequence. Scatchard analysis of 125I-vinculin binding to talin also indicates three sites, each with a similar affinity (Kd = 2- 6 x 10(-7) M). We also detect a substoichiometric interaction of higher affinity (Kd = 3 x 10(-8) M) which remains unexplained. By expressing regions of the chicken talin molecule in heterologous cells, we have shown that the sequences required to target talin to focal contacts overlap those which bind vinculin.     

Structural and dynamic characterization of a vinculin binding site in the talin rod

Talin is a key protein involved in linking integrins to the actin cytoskeleton. The long flexible talin rod domain contains a number of binding sites for vinculin, a cytoskeletal protein important in stabilizing integrin-mediated cell-matrix junctions. Here we report the solution structure of a talin rod polypeptide (residues 1843-1973) which contains a single vinculin binding site (VBS; residues 1944-1969). Like other talin rod polypeptides, it consists of a helical bundle, in this case a four-helix bundle with a right-handed topology. The residues in the VBS important for vinculin binding were identified by studying the binding of a series of VBS-related peptides to the vinculin Vd1 domain. The key binding determinants are buried in the interior of the helical bundle, suggesting that a substantial structural change in the talin polypeptide is required for vinculin binding. Direct evidence for this was obtained by NMR and EPR spectroscopy. [1H,15N]-HSQC spectra of the talin fragment indicate that vinculin binding caused approximately two-thirds of the protein to adopt a flexible random coil. For EPR spectroscopy, nitroxide spin labels were attached to the talin polypeptide via appropriately located cysteine residues. Measurements of inter-nitroxide distances in doubly spin-labeled protein showed clearly that the helical bundle is disrupted and the mobility of the helices, except for the VBS helix, is markedly increased. Binding of vinculin to talin is thus a clear example of the unusual phenomenon of protein unfolding being required for protein/protein interaction.     

Purification of a 190 kDa protein from smooth muscle: relationship to talin

Several studies of vinculin-binding proteins have described a 190 kDa protein in chicken gizzard smooth muscle which binds radioiodinated vinculin. We have purified and studied the 190 kDa protein from chicken gizzard smooth muscle. By indirect immunofluorescence, an antiserum raised against the 190 kDa protein stains adhesion plaques (focal contacts), ruffling membranes, and fibrillar streaks on the dorsal and ventral surfaces of fibroblasts. Both the binding to vinculin and the location of the protein in fibroblasts are properties shared with talin, a 215 kDa protein in smooth muscle and fibroblasts. Because antisera against talin and the 190 kDa cross-react the relationship of these two proteins has been investigated further. Upon prolonged storage at 4 degrees C, purified talin degrades into a 190 kDa fragment. A 190 kDa fragment is also generated from talin by the Staphylococcus aureus V-8 proteinase and by trypsin. Comparison of partial peptide maps of talin and the 190 kDa protein reveal that the proteins are very similar and when the 190 kDa fragment of talin is compared with the purified 190 kDa protein by partial proteolytic digestion no differences are found in the pattern of peptides generated. In addition, the amount of 190 kDa protein detected in muscle tissues excised from chick embryos can be drastically reduced if proteinase inhibitors are added to the tissue homogenates. We conclude that the purified 190 kDa dalton protein is a proteolytic fragment of talin. Although markedly reduced by proteinase inhibitors, detection of the 190 kDa protein is not completely abolished, suggesting that some talin may already be cleaved within living cells.     

Degraded collagen fragments promote rapid disassembly of smooth muscle focal adhesions that correlates with cleavage of pp125(FAK), paxillin, and talin.

Active matrix metalloproteinases and degraded collagen are observed in disease states, such as atherosclerosis. To examine whether degraded collagen fragments have distinct effects on vascular smooth muscle cells (SMC), collagenase-digested type I collagen was added to cultured human arterial SMC. After addition of collagen fragments, adherent SMC lose their focal adhesion structures and round up. Analysis of components of the focal adhesion complex demonstrates rapid cleavage of the focal adhesion kinase (pp125(FAK)), paxillin, and talin. Cleavage is suppressed by inhibitors of the proteolytic enzyme, calpain I. In vitro translated pp125(FAK) is a substrate for both calpain I- and II-mediated processing. Mapping of the proteolytic cleavage fragments of pp125(FAK) predicts a dissociation of the focal adhesion targeting (FAT) sequence and second proline-rich domain from the tyrosine kinase domain and integrin-binding sequence. Coimmunoprecipitation studies confirm that the ability of pp125(FAK) to associate with paxillin, vinculin, and p130cas is significantly reduced in SMC treated with degraded collagen fragments. Further, there is a significant reduction in the association of intact pp125(FAK) with the cytoskeletal fraction, while pp125(FAK) cleavage fragments appear in the cytoplasm in SMC treated with degraded collagen fragments. Integrin-blocking studies indicate that integrin-mediated signals are involved in degraded collagen induction of pp125(FAK) cleavage. Thus, collagen fragments induce distinct integrin signals that lead to initiation of calpain-mediated cleavage of pp125(FAK), paxillin, and talin and dissolution of the focal adhesion complex.     

A vinculin binding domain from the talin rod unfolds to form a complex with the vinculin head

The cytoskeletal protein talin plays a key role in activating integrins and in coupling them to the actin cytoskeleton. Its N-terminal globular head, which binds beta integrins, is linked to an extended rod having a C-terminal actin binding site and several vinculin binding sites (VBSs). The NMR structure of residues 755-889 of the rod (containing a VBS) is shown to be an amphipathic four-helix bundle with a left-handed topology. A talin peptide corresponding to the VBS binds the vinculin head; the X-ray crystallographic structure of this complex shows that the residues which interact with vinculin are buried in the hydrophobic core of the talin fragment. NMR shows that the interaction involves a major structural change in the talin fragment, including unfolding of one of its helices, making the VBS accessible to vinculin. Interestingly, the talin 755-889 fragment binds more than one vinculin head molecule, suggesting that the talin rod may contain additional as yet unrecognized VBSs.     

Partial cleavage mapping of the cytoskeletal protein vinculin. Antibody and talin binding sites.

Vinculin is a 1066-amino acid protein found at several types of actin-membrane junction. To locate sites of interest in the primary structure, a map was derived using partial cleavage reactions. Of several different types of cleavage tested, the most useful was the 5-5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) reaction which cuts at cysteine residues. About 30 well defined fragments were obtained from vinculin, and several methods were used to locate these products in the sequence. Comparison of the peptides generated from whole vinculin with those from the 90-kDa amino-terminal proteolytic fragment revealed which originated there. The use of [14C]cyanide in conjunction with DTNB showed which peptides contained the original amino terminus. Secondary cleavage with N-chlorosuccinimide, a tryptophan-specific reagent, helped locate fragments, although it led to apparent increases in molecular weight of the products. These experiments revealed the location of 10 of the major DTNB fragments on the sequence. This map was used to locate binding sites. The site of interaction between vinculin and the focal contact protein talin was mapped by binding labeled talin to the separated fragments. The binding site was found to be in the amino-terminal 325 amino acids. The binding site of a commercially obtained monoclonal antivinculin antibody was mapped using Western blotting of cleaved vinculin. It proved to bind in the central area of the molecule between amino acid residues 545 and 737. Thus the cysteine cleavage reaction products provide a map of general utility for locating features on the vinculin molecule.     

Structural and biophysical properties of the integrin-associated cytoskeletal protein talin

Talin is a large cytoskeletal protein (2541 amino acid residues) which plays a key role in integrin-mediated events that are crucial for cell adhesion, migration, proliferation and survival. This review summarises recent work on the structure of talin and on some of the structurally better defined interactions with other proteins. The N-terminal talin head (approx. 50 kDa) consists of an atypical FERM domain linked to a long flexible rod (approx. 220 kDa) made up of a series of amphipathic helical bundle domains. The F3 FERM subdomain in the head binds the cytoplasmic tail of integrins, but this interaction can be inhibited by an interaction of F3 with a helical bundle in the talin rod, the so-called “autoinhibited form” of the molecule. The talin rod contains a second integrin-binding site, at least two actin-binding sites and a large number of binding sites for vinculin, which is important in reinforcing the initial integrin–actin link mediated by talin. The vinculin binding sites are defined by hydrophobic residues buried within helical bundles, and these must unfold to allow vinculin binding. Recent experiments suggest that this unfolding may be mediated by mechanical force exerted on the talin molecule by actomyosin contraction.