Vinculin interaction with permeabilized cells: disruption and reconstitution of a binding site

Force transduction by Triton cytoskeletons

Force-initiated signal transduction can occur either via membrane-based ionic mechanisms or through changes in cytoskeletal-matrix linkages. We report here the stretch-dependent binding of cytoplasmic proteins to Triton X-100 cytoskeletons of L-929 cells grown on collagen-coated silicone. Triton X-100-insoluble cytoskeletons were stretched by 10% and incubated with biotinylated cytoplasmic proteins. Analysis with two-dimensional gel electrophoresis showed stretch-dependent binding of more than 10 cytoplasmic protein spots. Bound cytoplasmic proteins were purified by a photocleavable biotin tag and stretch-dependent binding of paxillin, focal adhesion kinase, and p130Cas was found, whereas the binding of vinculin was unchanged and actin binding decreased with stretch. Paxillin binding upon stretch was morphologically and biochemically similar in vitro and in vivo, that is, enhanced in the periphery and inhibited by the tyrosine phosphatase inhibitor, phenylarsine oxide. Thus, we suggest that transduction of matrix forces occurs through force-dependent conformation changes in the integrated cytoskeleton.

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.

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

The vinculin head to tail intramolecular self-association controls its binding sites for other components of focal adhesions. To study this interaction, the head and tail domains were expressed, purified, and assayed for various characteristics of complex formation. Analytical centrifugation demonstrated a strong interaction in solution and formation of a complex more asymmetric than either of the individual domains. A survey of binding conditions using a solid-phase binding assay revealed characteristics of both electrostatic and hydrophobic forces involved in the binding. In addition, circular dichroism of the individual domains and the complex demonstrated that conformational changes likely occur in both domains during association. The interaction sites were more closely mapped on the protein sequence by deletion mutagenesis. Amino acids 181-226, a basic region within the acidic head domain, were identified as a binding site for the vinculin tail, and residues 1009-1066 were identified as sufficient for binding the head. Moreover, mutation of an acidic patch in the tail (residues 1013-1015) almost completely eliminated its ability to interact with the head domain further supporting the significance of ionic interactions in the binding. Our data indicate that the interaction between the head and tail domains of vinculin occurs through oppositely charged contact sites and results in conformational changes in both domains.

Vinculin and talin: kinetics of entry and exit from the cytoskeletal pool

Vinculin and talin, two major components of focal contacts, exist in cytosolic and cytoskeletal pools. The kinetics of entry and exit of the two proteins between the two pools were investigated in normal and transformed cells. In cultured chick embryo fibroblasts, a fraction (2-5%) of the newly synthesized vinculin and talin reached maximal levels in the cytoskeleton in 30-45 min. Both proteins had 2-3 times shorter half-lives in the cytoskeletal pool (t1/2 = 6-7 h) than in the cytosolic pool (t1/2 = 14-15 h), which suggests that the incorporation of cytosolic vinculin and talin into the cytoskeleton does not involve a simple equilibrium between the two pools. However, after disruption of cell-to-substrate adhesion by trypsinization, an equilibrium in the incorporation between the two pools was transiently established, resulting in the use of the preexisting cytosolic pools of the two proteins during re-establishment of cell-to-matrix contacts. Viral transformation did not cause a significant change in the incorporation rates into the cytoskeleton. However, it decreased the half-lives of both proteins in the cytoskeletal pool (t1/2 = approximately 4 h) and in the cytosolic pool (t1/2 = 9-10 h). The increased turn-over rates of vinculin and talin in the cytoskeletal pool in transformed cells may contribute to the enhanced motility of transformed cells.

A cell-free system to study regulation of focal adhesions and of the connected actin cytoskeleton

Assembly and modulation of focal adhesions during dynamic adhesive processes are poorly understood. We describe here the use of ventral plasma membranes from adherent fibroblasts to explore mechanisms regulating integrin distribution and function in a system that preserves the integration of these receptors into the plasma membrane. We find that partial disruption of the cellular organization responsible for the maintenance of organized adhesive sites allows modulation of integrin distribution by divalent cations. High Ca2+ concentrations induce quasi-reversible diffusion of beta1 integrins out of focal adhesions, whereas low Ca2+ concentrations induce irreversible recruitment of beta1 receptors along extracellular matrix fibrils, as shown by immunofluorescence and electron microscopy. Both effects are independent from the presence of actin stress fibers in this system. Experiments with cells expressing truncated beta1 receptors show that the cytoplasmic portion of beta1 is required for low Ca2+-induced recruitment of the receptors to matrix fibrils. Analysis with function-modulating antibodies indicates that divalent cation-mediated receptor distribution within the membrane correlates with changes in the functional state of the receptors. Moreover, reconstitution experiments show that purified alpha-actinin colocalizes and redistributes with beta1 receptors on ventral plasma membranes depleted of actin, implicating binding of alpha-actinin to the receptors. Finally, we found that recruitment of exogenous actin is specifically restricted to focal adhesions under conditions in which new actin polymerization is inhibited. Our data show that the described system can be exploited to investigate the mechanisms of integrin function in an experimental setup that permits receptor redistribution. The possibility to uncouple, under cell-free conditions, events involved in focal adhesion and actin cytoskeleton assembly should facilitate the comprehension of the underlying molecular mechanisms.

Isolation and contraction of the stress fiber

Stress fibers were isolated from cultured human foreskin fibroblasts and bovine endothelial cells, and their contraction was demonstrated in vitro. Cells in culture dishes were first treated with a low-ionic-strength extraction solution and then further extracted using detergents. With gentle washes by pipetting, the nucleus and the apical part of cells were removed. The material on the culture dish was scraped, and the freed material was forced through a hypodermic needle and fractionated by sucrose gradient centrifugation. Isolated, free-floating stress fibers stained brightly with fluorescently labeled phalloidin. When stained with anti-alpha-actinin or anti-myosin, isolated stress fibers showed banded staining patterns. By electron microscopy, they consisted of bundles of microfilaments, and electron-dense areas were associated with them in a semiperiodic manner. By negative staining, isolated stress fibers often exhibited gentle twisting of microfilament bundles. Focal adhesion-associated proteins were also detected in the isolated stress fiber by both immunocytochemical and biochemical means. In the presence of Mg-ATP, isolated stress fibers shortened, on the average, to 23% of the initial length. The maximum velocity of shortening was several micrometers per second. Polystyrene beads on shortening isolated stress fibers rotated, indicating spiral contraction of stress fibers. Myosin regulatory light chain phosphorylation was detected in contracting stress fibers, and a myosin light chain kinase inhibitor, KT5926, inhibited isolated stress fiber contraction. Our study demonstrates that stress fibers can be isolated with no apparent loss of morphological features and that they are truly contractile organelle.

Tyrosine phosphorylation and cytoskeletal tension regulate the release of fibroblast adhesions

We have investigated the mechanisms by which fibroblasts release their adhesions to the extracellular matrix substrata using a permeabilized cell system in which the adhesions remain relatively stable. A large number of different molecules were assayed for their effect on focal adhesion stability using immunofluorescence with antibodies against different focal adhesion constituents. ATP uniquely stimulates a rapid breakdown of focal adhesions, and at high ATP concentrations (> 5 mM), many cells are released from the dish. The remaining cells appear contracted with talin, alpha-actinin, and vinculin localized diffusely throughout the cell. Integrin containing tracks of variable intensity outline the regions where cells had resided before they detached from the substratum. At lower ATP concentrations (0.5-5 mM) the cells remain spread; however the focal adhesion components, including integrin, show an array of phenotypes ranging from diffusely localized throughout the cell to a localization in small, thin focal adhesions. Okadaic acid, a serine, threonine phosphatase inhibitor, enhances the contracted phenotype, even at low concentrations (0.5 mM) of ATP. The localization of focal adhesion components is different in okadaic acid-treated cells. In highly contracted cells, integrin is present in tracks where the cells resided before the contraction; however focal adhesions are no longer apparent. Talin, vinculin, and alpha-actinin localize in trabecular networks toward the periphery of the cell. Interestingly, phosphotyrosine staining as well as nascent, intracellular integrin precedes the recruitment of focal adhesion constituents into the trabecular network. The ATP-stimulated focal adhesion breakdown appears to operate through two mechanisms. First, ATP stimulates the tyrosine phosphorylation of several cytoskeletally associated proteins. These tyrosine phosphorylations correlated well with focal adhesion breakdown. Furthermore, addition of a recombinant, constitutively active tyrosine phosphatase inhibits both the tyrosine phosphorylations and the breakdown of the focal adhesions. None of the major tyrosine phosphoproteins are FAK, integrin, tensin, paxillin, or other phosphoproteins implicated in focal adhesion assembly. The second mechanism is cell contraction. High ATP concentrations, or lower ATP concentrations in the presence of okadaic acid induce cell contraction. Inhibiting the contraction by addition of a heptapeptide IRICRKG, which blocks the actin-myosin interaction, also inhibits focal adhesion breakdown. Neither the peptide nor the phosphatase inhibits focal adhesion breakdown under all conditions suggesting that both tension and tyrosine phosphorylations mediate the release of adhesions.

Convergence of integrin and growth factor receptor signaling pathways within the focal adhesion complex

Extracellular matrix controls capillary endothelial cell sensitivity to soluble mitogens by binding integrin receptors and thereby activating a chemical signaling response that rapidly integrates with growth factor-induced signaling mechanisms. Here we report that in addition to integrins, growth factor receptors and multiple molecules that transduce signals conveyed by both types of receptors are immobilized on the cytoskeleton (CSK) and spatially integrated within the focal adhesion complex (FAC) at the site of integrin binding. FACs were rapidly induced in round cells and physically isolated from the remainder of the CSK after detergent-extraction using magnetic microbeads coated with fibronectin or a synthetic RGD-containing peptide. Immunofluorescence microscopy revealed that multiple signaling molecules (e.g., pp60c-src, pp125FAK, phosphatidylinositol-3-kinase, phospholipase C-gamma, and Na+/H+ antiporter) involved in both integrin and growth factor receptor signaling pathways became associated with the CSK framework of the FAC within 15 min after binding to beads coated with integrin ligands. Recruitment of tyrosine kinases to the FAC was also accompanied by a local increase in tyrosine phosphorylation, as indicated by enhanced phosphotyrosine staining at the site of integrin binding. In contrast, neither recruitment of signaling molecules nor increased phosphotyrosine staining was observed when cells bound to beads coated with a control ligand (acetylated low density lipoprotein) that ligates transmembrane scavenger receptors, but does not induce FAC formation. Western blot analysis confirmed that FACs isolated using RGD-beads were enriched for pp60c-src, pp125FAK, phospholipase C-gamma, and the Na+/H+ antiporter when compared with intact CSK or basal cell surface preparations that retained lipid bilayer. Isolated FACs were also greatly enriched for the high affinity fibroblast growth factor receptor flg. Most importantly, isolated FACs continued to exhibit multiple chemical signaling activities in vitro, including protein tyrosine kinase activities (pp60c-src and pp125FAK) as well as the ability to undergo multiple sequential steps in the inositol lipid synthesis cascade. These data suggest that many of the chemical signaling events that are induced by integrins and growth factor receptors in capillary cells may effectively function in a "solid-state" on insoluble CSK scaffolds within the FAC and that the FAC may represent a major site for signal integration between these two regulatory pathways. Future investigations into the biochemical and biophysical basis of signal transduction may be facilitated by this method, which results in isolation of FACs that retain the CSK framework as well as multiple associated chemical signaling activities.

Characterization of an F-actin-binding domain in the cytoskeletal protein vinculin

Vinculin, a major structural component of vertebrate cell-cell and cell-matrix adherens junctions, has been found to interact with several other junctional components. In this report, we have identified and characterized a binding site for filamentous actin. These results included studies with gizzard vinculin, its proteolytic head and tail fragments, and recombinant proteins containing various gizzard vinculin sequences fused to the maltose binding protein (MBP) of Escherichia coli. In cosedimentation assays, only the vinculin tail sequence mediated a direct interaction with actin filaments. The binding was saturable, with a dissociation constant value in the micromolar range. Experiments with deletion clones localized the actin-binding domain to a region confined by residues 893-1016 in the 170-residue-long carboxyterminal segment, while the proline-rich hinge connecting the globular head to the rodlike tail was not required for this interaction. In fixed and permeabilized cells (cell models), as well as after microinjection, proteins containing the actin-binding domain specifically decorated stress fibers and the cortical network of fibroblasts and epithelial cells, as well as of brush border type microvilli. These results corroborated the sedimentation experiments. Our data support and extend previous work showing that vinculin binds directly to actin filaments. They are consistent with a model suggesting that in adhesive cells, the NH2-terminal head piece of vinculin directs this molecule to the focal contact sites, while its tail segment causes bundling of the actin filament ends into the characteristic spear tip-shaped structures.

Suppression of tumorigenicity in transformed cells after transfection with vinculin cDNA

Transfection of chicken vinculin cDNA into two tumor cell lines expressing diminished levels of the endogenous protein, brought about a drastic suppression of their tumorigenic ability. The SV-40-transformed Balb/c 3T3 line (SVT2) contains four times less vinculin than the parental 3T3 cells, and the rat adenocarcinoma BSp73ASML has no detectable vinculin. Restoration of vinculin in these cells, up to the levels found in 3T3 cells, resulted in an apparent increase in substrate adhesiveness, a decrease in the ability to grow in soft agar, and suppression of their capacity to develop tumors after injection into syngeneic hosts or nude mice. These results suggest that vinculin, a cytoplasmic component of cell-matrix and cell-cell adhesions, may have a major suppressive effect on the transformed phenotype.

Accumulation of talin in nodes at the edge of the lamellipodium and separate incorporation into adhesion plaques at focal contacts in fibroblasts

The focal contact forms beneath F-actin-rich ribs, or cytoplasmic precursors, present in the lamellipodia of fibroblasts. The basal part of the precursor is retained at the contact as the initial adhesion plaque. We have examined the distribution of talin in the lamellipodia and adhesion plaques of chicken embryo fibroblasts relative to the process of focal contact formation. Motility of single cells was recorded with differential interference contrast or interference reflection microscopy before fixation and fluorescent staining for talin, F-actin, and vinculin. Talin is present along the extreme edge of the lamellipodium, where it is further concentrated into a series of nodes. The nodes of talin are present at the tips of both larger and finer F-actin-rich ribs and at small structural nodes at the edge of the lamellipodium. We suggest that the talin in the nodes functions, via a cross-linking activity, in the convergence of actin filaments at the membrane during development of the ribs. Talin accumulates de novo in the adhesion plaque, independent of that at the tip of the precursor, in response to contact with the substrate. This second accumulation of talin at the focal contact starts before vinculin, consistent with a sequential binding of talin at the membrane and of vinculin to talin. The results imply that talin functions independently at two steps during formation of the focal contact: the development of the F-actin-rich precursor of the contact; and development of the contact-associated adhesion plaque, both involving organization of F-actin at the membrane.

A 165 kDa membrane antigen mediating fibronectin-vinculin interaction is involved in murine odontoblast differentiation

Membrane-mediated matrix-microfilament interactions are involved in odontoblast differentiation. In this study, we analyzed the interactions of vinculin and fibronectin with plasma membrane proteins separated by sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis, and then transferred onto polyvinylidene-difluoride (PVDF) paper. Vinculin was found to interact with 58, 63 and 165 kDa plasma membrane proteins. Fibronectin interacted with three high molecular weight (145, 165, and 185 kDa) membrane proteins. Attempts were made to characterize the 165 kDa protein which interacted with vinculin and with fibronectin. The interaction of the 165 kDa protein with fibronectin was not competitively inhibited by synthetic peptides such as GRGDS or GRGDSP, suggesting that the protein was not related to integrins. Antibodies directed against the 165 kDa protein allowed the identification of the precise localization and biological role of this membrane antigen. The data presented in this paper and previous observations indicate that the 165 kDa protein, involved in odontoblast elongation and polarization, mediates a fibronectin-vinculin transmembrane interaction.

Purification and characterization of an 85 kDa talin-binding fragment of vinculin

Vinculin and talin are adhesion plaque proteins which have been shown to interact with each other in vitro. In order to begin to investigate where the talin-binding domain is in vinculin, vinculin was digested with Staphylococcus aureus V8 protease to generate two major fragments of 85 and 30 kDa, and these fragments were purified. Nitrocellulose overlays with 125I-talin and the 125I-85 kDa vinculin fragment and sucrose density gradient centrifugation demonstrated that the talin-binding domain was localized to the 85 kDa vinculin fragment. Quantification of 125I-talin binding in the overlays showed that four times more talin bound to the 85 kDa fragment as compared to intact vinculin. Competitive immunoprecipitation experiments demonstrated that unlabeled 85 kDa fragment was about three-fold more effective at competing for 125I-85 kDa binding to talin than was unlabeled vinculin. These results suggest that the 30 kDa fragment inhibits the vinculin-talin interaction even though the talin-binding domain is localized in the 85 kDa fragment.

Vinculin

Vinculin is clearly a key element in the transmembrane assemblages that link cells to each other or to the substrate. However, despite all the studies that have been done on the protein, we still do not know its function within these assemblages. The bulk of the biochemical and cell biological evidence suggests that, in some unknown way, its presence in the junctions may be involved in the stable association of actin with the membrane, yet vinculin by itself does not appear to interact with actin. In the future, identification of additional junctional molecules that interconnect actin and vinculin may resolve this dilemma. Alternatively, studies with vinculin that is phosphorylated or acylated may yield clues to its function. Perhaps the complexity of the protein composition of microfilament-containing junctions suggests that protein assemblages rather than individual proteins provide novel functions. As new proteins belonging to these junctions are discovered, it will be important to assess their interaction with already known components such as vinculin and to ask if the protein combination has a particular function.

Identification of two distinct functional domains on vinculin involved in its association with focal contacts

We report here on the identification of two distinct functional domains on chicken vinculin molecule, which can, independently, mediate its interaction with focal contacts in living cells. These findings were obtained by immunofluorescent labeling of COS cells transfected with a series of chicken vinculin-specific cDNA constructs derived from clones cVin1 and cVin5 (Bendori, R., D. Salomon, and B. Geiger. 1987. EMBO [Eur. Mol. Biol. Organ.] J. 6:2897-2905). These included a chimeric construct consisting of 5' sequences of cVin1 attached to the complementary 3' region of cVin5, as well as several constructs of either cVin1 or cVin5 from which 3' or 5' sequences were deleted. We show here that the products of both cVin1 and cVin5, and of the cVin1/cVin5 chimera, readily associated with focal contacts in transfected COS cells. Furthermore, 78 and 45 kD NH2-terminal fragments encoded by a deleted cVin1 and the 78-kD COOH-terminal portion of vinculin encoded by cVin5 were capable of binding specifically to focal contact areas. In contrast 3'-deletion mutants prepared from clone cVin5 and a 5'-deletion mutant of cVin1, lacking both NH2- and COOH-terminal sequences, failed to associate with focal contacts in transfected cells. The loss of binding was accompanied by an overall disarray of the microfilament system. These results, together with previous in vitro binding studies, suggest that vinculin contains at least two independent sites for binding to focal contacts; the NH2-terminal domain may contain the talin binding site while the COOH-terminal domain may mediate vinculin-vinculin interaction. Moreover, the disruptive effect of the double-deleted molecule (lacking the two focal-contact binding sites) on the organization of actin suggests that a distinct region involved in the binding of vinculin to the microfilament system is present in the NH2-terminal 45-kD region of the molecule.

Focal contacts: transmembrane links between the extracellular matrix and the cytoskeleton

The sites of tightest adhesion that form between cells and substrate surfaces in tissue culture are termed focal contacts. The external faces of focal contacts include specific receptors, belonging to the integrin family of proteins, for fibronectin and vitronectin, two common components of extracellular matrices. On the internal (cytoplasmic) side of focal contacts, several proteins, including talin and vinculin, mediate interactions with the actin filament bundles of the cytoskeleton. The changes that occur in focal contacts as a result of viral transformation are discussed.

Clostridium difficile toxin B induces reorganization of actin, vinculin, and talin in cultured cells

Clostridium difficile toxin B is a powerful cytopathic agent which causes animal cells in culture to become rounded and arborized, an effect similar to that induced by the cytochalasins. In this study, we demonstrated that the morphological effects of the toxin are directed specifically against the actin and related components of the cytoskeleton. Dramatic disruption and reorganization of the actin stress fibers were detectable prior to significant changes in cell shape and alterations in the microtubular and intermediate filament networks. Along with F-actin, the adhesion plaque proteins, vinculin and talin were localized in intoxicated cells in a patchy pattern reminiscent of that seen in cells treated with phorbol esters or transformed by oncogenic viruses. A quantitative fluorescence assay for cellular F-actin showed that these morphological changes were accompanied by a modest net depolymerization of only 15 to 20% of the actin filaments in the cell, and that depolymerization was closely correlated with changes in cell shape. In complementary studies on cells spreading on a substrate, we found that the toxin affected the actin content and the shape of the processes extended from the cell body. As in cells treated with cytochalasin, there was a differential response between normal and virally transformed cells spreading in the presence of the toxin. The results of this study support the view that C. difficile toxin B affects one or more cellular components that regulate the structure and function of the actin cytoskeleton, and that its predominant effect is to cause a dramatic disruption of stress fibers and relocalization of the F-actin.+

Age and growth-related changes in cyclopiazonic acid-potentiated lipophilic cation accumulation by cultured cells and binding to freeze-thaw lysed cells

In a previous study (1) we demonstrated that increased tetraphenylphosphonium (TPP) uptake by renal epithelial cells (LLC-PK1) exposed to the fungal metabolite cyclopiazonic acid (CPA) was not a result of hyperpolarization across the plasma membrane even though CPA-potentiated TPP uptake could be totally inhibited by the depolarizing agent carbonylcyanide-m-chlorophenylhydrazone (CCCP). We now demonstrate that CPA potentiates TPP accumulation by proliferating skeletal muscle (L6) and LLC-PK1 cells but not by nonproliferating primary rat hepatocytes. In LLC-PK1 cells, CPA-potentiated TPP accumulation is observed in cells at all ages. In L6 cells, CPA-potentiated TPP accumulation is maximal soon after subculturing, and as the cells age they become less sensitive to CPA until TPP accumulation by CPA-treated cells approaches that of untreated cells. The temporal change in sensitivity of L6 cells to CPA may be related to biochemical and/or metabolic changes which occur as the cells age in culture. Hepatocytes, LLC-PK1 cells, and L6 cells permeabilized by freeze-thaw lysis, all exhibit CPA-potentiated TPP partitioning, even in the presence of CCCP. This result indicates that both TPP and CPA must have access to the intracellular space in order for potentiated TPP partitioning to be observed. We hypothesize that the site of interaction between CPA and TPP is intracellular and probably associated with the cytoplasmic side of the plasma membrane and possibly the mitochondria.