A re-examination of the interaction of vinculin with actin

TNS1: Emerging Insights into Its Domain Function, Biological Roles, and Tumors

Tensins are a family of cellular-adhesion constituents that have been extensively studied. They have instrumental roles in the pathogenesis of numerous diseases. The mammalian tensin family comprises four members: tensin1 (TNS1), tensin2, tensin3, and tensin4. Among them, TNS1 has recently received attention from researchers because of its structural properties. TNS1 engages in various biological processes, such as cell adhesion, polarization, migration, invasion, proliferation, apoptosis, and mechano-transduction, by interacting with various partner proteins. Moreover, the abnormal expression of TNS1 in vivo is associated with the development of various diseases, especially tumors. Interestingly, the role of TNS1 in different tumors is still controversial. Here, we systematically summarize three aspects of TNS1: the gene structure, the biological processes underlying its action, and the dual regulatory role of TNS1 in different tumors through different mechanisms, of which we provide the first overview.

Tensins - emerging insights into their domain functions, biological roles and disease relevance

Tensins are a family of focal adhesion proteins consisting of four members in mammals (TNS1, TNS2, TNS3 and TNS4). Their multiple domains and activities contribute to the molecular linkage between the extracellular matrix and cytoskeletal networks, as well as mediating signal transduction pathways, leading to a variety of physiological processes, including cell proliferation, attachment, migration and mechanical sensing in a cell. Tensins are required for maintaining normal tissue structures and functions, especially in the kidney and heart, as well as in muscle regeneration, in animals. This Review discusses our current understanding of the domain functions and biological roles of tensins in cells and mice, as well as highlighting their relevance to human diseases.

Molecular physiology of the tensin brotherhood of integrin adaptor proteins

Numerous proteins have been identified as constituents of the adhesome, the totality of molecular components in the supramolecular assemblies known as focal adhesions, fibrillar adhesions and other kinds of adhesive contact. The transmembrane receptor proteins called integrins are pivotal adhesome members, providing a physical link between the extracellular matrix (ECM) and the actin cytoskeleton. Tensins are ever more widely investigated intracellular adhesome constituents. Involved in cell attachment and migration, cytoskeleton reorganization, signal transduction and other processes relevant to cancer research, tensins have recently been linked to functional properties of deleted in liver cancer 1 (DLC1) and a mitogen-activated protein kinases (MAPK), to cell migration in breast cancer, and to metastasis suppression in the kidney. Tensins are close relatives of phosphatase homolog/tensin homolog (PTEN), an extensively studied tumor suppressor. Such findings are recasting the earlier vision of tensin (TNS) as an actin-filament (F-actin) capping protein in a different light. This critical review aims to summarize current knowledge on tensins and thus to highlight key points concerning the expression, structure, function, and evolution of the various members of the TNS brotherhood. Insight is sought by comparisons with homologous proteins. Some historical points are added for perspective.

The interaction of vinculin with actin

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.

The interface between a cell and its substrate is strengthened by the formation of focal adhesions. In this study molecular dynamics simulations are used to explore the connectivity of one focal adhesion forming protein, vinculin, and the cytoskeletal filament, F-actin. The simulations demonstrate: (1) that vinculin can link along F-actin at these focal adhesions when it adopts an open conformation, (2) that the vinculin tail (Vt) can bind F-actin at its barbed-end preventing actin polymerization, (3) that vinculin can adopt two open conformations, and (4) that the second open conformation is necessary for vinculin to cap the actin filament. The results suggest that vinculin can act as a variable switch, changing its shape and the nature of its interaction with F-actin depending on the level of stress seen at a focal adhesion. Under the highest stress vinculin would adopt the open II conformation and link anywhere on F-actin, even its barbed-end. Under less stress vinculin could adopt the open I conformation and bind along F-actin. And under minimal stress vinculin could adopt its closed conformation. This variability allows for vinculin to truly function as the cell's mechanical reinforcing agent.

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.

Vinculin nucleates actin polymerization and modifies actin filament structure

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.

Cleavage of tensin during cytoskeleton disruption in YTX-induced apoptosis

Yessotoxin (YTX) is a marine algal toxin previously shown to induce apoptosis in L6 and BC3H1 myoblast cell lines. Disassembly of the F-actin cytoskeleton and cleavage of tensin, a cytoskeletal protein localised at the focal adhesion contacts, appear during this apoptotic process. Tensin binds to actin filaments at the focal adhesion contacts and it links the actin cytoskeleton to the extracellular matrix (ECM). This binding occurs via integrin receptors and it makes tensin a potential link between the actin cytoskeleton and signal transduction. This study evaluates disruption in the F-actin cytoskeleton and change of tensin in myoblast cell lines exposed to 100 nM YTX up to 72 h. YTX treatment cleaves tensin and makes it translocate to the cell centre. Tensin has normally a role in the maintenance of cell shape and YTX-treatment may therefore alter the shape of the cells. YTX exposure also induces formation of lamellas associated with pseudopodia. Alternative linkages and cytoskeletal proteins anchoring the actin filaments to focal contacts remain to be identified.

Cloning and characterization of betaCAP73, a novel regulator of beta-actin assembly

In non-muscle cells, the isoactins are differentially localized, with beta-actin specifically enriched at the cell cortex within motile structures, such as lamellae, while gamma-actin shows no specific localization. To understand the sorting and regulation of beta-actin within moving cells, we previously isolated betaCAP73, a novel beta-actin-specific binding protein (Cell Motil. Cytoskel. 35 (1996) 175). Here, we have cloned and characterized the 4718 nucleotide betaCAP73 cDNA from an endothelial cell library. betaCAP73 cDNA contains six predicted ankyrin-like repeats at the amino terminus and is partially homologous to three previously reported sequences of unknown function. Northern analysis reveals betaCAP73 expression in all tissues tested, with highest levels in skeletal muscle. Consistent with previously demonstrated interactions between native betaCAP73 and beta-actin filament barbed-ends, recombinant betaCAP73 inhibits pyrene-actin assembly in an isoactin-specific manner. Compared to stationary cells betaCAP73 mRNA is down regulated in crawling cells. Similarly, motility-defective cells have increased betaCAP73 protein. Overexpression of full-length betaCAP73 induces the formation of novel membrane protrusions that are enriched in betaCAP73, while overexpression of betaCAP73 domains alters cell morphology. Combined, these results indicate that betaCAP73 modulates isoactin dynamics to regulate the morphological alterations required for cell growth and motility.

Alpha v integrin antagonists induce the disassembly of focal contacts in melanoma cells

In recent years, several antagonists of alpha(v)beta3 have been used to develop therapeutic approaches to the treatment of melanoma neoplasia. We studied the effects of anti-alpha(v)-integrin-blocking antibodies on attached M21 melanoma cells, the cellular distribution of alpha(v)-integrin and the molecular organization of focal structures. Anti-alpha(v)-integrin-blocking antibodies 17E6 and LM609, and an anti-alpha(v)beta3-integrin antagonist peptide cRGD 85189 induced detachment of M21 melanoma cells cultured for 24 hours on various substrates. cRGD was the most effective antagonist, reducing the number of adherent cells by 80%, while 17E6 reduced adhesion by only 30%. Light- and electron microscopy revealed attached cells with a flat shape and well-formed actin cytoskeleton. After treatment, cells became rounded and detached from the culture dish. alpha(v)-Integrins and focal-contact proteins were observed at adhesion sites in focal structures by immunocytochemistry. After treatment, however, cell rounding was accompanied by disorganization of the actin filaments and redistribution of alpha(v)-integrins and most of the focal proteins studied, except vinculin and tensin. Our results indicate that treatment of M21 melanoma cells with a(v)-integrin antagonists disrupts the actin cytoskeleton, redistributes a(v)-integrin and induces molecular disassembly of focal contacts.

Derivation of insertin

Insertin is an actin-binding protein that has been isolated from chicken gizzard smooth muscle that has been shown to be highly homologous to amino acids 962-1292 of tensin [Weigt et al., 1992]. Because of the high homology, we investigated the question whether the mRNAs of insertin and of tensin are derived from the same gene by alternative splicing, whether insertin and tensin are encoded by two different genes, or whether insertin is a proteolytic fragment of tensin. In a Northern blot analysis, mRNA from chicken gizzard was hybridized with oligonucleotides specific for tensin and for the insertin domain of tensin. The tensin-specific oligonucleotide hybridized only with the previously reported 8- and 10-kbp RNAs. However, the insertin domain-specific oligonucleotide hybridized with a 1.2 and a 1.6 kbp RNA in addition to the 8 and 10 kbp RNA. The 1.2- and 1.6-kbp RNA occurred in small amounts, as compared with the 8- and 10-kbp RNA. Southern blot analysis of DNA cleaved by the restriction endonucleases BamH1 and HindIII demonstrated that only one gene for the insertin and tensin exists. Insertin isolated from chicken gizzard smooth muscle was investigated by mass spectrometry. The N-termini of three isolated peptides were found to begin at adjacent amino acids and were likely to be formed from tensin by proteolysis. The results suggest that, for insertin, an mRNA exists that is derived from one gene common for insertin and tensin. However, the insertin-specific mRNA contributes relatively little to expression of insertin domains in cells. Insertin preparations from chicken gizzard contain mainly insertin domains formed from tensin by proteolysis.

Vinculin is associated with the E-cadherin adhesion complex

Cadherins mediate calcium-dependent cell-cell adhesion, and this activity is regulated by cytoplasmic interactions between cadherins, catenins, and the actin-based cytoskeleton. alpha-Catenin plays a critical role in the transmembrane anchorage of cadherins, and deletion of alpha-catenin has been shown to inactivate cadherin-mediated adhesion, resulting in a nonadhesive phenotype. Here we show that serum starvation increases E-cadherin expression and induces E-cadherin-dependent adhesion in the MDA-MB-468 breast cancer cell line. This adhesion occurred despite a lack of alpha-catenin expression, which was caused by mutations in the alpha-catenin gene. Coprecipitation analysis suggests that this adhesion may be mediated by cytoplasmic connections from cadherins to the cytoskeleton involving vinculin. A high level of vinculin associated with E-cadherin immunoprecipitates was observed in MDA-MB-468 cells. In contrast, vinculin was not detected in E-cadherin complexes in the A431 and MCF-7 epithelial carcinoma cell lines, which express alpha-catenin. However, in reciprocal immunoprecipitations using anti-vinculin antibodies, E-cadherin associated strongly with vinculin in MDA-MB-468 cells and, to a lesser extent, in A431 and MCF-7 cells. These results suggest that both alpha-catenin and vinculin may be present in the adhesion complex. To test the hypothesis that vinculin associates with E-cadherin complexes via beta-catenin, excess recombinant beta-catenin or alpha-catenin fusion protein was added to MDA-MB-468 cell lysates. Both specifically inhibited the coprecipitation of E-cadherin with vinculin, suggesting competition for the same binding site. These results suggest that vinculin plays a role in the establishment or regulation of the cadherin-based cell adhesion complex by direct interaction with beta-catenin.

The N-terminal domains of tensin and auxilin are phosphatase homologues

Tensin, an actin filament capping protein, and auxilin, a component of receptor-mediated endocytosis, are known to have 350 residue regions of significant sequence similarity near their N-termini (Schröder et al., 1995, Eur J Biochem 228:297-304). Here we demonstrate that these regions are homologous, not only to each other, but also to the catalytic domain of a putative protein tyrosine phosphatase (PTP) from Saccharomyces cerevisiae and to other PTPs. We propose that the PTP-like portion of the homology region of tensin and auxilin represents a distinct domain. A detailed sequence comparison indicates that the PTP-like domain in tensin is unlikely to exhibit phosphatase activity, whereas in auxilin it may possess a different phosphatase specificity from tyrosine phosphatases. It is probable that the PTP-like domains in tensin and auxilin mediate binding interactions with phosphorylated polypeptides; they may therefore represent members of a distinct class of phosphopeptide recognition domain.

H2O2-treated actin: assembly and polymer interactions with cross-linking proteins

During inflammation, hydrogen peroxide, produced by polymorphonuclear leukocytes, provokes cell death mainly by disarranging filamentous (polymerized) actin (F-actin). To show the molecular mechanism(s) by which hydrogen peroxide could alter actin dynamics, we analyzed the ability of H2O2-treated actin samples to polymerize as well as the suitability of actin polymers (from oxidized monomers) to interact with cross-linking proteins. H2O2-treated monomeric (globular) actin (G-actin) shows an altered time course of polymerization. The increase in the lag phase and the lowering in both the polymerization rate and the polymerization extent have been evidenced. Furthermore, steady-state actin polymers, from oxidized monomers, are more fragmented than control polymers. This seems to be ascribable to the enhanced fragility of oxidized filaments rather than to the increase in the nucleation activity, which markedly falls. These facts; along with the unsuitability of actin polymers from oxidized monomers to interact with both filamin and alpha-actinin, suggest that hydrogen peroxide influences actin dynamics mainly by changing the F-actin structure. H2O2, via the oxidation of actin thiols (in particular, the sulfhydryl group of Cys-374), likely alters the actin C-terminus, influencing both subunit/subunit interactions and the spatial structure of the binding sites for cross-linking proteins in F-actin. We suggest that most of the effects of hydrogen peroxide on actin could be explained in the light of the "structural connectivity," demonstrated previously in actin.

Molecular cloning, expression, and mapping of the high affinity actin-capping domain of chicken cardiac tensin

Tensin, an actin filament capping protein first purified from chicken gizzard, is localized to various types of adherens junctions in muscle and nonmuscle cells. In this paper, we describe the isolation and sequencing of tensin cDNA from a chicken cardiac library. The 6.3-kb chicken cardiac tensin cDNA encodes an open reading frame of 1,792 amino acids. Mammalian cells transfected with the chicken tensin cDNA expressed a polypeptide of approximately 200 kD recognizable by antibodies to chicken gizzard tensin. The expressed protein was incorporated into focal adhesions and other actin-containing structures in the transfected cells. To map the domain associated with tensin's high affinity, barbed-end F-actin-capping activity, bacterially expressed recombinant fusion proteins containing various segments of tensin were prepared and assayed for activity. The results of these experiments show that the high affinity capping domain (kD = 1.3 nM) lies within amino acid residues R1037-V1169. Additional studies on a shorter construct, S1061-H1145, showed that these 85 residues were sufficient for producing complete inhibition of actin polymerization and depolymerization. While this active domain is located within that of the "insertin" sequence (Weigt, C., A. Gaertner, A. Wegner, H. Korte, and H. E. Meyer. 1992. J. Mol. Biol. 227:593-595), our data showing complete inhibition of polymerization and shift in critical concentration are consistent with a simple barbed-end capping mechanism rather than the "insertin model." Our results also differ from those of a recent report (Lo, S. H., P. A. Janmey, J. H. Hartwig, and L. B. Chen. 1994. J. Cell Biol. 125:1067-1075), which concluded that their recombinant tensin has an "insertin-like" inhibitory effect on barbed-end actin polymerization, and that this activity is attributed to residues T936-R1037 (residues 888-989 in their numbering system). In our study, a fusion construct (N790-K1060) encompassing T936-R1037 had no significant effect on actin polymerization and depolymerization, even at high concentrations.

F-actin binding site masked by the intramolecular association of vinculin head and tail domains

Although vinculin is present at all sites of F-actin attachment to plasma membranes and is required for linkage of myofibrils to sarcolemma, it is unclear how it promotes attachment of actin to membranes. Because biochemical evidence for a direct interaction of vinculin with F-actin is controversial, current models of actin-membrane linkages depict only an indirect role for vinculin, as a tether for alpha-actinin. We demonstrate here that an intramolecular association between the 95K head and 30K tail domains of vinculin masks an F-actin binding site present in the carboxy-terminal tail domain. Cosedimentation and crosslinking assays, and direct visualization by transmission electron microscopy, reveal an interaction between F-actin and a bacterially expressed fusion protein containing amino acids 811-1066 of vinculin, and between F-actin and a proteolytic fragment of vinculin containing amino acids 858-1066. Vinculin itself neither cosediments with nor crosslinks F-actin. The amino-terminal 95K head fragment of vinculin, but not intact vinculin, inhibits both cosedimentation and crosslinking. We propose that assembly of vinculin into an adherens junction involves disruption of the head-tail interaction, revealing a site that mediates microfilament attachment.

N-ethylmaleimide-modified actin filaments do not bundle in the presence of alpha-actinin

We show that the modification of actin subdomain 1 by N-ethylmaleimide (NEM), which binds Cys-374 close to the C-terminus of the molecule, inhibits the alpha-actinin-induced bundling of actin filaments. This effect is not merely related to the block of Cys-374, since N-(1-pyrenyl)iodoacetamide (pyrene-IA) is unable to prevent bundling. Considering that NEM (but not pyrene-IA) influences actin assembly, we suggest that the inhibition of the actin-alpha-actinin interaction is due to the chemical modification of actin Cys-374 which, by inducing a marked spatial reorganization of actin monomers, is able to modify both the intra- and inter-molecular interactions of this protein. Finally, NEM-modified actin filaments form bundles in the presence of polyethylene glycol 6000 since, in this case, the side by side association of actin filaments does not depend on the accessibility of binding sites nor on the formation of chemical bonds.

Tensin: a potential link between the cytoskeleton and signal transduction

Cytoskeletal proteins provide the structural foundation that allows cells to exist in a highly organized manner. Recent evidence suggests that certain cytoskeletal proteins not only maintain structural integrity, but might also be associated with signal transduction and suppression of tumorigenesis. Since the time of the discovery of tensin, a fair amount of data has been gathered which supports the notion that tensin is one such protein possessing these characteristics. In this review, we discuss recent studies that: (1) elucidate a role for tensin in maintenance of cellular structure and signal transduction; (2) implicate tensin as the anchor for actin filaments at the focal adhesion; (3) describe the phosphorylation of tensin; (4) describe potential targets for its Src homology region 2 domain; (5) describe the association between tensin and the nuclear protein p130; and (6) demonstrate that increased tensin expression in a cell line appears to reduce its transformation potential.

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.

Interactions of tensin with actin and identification of its three distinct actin-binding domains

Tensin, a 200-kD phosphoprotein of focal contacts, contains sequence homologies to Src (SH2 domain), and several actin-binding proteins. These features suggest that tensin may link the cell membrane to the cytoskeleton and respond directly to tyrosine kinase signalling pathways. Here we identify three distinct actin-binding domains within tensin. Recombinant tensin purified after overexpression by a baculovirus system binds to actin filaments with Kd = 0.1 microM, cross-links actin filaments at a molar ratio of 1:10 (tensin/actin), and retards actin assembly by barbed end capping with Kd = 20 nM. Tensin fragments were constructed and expressed as fusion proteins to map domains having these activities. Three regions from tensin interact with actin: two regions composed of amino acids 1 to 263 and 263 to 463, cosediment with F-actin but do not alter the kinetics of actin assembly; a region composed of amino acids 888-989, with sequence homology to insertin, retards actin polymerization. A claw-shaped tensin dimer would have six potential actin-binding sites and could embrace the ends of two actin filaments at focal contacts.

Immunoblotting of contractile and cytoskeletal proteins of canine basilar artery in vasospasm

Vasospasm was produced in the canine basilar arteries by a two-hemorrhage method, and voltage- and receptor-dependent contractions of the normal canine basilar arteries were induced by local applications of potassium chloride (KCI) and serotonin, respectively, after transclival exposure. Actin, myosin, desmin, filamin, talin, vinculin, and alpha-actinin in the basilar artery were studied by immunoblotting. The immunoblots showed a decrease or loss in immunoreactivity of some native proteins and generation of protein fragments, smaller in size than native proteins, in spastic, KCI, and serotonin groups, indicating a proteolytic degradation. In the spastic group on Day 2, actin, desmin, and filamin were usually degraded slightly; myosin moderately; and talin and alpha-actinin substantially. Vinculin and metavinculin remained intact. In the spastic group on Day 7, actin and desmin were usually decomposed slightly; myosin, filamin, and vinculin substantially; and talin, metavinculin, and alpha-actinin markedly. In the KCI and serotonin groups, slight degradation was usually observed in filamin, often in alpha-actinin, and occasionally in actin, whereas desmin, vinculin, and metavinculin were not degraded. In addition, myosin was usually degraded moderately in the KCI group and slightly in the serotonin group, and talin was generally decomposed slightly in the KCI group and moderately in the serotonin group. The degraded fragments, although variable in number and immunoreactivity, were similar in size in the three groups. We suggest that the intracellular devices responsible for contraction of the basilar arteries are degraded more severely in the spastic group than in the KCI or serotonin group, probably by similar proteolytic mechanism and progressively with the passage of time after subarachnoid hemorrhage in vasospasm.

Structures linking microfilament bundles to the membrane at focal contacts

We used quick-freeze, deep-etch, rotary replication and immunogold cytochemistry to identify a new structure at focal contacts. In Xenopus fibroblasts, elongated aggregates of particles project from the membrane to contact bundles of actin microfilaments. Before terminating, a single bundle of microfilaments interacts with several aggregates that appear intermittently over a distance of several microns. Aggregates are enriched in proteins believed to mediate actin-membrane interactions at focal contacts, including beta 1-integrin, vinculin, and talin, but they appear to contain less alpha-actinin and filamin. We also identified a second, smaller class of aggregates of membrane particles that contained beta 1-integrin but not vinculin or talin and that were not associated with actin microfilaments. Our results indicate that vinculin, talin, and beta 1-integrin are assembled into distinctive structures that mediate multiple lateral interactions between microfilaments and the membrane at focal contacts.

Purification and properties of transgelin: a transformation and shape change sensitive actin-gelling protein

We have purified the transformation and shape change sensitive isoform of an actin associated polypeptide doublet previously described by us (Shapland, C., P. Lowings, and D. Lawson. 1988. J. Cell Biol. 107:153-161) and have shown that it is evolutionarily conserved as far back as yeast. The purified protein: (a) binds directly to actin filaments at a ratio of 1:6 actin monomers, with a binding constant (Ka) of approximately 7.5 x 10(5) M-1; and (b) causes actin filament gelation within 2 min. Although these activities are controlled by ionic strength (and may be mediated by positively charged amino acid residues) the molecule remains as a monomer irrespective of ionic conditions. EM reveals that the addition of this protein to actin filaments converts them from a loose, random distribution into a tangled, cross-linked meshwork within 1 min, and discrete tightly aggregated foci after 10 min. By use of an "add-back" cell permeabilization system we can rebind this molecule specifically to actin filaments in cells from which it has previously been removed. Since the protein is transformation sensitive and gels actin, we have named it transgelin.

Actin, alpha-actinin, and vinculin are associated with septate junctions in Insecta

Cytoskeletal elements associated with the smooth septate junctions linking the midgut columnar cells of Manduca sexta larvae (Insecta, Lepidoptera) were characterized. Myosin subfragment 1 decoration and immunostaining for actin demonstrated that the filaments associated with the septate junctions were constituted of actin. Moreover, using a combination of immunochemical and immunolocalization techniques, evidence is presented that alpha-actinin, myosin II, and vinculin are localized close to the specialized plasma membranes. The insertion of microfilament bundles into submembranous F-actin/alpha-actinin/vinculin complexes, previously described in vertebrate junctions of adherens type, appears to be a more general organization, including the insect septate junction here examined.

Phosphorylation of vinculin in human platelets spreading on a solid surface

Vinculin is a cytoskeletal protein believed to be involved in linking microfilaments to the cell membrane. It is a substrate for the Ca(2+)- and phospholipid-dependent protein kinase C. We show here that when human platelets attach and spread on a solid surface, the alpha isoforms of vinculin become phosphorylated at serine and/or threonine residues. Phosphorylation is dependent on adhesion to a surface, since suspended, unattached platelets can produce filopodia but no phosphorylation of vinculin. Phosphorylation is also dependent on actin polymerization, as it does not occur when platelets had been pretreated with cytochalasin B. Most likely, protein kinase C is responsible for the phosphorylation of vinculin, since phosphorylation also occurs when platelets are treated with a phorbol ester, which activates protein kinase C, and is blocked by treatment with a staurosporine derivative which inhibits this enzyme. These results suggest that phosphorylation plays a role in anchoring vinculin at sites of microfilament-membrane interaction.

Interaction of cardiac alpha-actinin and actin in the presence of doxorubicin

The therapeutic use of doxorubicin (an antitumoral antibiotic belonging to the anthracycline group) is limited by its cardiotoxicity. Adriamycin (DXR) causes myocardial subcellular damage, such as myocytolysis, disarray of actin filaments, and alterations in the Z-band with loss of sarcomeric organization. We studied the effect of stoichiometrical concentrations of DXR on the interaction between cardiac actin and alpha-actinin in solution. Doxorubicin inhibits the formation of alpha-actinin/actin tridimensional networks and bundles. The main effect of the drug seems to be on the size of the actin polymers.

Cytoskeletal assembly and vinculin-cytoskeleton interaction in different phases of the activation of bovine platelets

Vinculin is an Mr 130 kDa protein that has been implicated in membrane-cytoskeleton interaction in various cell types. It has been demonstrated that vinculin is not a cytoskeletal component in resting platelets, but part of it becomes associated with the cytoskeleton during thrombin-induced activation. In this study, using a quantitative immunoblotting technique, the relation of vinculin to the cytoskeleton in different phases of activation of bovine platelets was explored, and the process of incorporation of vinculin into the cytoskeleton was related to that of cytoskeletal assembly. The assembly of cytoskeleton proceeded at a significantly faster rate than the association of vinculin with it, which shows that the latter process is not due to passive trapping of vinculin into the Triton-insoluble residue, but certain biochemical changes had to occur before such an interaction became possible. When the formation of pseudopodia was prevented by cytochalasin B, but neither aggregation nor the release reaction induced by thrombin were inhibited, the recovery of vinculin in the Triton-insoluble residue even increased. In both time- and thrombin-concentration-dependent studies, poor correlation was found between vinculin-cytoskeleton association and the extent of aggregation. Activation with phorbol-myristate-acetate, which is a strong stimulus for aggregation but produces only a slight release in the granular content, resulted in the association of only a negligible amount of vinculin with the cytoskeletal fraction. The incorporation of vinculin into the cytoskeletal fraction of thrombin activated platelets started with the release reaction but still proceeded, and the greatest part of the reaction occurred after secretion had gone to completion.(ABSTRACT TRUNCATED AT 250 WORDS)

The 102 kd cadherin-associated protein: similarity to vinculin and posttranscriptional regulation of expression

The E-cadherin cell adhesion molecule is associated with cytoplasmic polypeptides, and this association is essential for its cell-binding function. Using isolated adherens junctions of the liver, we purified a 102 kd protein that can associate with E-cadherin (CAP102) and isolated cDNAs encoding this protein. Sequence analysis of the cDNAs revealed that this protein has a similarity to vinculin. L cells not expressing endogenous cadherin express the mRNA for CAP102 but have only a trace amount of CAP102 protein. Introducing exogenous E-cadherin into these cells, however, induced a high expression of CAP102 protein without affecting the amount of its mRNA, suggesting that there is a posttranscriptional regulatory mechanism for this molecule. The same effect was observed by introducing N- or P-cadherin into L cells.

Simple and rapid method for purification of vinculin from bovine aorta

A new rapid purification procedure has been developed for mammalian smooth muscle vinculin. Bovine aorta vinculin has been purified by a two-step procedure employing hydrophobic chromatography on phenyl-Sepharose and gel filtration. This procedure should be particularly useful for purifying vinculin from tissues in which it is present in a relatively low concentration. In addition, the new purification protocol provides highly pure vinculin free from active contaminants which reduce the low-shear viscosity of F-actin solution.

Direct interactions between talin and actin

Talin was purified from chicken gizzard by a modification of the method of L. Molony et al. [J. Biol. Chem.(1987) 262, 7790-7795]. Unlike the talin purified by the previous method, the talin purified by the new method was found to bind to both F- and G-actin: Talin cosedimented with F-actin. On gel filtration of a mixture of talin and G-actin, a complex of talin and action was obtained. Talin stimulated the polymerization rate of G-actin. A major proteolytic fragment of talin that retained the binding ability to F-actin was also identified. These results indicate that talin can bind directly to actin and suggest that talin plays a key role in the organization of actin filaments at the actin-membrane attachment sites in vivo also.

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.

Complete sequence of human vinculin and assignment of the gene to chromosome 10

We have determined the complete sequence of human vinculin, a cytoskeletal protein associated with cell-cell and cell-matrix junctions. Comparison of human and chicken embryo vinculin sequences shows that both proteins contain 1066 amino acids and exhibit a high level of sequence identity (greater than 95%). The region of greatest divergence falls within three 112-amino acid repeats spanning residues 259-589. Interestingly, nematode vinculin lacks one of these central repeats. The regions of human vinculin that are N- and C-terminal to the repeats show 54% and 61% sequence identity, respectively, to nematode vinculin. Southern blots of human genomic DNA hybridized with short vinculin cDNA fragments indicate that there is a single vinculin gene. By using a panel of human-rodent somatic cell hybrids, the human vinculin gene was mapped to chromosome 10q11.2-qter.

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.

Kinetic evidence for insertion of actin monomers between the barbed ends of actin filaments and barbed end-bound insertin, a protein purified from smooth muscle

An actin polymerization-retarding protein was isolated from chicken gizzard smooth muscle. This protein copurified with vinculin on DEAE-cellulose and gel filtration columns. The polymerization-retarding protein could be separated from vinculin by hydroxylapatite chromatography. The isolated polymerization-retarding protein lost its activity within a few days, but was stable for weeks when it was not separated from vinculin. We termed the polymerization-retarding protein "insertin". Because of the instability of the isolated insertin, we investigated the effect of insertin-vinculin on actin polymerization. Insertin-vinculin retarded nucleated actin polymerization maximally fivefold. Polymerization at the pointed ends of gelsolin-capped actin filaments was not affected by insertin-vinculin, suggesting that insertin-vinculin binds to the barbed ends, but not to the pointed ends, of actin filaments. Retarded polymerization was observed even if the actin monomer concentration was between the critical concentrations of the ends of treadmilling actin filaments. As at this low monomer concentration the pointed ends depolymerize, monomers appeared to be inserted at the barbed ends between the terminal subunit and barbed end-bound insertin molecules. Insertin-vinculin was found not to increase the actin monomer concentration to the value of the pointed ends. These observations support the conclusion that insertin is not a barbed end-capping protein but an actin monomer-inserting protein. According to a quantitative analysis of the kinetic data, all observations could be explained by a model in which two insertin molecules were assumed to bind co-operatively to the barbed ends of actin filaments. Actin monomers were found to be inserted between the barbed ends and barbed end-bound insertin molecules at a rate of about 1 x 10(6) M-1 s-1. Insertin may be an essential part of the machinery of molecules that permit treadmilling of actin filaments in living cells by insertion of actin molecules between membranes and actin filaments.

Enhancement of vinculin synthesis by migrating stratified squamous epithelium

A 110-115-kD protein is present at levels 27-fold higher in migratory epithelium in the rat cornea than in stationary epithelium. This protein represents 2.7% of the total protein in migratory epithelium 6-h postabrasion wound and 0.1% of the total protein in stationary epithelium. Our findings demonstrate that this 110-115-kD protein is vinculin. In Western blots comparing proteins from migratory and control epithelium, antibody against vinculin cross-reacted with the 110-115-kD protein. Using immunoslot blots, vinculin was determined to be present at maximal levels 6 h postabrasion wound, at levels 22- and 8-fold higher than control at 18 and 48 h, respectively, returning to control levels 72 h postwounding. Vinculin was also localized by indirect immunohistochemistry in migrating corneal epithelium. 3-mm scrape wounds were allowed to heal in vivo for 20 h. In flat mounts of these whole wounded corneas, vinculin was localized as punctate spots in the leading edge of migrating epithelium. In cryostat sections, vinculin was localized as punctate spots along the basal cell membranes of the migrating sheet adjacent to the basement membrane and in patches between cells as well as diffusely throughout the cell. Only very diffuse localization with occasional punctate spots between adjacent superficial cells was present in stationary epithelium. The increased synthesis of vinculin during migration and the localization of vinculin at the leading edge of migratory epithelium suggest that vinculin may be involved in cell-cell and cell-substrate adhesion as the sheet of epithelium migrates to cover a wound.

A new 82-kD barbed end-capping protein (radixin) localized in the cell-to-cell adherens junction: purification and characterization

An 82-kD protein has been purified from the undercoat of the adherens junction isolated from the rat liver. The purification scheme includes low salt extraction followed by DEAE-cellulose ion exchange, DNase I-actin affinity, and carboxyl methyl-cellulose ion exchange chromatographies. The purified 82-kD protein was essentially free of contaminants as judged by SDS-PAGE combined with silver staining. The substoichiometric 82-kD protein largely inhibited the actin filament assembly; when the molar ratio of the 82-kD protein to G-actin was 1:1,000, the viscosity was reduced to 28% of the control value. Direct electron microscopic studies revealed that the 82-kD protein selectively inhibited monomer addition at the barbed ends of actin filaments. By use of the antibody raised against the 82-kD protein, this protein was shown by immunofluorescence microscopy to be localized at the cell-to-cell adherens junction in various types of cells. In contrast, the 82-kD protein was not concentrated at the cell-to-substrate adherens junctions (focal contacts). These findings have led us to conclude that the 82-kD protein is a barbed end-capping protein which is associated with the undercoat of the cell-to-cell adherens junction. Hence, we have tentatively designated the 82-kD protein as radixin (from the Latin word radix meaning root).

Cytoskeletal architecture of neuromuscular junction: localization of vinculin

Cytoskeletons underneath the postsynaptic membrane of neuromuscular junctions were studied by using a quick-freeze deep-etch method and immunoelectron microscopy of ultrathin frozen sections. In a quick-freeze deep-etched replica of fresh, unfixed muscles, 8.9 +/- 1.5-nm particles were present on the true postsynaptic membrane surface. Underneath this receptor-rich postsynaptic membrane, networks of fine filaments were observed. These cytoskeletal networks were more clearly observed in extracted samples. In these samples, diameters of the filaments which formed networks were measured. In the platinum replica, three kinds of filament were recognized--12 nm, 9 nm, and 7 nm in diameter. The 12-nm filament seemed to correspond to the intermediate filament. The other two filaments formed meshworks between intermediate filaments and plasma membrane. In ultrathin frozen sections vinculin label was localized just beneath the plasma membrane. Thirty-six percent of the label was within 18 nm from the cytoplasmic side of the plasma membrane and 50% was within 30 nm. Taking the size of the vinculin molecule into account, it was concluded that vinculin is localized just beneath the plasma membrane and might play some role in anchoring filaments which formed meshworks underneath the plasma membrane.

Characterization of synapsin I fragments produced by cysteine-specific cleavage: a study of their interactions with F-actin

Synapsin I is a neuron-specific phosphoprotein that is concentrated in the presynaptic nerve terminal in association with the cytoplasmic surface of synaptic vesicles. It has been demonstrated to bundle F-actin in a phosphorylation-dependent manner in vitro, a property consistent with its proposed role in linking synaptic vesicles to the cytoskeleton and its involvement in the regulation of neurotransmitter release. Synapsin I is composed of two distinct domains, a COOH terminal, collagenase-sensitive, hydrophilic, and strongly basic tail region, and an NH2 terminal, collagenase-resistant head region relatively rich in hydrophobic amino acids. To elucidate the structural basis for the interactions between synapsin I and F-actin and how it relates to other characteristics of synapsin I, we have performed a structure-function analysis of fragments of synapsin I produced by cysteine-specific cleavage with 2-nitro-5-thiocyanobenzoic acid. The fragments were identified and aligned with the parent molecule using the deduced primary structure of synapsin I and the known phosphorylation sites as markers. We have purified these fragments and examined their interactions with F-actin. Two distinct fragments, a 29-kD NH2-terminal fragment and a 15-kD middle fragment, were shown to contain F-actin binding sites. A 51/54-kD middle/tail fragment retained the F-actin binding and bundling activity of synapsin I, but the isolated tail fragment did not retain either activity. In contrast to phosphorylation of sites two and three in intact synapsin I, which abolishes F-actin bundling activity, phosphorylation of these sites in the middle/tail fragment failed to abolish this activity. In conclusion, three domains of synapsin I appear to be involved in F-actin binding and bundling.

Actin-associated proteins and fibronectin in the fetal human inner ear

The distribution of alpha-actinin, vinculin, alpha-spectrin, beta-spectrin and fibronectin was analyzed in 14- to 21-week-old fetal human inner ears using immunofluorescence microscopy. Staining for alpha-actinin was fairly evenly distributed at the epithelial surfaces of all five vestibular organs, whereas in the cochlea it was mainly at the surface of the receding greater epithelial ridge and in some foci apically at the lesser epithelial ridge. Fluorescence for vinculin was observed mainly at the surface of vestibular organs, but was lacking in the LER. Intense fluorescence for alpha-spectrin was found at the apical surface of individual cells of the cristae and maculae. Antibodies against beta-spectrin mainly stained the endothelial cells of blood vessels, but faint staining of the epithelial cell surfaces of the vestibular organs was also detected. The fluorescence pattern of the actin-associated proteins is indicative of structural differences between cochlear and vestibular hair cells. Fibronectin was identified only between mesenchymal cells and its functional importance in the mature inner ear epithelia can be discounted.

Characterization of an inhibitor of actin polymerization in vinculin-rich fraction of turkey gizzard smooth muscle

We report here on the purification and characterization of a new 25-kDa inhibitor of actin polymerization from turkey gizzard smooth muscle. The protein was purified by chromatography on DEAE-cellulose and hydroxyapatite, as well as by affinity chromatography on an immobilized-antibody column. The purified polypeptide reduced the low-shear viscosity of actin, apparently due to its inhibitory effect on actin polymerization. We demonstrate that this protein is largely responsible for the apparent inhibitory activity previously reported to be associated with smooth muscle vinculin preparations. Three independent monoclonal antibodies prepared against the 25-kDa inhibitor of actin polymerization can effectively adsorb the inhibiting activity of actin polymerization from the crude vinculin preparation or inhibit it. We also show here that the 25-kDa inhibitor of actin polymerization tends to undergo dimerization when maintained in non-reducing buffers, concomitant with the loss of its inhibitory activity. Immunohistochemical labeling of frozen sections, as well as immunoblotting analyzes, indicated that the 25-kDa inhibitor of actin polymerization is particularly enriched in smooth muscle cells and that its distribution is apparently homogenous throughout the cytoplasm showing no apparent enrichment in the vinculin-rich dense plaques located along the endofacial surface of the plasma membrane.

Isolation of 110K actin binding protein from mammalian brain and its immunocytochemical localization within cultured cells

Crude extract of young rat brain forms actin-based gels upon incubation at 25 degrees C. After boiling the gelled material, a protein fraction composed mostly of a major band of 110 kDa and a minor band of 120 kDa in SDS-PAGE was obtained by hydroxyapatite column chromatography. When the same protein fraction was prepared from bovine brains using the same procedure with two additional column chromatographies, the amounts of both proteins were nearly the same. Both proteins cosedimented with actin filaments upon centrifugation. Antibody was produced in a rabbit against the bovine fraction and affinity purified using a nitrocellulose paper onto which these proteins were transferred electrophoretically. Immunoblot analysis showed that both proteins are immunologically similar, and we refer to both proteins as 110K protein, collectively. The immunoblot analysis also revealed that the 110K protein is contained in cultured cells such as BHK, 3Y1, NRK, and MDBK. Analysis of various tissue extracts showed that brain is rich in this protein but liver, kidney, and lung contain negligible amounts. Indirect immunofluorescent analysis using cells during spreading showed preferential localization in the leading edge region and no fluorescence was detected in the stress fiber. Double immunostaining using monoclonal anti-vinculin and anti-110K protein antibodies revealed that the distribution patterns of both proteins are different from each other.