Identification of a repeated domain within mammalian alpha-synemin that interacts directly with talin

The type VI intermediate filament (IF) protein synemin is a unique member of the IF protein superfamily. Synemin associates with the major type III IF protein desmin forming heteropolymeric intermediate filaments (IFs) within developed mammalian striated muscle cells. These IFs encircle and link all adjacent myofibrils together at their Z-lines, as well as link the Z-lines of the peripheral layer of cellular myofibrils to the costameres located periodically along and subjacent to the sarcolemma. Costameres are multi-protein assemblies enriched in the cytoskeletal proteins vinculin, alpha-actinin, and talin. We report herein a direct interaction of human alpha-synemin with the cytoskeletal protein talin by protein-protein interaction assays. The 312 amino acid insert (SNTIII) present only within alpha-synemin binds to the rod domain of talin in vitro and co-localizes with talin at focal adhesion sites within mammalian muscle cells. Confocal microscopy studies showed that synemin co-localizes with talin within the costameres of human skeletal muscle cells. Analysis of the primary sequences of human alpha- and beta-synemins revealed that SNTIII is composed of seven tandem repeats, each containing a specific Ser/Thr-X-Arg-His/Gln (S/T-X-R-H/Q) motif. Our results suggest human alpha-synemin plays an essential role in linking the heteropolymeric IFs to adherens-type junctions, such as the costameres within mammalian striated muscle cells, via its interaction with talin, thereby helping provide mechanical integration for the muscle cell cytoskeleton.     

Interactions of intermediate filament protein synemin with dystrophin and utrophin

Synemin is a unique, very large intermediate filament (IF) protein present in all types of muscle cells, which forms heteropolymeric intermediate filaments (IFs) with the major IF proteins desmin and/or vimentin. We show herein that tissue-purified avian synemin directly interacts with both dystrophin and utrophin, and that specific expressed regions of both of the mammalian (human) synemin isoforms (alpha-synemin and beta-synemin) directly interact with specific expressed domains/regions of the dystrophin and utrophin molecules. Mammalian synemin is also shown to colocalize with dystrophin within muscle cell cultures. These results indicate that synemin is an important IF protein in muscle cells that helps fortify the linkage between the peripheral layer of cellular myofibrils and the costameric regions located along the sarcolemma and the sarcolemma region located within the neuromuscular and myotendinous junctions (NMJs and MTJs).     

Synemin may function to directly link muscle cell intermediate filaments to both myofibrillar Z-lines and costameres.

Synemin is a large intermediate filament (IF) protein that has been identified in all types of muscle cells in association with desmin- and/or vimentin-containing IFs. Our previous studies (Bellin, R. M., Sernett, S. W., Becker, B., Ip, W., Huiatt, T. W., and Robson, R. M. (1999) J. Biol. Chem. 274, 29493-29499) demonstrated that synemin forms heteropolymeric IFs with major IF proteins and contains a binding site for the myofibrillar Z-line protein alpha-actinin. By utilizing blot overlay assays, we show herein that synemin also interacts with the costameric protein vinculin. Furthermore, extensive assays utilizing the Gal4 yeast two-hybrid system demonstrate interactions of synemin with desmin and vimentin and additionally define more precisely the protein subdomains involved in the synemin/alpha-actinin and synemin/vinculin interactions. The C-terminal approximately 300-amino acid region of synemin binds to the N-terminal head and central rod domains of alpha-actinin and the approximately 150-amino acid C-terminal tail of vinculin. Overall, these interactions indicate that synemin may anchor IFs to myofibrillar Z-lines via interactions with alpha-actinin and to costameres at the sarcolemma via interactions with vinculin and/or alpha-actinin. These linkages would enable the IFs to directly link all cellular myofibrils and to anchor the peripheral layer of myofibrils to the costameres.     

Molecular characteristics and interactions of the intermediate filament protein synemin. Interactions with alpha-actinin may anchor synemin-containing heterofilaments.

Synemin is a cytoskeletal protein originally identified as an intermediate filament (IF)-associated protein because of its colocalization and copurification with the IF proteins desmin and vimentin in muscle cells. Our sequencing studies have shown that synemin is an unusually large member (1,604 residues, 182,187 Da) of the IF protein superfamily, with the majority of the molecule consisting of a long C-terminal tail domain. Molecular interaction studies demonstrate that purified synemin interacts with desmin, the major IF protein in mature muscle cells, and with alpha-actinin, an integral myofibrillar Z-line protein. Furthermore, expressed synemin rod and tail domains interact, respectively, with desmin and alpha-actinin. Analysis of endogenous protein expression in SW13 clonal lines reveals that synemin is coexpressed and colocalized with vimentin IFs in SW13.C1 vim+ cells but is absent in SW13.C2 vim- cells. Transfection studies indicate that synemin requires the presence of another IF protein, such as vimentin, in order to assemble into IFs. Taken in toto, our results suggest synemin functions as a component of heteropolymeric IFs and plays an important cytoskeletal cross-linking role by linking these IFs to other components of the cytoskeleton. Synemin in striated muscle cells may enable these heterofilaments to help link Z-lines of adjacent myofibrils and, thereby, play an important role in cytoskeletal integrity.     

Keratin 8/18 Modulation of Protein Kinase C-mediated Integrin-dependent Adhesion and Migration of Liver Epithelial Cells

Keratins are intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatocyte and hepatoma cell IFs are made solely of keratins 8/18 (K8/K18), the hallmark of all simple epithelia. Cell attachment/spreading (adhesion) and migration involve the formation of focal adhesions at sites of integrin interactions with extracellular matrix, actin adaptors such as talin and vinculin, and signaling molecules such as focal adhesion kinase (FAK) and member(s) of the protein kinase C (PKC) family. Here, we identify the novel PKCδ as mediator of the K8/K18 modulation of hepatoma cell adhesion and migration. We also demonstrate a K8/K18-dependent relationship between PKCδ and FAK activation through an integrin/FAK-positive feedback loop, in correlation with a reduced FAK time residency at focal adhesions. Notably, a K8/K18 loss results to a time course modulation of the receptor of activated C-kinase-1, β1-integrin, plectin, PKC, and c-Src complex formation. Although the K8/K18 modulation of hepatocyte adhesion also occurs through a PKC mediation, these differentiated epithelial cells exhibit minimal migrating ability, in link with marked differences in protein partner content and distribution. Together, these results uncover a key regulatory function for K8/K18 IFs in the PKC-mediated integrin/FAK-dependent adhesion and migration of simple epithelial cells.     

Characterization of mammalian synemin,an intermediate filament protein present in all four classes of muscle cells and some neuroglial cells: co-localization and interaction with type III intermediate filament proteins and keratins

Using a monoclonal antibody, we have detected a high molecular weight muscle protein, co-localized and co-isolating with desmin. Searching a human cDNA database with partial amino acid sequences of the protein, we found a cDNA clone encoding a 1565-amino-acid polypeptide, identified as a mammalian (human) synemin, a member of the intermediate filament (IF) protein family. Immunoblotting showed the presence of a 180-kDa polypeptide in skeletal muscle and 180- and 200-kDa polypeptides in cardiac and smooth muscles. Interestingly, synemin was also found in myoepithelial cells, which have keratin filaments instead of desmin. Moreover, synemin was also found in astrocytes of optic nerves and non-myelin-forming Schwann cells, together with glial fibrillary acidic protein (GFAP) and vimentin. Blot overlays pointed to molecular interactions of synemin with desmin, vimentin, GFAP and keratin 5 and 6, but not with keratin 14. The experimental data also suggested a possible link with nebulin, a skeletal muscle protein. Purified synemin was coassembled with desmin in different molar ratios, and at 1:25, as typically found in vivo, IFs were formed which were comparable in length to desmin filaments. However, at molar ratios of 3:25 and 6:25, much shorter and irregular shaped filamentous polymers were generated. The fact that synemin is present in all four classes of muscle cells and a specific type of glial cells is indicative of important functions. Its incorporation may give structural and functional versatility to the IF cytoskeleton.This work was supported by grants from the Ministry of Education, Science, and Culture of Japan.     

Metavinculin: New insights into functional properties of a muscle adhesion protein

Metavinculin is a muscle-specific splice variant of the ubiquitously expressed cytoskeletal adaptor protein vinculin. Both proteins are thought to be co-expressed in all muscle types where they co-localize to microfilament-associated adhesion sites. It has been shown that a metavinculin-specific insertion of 68 amino acids alters the biochemical properties of the five-helix bundle in the tail domain. Here, we demonstrate that the metavinculin-specific helix H1′ plays an important role for protein stability of the tail domain, since a point mutation in this helix, R975W, which is associated with the occurrence of dilated cardiomyopathy in man, further decreases thermal stability of the metavinculin tail domain. In striated muscle progenitor cells (myoblasts), both, metavinculin and the R975W mutant show significantly reduced, albeit distinctive residency and exchange rates in adhesion sites as compared to vinculin. In contrast to previous studies, we show that metavinculin is localized in a muscle fiber type-dependent fashion to the costameres of striated muscle, reflecting the individual metabolic and physiological status of a given muscle fiber. Metavinculin expression is highest in fast, glycolytic muscle fibers and virtually absent in M. diaphragmaticus, a skeletal muscle entirely lacking fast, glycolytic fibers. In summary, our data suggest that metavinculin enrichment in attachment sites of muscle cells leads to higher mechanical stability of adhesion complexes allowing for greater shear force resistance.     

Molecular domain structure of porcine vinculin and metavinculin

Summary Metavinculin is a higher molecular weight variant of vinculin expressed only in cardiac and smooth muscle. Using microsequencing methods on the intact molecules and their proteolytic subfragments we have been able to map the common and different parts of these closely related proteins. Both vinculin and metavinculin, from mammals and birds exhibit a relatively protease resistant 90 kD core fragment. N-terminal sequencing analysis of the avian and mammalian core fragments as well as of major core subfragments obtained by extended proteolysis placed the core domain at the N-terminus of the intact molecules and revealed identity between metavinculin and vinculin as well as between species. Limited chymotryptic digestion of porcine vinculin and metavinculin yielded a common 16 kD fragment which could be placed at the C-terminus of the cDNA sequence derived from chick fibroblast vinculin (G. J.Price, P.Jones, M. D.Davison, R.Bendori, S.Griffiths, B.Patel, B.Geiger and D. R.Critchley 1988, in press). From additional sequence data the metavinculin specific fragment could be placed at the metavinculin C-terminus. Using a polyclonal antibody specific for porcine metavinculin a peptide unique to metavinculin could be identified. Direct sequencing of this, as well as of related, overlapping fragments, purified by reversed phase HPLC revealed a 68 amino acid insert in procine metavinculin, between the core fragment and the C-terminal piece, common to vinculin and metavinculin. The domain organizazions of vinculin and metavinculin and their possible functional implications are discussed.Abbreviations SDS sodium dodecyl sulfate - EDTA ethylendinitrilotetra acetic acid - HPLC high pressure liquid chromatography     

Keratin 8/18 regulation of cell stiffness-extracellular matrix interplay through modulation of Rho-mediated actin cytoskeleton dynamics

Cell mechanical activity generated from the interplay between the extracellular matrix (ECM) and the actin cytoskeleton is essential for the regulation of cell adhesion, spreading and migration during normal and cancer development. Keratins are the intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatic epithelial cell IFs are made solely of keratins 8/18 (K8/K18), hallmarks of all simple epithelia. Notably, our recent work on these epithelial cells has revealed a key regulatory function for K8/K18 IFs in adhesion/migration, through modulation of integrin interactions with ECM, actin adaptors and signaling molecules at focal adhesions. Here, using K8-knockdown rat H4 hepatoma cells and their K8/K18-containing counterparts seeded on fibronectin-coated substrata of different rigidities, we show that the K8/K18 IF-lacking cells lose their ability to spread and exhibit an altered actin fiber organization, upon seeding on a low-rigidity substratum. We also demonstrate a concomitant reduction in local cell stiffness at focal adhesions generated by fibronectin-coated microbeads attached to the dorsal cell surface. In addition, we find that this K8/K18 IF modulation of cell stiffness and actin fiber organization occurs through RhoA-ROCK signaling. Together, the results uncover a K8/K18 IF contribution to the cell stiffness-ECM rigidity interplay through a modulation of Rho-dependent actin organization and dynamics in simple epithelial cells.     

Synemin isoforms differentially organize cell junctions and desmin filaments in neonatal cardiomyocytes

Intermediate filaments (IFs) in cardiomyocytes consist primarily of desmin, surround myofibrils at Z disks, and transmit forces from the contracting myofilaments to the cell surface through costameres at the sarcolemma and desmosomes at intercalated disks. Synemin is a type IV IF protein that forms filaments with desmin and also binds α-actinin and vinculin. Here we examine the roles and expression of the α and β forms of synemin in developing rat cardiomyocytes. Quantitative PCR showed low levels of expression for both synemin mRNAs, which peaked at postnatal day 7. Synemin was concentrated at sites of cell-cell adhesion and at Z disks in neonatal cardiomyocytes. Overexpression of the individual isoforms showed that α-synemin preferentially localized to cell-cell junctions, whereas β-synemin was primarily at the level of Z disks. An siRNA targeted to both synemin isoforms reduced protein expression in cardiomyocytes by 70% and resulted in a failure of desmin to align with Z disks and disrupted cell-cell junctions, with no effect on sarcomeric organization. Solubility assays showed that β-synemin was soluble and interacted with sarcomeric α-actinin by coimmunoprecipitation, while α-synemin and desmin were insoluble. We conclude that β-synemin mediates the association of desmin IFs with Z disks, whereas α-synemin stabilizes junctional complexes between cardiomyocytes.     

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.     

Self-assembly incompetence of synemin is related to the property of its head and rod domains

The mechanisms regulating the intermediate filament (IF) protein assembly are complex and not yet fully understood. All vertebrate cytoplasmic IF proteins have a central alpha-helical rod domain flanked by variable head and tail domains. The IF protein synemin cannot homopolymerize to form filament networks; it needs an appropriate copolymerization partner. To elucidate the roles of the vimentin head domain, the TAAL motif in the 2A region, and the TYRKLLEGEE motif in the 2B region of the rod domain in synemin filament formation, we have prepared a series of synemin constructs by site-directed mutagenesis and chimeric synemins having the vimentin head domain. The assembly properties of synemin constructs were assessed by the immunofluorescence of transient transfection into cultured SW13 cells without endogenous IFs. Our data showed that the formation of a filamentous network required at least the vimentin-like head domain and both the 2A and 2B regions of the rod domain.     

Comparative biochemical analysis suggests that vinculin and metavinculin cooperate in muscular adhesion sites

Metavinculin, the muscle-specific splice variant of the cell adhesion protein vinculin, is characterized by a 68-amino acid insert within the C-terminal tail domain. The findings that mutations within this region correlate with hereditary idiopathic dilated cardiomyopathy in man suggest a specific contribution of metavinculin to the molecular architecture of muscular actin-membrane attachment sites, the nature of which, however, is still unknown. In mice, metavinculin is expressed in smooth and skeletal muscle, where it co-localizes with vinculin in dense plaques and costameres, respectively, but is of conspicuously low abundance in the heart. Immunoprecipitates suggest that both isoforms are present in the same complex. On the molecular level, both vinculin isoforms are regulated via an intramolecular head-tail interaction, with the metavinculin tail domain having a lower affinity for the head as compared with the vinculin tail. In addition, metavinculin displays impaired binding to acidic phospholipids and reduced homodimerization. Only in the presence of phospholipid-activated vinculin tail, the metavinculin tail domain is readily incorporated into heterodimers. Mutational analysis revealed that the metavinculin insert significantly alters binding of the C-terminal hairpin loop to acidic phospholipids. In summary, our data lead to a model in which unfurling of the metavinculin tail domain is impaired by the negative charges of the 68-amino acid insert, thus requiring vinculin to fully activate the metavinculin molecule. As a consequence, microfilament anchorage may be modulated at muscular adhesion sites through heterodimer formation.     

Porcine vinculin and metavinculin differ by a 68-residue insert located close to the carboxy-terminal part of the molecule.

Metavinculin is a higher mol. wt variant of vinculin expressed only in muscle tissue. Using amino acid sequencing methods on the intact molecules and their proteolytic subfragments, together with a polyclonal antibody specific only for metavinculin from porcine stomach, we have been able to identify and sequence the difference peptide in the porcine metavinculin molecule. By alignment with the complete sequence of chick fibroblast vinculin (communicated by G.J. Price, P. Jones, M.D. Davison, R. Bendori, S. Griffiths, B. Patel, B. Geiger and D.R. Critchley, prior to publication) the exact location of the insert could be determined. In porcine metavinculin, this insert lies between the 90-kd protease-resistant amino-terminal core and the carboxy terminus of the molecule. It contains 68 amino acids and is flanked by KWSSK sequences, one of which is present in vinculin. The identity of the mapped vinculin and metavinculin sequences outside this difference peptide is consistent with the two proteins arising via alternative splicing at the mRNA level. The lack of reactivity of the porcine metavinculin antibody with metavinculin from chicken as well as the finding of different proteolytic cleavage sites in avian metavinculin indicate a species-specific amino acid sequence in the difference piece of the metavinculin molecule.     

Raver1, a dual compartment protein, is a ligand for PTB/hnRNPI and microfilament attachment proteins.

By screening a yeast two-hybrid library with COOH-terminal fragments of vinculin/metavinculin as the bait, we identified a new protein termed raver1. Raver1 is an 80-kD multidomain protein and widely expressed but to varying amounts in different cell lines. In situ and in vitro, raver1 forms complexes with the microfilament-associated proteins vinculin, metavinculin, and alpha-actinin and colocalizes with vinculin/metavinculin and alpha-actinin at microfilament attachment sites, such as cell-cell and cell matrix contacts of epithelial cells and fibroblasts, respectively, and in costameres of skeletal muscle. The NH2-terminal part of raver1 contains three RNA recognition motifs with homology to members of the heterogeneous nuclear RNP (hnRNP) family. Raver1 colocalizes with polypyrimidine tract binding protein (PTB)/hnRNPI, a protein involved in RNA splicing of microfilament proteins, in the perinucleolar compartment and forms complexes with PTB/hnRNPI. Hence, raver1 is a dual compartment protein, which is consistent with the presence of nuclear location signal and nuclear export sequence motifs in its sequence. During muscle differentiation, raver1 migrates from the nucleus to the costamere. We propose that raver1 may coordinate RNA processing and targeting as required for microfilament anchoring in specific adhesion sites.     

Review on intermediate filaments of the nervous system and their pathological alterations

Intermediate filaments (IFs) of the nervous system, including neurofilaments, α-internexin, glial fibrillary acidic protein, synemin, nestin, peripherin and vimentin, are finely expressed following elaborated cell, tissue and developmental specific patterns. A common characteristic of several neurodegenerative diseases is the abnormal accumulation of neuronal IFs in cell bodies or along the axon, often associated with impairment of the axonal transport and degeneration of neurons. In this review, we also present several perturbations of IF metabolism and organization associated with neurodegenerative disorders. Such modifications could represent strong markers of neuronal damages. Moreover, recent data suggest that IFs represent potential biomarkers to determine the disease progression or the differential stages of a neuronal disorder. Finally, recent investigations on IF expression and function in cancer provide evidence that they may be useful as markers, or targets of brain tumours, especially high-grade glioma. A better knowledge of the molecular mechanisms of IF alterations, combined to neuroimaging, is essential to improve diagnosis and therapeutic strategies of such neurodegenerative diseases and glioma.     

Obstructive hypertrophic cardiomyopathy is associated with reduced expression of vinculin in the intercalated disc

Mutations in the cardiac-specific insert of vinculin, metavinculin, rarely cause hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). Subsequently, a missense mutation in the ubiquitously expressed vinculin was discovered in a patient with obstructive HCM. Microscopic examination of both myectomy specimens from patients bearing genetic defects in metavinculin and vinculin showed a marked reduction of vinculin/metavinculin expression in the intercalated disc, but normal expression in the Z-disc. Given that distinct functional domains were altered by the metavinculin and vinculin mutations, we hypothesized that the intercalated disc-specific reduction of vinculin may stem from left ventricular tract obstruction in general rather than rarely observed perturbations in VCL-encoded vinculin. To test this hypothesis, we examined the localization of vinculin/metavinculin in hypertrophied human heart tissue from patients with cardiovascular conditions associated with obstruction and hemodynamic overload using an immunohistochemistry approach. Tissue specimens derived from patients with obstructive HCM and aortic stenosis (AS) showed a universal defect of vinculin/metavinculin expression in the intercalated disc but preserved expression in the cardiac Z-disc, whereas tissue specimens derived from patients with either DCM, hypertensive heart disease (HTN), or pulmonary hypertension (PHTN) exhibited normal expression of vinculin/metavinculin in both the Z- and the intercalated disc despite being associated with hypertrophy. Results of this study suggest that cardiac hypertrophy may be associated with different expression of the marker vinculin/metavinculin depending on the underlying pathophysiology; hemodynamic overload may not affect the localization whereas obstructive disease substantially reduces the expression of vinculin preferentially in the intercalated disc.     

Spatial and temporal relationships between vinculin and talin in the developing chicken gizzard smooth muscle

The spatiotemporal relationships between vinculin and talin in developing chicken gizzard smooth muscle were investigated. Immunofluorescence and immunoelectron-microscopic labeling revealed that both proteins are associated with membrane-bound dense plaques in muscle cells; however, the most intense labeling for vinculin was located rather closer to the membrane than that for talin. The localization of vinculin and talin in embryonic chicken gizzards indicated that both are primarily cytoplasmic during the first 2 embryonic weeks. Only around days 16-18 does talin apparently become associated with the plasma membrane, this being concomitant with the appearance of distinct myofilament-bound dense plaques. Vinculin, on the other hand, remains primarily cytoplasmic and appears in the plaques only 1-3 days after hatching. It is thus proposed that the interactions of the dense plaque with myofilaments or with the membrane do not depend on the presence of vinculin in the plaque. Electrophoretic analyses indicated that, during development, there is no major change in the differential expression of specific vinculin isoforms. Quantitative immunoblotting analysis indicated that the vinculin content (relative to total extracted protein) is virtually constant during the last week of embryonic life. However, within 3 days of hatching, the vinculin concentration increases remarkably to over twice the embryonic level, and then slowly increases until it reaches the adult levels, which are three to four times higher than the embryonic level. The concentration of metavinculin (a 160-Kd vinculin-related protein) showed only a limited increase after hatching. We discuss the possible roles of vinculin and talin in the assembly of membrane-bound dense plaques during the different phases of smooth-muscle development.     

A helping hand: How vinculin contributes to cell-matrix and cell-cell force transfer

Vinculin helps cells regulate and respond to mechanical forces. It is a scaffolding protein that tightly regulates its interactions with potential binding partners within adhesive structures—including focal adhesions that link the cell to the extracellular matrix and adherens junctions that link cells to each other—that physically connect the force-generating actin cytoskeleton (CSK) with the extracellular environment. This tight control of binding partner interaction—mediated by vinculin's autoinhibitory head–tail interaction—allows vinculin to rapidly interact and detach in response to changes in the dynamic forces applied through the cell. In doing so, vinculin modulates the structural composition of focal adhesions and the cell's ability to generate traction forces and adhesion strength. Recent evidence suggests that vinculin plays a similar role in regulating the fate and function of cell–cell junctions, further underscoring the importance of this protein. Using our lab's recent work as a starting point, this commentary explores several outstanding questions regarding the nature of vinculin activation and its function within focal adhesions and adherens junctions.