Vinculin's C-terminal region facilitates phospholipid membrane insertion

Fluorescence Correlation Spectroscopy Reveals Interaction of Some Microdomain-Associated Lipids with Cellular Focal Adhesion Sites

Cells adhere to the extracellular matrix at distinct anchoring points, mostly focal adhesions. These are rich in immobile transmembrane- and cytoskeletal-associated proteins, some of which are known to interact with lipids of the plasma membrane. To investigate their effect on lipid mobility and molecular interactions, fluorescently labeled lipids were incorporated into the plasma membranes of primary myofibroblasts using fusogenic liposomes. With fluorescence correlation spectroscopy, we tested mobilities of labeled microdomain-associated lipids such as sphingomyelin (SM), ganglioside (GM1), and cholesterol as well as of a microdomain-excluded phospholipid (PC) and a lipid-like molecule (DiIC₁₈(7)) in focal adhesions (FAs) and in neighboring non-adherent membrane areas. We found significantly slower diffusion of SM and GM1 inside FAs but no effect on cholesterol, PC, and DiIC₁₈(7). These data were compared to the molecular behavior in L_o/L_d-phase separated giant unilamellar vesicles, which served as a model system for microdomain containing lipid membranes. In contrast to the model system, lipid mobility changes in FAs were molecularly selective, and no particle enrichment occurred. Our findings suggest that lipid behavior in FAs cannot be described by L_o/L_d-phase separation. The observed slow-down of some molecules in FAs is potentially due to transient binding between lipids and some molecular constituent(s).

Molecular interactions between vinculin and phospholipids

The focal adhesion protein vinculin has been implicated in associating with soluble and membranous phospholipids. Detailed investigations over the past ten years describe the intermolecular interactions of the vinculin tail domain with soluble and membrane phospholipids. Previous studies have implied that the tail's unstructured C-terminal region affects the mechanical behavior of cells and that the same region, at the molecular level, has bi-stable behavior sensitive to different protonation states. The aim of this short communication is to discuss whether the C-terminal vinculin tail (Vt) domain interacts favorably with membrane-embedded phospholipids such as PIP₂ and that the region is also an anchor for lipid membranes.

Vinculin in cell-cell and cell-matrix adhesions

Vinculin was identified as a component of focal adhesions and adherens junctions nearly 40 years ago. Since that time, remarkable progress has been made in understanding its activation, regulation and function. Here we discuss the current understanding of the roles of vinculin in cell–cell and cell–matrix adhesions. Emphasis is placed on the how vinculin is recruited, activated and regulated. We also highlight the recent understanding of how vinculin responds to and transmits force at integrin- and cadherin-containing adhesion complexes to the cytoskeleton. Furthermore, we discuss roles of vinculin in binding to and rearranging the actin cytoskeleton.

A Structural Model for Vinculin Insertion into PIP2-Containing Membranes and the Effect of Insertion on Vinculin Activation and Localization

Vinculin, a scaffolding protein that localizes to focal adhesions (FAs) and adherens junctions, links the actin cytoskeleton to the adhesive super-structure. While vinculin binds to a number of cytoskeletal proteins, it can also associate with phosphatidylinositol 4,5-bisphosphate (PIP₂) to drive membrane association. To generate a structural model for PIP₂-dependent interaction of vinculin with the lipid bilayer, we conducted lipid-association, nuclear magnetic resonance, and computational modeling experiments. We find that two basic patches on the vinculin tail drive membrane association: the basic collar specifically recognizes PIP₂, while the basic ladder drives association with the lipid bilayer. Vinculin mutants with defects in PIP₂-dependent liposome association were then expressed in vinculin knockout murine embryonic fibroblasts. Results from these analyses indicate that PIP₂ binding is not required for localization of vinculin to FAs or FA strengthening, but is required for vinculin activation and turnover at FAs to promote its association with the force transduction FA nanodomain.

Mechanisms and Functions of Vinculin Interactions with Phospholipids at Cell Adhesion Sites

The cytoskeletal protein vinculin is a major regulator of cell adhesion and attaches to the cell surface by binding to specific phospholipids. Structural, biochemical, and biological studies provided much insight into how vinculin binds to membranes, what components it recognizes, and how lipid binding is regulated. Here we discuss the roles and mechanisms of phospholipids in regulating the structure and function of vinculin and of its muscle-specific metavinculin splice variant. A full appreciation of these processes is necessary for understanding how vinculin regulates cell motility, migration, and wound healing, and for understanding of its role in cancer and cardiovascular diseases.

Lipid-directed vinculin dimerization

Vinculin localizes to cellular adhesions where it regulates motility, migration, development, wound healing, and response to force. Importantly, vinculin loss results in cancer phenotypes, cardiovascular disease, and embryonic lethality. At the plasma cell membrane, the most abundant phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP2), binds the vinculin tail domain, Vt, and triggers homotypic and heterotypic interactions that amplify binding of vinculin to the actin network. Binding of PIP2 to Vt is necessary for maintaining optimal focal adhesions, for organizing stress fibers, for cell migration and spreading, and for the control of vinculin dynamics and turnover of focal adhesions. While the recently determined Vt/PIP2 crystal structure revealed the conformational changes occurring upon lipid binding and oligomerization, characterization of PIP2-induced vinculin oligomerization has been challenging in the adhesion biology field. Here, via a series of novel biochemical assays not performed in previous studies that relied on chemical cross-linking, we characterize the PIP2-induced vinculin oligomerization. Our results show that Vt/PIP2 forms a tight dimer with Vt or with the muscle-specific vinculin isoform, metavinculin, at sites of adhesion at the cell membrane. Insight into how PIP2 regulates clustering and into mechanisms that regulate cell adhesion allows the development for a more definite sensor for PIP2, and our developed techniques can be applied generally and thus open the door for the characterization of many other protein/PIP2 complexes under physiological conditions.

Lipid binding promotes oligomerization and focal adhesion activity of vinculin

PIP₂ binds vinculin and directs its oligomerization, which promotes proper focal adhesion structure and function.

Adherens junctions (AJs) and focal adhesion (FA) complexes are necessary for cell migration and morphogenesis, and for the development, growth, and survival of all metazoans. Vinculin is an essential regulator of both AJs and FAs, where it provides links to the actin cytoskeleton. Phosphatidylinositol 4,5-bisphosphate (PIP₂) affects the functions of many targets, including vinculin. Here we report the crystal structure of vinculin in complex with PIP₂, which revealed that PIP₂ binding alters vinculin structure to direct higher-order oligomerization and suggests that PIP₂ and F-actin binding to vinculin are mutually permissive. Forced expression of PIP₂-binding–deficient mutants of vinculin in vinculin-null mouse embryonic fibroblasts revealed that PIP₂ binding is necessary for maintaining optimal FAs, for organization of actin stress fibers, and for cell migration and spreading. Finally, photobleaching experiments indicated that PIP₂ binding is required for the control of vinculin dynamics and turnover in FAs. Thus, through oligomerization, PIP₂ directs a transient vinculin sequestration at FAs that is necessary for proper FA function.

Phosphorylation at Y1065 in vinculin mediates actin bundling, cell spreading, and mechanical responses to force

Vinculin is an essential structural adaptor protein that localizes to sites of adhesion and is involved in a number of cell processes including adhesion, spreading, motility, force transduction, and cell survival. The C-terminal vinculin tail domain (Vt) contains the necessary structural components to bind and cross-link actin filaments. Actin binding to Vt induces a conformational change that promotes dimerization through the C-terminal hairpin of Vt and enables actin filament cross-linking. Here we show that Src phosphorylation of Y1065 within the C-terminal hairpin regulates Vt-mediated actin bundling and provide a detailed characterization of Y1065 mutations. Furthermore, we show that phosphorylation at Y1065 plays a role in cell spreading and the response to the application of mechanical force.

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.

Phosphorylation primes vinculin for activation

Vinculin phosphorylation has been implicated as a potential mechanism for focal adhesion growth and maturation. Four vinculin residues-Y100, S1033, S1045, and Y1065-are phosphorylated by kinases during focal adhesion maturation. In this study, phosphorylation at each of these residues is simulated using molecular dynamics models. The simulations demonstrate that once each phosphorylated vinculin structure is at equilibrium, significant local conformational changes result that may impact either vinculin activation or vinculin binding to actin and PIP2. Simulation of vinculin activation after phosphorylation shows that the added phosphoryl groups can prime vinculin for activation. It remains to be seen if vinculin can be phosphorylated at S1033 in vivo, but these simulations highlight that in the event of a S1033 phophorylation vinculin will likely be primed for activation.

Tyrosine phosphorylation of vinculin at position 1065 modifies focal adhesion dynamics and cell tractions

Focal adhesions (FAs) connect the cellular actin cytoskeleton via integrin with the extracellular matrix. They comprise of many structural and signaling proteins which are highly dynamic, well regulated, and responsible for the sensing of physical properties from the environment. Vinculin is a protein that incorporates all these functions. Here, we investigated the phosphorylation of Y1065 in the activation/regulation of vinculin. We used different vinculin mutants mimicking either a permanently activated or inhibited phosphorylation site at position 1065. Using these mutants, we determined their influence on the exchange dynamics and cell forces using fluorescence recovery after photobleaching and traction microscopy. The results indicate that phosphorylation at Y1065 significantly increases the amount of freely exchanging vinculin within FAs whereas inhibition of this phosphorylation site leads to an uncontrolled exchange of vinculin and reduced adhesive cell forces. In conclusion, we show that phosphorylation on position Y1065 is essential for accurate incorporation of vinculin into FAs and mechanical behavior of cells.