Interaction in situ of the cytoskeletal protein vinculin with bilayers studied by introducing a photoactivatable fatty acid into living chicken embryo fibroblasts

Binding of Vinculin to Lipid Membranes in Its Inhibited and Activated States

Phosphoinositols are an important class of phospholipids that are involved in a myriad of cellular processes, from cell signaling to motility and adhesion. Vinculin (Vn) is a major adaptor protein that regulates focal adhesions in conjunction with PIP₂ in lipid membranes and other cytoskeletal components. The binding and unbinding transitions of Vn at the membrane interface are an important link to understanding the coordination of cell signaling and motility. Using different biophysical tools, including atomic force microscopy combined with confocal fluorescence microscopy and Fourier transform infrared spectroscopy, we studied the nanoscopic interactions of activated and autoinhibited states of Vn with lipid membranes. We hypothesize that a weak interaction occurs between Vn and lipid membranes, which leads to binding of autoinhibited Vn to supported lipid bilayers, and to unbinding in freestanding lipid vesicles. Likely driving forces may include tethering of the C-terminus to the lipid membrane, as well as hydrophobic helix-membrane interactions. Conversely, activated Vn binds strongly to membranes through specific interactions with clusters of PIP₂ embedded in lipid membranes. Activated Vn harbored on PIP₂ clusters may form small oligomeric interaction platforms for further interaction partners, which is necessary for the proper function of focal adhesion points.

Vinculin, cell mechanics and tumour cell invasion

The focal adhesion protein, vinculin, is important for transmitting mechanical forces and orchestrating mechanical signalling events. Deregulation of vinculin results in altered cell adhesion, contractility, motility and growth, all of which are important processes in cancer metastasis. This review summarises recent reports on the role of vinculin in cellular force generation and signalling, and discusses implications for a role of vinculin in promoting cancer cell migration in 3D environments.

Is DRM lipid composition relevant in cell-extracellular matrix adhesion structures?

Focal adhesions mediate cell-extracellular matrix adhesion. They are inserted in detergent-resistant membrane microdomains enriched in phosphatidylinositol-4,5-bisphosphate. In spite of the relevance that membrane lipids appear to have on cell adhesion structures, to our knowledge, there are no previous reports on the membrane lipid composition where focal adhesions are located in vivo or on how changes in local membrane composition contribute to focal adhesion maintenance. This may be due to the fact that the explosion of information in the fields of genomics and proteomics has not been matched by a corresponding advancement of knowledge in the field of lipids. The physiological importance of lipids is illustrated by the numerous diseases to which lipid abnormalities contribute. To gain insight into the role of membrane lipid composition in the preservation of epithelial cell adhesion to the substratum, how specific changes in the membrane lipid composition in vivo affect the maintenance of focal adhesions in renal papillae collecting duct cells has been previously studied. It is currently considered that phosphatidylinositol-4,5-bisphosphate plays a crucial role in the maintenance of assembled focal adhesion. However, such pool of polyphosphoinositides has to be part of a domain of a specific lipid composition to serve as a membrane lipid stabilizing the focal adhesion plaque.

Protein-lipid interactions: correlation of a predictive algorithm for lipid-binding sites with three-dimensional structural data

Background

Over the past decade our laboratory has focused on understanding how soluble cytoskeleton-associated proteins interact with membranes and other lipid aggregates. Many protein domains mediating specific cell membrane interactions appear by fluorescence microscopy and other precision techniques to be partially inserted into the lipid bilayer. It is unclear whether these protein-lipid-interactions are dependent on shared protein motifs or unique regional physiochemistry, or are due to more global characteristics of the protein.

Results

We have developed a novel computational program that predicts a protein's lipid-binding site(s) from primary sequence data. Hydrophobic labeling, Fourier transform infrared spectroscopy (FTIR), film balance, T-jump, CD spectroscopy and calorimetry experiments confirm that the interfaces predicted for several key cytoskeletal proteins (alpha-actinin, Arp2, CapZ, talin and vinculin) partially insert into lipid aggregates. The validity of these predictions is supported by an analysis of the available three-dimensional structural data. The lipid interfaces predicted by our algorithm generally contain energetically favorable secondary structures (e.g., an amphipathic alpha-helix flanked by a flexible hinge or loop region), are solvent-exposed in the intact protein, and possess favorable local or global electrostatic properties.

Conclusion

At present, there are few reliable methods to determine the region of a protein that mediates biologically important interactions with lipids or lipid aggregates. Our matrix-based algorithm predicts lipid interaction sites that are consistent with the available biochemical and structural data. To determine whether these sites are indeed correctly identified, and whether use of the algorithm can be safely extended to other classes of proteins, will require further mapping of these sites, including genetic manipulation and/or targeted crystallography.

Structural properties of lipid-binding sites in cytoskeletal proteins

Several cytoskeletal proteins have been shown to interact in vitro with, and in some cases are regulated by, specific membrane lipids. In some cases, evidence for in situ interactions has been provided. The molecular basis for such interactions is now being unravelled. At least five structurally distinct types of lipid-binding sites in cytoskeletal proteins have been identified. However, our understanding of the physiological role of such interactions is still limited. Precise knowledge about the binding-site structures and the actual amino acid residues involved should now enable the expression of mutant proteins that specifically lack the ability to interact with lipids. The impact of these mutations on protein location and function can then be assessed.

Amphitropic proteins: regulation by reversible membrane interactions (review)

What do Src kinase, Ras-guanine nucleotide exchange factor, cytidylyltransferase, protein kinase C, phospholipase C, vinculin, and DnaA protein have in common? These proteins are amphitropic, that is, they bind weakly (reversibly) to membrane lipids, and this process regulates their function. Proteins functioning in transduction of signals generated in cell membranes are commonly regulated by amphitropism. In this review, the strategies utilized by amphitropic proteins to bind to membranes and to regulate their membrane affinity are described. The recently solved structures of binding pockets for specific lipids are described, as well as the amphipathic alpha-helix motif. Regulatory switches that control membrane affinity include modulation of the membrane lipid composition, and modification of the protein itself by ligand binding, phosphorylation, or acylation. How does membrane binding modulate the protein's function? Two mechanisms are discussed: (1) localization with the substrate, activator, or downstream target, and (2) activation of the protein by a conformational switch. This paper also addresses the issue of specificity in the cell membrane targetted for binding.

A conserved motif in the tail domain of vinculin mediates association with and insertion into acidic phospholipid bilayers

The tail domain of vinculin (Vt) contains a salt-insensitive binding site for acidic phospholipids which is masked by the intramolecular head-tail interaction in native vinculin [Johnson, R. P., and Craig, S. W. (1995) Biochem. Biophys. Res. Commun. 210, 159-164]. To characterize further this phospholipid binding site, we have used hydrophobic photolabeling with a photoactivatable phosphatidylcholine analogue to detect insertion of protein into the lipid bilayer. We show here that, although the properties of binding to acidic phospholipid vesicles and spontaneous insertion into the bilayer are cryptic and inactive in vinculin at physiologic ionic strength, these activities of the purified tail domain can be activated by physical and chemical disruption of the intramolecular interaction between the head and tail domains. By analyzing the lipid binding and insertion activity of a series of GST-Vt fusion proteins, we defined 55 amino acids, comprising vinculin residues 916-970, that mimic the lipid-binding and insertion activity of Vt. Predictions of secondary structure suggest that these 55 amino acids form a basic, amphipathic helical hairpin. This prediction is supported by circular dichroism analysis, which indicates that at least 80% of the residues in residues 916-970 are in a helical conformation. This predicted helical hairpin motif, which is conserved in all vinculins and is present in an acidic phospholipid-binding region of alpha-catenin, is distinct from C2 and PH domains, and likely represents a third type of acidic phospholipid-binding structure.

Rescue of the mutant phenotype by reexpression of full-length vinculin in null F9 cells; effects on cell locomotion by domain deleted vinculin

Vinculin plays a role in signaling between integrins and the actin cytoskeleton. We reported earlier that F9-derived cells lacking vinculin are less spread, less adhesive, and move two times faster than wild-type F9 cells. Expression of intact vinculin in null cells restored all wild-type characteristics. In contrast, expression of the head (90 kDa) fragment exaggerated mutant characteristics, especially locomotion, which was double that of vinculin null cells. Expression of the tail domain also had a marked effect on locomotion in the opposite direction, reducing it to very low levels. The expression of the head plus tail domains together (no covalent attachment) effected a partial rescue towards wild-type phenotype, thus indicating that reexpressed polypeptides may be in their correct location and are interacting normally. Therefore, we conclude that: (1) the head domain is part of the locomotory force of the cell, modulated by the tail, and driven by the integrin/matrix connection; (2) intact vinculin is required for normal regulation of cell behavior, suggesting that vinculin head-tail interactions control cell adhesion, spreading, lamellipodia formation and locomotion.

Protein kinase C and the cytoskeleton

The protein kinase C family of serine-threonine kinases are important signal transducers participating in many different agonist-induced signalling cascades. PKC is activated by increases in diacylglycerol produced in response to agonist-induced hydrolysis of inositol phospholipids. PKC is thought to reside in the cytosol in an inactive conformation and translocate to the plasma membrane upon cell activation where it modifies various cellular functions through phosphorylation of target substrates. Increasing evidence has illustrated that this family of enzymes is capable of translocating to other subcellular sites than the plasma membrane. A key to understanding the functions of the members of this family is identifying their physiological substrates and their relationship with those target substrates. The idea that PKC may be an important regulator of cytoskeletal function has been suggested by numerous studies. Activation of PKC in a variety of different cell types leads to changes in the cell cytoskeleton including lymphocyte surface receptor capping, smooth muscle contraction and actin rearrangement in T cells and neutrophils. Given the ubiquitous expression of PKC and the diversity of cytoskeletons in different cell types it is not surprising that PKC has been shown to be associated with and/or phosphorylate a wide range of cytoskeletal components. This review examines the interaction of PKC with the cytoskeleton and discusses some of the cytoskeletal functions ascribed to PKC to date.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Acidic phospholipids inhibit the intramolecular association between the N- and C-terminal regions of vinculin, exposing actin-binding and protein kinase C phosphorylation sites

Chick vinculin polypeptides expressed in Escherichia coli as glutathione S-transferase (GST) fusion proteins have been used to identify the sites involved in the intramolecular association between the 90 kDa N-terminal head and the 30 kDa C-terminal tail region of the vinculin molecule. Fusion proteins spanning vinculin residues 1-258 and 1-398, immobilized on glutathione-agarose beads, were shown to bind a C-terminal vinculin polypeptide spanning residues 881-1066 (liberated from GST by thrombin cleavage). However, the C-terminal polypeptide did not bind to a fusion protein spanning residues 399-881 or to itself. Binding was dependent on residues 167-207 within the N-terminal polypeptide, a sequence also essential for talin binding. Conversely, the 90 kDa head polypeptide was shown to bind to residues 1029-1036 in the tail region of vinculin. The association of the head and tail was inhibited by acidic, but not neutral, phospholipids. Pre-incubation of vinculin with acidic phospholipids exposed the binding site for F-actin and a phosphorylation site for protein kinase C. The phosphorylation site was located in the tail region of the vinculin molecule. These results raise the possibility that acidic phospholipids play a role in regulating the activity of vinculin and therefore the assembly of both cell-cell and cell-matrix adherens-type junctions.

Identification of a phosphatidylinositol-4,5-bisphosphate-binding domain in the N-terminal region of ezrin

Purified human recombinant ezrin cosediments with large liposomes containing phosphatidylserine (PS). This interaction is optimal at low ionic strength. At physiological ionic strength (130 mM KCl) ezrin interacts strongly with liposomes containing > or = 5% phosphatidylinositol-4,5-bisphosphate (PIP2), the residual being phosphatidylcholine (PC). When PIP2 is replaced by phosphatidylinositol-4-monophosphate (PIP), phosphatidylinositol (PI) or PS, the interaction is markedly reduced. Furthermore we show, that a purified N-terminal glutathione S-transferase (GST) fusion protein of ezrin (1-309) still has retained the capacity to interact with PIP2-containing liposomes, whereas a C-terminal fusion protein (310-586) has lost this ability.

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

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

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.

Evidence for a ternary interaction between alpha-actinin, (meta)vinculin and acidic-phospholipid bilayers

The cytoskeletal component vinculin has been demonstrated by hydrophobic photoradiolabelling, to insert into bilayers containing acidic phospholipids and trace amounts of a photoactivatable analogue of lecithin. It is shown in this study that the higher-molecular-mass variant metavinculin and alpha-actinin, also share this property. alpha-Actinin and vinculin were also shown to associate with phosphatidylserine liposomes by chromatography of protein/lipid mixtures on a Bio-Gel A-5m column. Furthermore, interesting differences in the behaviour of binary mixtures of these proteins, in the presence of phosphatidylserine liposomes, are shown. Thus, incubation of alpha-actinin with vinculin or metavinculin, prior to the addition of liposomes, strongly inhibited the photoradiolabelling of alpha-actinin under conditions in which the liposome surface was non-limiting, but enhanced the labelling of vinculin. In contrast, vinculin and metavinculin did not mutually influence their labelling. Using gel-filtration chromatography, it was shown that alpha-actinin still bound to the vinculin-liposome complex, under conditions similar to those used for hydrophobic photolabelling with a non-limiting lipid surface. In the presence of limiting amounts of liposomes, the alpha-actinin/vinculin ratio was markedly decreased in the liposome fractions. Our results suggest the formation of a ternary complex consisting of vinculin, alpha-actinin and phospholipids. In this complex, both proteins interact at the bilayer, resulting in an altered conformation of the two proteins and, as a consequence, in modified bilayer interactions.

Pattern formation and handedness in the cytoskeleton of human platelets

The cytoskeletal patterns of human platelets spread on a glass surface are analyzed. F-actin is arranged in patterns of parallel microfilaments, microfilaments forming triangles, or microfilaments radiating tangentially from a central ellipse or circle. Vinculin, a cytoskeletal protein, is located at both ends of the filaments. In platelets with tangentially radiating microfilaments, vinculin patches are aligned on the branches of a two-armed spiral. The spirals are always left-handed. Talin and two integrins (gpIIb-IIIa, vitronectin receptor), proteins usually associated with focal contacts in tissue culture cells, are not concentrated at the ends of microfilaments in human platelets. It is suggested that the distribution of vinculin is due to competitive aggregation of vinculin close to the inner leaflet of the ventral plasma membrane and that sites of cytoskeleton-membrane linkage are important for generating supramolecular asymmetries of biological systems.

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.

Vinculin is one of the major endogenous substrates for intrinsic tyrosine kinases in neuronal growth cones isolated from fetal rat brain

Neuronal growth cones, the motile tips of growing neuronal processes, are responsible for the exact guidance of extending neurites. To elucidate the mechanisms of their biochemical signal transduction in growth cones, the growth-cone-enriched fraction was isolated biochemically from fetal rat brain and the endogenous protein phosphorylation in the fraction was analyzed under the conditions where tyrosine residues were preferentially phosphorylated. One of the major phosphoproteins was a 130-kDa slightly acidic protein which reacted with antiphosphotyrosine antibody. Its phosphoryl residues were alkali-stable. Thus, the 130-kDa protein was concluded to be susceptible to tyrosine phosphorylation. This protein was a component of cytoskeletal proteins thought to be associated indirectly with membranes. All the behavior of the 130-kDa protein was compatible with the properties of vinculin, a component of focal contacts which are responsible for the stable or motile adhesion between cells or between a cell and the substratum. Immunochemical analyses showed that the 130-kDa protein was specifically recognized by anti-vinculin antibody. Therefore, the 130-kDa protein was concluded to be vinculin. Tyrosine phosphorylation of the protein appeared to be relatively more pronounced in the growth-cone-enriched fraction than in adult synaptosomes. The results suggest that tyrosine phosphorylation of vinculin may be regulated developmentally and it may be involved in the functions of growth cones.