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

Suppression of focal adhesion formation may account for the suppression of cell migration, invasion and growth of non-small cell lung cancer cells following treatment with polyisoprenylated cysteinyl amide inhibitors

Migratory cells form extracellular matrix attachments called focal-adhesions. Focal adhesion assembly and disassembly are regulated by the Rho family of small GTPases. We previously reported that polyisoprenylated cysteinyl amide inhibitors (PCAIs) suppress Rho protein levels, disrupting F-actin cytoskeleton remodeling in the formation of lamellipodia and filopodia. In this study, we investigated whether these observations effect focal adhesion formation, which involves cell surface receptors known as integrins and several signaling/adaptor proteins such as vinculin, α-actinin, Rock kinases and phospho-Myosin Light Chain-2 (p-MLC-2), that foster the linkage of the actin cytoskeleton to the extracellular matrix. We observed that treatment of H1299 cells with 5 μM PCAIs for 24 h markedly diminished the level of full-length integrin α4 by at least 24% relative to controls. PCAIs at 5 μM, diminished the levels of vinculin by at least 50%. Immunofluorescent analysis showed at least a 76% decrease in the number of vinculin-focal adhesion punctates. In addition, PCAIs diminished Rock1 levels by 25% and its substrate, p-MLC-2 by 75%. PCAIs did not significantly alter the levels of integrin β5, α-actinin, and Rock2, suggesting that the effects of the PCAIs are target specific. Our data indicate that the PCAIs alter the levels of the Rho proteins and their effectors to abrogate their functions in cytoskeleton remodeling thereby suppressing focal adhesion formation. This in turn results in a PCAIs-induced decrease in cell invasion, thus making the PCAIs propitious agents for the inhibition of cancer growth and metastasis.

The Calponin Family Member CHDP-1 Interacts with Rac/CED-10 to Promote Cell Protrusions

Eukaryotic cells extend a variety of surface protrusions to direct cell motility. Formation of protrusions is mediated by coordinated actions between the plasma membrane and the underlying actin cytoskeleton. Here, we found that the single calponin homology (CH) domain-containing protein CHDP-1 induces the formation of cell protrusions in C. elegans. CHDP-1 is anchored to the cortex through its amphipathic helix. CHDP-1 associates through its CH domain with the small GTPase Rac1/CED-10, which is a key regulator of the actin cytoskeleton. CHDP-1 preferentially binds to the GTP-bound active form of the CED-10 protein and preserves the membrane localization of GTP-CED-10. Hence, by coupling membrane expansion to Rac1-mediated actin dynamics, CHDP-1 promotes the formation of cellular protrusions in vivo.

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.

Membrane-induced structural rearrangement and identification of a novel membrane anchor in talin F2F3

Experimental challenges associated with characterization of the membrane-bound form of talin have prevented us from understanding the molecular mechanism of its membrane-dependent integrin activation. Here, utilizing what we believe to be a novel membrane mimetic model, we present a reproducible model of membrane-bound talin observed across multiple independent simulations. We characterize both local and global membrane-induced structural transitions that successfully reconcile discrepancies between biochemical and structural studies and provide insight into how talin might modulate integrin function. Membrane binding of talin, captured in unbiased simulations, proceeds through three distinct steps: initial electrostatic recruitment of the F2 subdomain to anionic lipids via several basic residues; insertion of an initially buried, conserved hydrophobic anchor into the membrane; and association of the F3 subdomain with the membrane surface through a large, interdomain conformational change. These latter two steps, to our knowledge, have not been observed or described previously. Electrostatic analysis shows talin F2F3 to be highly polarized, with a highly positive underside, which we attribute to the initial electrostatic recruitment, and a negative top face, which can help orient the protein optimally with respect to the membrane, thereby reducing the number of unproductive membrane collision events.

Efficient Exploration of Membrane-Associated Phenomena at Atomic Resolution

Biological membranes constitute a critical component in all living cells. In addition to providing a conducive environment to a wide range of cellular processes, including transport and signaling, mounting evidence has established active participation of specific lipids in modulating membrane protein function through various mechanisms. Understanding lipid-protein interactions underlying these mechanisms at a sufficiently high resolution has proven extremely challenging, partly due to the semi-fluid nature of the membrane. In order to address this challenge computationally, multiple methods have been developed, including an alternative membrane representation termed highly mobile membrane mimetic (HMMM) in which lateral lipid diffusion has been significantly enhanced without compromising atomic details. The model allows for efficient sampling of lipid-protein interactions at atomic resolution, thereby significantly enhancing the effectiveness of molecular dynamics simulations in capturing membrane-associated phenomena. In this review, after providing an overview of HMMM model development, we will describe briefly successful application of the model to study a variety of membrane processes, including lipid-dependent binding and insertion of peripheral proteins, the mechanism of phospholipid insertion into lipid bilayers, and characterization of optimal tilt angle of transmembrane helices. We conclude with practical recommendations for proper usage of the model in simulation studies of membrane processes.

Dynamic shaping of cellular membranes by phospholipids and membrane-deforming proteins

All cellular compartments are separated from the external environment by a membrane, which consists of a lipid bilayer. Subcellular structures, including clathrin-coated pits, caveolae, filopodia, lamellipodia, podosomes, and other intracellular membrane systems, are molded into their specific submicron-scale shapes through various mechanisms. Cells construct their micro-structures on plasma membrane and execute vital functions for life, such as cell migration, cell division, endocytosis, exocytosis, and cytoskeletal regulation. The plasma membrane, rich in anionic phospholipids, utilizes the electrostatic nature of the lipids, specifically the phosphoinositides, to form interactions with cytosolic proteins. These cytosolic proteins have three modes of interaction: 1) electrostatic interaction through unstructured polycationic regions, 2) through structured phosphoinositide-specific binding domains, and 3) through structured domains that bind the membrane without specificity for particular phospholipid. Among the structured domains, there are several that have membrane-deforming activity, which is essential for the formation of concave or convex membrane curvature. These domains include the amphipathic helix, which deforms the membrane by hemi-insertion of the helix with both hydrophobic and electrostatic interactions, and/or the BAR domain superfamily, known to use their positively charged, curved structural surface to deform membranes. Below the membrane, actin filaments support the micro-structures through interactions with several BAR proteins as well as other scaffold proteins, resulting in outward and inward membrane micro-structure formation. Here, we describe the characteristics of phospholipids, and the mechanisms utilized by phosphoinositides to regulate cellular events. We then summarize the precise mechanisms underlying the construction of membrane micro-structures and their involvements in physiological and pathological processes.

Prediction of fatty acid-binding residues on protein surfaces with three-dimensional probability distributions of interacting atoms

Protein-fatty acid interaction is vital for many cellular processes and understanding this interaction is important for functional annotation as well as drug discovery. In this work, we present a method for predicting the fatty acid (FA)-binding residues by using three-dimensional probability density distributions of interacting atoms of FAs on protein surfaces which are derived from the known protein-FA complex structures. A machine learning algorithm was established to learn the characteristic patterns of the probability density maps specific to the FA-binding sites. The predictor was trained with five-fold cross validation on a non-redundant training set and then evaluated with an independent test set as well as on holo-apo pair's dataset. The results showed good accuracy in predicting the FA-binding residues. Further, the predictor developed in this study is implemented as an online server which is freely accessible at the following website, http://ismblab.genomics.sinica.edu.tw/.

Global approaches for the elucidation of phosphoinositide-binding proteins

Phosphoinositide lipids (PIPns) control numerous critical biological pathways, typically through the regulation of protein function driven by non-covalent protein-lipid binding interactions. Despite the importance of these systems, the unraveling of the full scope of protein-PIPn interactions has represented a significant challenge due to the massive complexity associated with these events, including the large number of diverse proteins that bind to these lipids, variations in the mechanisms by which proteins bind to lipids, and the presence of multiple distinct PIPn isomers. As a result of this complexity, global methods in which numerous proteins that bind PIPns can be identified and characterized simultaneously from complex samples, which have been enabled by key technological advancements, have become popular as an efficient means for tackling this challenge. This review article provides an overview of advancements in large-scale methods for profiling protein-PIPn binding, including experimental methods, such as affinity enrichment, microarray analysis and activity-based protein profiling, as well as computational methods, and combined computational/experimental efforts.

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.

Vinculin's C-terminal region facilitates phospholipid membrane insertion

The focal adhesion protein vinculin has been implicated in associating with soluble and membranous phospholipids. Here, we investigated the intermolecular interactions of two vinculin tail domains with membrane phospholipids. Previous studies have shown that the tail's unstructured C-terminus affects the mechanical behavior of cells, but not the H3 region. The aim of this work was to establish whether the C-terminal or the H3 region either associate favorably with or anchor in lipid membranes. This work characterizes the energetics and dynamics of phospholipid interactions using differential scanning calorimetry (DSC) as well as circular dichroism (CD) spectroscopy. Biochemical data from tryptophan quenching and SDS-PAGE experiments support calorimetric and CD spectroscopic findings insofar that only vinculin's C-terminus inserts into lipid membranes. These in vitro results provide further insight into the mechanical behavior of vinculin tail regions in cells and contribute to the understanding of their structure and function.

Biochemical characterization of distinct regions of SPEC molecules and their role in phagocytosis

Cdc42 signaling pathways play important roles in immune cell polarization and cytoskeletal changes. Although the small Cdc42-binding proteins SPEC1 and SPEC2 play a role in F-actin accumulation in activated T lymphocytes, little is known about their precise activities in other cell types. Here, we mapped the Cdc42-binding activity of SPEC1 to the CRIB sequence and a downstream alpha helical region. Biochemical studies revealed that SPEC1 did not interact with a Rac1 switch-of-function mutant capable of inducing Cdc42-like filopodia, potentially eliminating a role for SPECs in this process. A phosphoinositide-binding region was identified within a basic region N-terminal to the CRIB sequence of SPEC1. Using an anti-SPEC2 antibody, we found that endogenous SPEC2 colocalized with Cdc42 at the phagocytic cup of macrophages internalizing zymosan A particles prior to significant F-actin accumulation. Overexpression studies of the related SPEC1 protein induced marked macrophage contraction and prevented particle binding and phagocytosis. Although a Cdc42-binding mutant of SPEC1 still caused macrophage contraction, mutations within the N-terminal cysteines and phosphoinositide-binding region reversed macrophage contraction but still resulted in impaired phagocytosis. These results identify three distinct structural and functional regions within SPECs and demonstrate their likely role in early contractile events in phagocytosis.

CapZ-lipid membrane interactions: a computer analysis

Background

CapZ is a calcium-insensitive and lipid-dependent actin filament capping protein, the main function of which is to regulate the assembly of the actin cytoskeleton. CapZ is associated with membranes in cells and it is generally assumed that this interaction is mediated by polyphosphoinositides (PPI) particularly PIP₂, which has been characterized in vitro.

Results

We propose that non-PPI lipids also bind CapZ. Data from computer-aided sequence and structure analyses further suggest that CapZ could become partially buried in the lipid bilayer probably under mildly acidic conditions, in a manner that is not only dependent on the presence of PPIs. We show that lipid binding could involve a number of sites that are spread throughout the CapZ molecule i.e., alpha- and beta-subunits. However, a beta-subunit segment between residues 134–151 is most likely to be involved in interacting with and inserting into lipid membrane due to a slighly higher ratio of positively to negatively charged residues and also due to the presence of a small hydrophobic helix.

Conclusion

CapZ may therefore play an essential role in providing a stable membrane anchor for actin filaments.

Opening of holes in liposomal membranes is induced by proteins possessing the FERM domain

The destabilization of vesicles caused by interactions between lipid bilayers and proteins was studied by direct, real-time observation using high-intensity dark-field microscopy. We previously reported that talin, a cytoskeletal submembranous protein, can reversibly open stable large holes in giant liposomes made of neutral and acidic phospholipids. Talin and other proteins belonging to the band 4.1 superfamily have the FERM domain at their N-terminal and interact with lipid membranes via that domain. Here, we observed that band 4.1, ezrin and moesin, members of the band 4.1 superfamily, are also able to open stable holes in liposomes. However, truncation of their C-terminal domains, which can interact with the N-terminal FERM domain, impaired their hole opening activities. Oligomeric states of ezrin affected the capability of the membrane hole formation. Phosphatidylinositol bisphosphate (PIP2), which binds to the FERM domain and disrupts the interaction between the N and C termini of the band 4.1 superfamily, down-regulates their membrane opening activity. These results suggest that the intermolecular interaction plays a key role in the observed membrane hole formation.

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.

Huntingtin associates with acidic phospholipids at the plasma membrane

We have identified a domain in the N terminus of huntingtin that binds to membranes. A three-dimensional homology model of the structure of the binding domain predicts helical HEAT repeats, which emanate a positive electrostatic potential, consistent with a charge-based mechanism for membrane association. An amphipathic helix capable of inserting into pure lipid bilayers may serve to anchor huntingtin to the membrane. In cells, N-terminal huntingtin fragments targeted to regions of plasma membrane enriched in phosphatidylinositol 4,5-bisphosphate, receptor bound-transferrin, and endogenous huntingtin. N-terminal huntingtin fragments with an expanded polyglutamine tract aberrantly localized to intracellular regions instead of plasma membrane. Our data support a new model in which huntingtin directly binds membranes through electrostatic interactions with acidic phospholipids.

Microviscoelasticity of the apical cell surface of human umbilical vein endothelial cells (HUVEC) within confluent monolayers

We studied the local viscoelasticity of the apical membrane of human umbilical vein endothelial cells within confluent layers by magnetic tweezers microrheometry. Magnetic beads are coupled to various integrins by coating with fibronectin or invasin. By analyzing the deflection of beads evoked by various force scenarios we demonstrate that the cell envelope behaves as a linear viscoelastic body if forces up to 2 nN are applied for short times (<20 s) but can respond in an adaptive way if stress pulses are applied longer (>30 s). The time-dependent shear relaxation modulus G(t) exhibits three time regimes: a fast response (t < 0.05 s) where the relaxation modulus G(t) obeys a power law G(t) approximately t(-0.82+/-0.02); a plateau-like behavior (at 0.05 s < t < 0.15 s); and a slow flow-like response which is, however, partially reversible. Strain field mapping experiments with colloidal probes show that local forces induce a strain field exhibiting a range of zeta = 10 +/- 1 microm, but which could only be observed if nonmagnetic beads were coupled to the cell surface by invasin. By application of the theory of elasticity of planar bodies we estimated a surface shear modulus of 2.5 x10(-4) N/m. By assuming a thickness of the actin cortex of approximately 0.5 microm we estimate a Young modulus micro approximately 400 Pa for the apical membrane. The value agrees with a plateau modulus of an entangled or weakly cross-linked actin network of an actin concentration of 100 microM (mesh size 0.2 microm). This result together with our observation of a strong reduction of the shear modulus by the actin destabilizing agent latrunculin A suggests that the shear modulus measured by our technique is determined by the actin cortex. The effect of two ligands inducing actin stress fiber formation and centripetal contraction of cells (associated with the formation of gaps in the confluent cell monolayer) on the viscoelastic responses were studied: histamine and lysophosphatidic acid (LPA). Histamine evoked a dramatic increase of the cell stiffness by >1 order of magnitude within <30 s, which is attributed to a transient rise of the intracellular Ca(2+) level, since DMSO exerted a similar effect. The stiffening is accompanied by a concomitant rounding of the cells as observed by microinterferometry and relaxes partially in the timescale of 5 min, whereas gaps between cells close after approximately 30 min. LPA did not exert a remarkable and reproducible effect other than an occasional very weak transient increase of the shear stiffness, which shows that the gap formation activated by LPA is mediated by a different mechanism than that induced by histamine.

Alpha-catenin: at the junction of intercellular adhesion and actin dynamics

Alpha-catenin has often been considered to be a non-regulatory intercellular adhesion protein, in contrast to beta-catenin, which has well-documented dual roles in cell-cell adhesion and signal transduction. Recently, however, alpha-catenin has been found to be important not only in connecting the E-cadherin-beta-catenin complex to the actin cytoskeleton, but also in coordinating actin dynamics and inversely correlating cell adhesion with proliferation. As the number of alpha-catenin-interacting partners increases, intriguing new connections imply even more complex regulatory functions for this protein.

Inhibition of phosphatidylinositol-4-phosphate 5-kinase Ialpha impairs localized actin remodeling and suppresses phagocytosis

Actin polymerization drives the extension of pseudopods required for phagocytosis. Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is thought to play a central role in this process, because it interacts with several actin-regulatory proteins and undergoes acute and localized changes at sites of phagocytosis. We therefore studied whether phosphatidylinositol-4-phosphate 5-kinase (PIPK), the enzyme responsible for the generation of PIP(2) from phosphatidylinositol 4-phosphate, is involved in the control of phagocytosis. PIPKIalpha was found to accumulate transiently on forming phagosomes. To test the functional involvement of PIPKIalpha in particle engulfment, we generated a double mutant (D309N/R427Q) that lacks kinase activity. When ectopically expressed in cultured cells, this mutant is targeted to the plasma membrane and accumulates at the phagosomal cup during particle engulfment. Expression of PIP5KIalpha D309N/R427Q impaired phagocytosis in RAW264.7 macrophages and in engineered phagocytes generated by transfection of Fc receptors in Chinese hamster ovary cells. Inhibition of phagocytosis could not be attributed to defects in particle binding or receptor clustering, which was monitored using green fluorescent protein-tagged Fcgamma receptors. Instead, expression of the inactive kinase diminished the accumulation of PIP(2) and of F-actin in the phagosomal cup. These data suggest that PIPKIalpha activity is involved in the actin remodeling that is a prerequisite for efficient phagocytosis. PIPKIalpha appears to contribute to the transient changes in PIP(2) levels that are associated with, and likely required for, the recruitment and regulation of actin-modulating proteins.

Membrane fusion induced by the major lipid-binding domain of the cytoskeletal protein talin

Secondary structure predictions have led to the identification of a major membrane-anchoring domain of the cytoskeletal protein talin spanning from amino acid 385 to 406. Using a synthetically derived peptide of this region, researchers have shown that it inserts into POPC/POPG phospholipid membranes with a partition coefficient of K(app)=1.1+/-0.2 x 10(5) M(-1) and has an average molar reaction enthalpy of DeltaH=-2.5 kcal/mol, as determined by monolayer expansion technique and isothermic titration calorimetry [J. Biol. Chem. 275, 17954]. We applied resonance energy transfer (RET) assays to analyze the fusogenic properties of this peptide by lipid mixing and used liposomes containing carboxyfluorescein to measure the contents leakage. We directly visualized talin peptide-induced vesicle membrane fusion using cryo-electron microscopy. This is the first example of a cytoskeletal protein domain that can trigger membrane fusion that might be of importance for understanding membrane targeting and motile events at the leading edge of the cell.

Phosphoinositides, key molecules for regulation of actin cytoskeletal organization and membrane traffic from the plasma membrane

Phosphoinositide plays a critical role not only in generating second messengers, such as inositol 1,4,5-trisphosphate and diacylglycerol, but also in modulating a variety of cellular functions including cytoskeletal organization and membrane trafficking. Many inositol lipid kinases and phosphatases appear to regulate the concentration of a variety of phosphoinositides in a specific area, thereby inducing spatial and temporal changes in their availability. For example, local concentration changes in phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) in response to extracellular stimuli cause the reorganization of actin filaments and a change in cell shape. PI(4,5)P(2) uncaps the barbed end of actin filaments and increases actin nucleation by modulating a variety of actin regulatory proteins, leading to de novo actin polymerization. PI(4,5)P(2) also plays a key role in membrane trafficking processes. In endocytosis, PI(4,5)P(2) targets clathrin-associated proteins to endocytic vesicles, leading to clathrin-coated pit formation. On the contrary, PI(4,5)P(2) must be dephosphorylated when they shed clathrin coats to fuse endosome. Thus, through regulating actin cytoskeleton organization and membrane trafficking, phosphoinositides play crucial roles in a variety of cell functions such as growth, polarity, movement, and pattern formation.

Conformational change in the vinculin C-terminal depends on a critical histidine residue (His-906)

A phospholipid-controlled interaction between the N-terminal and C-terminal domains of vinculin is thought to be a major mechanism that regulates binding activities of the protein. To probe the mechanisms underlying these interactions we used chemical modification and site-directed mutagenesis directed at histidine residues. Diethylpyrocarbonate (DEPC) modification of the C-terminal, but not the N-terminal, domain greatly decreased affinity of the N-terminal-C-terminal binding, implicating histidine residues in the C-terminal. Mutation of either or both C-terminal histidines (at positions 906 and 1026), however, did not affect N-C binding at neutral pH. The H906A mutation did prevent DEPC effects and also prevented the normal decrease in binding affinity for the N-terminal at lower pH. We found that the wild type C-terminal domain, but not the H906A mutant, underwent a conformational change at pH 6.5, reflected in an altered circular dichroism spectrum and apparent oligomerization. Phospholipid also induced conformational changes in the wild type C-terminal domain but not in the H906A mutant, even though the mutant protein did bind to the phospholipid. Finally, the sensitivity of the N-C interaction to phospholipid was much reduced by the H906A mutation. These results show that H906 plays a key role in the conformational dynamics of the C-terminal domain and thus the regulation of vinculin.

The actin cytoskeleton and plasma membrane connection: PtdIns(4,5)P(2) influences cytoskeletal protein activity at the plasma membrane

The co-ordination of rearrangements of the actin cytoskeleton depends on its tight connection to the plasma membrane. Phosphatidylinositol 4,5-bisphosphate is thought to transmit signals originating at the plasma membrane to the underlying actin cytoskeleton. This lipid binds to, and influences the activity of, several actin-associated proteins in vitro that regulate the architecture of the actin cytoskeleton. Signalling intermediates in this process include focal adhesion molecules such as vinculin and members of two families of proteins, ERM and WASP. These proteins interact with phosphatidylinositol 4,5-bisphosphate and appear to be regulated by interplay between small GTPases and phosphatidylinositol 4,5-bisphosphate metabolism, and thus link the plasma membrane with cytoskeletal remodelling.

Phospholipid binding of synthetic talin peptides provides evidence for an intrinsic membrane anchor of talin

Talin, an actin-binding protein, is assumed to anchor at the membrane via an intrinsic amino acid sequence. Three N-terminal talin fragments, 21-39 (S19), 287-304 (H18), and 385-406 (H17) have been proposed as potential membrane anchors. The interaction of the corresponding synthetic peptides with lipid model systems was investigated with CD spectroscopy, isothermal titration calorimetry, and monolayer expansion measurements. The membrane model systems were neutral or negatively charged small unilamellar vesicles or monolayers with a lateral packing density of bilayers (32 mN/m). S19 partitions into charged monolayers/bilayers with a penetration area A(p) = 140 +/- 30 A(2) and a free energy of binding of DeltaG(0) = -5.7 kcal/mol, thereby forming a partially alpha-helical structure. H18 does not interact with lipid monolayers or bilayers. H17 penetrates into neutral and charged monolayers/bilayers with A(p) = 148 +/- 23 A(2) and A(p) = 160 +/- 15 A(2), respectively, forming an alpha-helix in the membrane-bound state. Membrane partitioning is mainly entropy-driven. Under physiological conditions the free energy of binding to negatively charged membranes is DeltaG(0) = -9. 4 kcal/mol with a hydrophobic contribution of DeltaG(h) = -7.8 kcal/mol, comparable to that of post-translationally attached membrane anchors, and an electrostatic contribution of DeltaG(h) = -1.6 kcal/mol. The latter becomes more negative with decreasing pH. We show that H17 provides the binding energy required for a membrane anchor.

Actin activates a cryptic dimerization potential of the vinculin tail domain

The tail domain of vinculin (V(t)) is an actin binding module containing two regions that interact with F-actin. Although intact V(t) purified from a bacterial expression system is a globular monomer, each actin binding region dimerizes when expressed individually, suggesting the presence of cryptic self-association sites whose exposure is regulated. We show that actin modulates V(t) self-association by inducing or stabilizing a conformational change in V(t) that allows dimerization. Chemical cross-linking studies implicate one of the actin binding regions in mediating dimerization in the presence of actin. Actin-induced V(t) dimers may play a role in the filament cross-linking activity of this protein. The V(t) dimers induced by actin are biochemically distinct from the V(t) dimers and higher oligomers induced by acidic phospholipids such as phosphatidylinositol 4,5-bisphosphate, suggesting structural differences in V(t) bound to these two ligands that may provide a mechanistic basis for inhibition of F-actin binding by phosphatidylinositol 4,5-bisphosphate. The ability of actin to regulate the dimerization state of an actin binding protein suggests that, rather than serving a passive structural role, actin filaments may directly participate in signal transduction and other cellular events that are known to depend on cytoskeletal integrity.

Fragments from alpha-actinin insert into reconstituted lipid bilayers

Recent experiments have indicated that alpha-actinin interacts with phospholipid membranes. Using computer analysis methods we determined two possible lipid binding sites capable of membrane attachment/insertion, residues 281-300 and 720-739 of the primary amino acid sequence on smooth muscle alpha-actinin. Having expressed these regions as fusion proteins with schistosomal GST (glutathione S-transferase), we used differential scanning calorimetry (DSC) to investigate their interaction with mixtures of zwitterionic (dimyristoyl-l-alpha-phosphatidylcholine, DMPC) and anionic (dimyristoyl-l-alpha-phosphatidylglycerol, DMPG) phospholipids in reconstituted lipid bilayers. Calorimetric measurements showed that as fusion protein concentration increased, the main chain transition enthalpy decreased and chain melting temperatures shifted, which is indicative of partial protein insertion into the hydrophobic region of the lipid membranes. Centrifugation assay and subsequent SDS/Page chromatography confirmed this finding.

Controlling cytoskeleton structure by phosphoinositide-protein interactions: phosphoinositide binding protein domains and effects of lipid packing

Cell movement and resistance to mechanical forces are largely governed by the cytoskeleton, a three-dimensional network of protein filaments that form viscoelastic networks within the cytoplasm. The cytoskeleton underlying the plasma membrane of most cells is rich in actin filaments whose assembly and disassembly are regulated by actin binding proteins that are stimulated or inhibited by signals received and transmitted at the membrane/cytoplasm interface. Inositol phospholipids, or phosphoinositides, are potent regulators of many actin binding proteins, and changes in the phosphorylation of specific phosphoinositide species or in their spatial localization are associated with cytoskeletal remodeling in vitro. This review will focus on recent studies directed at defining the structural features of phosphoinositide binding sites in actin binding proteins and on the influence of the physical state of phosphoinositides on their ability to interact with their target proteins.

Polyphosphoinositides inhibit the interaction of vinculin with actin filaments

Binding of vinculin to adhesion plaque proteins is restricted by an intramolecular association of vinculin's head and tail regions. Results of previous work suggest that polyphosphoinositides disrupt this interaction and thereby promote binding of vinculin to both talin and actin. However, data presented here show that phosphatidylinositol 4,5-bisphosphate (PI4,5P2) inhibits the interaction of purified tail domain with F-actin. Upon re-examining the effect of PI4,5P2 on the actin and talin-binding activities of intact vinculin, we find that when the experimental design controls for the effect of magnesium on aggregation of PI4,5P2 micelles, polyphosphoinositides promote interactions with the talin-binding domain, but block interactions of the actin-binding domain. In contrast, if vinculin is trapped in an open confirmation by a peptide specific for the talin-binding domain of vinculin, actin binding is allowed. These results demonstrate that activation of the actin-binding activity of vinculin requires steps other than or in addition to the binding of PI4,5P2.

Fragments from actin binding protein (ABP-280; filamin) insert into reconstituted lipid layers

Previous computer analyses suggested two possible lipid binding sites, residues 49-71 and 131-155, of the primary amino acid sequence on ABP-280 (filamin), which could facilitate membrane attachment/insertion. We expressed these regions as fusion proteins with schistosomal GST and investigated their interaction with mixtures of zwitterionic (dimyristoyl-l-alpha-phosphatidylcholine, DMPC) and anionic (dimyristoyl-l-alpha-phosphatidylglycerol, DMPG) phospholipids in reconstituted lipid bilayers by differential scanning calorimetry (DSC). Using vesicles of mixed DMPC/DMPG with increasing fusion protein concentrations, we established in calorimetric assays a decrease of the main chain transition enthalpy, DeltaH, and a shift in chain melting temperature. This is indicative of the insertion of these fragments into the hydrophobic region of lipid membranes. We confirmed these findings by the film balance technique using lipid monolayers (DMPG). The binding judged from both methods was of moderate affinity.

The effect of intact talin and talin tail fragment on actin filament dynamics and structure depends on pH and ionic strength

We employed quasi-elastic light scattering and electron microscopy to investigate the influence of intact talin and talin tail fragment on actin filament dynamics and network structure. Using these methods, we confirm previous reports that intact talin induces cross-linking as well as filament shortening on actin networks. We now show that the effect of intact talin as well as talin tail fragment on actin networks is controlled by pH and ionic strength. At pH 7.5, actin filament dynamics in the presence of intact talin and talin tail fragment are characterized by a rapid decay of the dynamic structure factor and by a square root power law for the stretched exponential decay which is in contrast with the theory for pure actin solutions. At pH 6 and low ionic strength, intact talin cross-links actin filaments more tightly than talin tail fragment. Talin head fragment showed no effect on actin networks, indicating that the actin binding sites reside probably exclusively within the tail domain.

Vinculin but not alpha-actinin is a target of PKC phosphorylation during junctional assembly induced by calcium

The establishment of the junctional complex in epithelial cells requires the presence of extracellular calcium, and is controlled by a network of reactions involving G-proteins, phospholipase C and protein kinase C. Since potential candidates for phosphorylation are the tight junction associated proteins ZO1, ZO2 and ZO3, in a previous work we specifically explored these molecules but found no alteration in their phosphorylation pattern. To continue the search for the target of protein kinase C, in the present work we have studied the subcellular distribution and phosphorylation of vinculin and alpha-actinin, two actin binding proteins of the adherent junctions. We found that during the junctional sealing induced by Ca2+, both proteins move towards the cell periphery and, while there is a significant increase in the phosphorylation of vinculin, alpha-actinin remains unchanged. The increased phosphorylation of vinculin is due to changes in phosphoserine and phosphothreonine content and seems to be regulated by protein kinase C, since: (1) DiC8 (a kinase C stimulator) added to monolayers cultured without calcium significantly increases the vinculin phosphorylation level; (2) H7 and calphostin C (both protein kinase C inhibitors) completely abolish this increase during a calcium switch; (3) inhibition of phosphorylation during a calcium switch blocks the subcellular redistribution of vinculin and alpha-actinin. These results therefore suggest that vinculin phosphorylation by protein kinase C is a crucial step in the correct assembly of the epithelial junctional complex.

Vinculin and alpha-catenin: shared and unique functions in adherens junctions

Vinculin and alpha-catenin are two functionally related proteins of adherens junctions, structures in which cells make contacts to neighboring cells or to the extracellular matrix. At these sites, the actin cytoskeleton of animal cells is anchored to the plasma membrane. Junction assembly and disassembly are coordinated in processes as different as mitosis, cell movement and tissue formation. Since adherens junctions are assembled from a large number of proteins, these molecules have to be coordinately activated and spatially regulated. Vinculin and alpha-catenin have been characterized as tumor suppressors, suggesting that they have a regulatory function in addition to their structural role. Several possible modes of vinculin and alpha-catenin regulation are discussed here, as the published data favor the concept that no single model fully explains the complexity of adherens junctions. Most probably, cells select from a variety of possibilities to solve the problem of making specific contacts.

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.

The cytoskeleton and cell signaling: component localization and mechanical coupling

The three-dimensional intracellular network formed by the filamentous polymers comprising the cytoskeletal affects the way cells sense their extracellular environment and respond to stimuli. Because the cytoskeleton is viscoelastic, it provides a continuous mechanical coupling throughout the cell that changes as the cytoskeleton remodels. Such mechanical effects, based on network formation, can influence ion channel activity at the plasma membrane of cells and may conduct mechanical stresses from the cell membrane to internal organelles. As a result, both rapid responses such as changes in intracellular Ca2+ and slower responses such as gene transcription or the onset of apoptosis can be elicited or modulated by mechanical perturbations. In addition to mechanical features, the cytoskeleton also provides a large negatively charged surface on which many signaling molecules including protein and lipid kinases, phospholipases, and GTPases localize in response to activation of specific transmembrane receptors. The resulting spatial localization and concomitant change in enzymatic activity can alter the magnitude and limit the range of intracellular signaling events.

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.

The interaction of the cell-contact proteins VASP and vinculin is regulated by phosphatidylinositol-4,5-bisphosphate

BACKGROUND

Focal adhesion sites are cell-matrix contacts that are regulated by phosphatidylinositol-4,5-bisphosphate (PIP2)-dependent pathways. Vinculin is a major structural component of these sites and is thought to be engaged in multiple ligand interactions at the cytoplasmic face of these contacts. Cytoplasmic vinculin is considered to be inactive due to its closed conformation involving intramolecular head-tail interactions. Recently, the vasodilator-stimulated phosphoprotein (VASP), a substrate of cyclic AMP-dependent or cyclic GMP-dependent kinases and a component of focal adhesion sites, was shown to bind to vinculin.

RESULTS

VASP-vinculin complexes could be immunoprecipitated from cell lysates and, using immunofluorescence, both proteins were found to colocalize in nascent focal adhesions. Consistent with the view that vinculin must be activated at these sites, we found that PIP2, levels of which are elevated during the early stages of adhesion, bound to two discrete regions in the vinculin tail, disrupting the intramolecular head-tail interaction and inducing vinculin oligomerization. Vinculin-VASP complex formation was greatly enhanced by PIP2 and both the EVH1 and EVH2 domains of VASP participated in vinculin binding.

CONCLUSIONS

Focal contact assembly involves interaction between VASP and vinculin, which is enhanced by PIP2-induced vinculin activation and oligomerization. Given that vinculin and VASP both bind to F-actin, vinculin-VASP complexes might bundle the distal ends of actin filaments in focal contacts. We propose that PIP2-dependent signalling modulates microfilament organization at cellular adhesion sites by regulating vinculin-VASP complexes.

Peptide-specific antibodies localize the major lipid binding sites of talin dimers to oppositely arranged N-terminal 47 kDa subdomains

Using ultrastructural analysis and labeling with polyclonal antibodies that recognize peptide sequences specific for phospholipid binding, we mapped the functional domain structure of intact platelet talin and its proteolytic fragments. The talin dimer, which is crucial for actin and lipid binding, is built of a backbone containing the 200 kDa rod portions, at both ends of which a 47 kDa globular domain is attached. Peptide-specific polyclonal antibodies were raised against three potential lipid binding sequences residing within the N-terminal 47 kDa domain (i.e. S19, amino acids 21-39; H18, amino acids 287-304; and H17, amino acids 385-406). Antibodies H17 and H18 localize these lipid binding sequences within the N-terminal 47 kDa globular talin subdomains opposed at the outer 200 kDa rod domains within talin dimers. Hence, we conclude that in its dimeric form, which is used in actin and lipid binding, talin is a dumbbell-shaped molecule built of two antiparallel subunits.

Differences in elasticity of vinculin-deficient F9 cells measured by magnetometry and atomic force microscopy

We have investigated a mouse F9 embryonic carcinoma cell line, in which both vinculin genes were inactivated by homologous recombination, that exhibits defective adhesion and spreading [Coll et al. (1995) Proc. Natl. Acad. Sci. USA 92, 9161-9165]. Using a magnetometer and RGD-coated magnetic microbeads, we measured the local effect of loss and replacement of vinculin on mechanical force transfer across integrins. Vinculin-deficient F9Vin(-/-) cells showed a 21% difference in relative stiffness compared to wild-type cells. This was restored to near wild-type levels after transfection and constitutive expression of increasing amounts of vinculin into F9Vin(-/-) cells. In contrast, the transfection of vinculin constructs deficient in amino acids 1-288 (containing the talin- and alpha-actinin-binding site) or substituting tyrosine for phenylalanine (phosphorylation site, amino acid 822) in F9Vin(-/-) cells resulted in partial restoration of stiffness. Using atomic force microscopy to map the relative elasticity of entire F9 cells by 128 x 128 (n = 16,384) force scans, we observed a correlation with magnetometer measurements. These findings suggest that vinculin may promote cell adhesions and spreading by stabilizing focal adhesions and transferring mechanical stresses that drive cytoskeletal remodeling, thereby affecting the elastic properties of the cell.

Opening-up of liposomal membranes by talin

Morphological changes of liposomes caused by interactions between liposomal membranes and talin, a cytoskeletal submembranous protein, were studied by direct, real-time observation by using high-intensity dark-field microscopy. Surprisingly, when talin was added to a liposome solution, liposomes opened stable holes and were transformed into cup-shaped liposomes. The holes became larger with increasing talin concentration, and finally the cup-shaped liposomes were transformed into lipid bilayer sheets. These morphological changes were reversed by protein dilution, i.e., the sheets could be transformed back into closed spherical liposomes. We demonstrated that talin was localized mainly along the membrane verges, presumably avoiding exposure of its hydrophobic portion at the edge of the lipid bilayer. This is the first demonstration that a lipid bilayer can stably maintain a free verge in aqueous solution. This finding refutes the established dogma that all lipid bilayer membranes inevitably form closed vesicles and suggests that talin is a useful tool for manipulating liposomes.

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.

Examining F-actin interaction with intact talin and talin head and tail fragment using static and dynamic light scattering

We examined the binding kinetics of intact talin and talin head and tail fragment with F-actin at pH 7.0 and at low ionic strength. We observed by a transient kinetic method a fast followed by a slower binding process for intact talin and talin tail fragment with filamentous actin. The latter can be attributed to F-actin cross-linking and/or bundling, which was observed in cosedimentation assays as well as by low shear viscometry and electron microscopy [Zhang, J., Robson, R. M., Schmidt, J. M. & Stromer, M. H. (1996) Biochem. Biophys. Res. Commun. 218, 530-537]. This finding is supported by dynamic light scattering measurements, indicating changes in internal actin filament dynamics due to cross-linking/bundling events with intact talin and talin tail fragment. No binding of the talin head fragment with F-actin was detected by either method.

Characterization of two F-actin-binding and oligomerization sites in the cell-contact protein vinculin

Vinculin, a structural protein of animal cells, is critically involved in the assembly of microfilament/plasma membrane junctions at cell contacts. To understand its role in organizing the distal portions of microfilaments into specific, morphologically distinct structures at these sites in more detail, we characterized its interaction with filamentous actin and with itself by means of in vitro assays. Using recombinant proteins comprising different parts of the vinculin tail fused to the maltose-binding protein of Escherichia coli, we show in sedimentation assays that this part of vinculin harbors two discrete sites that can bind to actin independently. They reside within amino acid residues 893-985 and 1016-1066 of the 1066-residue polypeptide chain. However, both sites are necessary to cross-link or bundle actin filaments, as demonstrated by low shear viscometry. Crosslinking and bundling are alternatives determined by the molar ratio of fusion protein to F-actin. Both actin-binding sequences are capable of oligomer formation, as shown in chemical-cross-linking and dot-overlay assays. These data allow us to propose a possible role for vinculin in organizing the distal ends of microfilaments at the plasma membrane into the point-like structure characteristic for cell-matrix contacts.

Energy-filtered electron microscopy reveals that talin is a highly flexible protein composed of a series of globular domains

Talin is a multidomain cytoskeletal protein containing discrete binding sites for acidic phospholipids, beta-integrin, actin and vinculin. Hence, it is thought to link microfilaments to the cytoplasmic membrane in cell-matrix adhesion sites, and this should critically depend on talin structure. To obtain more information on the latter, we used energy-filtered transmission electron microscopy of negatively stained talin purified from chicken smooth muscle. We show that in buffers of physiological ionic strength, talin adopts an elongated shape (56 +/- 7 nm in length), consisting of a series of globular masses. While these compact elements, arranged like beads on a string, were of rather uniform dimensions (3.8 nm in diameter), their center-to-center spacings varied, indicating the flexibility of the connecting strands. The ends of the elongated molecules frequently formed loops. The images obtained are consistent with the assumption that, under the conditions used, the majority of the talin molecules are monomeric. A minor fraction appeared as dimers, composed of two chains only partially intertwined, thus giving rise to Y-shaped particles. Electron micrographs revealed that the biochemically defined 50-kDa N-terminal talin head domain is composed of two globular subunits, while chemical cross-linking provided evidence that the C-terminal 220-kDa fragment is solely responsible for dimerization. These results imply that in the dimeric molecules, the polypeptide chains are arranged in parallel, in contrast to what has been described for human-platelet talin. In buffers of low ionic strength (0.02 M instead of 0.15 M KCl), the molecules collapsed into a compact shape. By showing the high flexibility and versatility of its morphology, our data favour the concept of talin as an important resilient link in microfilament-plasma-membrane attachment.

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.

Determination of the affinity of talin and vinculin to charged lipid vesicles: a light scatter study

Recent experimental findings have demonstrated that both talin and vinculin bind to phospholipids and insert into the hydrophobic region of lipid membranes. Here, we show that the light scatter method can be used for measuring the affinity of proteins to phospholipid membranes. Large unilamellar DMPC/DMPG vesicles were produced by the extrusion technique (LUVETs). We have used repeated heating/cooling scans between 15 degrees C and 35 degrees C to ensure protein-lipid interaction/insertion. A molar affinity of talin, K = 2.9 x 10(6) M-1 and of vinculin, K = 3.3 x 10(5) M-1 to lipid vesicles, respectively, was determined from the plot; light scatter signal at 380 nm against protein concentrations by fitting the term, ln (Io/I-1) = A-K x c to the data.