How force might activate talin's vinculin binding sites: SMD reveals a structural mechanism

Upon cell adhesion, talin physically couples the cytoskeleton via integrins to the extracellular matrix, and subsequent vinculin recruitment is enhanced by locally applied tensile force. Since the vinculin binding (VB) sites are buried in the talin rod under equilibrium conditions, the structural mechanism of how vinculin binding to talin is force-activated remains unknown. Taken together with experimental data, a biphasic vinculin binding model, as derived from steered molecular dynamics, provides high resolution structural insights how tensile mechanical force applied to the talin rod fragment (residues 486–889 constituting helices H1–H12) might activate the VB sites. Fragmentation of the rod into three helix subbundles is prerequisite to the sequential exposure of VB helices to water. Finally, unfolding of a VB helix into a completely stretched polypeptide might inhibit further binding of vinculin. The first events in fracturing the H1–H12 rods of talin1 and talin2 in subbundles are similar. The proposed force-activated α-helix swapping mechanism by which vinculin binding sites in talin rods are exposed works distinctly different from that of other force-activated bonds, including catch bonds.

For cell survival, most eukaryotic cells need to be mechanically anchored to their environment. This is done by transmembrane proteins, including integrins, which externally bind to the extracellular matrix and on the cell interior to the contractile cytoskeleton via scaffolding proteins. One essential scaffolding protein is talin, which binds to integrins via its head and to the cytoskeletal filament f-actin via its rodlike tail. As cells apply tensile forces to newly formed adhesion sites, proteins that are part of such force-bearing networks get stretched and might change their structure and thus function. One of many proteins that are recruited to newly formed adhesions is vinculin, and vinculin recruitment is upregulated by tensile mechanical force—but how? Since talin's vinculin binding sites are buried in its native structure, we used steered molecular dynamics here to derive a high resolution structural model of how tensile mechanical forces might activate talin's vinculin binding sites. Once tensile forces break up the talin rod into helix subbundles, an event that we find here to constitute the main energy barrier, we propose how the strain-induced gradual exposure of the vinculin-binding helices finally allows for their activation and enables helix swapping with the vinculin head.

Interaction of talin with actin: sensitive modulation of filament crosslinking activity

Talin is an adhesion plaque protein believed important in linking actin filaments to the plasma membrane. The nature of a direct talin-actin interaction, however, is complex and has remained unclear. We have systematically characterized the effects of pH, ionic strength, temperature, and protein molar ratio on the interaction between highly purified talin and actin. The ability of talin to increase viscosity of F-actin at 25 degrees C and low ionic strength increased with decreasing pH from 7.3 to 6.4 and increasing molar ratio of talin to actin. At pH 6.4 and low ionic strength, talin could extensively crosslink actin filaments into ordered bundles as shown by negative staining and could cosediment with F-actin at molar ratios as high as one talin to two actin monomers. Talin crosslinked prepolymerized actin filaments to a similar extent as actin filaments polymerized in its presence. The 190-kDa calpain-generated proteolytic fragment of talin bound poorly to actin under conditions favorable for intact talin, but was able to crosslink actin filaments at a lower pH. Increasing the ionic strength within a relatively narrow range significantly decreased ability of talin to bind to actin, regardless of pH. The effects of pH and ionic strength on the talin-actin interaction were rapid and reversible. Low-shear-viscosity studies revealed a strong temperature dependence in the talin-actin interaction with significant crosslinking activity at physiological-like ionic conditions and temperature (37 degrees C). Our results consistently demonstrated that talin crosslinks actin filaments and that this direct interaction is highly sensitive to, and dependent upon, ionic conditions and temperature.

Thrombin-mediated focal adhesion plaque reorganization in endothelium: role of protein phosphorylation

Endothelial cell (EC) gap formation and barrier function are subject to dual regulation by (1) axial contractile forces, regulated by myosin light chain kinase activity, and (2) tethering forces, represented by cell-cell and cell-substratum adhesions. We examined whether focal adhesion plaque proteins (vinculin and talin) and focal adhesion kinase, p125FAK (FAK), represent target regulatory sites involved in thrombin-mediated EC barrier dysfunction. Histologically, thrombin produced dramatic rearrangement of EC actin, vinculin, and FAK in parallel with the evolution of gap formation and barrier dysfunction. Vinculin and talin were in vitro substrates for phosphorylation by EC PKC, a key effector enzyme involved in thrombin-induced EC barrier dysfunction. Although vinculin and talin were phosphorylated in situ under basal conditions in 32P-labeled EC, thrombin failed to alter the basal level of phosphorylation of these proteins. Phosphotyrosine immunoblotting showed that neither vinculin nor talin was significantly phosphorylated in situ on tyrosine residues in unstimulated ECs, and this was not further increased after thrombin. In contrast, both thrombin and the thrombin receptor-activating peptide (TRAP) produced an increase in FAK phosphotyrosine levels (corrected for immunoreactive FAK content) present in EC immunoprecipitates. Ionomycin, which produces EC barrier dysfunction in a myosin light chain kinase-independent manner, was used to increase intracellular Ca2+ and evaluate the Ca2+ sensitivity of this observation. In contrast to thrombin, ionomycin effected a dramatic decrease in the phosphotyrosine-to-immunoreactive FAK ratios, suggesting distinct effects of the two agents on FAK phosphorylation and function. These data indicate that modulation of cell tethering via phosphorylation of focal adhesion proteins is complex, agonist-specific, and may be a relevant mechanism of EC barrier dysfunction in permeability models that do not depend on an increase in myosin 20-kD regulatory light chain phosphorylation.

NPXY motifs control the recruitment of the alpha5beta1 integrin in focal adhesions independently of the association of talin with the beta1 chain

With the exception of the divergent beta4 and beta8 chains, the integrin beta subunit cytoplasmic domains are short and highly conserved sequences. Consensus motifs are found among the different cytoplasmic beta chains. Experiments using chimeric receptors demonstrated that the 47 amino acids of the beta1 subunit cytoplasmic domain contain sufficient information to target integrins to adhesion plaques. Three clusters of amino acids, named cyto-1, cyto-2 and cyto-3, seem to contribute to this localization. Cyto-2 and cyto-3 exhibit NPXY motifs. At present, the exact function of these motifs remains unknown but it is likely that these sequences are involved in protein-protein interactions. Although NPXY motifs often act as internalization signals at the cytoplasmic tail of membrane receptors, our previous results showed that the two NPXY motifs are not responsible for the alpha5beta1 integrin endocytosis. Herein, we address the question of the role of the two highly conserved NPXY motifs found in the beta1 cytoplasmic domain, and which correspond to the conserved domains cyto-2 and cyto-3. We demonstrate that, within the integrin beta1 cytoplasmic tail, the two NPXY motifs are required for the recruitment of the integrin in focal adhesions. In addition, our results indicate that these two motifs control but do not belong to the talin-binding sites. Finally, the analysis of the phenotypes of NPXY mutants reveals that the interaction of talin with the beta1 cytosolic domain is not sufficient to target the integrins to focal adhesions.

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.

Native talin is a dumbbell-shaped homodimer when it interacts with actin

Electron microscopy of glycerol-sprayed and rotary metal-shadowed talin from human platelets reveals a dumbbell-shaped molecule with an average length of approximately 51 nm. Analytical ultracentrifugation of native talin yields a single molecular species with an apparent molecular mass of 412 (+/- 28.6) kDa and a sedimentation coefficient of S20w = 11.2. Chemical cross-linking with glutaraldehyde (GA) and corresponding SDS-PAGE analysis show that the monomer band of talin can be quantitatively converted to a dimer band at GA concentrations > or = 0.45%, indicating that there is no significant amount of monomer present in solution. These structural and biophysical data are compatible with native talin being an antiparallel homodimer. Length measurements and viscometric and fluorescent assays of actin filaments polymerized in the presence of native talin and of covalently cross-linked talin dimers all yield similar effects: namely, increased nucleation and polymerization rates and an overall reduction of actin filament length. Hence, we conclude that talin in its native biological state is a dimer when promoting nucleation of actin filaments.

The marked pH dependence of the talin-actin interaction

The effect of pH on the interaction between talin and actin was examined by using cosedimentation assays, low shear viscometry, and electron microscopy. When the pH at which the interaction was tested was lowered from 7.5 to 7.0 and below, the ability of talin to cosediment with F-actin was greatly enhanced, with maximal binding at pH 6.6. Talin's ability to increase the low shear viscosity of F-actin solutions was also significantly elevated as pH was decreased from 7.4 to 6.6. Electron microscope observations of negatively stained actin controls and talin-actin mixtures supported these results by demonstrating an increase in the degree of crosslinking of actin filaments in the presence of talin at pH values of 7 and lower.

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.