Flexibility and fine structure of smooth-muscle alpha-actinin

The microfilament protein alpha-actinin exists as a dimer. The N-terminal regions of both polypeptides, arranged in antiparallel orientation, comprise the actin-binding regions, while the C-terminal, larger parts consist of four spectrin-like repeats that interact to form a rod-like structure. To elucidate the fine structure of smooth-muscle alpha-actinin, we used energy-filtered transmission electron microscopy in conjunction with negative staining. Survey pictures of the protein purified from chicken gizzard revealed discrete, elongated particles whose length and width varied with the ionic strength of the buffer. It was determined to to 29.3 nm x 4.8 nm in 0.05 M KCl and 32.6 nm x 4.4 nm in 0.15 M KCl. Both ends of the molecule displayed hook-like structures consisting of globular domains, which were highly variable in their orientation with respect to the long axis of the molecule. Their location at the ends of the molecule, and the finding that these hooks were missing from particles obtained by thermolysin digestion indicated that they probably correspond to the N-terminal actin-binding regions. The rod-like center of the molecule revealed discrete globular masses which probably comprise the spectrin-like repeats. Their arrangement was compatible with the interpretation that three spectrin repeats of each polypeptide chain can form pairs with the respective sequences of the other chain. The rod-like 53-kDa fragment obtained after thermolysin digestion largely retained this structural organization but appeared wider (22.5 nm x 5.9 nm). Our results help to clarify previous discrepancies on the quatenary organization of alpha-actinin and suggest that effective actin-binding and cross-linking of alpha-actinin is based on the high flexibility of the terminal hooks.

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.

Birch pollen profilin: structural organization and interaction with poly-(L-proline) peptides as revealed by NMR

The secondary structure of birch pollen profilin, a potent human allergen, was elucidated by multidimensional nuclear magnetic resonance (NMR), as a prerequisite to study the interaction of this profilin with ligands for its poly-(L-proline) (PLP)-binding site. The chemical shifts of the 15N-labeled backbone amide groups were used to monitor complex formation with various PLP peptides. Titration with deca-L-proline (P10) yielded a KD of 0.2 mM. P8 was the shortest PLP to provoke a significant reaction. (GP5)3G bound significantly, confirming the interaction between profilins and the protein VASP containing this motif. Birch profilin interacted also with GP6GP5, found in the cyclase-associated protein (CAP), a suspected profilin ligand.

Ligand recruitment by vinculin domains in transfected cells

Vinculin, a prominent protein component of microfilament-membrane attachment sites, consists of three major domains: an N-terminal, compact head and a C-terminal rod-like tail that are connected by a flexible, proline-rich hinge. In vitro, the protein has been shown to interact with numerous ligands, including other components of the microfilament system. To characterize the ligand recruitment ability of the different vinculin domains in a cellular environment, we used a novel approach of comprising chimeric proteins of either the vinculin head, hinge or tail regions, fused to the membrane anchor sequence of ActA, a surface protein of the intracellular bacterial pathogen Listeria monocytogenes. When PtK2 cells were transfected with the corresponding constructs, the ActA membrane anchor directed the chimeric polypeptides to mitochondrial membranes. In this position, they accumulated microfilament proteins, as seen by immunofluorescence analysis. A chimera comprising the full length vinculin clone recruited a substantial amount of the cellular F-actin, the vasodilator stimulated phosphoprotein (VASP) and paxillin, but little alpha-actinin and talin. The presence of only the vinculin head directed some of the fusion protein to focal contacts, and alpha-actinin recruitment was still ineffective. Prominent recruitment of F-actin and of VASP required the presence of the tail and proline-rich hinge, respectively. Reducing the vinculin tail to short pieces harboring only one of the two F-actin binding sequences, which were defined by in vitro experiments, resulted in loss of activity, possibly by incorrect polypeptide folding. The proline-rich hinge domain could be exchanged for the analogous region of the ActA protein, and the number of such proline-clusters, containing an FPPPP motif, correlated with the extent of VASP recruitment. The results show that this system can be used to analyze in vivo the activity of vinculin domains responsible for the assembly of various cytoskeletal ligands.

p140mDia, a mammalian homolog of Drosophila diaphanous, is a target protein for Rho small GTPase and is a ligand for profilin

Rho small GTPase regulates cell morphology, adhesion and cytokinesis through the actin cytoskeleton. We have identified a protein, p140mDia, as a downstream effector of Rho. It is a mammalian homolog of Drosophila diaphanous, a protein required for cytokinesis, and belongs to a family of formin-related proteins containing repetitive polyproline stretches. p140mDia binds selectively to the GTP-bound form of Rho and also binds to profilin. p140mDia, profilin and RhoA are co-localized in the spreading lamellae of cultured fibroblasts. They are also co-localized in membrane ruffles of phorbol ester-stimulated sMDCK2 cells, which extend these structures in a Rho-dependent manner. The three proteins are recruited around phagocytic cups induced by fibronectin-coated beads. Their recruitment is not induced after Rho is inactivated by microinjection of botulinum C3 exoenzyme. Overexpression of p140mDia in COS-7 cells induced homogeneous actin filament formation. These results suggest that Rho regulates actin polymerization by targeting profilin via p140mDia beneath the specific plasma membranes.

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.

VASP interaction with vinculin: a recurring theme of interactions with proline-rich motifs

VASP (vasodilator-stimulated phosphoprotein), a protein associated with microfilaments at cellular contact sites, has been identified as a ligand for profilin and zyxin, two proteins also involved in microfilament dynamics and organization at these regions. Here, we report that VASP also directly binds to vinculin, another component of adherens junctions. Competition experiments with a vinculin-derived peptide showed that a proline-rich motif, located in the hinge region that connects vinculin's head and tail domains, is involved in VASP binding. The same motif is present in zyxin but the interactions of VASP with vinculin and zyxin differ in detail. Hence, this motif may be recognized by VASP in different ways when presented in distinct cellular sites.

Light microscopic analysis of ligand-induced actin filament suprastructures

In this study, we describe a simple light microscopic assay which allows to rapidly determine the ligand-induced organization of actin filaments into specific suprastructures, such as web-like arrangements or bundles. The validity of this assay is demonstrated by accompanying low shear (falling ball) viscometry. While the visually identified webs demonstrated viscosity values significantly higher than the F-actin control, the bundles were characterized by viscosities distinctly lower than that of the control. In addition, we show that at least in some cases, the type of actin suprastructure formed depends on the molar ratio between the ligand and actin filaments. The assay should be useful to assess the conditions under which a particular ligand leads to a specific actin filament organization, to determine quickly the biological activity of recombinant proteins or isolated actin-binding domains, and to define new F-actin ligands.

Crosstalk between cell adhesion molecules: vinculin as a paradigm for regulation by conformation

With the identification of ever more protein components associated with cellular adhesion sites, the nature of the mechanisms underlying assembly and maintenance of these important cellular structures was in danger of becoming completely intangible. However, new information on how the interaction between the different proteins can be regulated is beginning to shed more light on this problem. In particular, recent biochemical and electron microscopic data on the overall structure and function of vinculin, one of the key structural proteins involved in cellular adhesion, leads to a novel model for the regulation of cellular adhesion.

Intramolecular interactions regulate serine/threonine phosphorylation of vinculin

Using protein kinase C, we have studied the influence of intramolecular interactions on phosphorylation in vinculin. We show that vinculin and its 90 kDa head and 29/27 kDa tail fragments, generated by V8 proteolytic cleavage, are differentially phosphorylated. While intact vinculin and the isolated head domain are only weakly labelled, the isolated tail fragment is much more strongly phosphorylated. In the presence of the tail, the head is fully protected from the kinase. These data are consistent with our observation that native vinculin is primarily phosphorylated within the tail domain and suggest a function of vinculin phosphorylation in the regulation of the vinculin conformation.

The ultrastructure of chicken gizzard vinculin as visualized by high-resolution electron microscopy

We have used high-resolution electron spectroscopic imaging to study the ultrastructure of negatively stained chicken gizzard vinculin. A careful examination of uranium salt-stained molecules revealed a high versatility of the overall shape of vinculin, for which an element of high flexibility is mainly responsible. This neck region links the vinculin head, probably consisting of the biochemically defined 90-kDa N-terminal fragment, to the rod-like tail. The hinge allows for sharp kinks in the molecule, so that head and tail structures can contact each other. By electron spectroscopic imaging, we were able to reveal substructural components in both head and tail regions. The head resembles a cloverleaf-like structure, consisting of three globular centers of mass, surrounding a protein-deficient center in a planar arrangement and of a short, stem-like fragment. The tail contains four spherical protein masses arranged like pearls on a string. Our data reveal a substructural organization of vinculin which is consistent with its presumed function as a structural component of microfilament attachment sites and support the concept of cryptic ligand-binding domains, previously based on biochemical evidence.

Plant and animal profilins are functionally equivalent and stabilize microfilaments in living animal cells

We have analyzed the degree of functional similarity between birth and mammalian profilins, two members of the profilin family which show only a moderate sequence homology (22%) in living animal cells. The plant profilin, derived from birch pollen, was stably expressed in BHK-21 cells. Plant and endogenous profilin synthesis and cellular distribution were monitored by specific monoclonal antibodies. Quantitation of profilin and actin on calibrated immunoblots showed that two stable clones contained in total 1.4 and 2.0 times as much profilin as the parental cells. Using double fluorescence and confocal laser scanning microscopy, it was seen that the endogenous and the plant profilin colocalized with dynamic microfilaments, in particular with F-actin-rich foci and cortical microfilament webs of spreading cells, with dynamic microfilament bundles induced by serum deprival, and with cytochalasin D- and latrunculin-induced transient F-actin aggregates. The increase in the overall profilin concentration correlated with a significantly higher resistance of actin filaments to these drugs. Our data indicate that even profilins of highly distant evolutionary origin can functionally substitute for each other and support the hypothesis that in animal cells, profilins are engaged in regulating either the stability or the kinetic properties of actin filaments.

The molecular architecture of focal adhesions

This article outlines the present knowledge of the architecture, molecular composition, and dynamics of focal contacts of adhesive animal cells. These structures, developed at the plasma membrane at sites where cells touch their substratum, are essential for cellular attachment in tissue formation during embryogenesis and wound healing. In tissue culture, they are particularly prominent and thus amenable to detailed investigation. Focal contacts consist of a cytoplasmic face, comprising cytoskeletal elements, a transmembrane connecting region, and a extracellular face composed of proteins of the extracellular matrix. The molecular anatomy of the numerous proteins involved, the basis for classifying them as structural or regulatory components, and their in vitro interactions are described. Based on this information, current models on the dynamics of their assembly and of possible regulatory mechanisms involving a variety of signal transduction pathways are discussed.

The proline-rich focal adhesion and microfilament protein VASP is a ligand for profilins

Profilins are small proteins that form complexes with G-actin and phosphoinositides and are therefore considered to link the microfilament system to signal transduction pathways. In addition, they bind to poly-L-proline, but the biological significance of this interaction is not yet known. The recent molecular cloning of the vasodilator-stimulated phosphoprotein (VASP), an established in vivo substrate of cAMP- and cGMP-dependent protein kinases, revealed the presence of a proline-rich domain which prompted us to investigate a possible interaction with profilins. VASP is a microfilament and focal adhesion associated protein which is also concentrated in highly dynamic regions of the cell cortex. Here, we demonstrate that VASP is a natural proline-rich profilin ligand. Human platelet VASP bound directly to purified profilins from human platelets, calf thymus and birch pollen. Moreover, VASP and a novel protein were specifically extracted from total cell lysates by profilin affinity chromatography and subsequently eluted either with poly-L-proline or a peptide corresponding to a proline-rich VASP motif. Finally, the subcellular distributions of VASP and profilin suggest that both proteins also interact within living cells. Our data support the hypothesis that profilin and VASP act in concert to convey signal transduction to actin filament formation.

A focal adhesion factor directly linking intracellularly motile Listeria monocytogenes and Listeria ivanovii to the actin-based cytoskeleton of mammalian cells

The surface-bound ActA polypeptide of the intracellular bacterial pathogen Listeria monocytogenes is the sole listerial factor needed for recruitment of host actin filaments by intracellularly motile bacteria. Here we report that following Listeria infection the host vasodilator-stimulated phosphoprotein (VASP), a microfilament- and focal adhesion-associated substrate of both the cAMP- and cGMP-dependent protein kinases, accumulates on the surface of intracytoplasmic bacteria prior to the detection of F-actin 'clouds'. VASP remains associated with the surface of highly motile bacteria, where it is polarly located, juxtaposed between one extremity of the bacterial surface and the front of the actin comet tail. Since actin filament polymerization occurs only at the very front of the tail, VASP exhibits properties of a host protein required to promote actin polymerization. Purified VASP binds directly to the ActA polypeptide in vitro. A ligand-overlay blot using purified radiolabelled VASP enabled us to identify the ActA homologue of the related intracellular motile pathogen, Listeria ivanovii, as a protein with a molecular mass of approximately 150 kDa. VASP also associates with actin filaments recruited by another intracellularly motile bacterial pathogen, Shigella flexneri. Hence, by the simple expedient of expressing surface-bound attractor molecules, bacterial pathogens effectively harness cytoskeletal components to achieve intracellular movement.

Intramolecular interactions in vinculin control alpha-actinin binding to the vinculin head

Using blot overlay techniques we have investigated the interaction of vinculin with alpha-actinin. We show that an alpha-actinin binding site is located in the 90 kDa vinculin head and confirm a vinculin binding site in the C-terminal rod of alpha-actinin, as recently reported by McGregor et al. [(1994) Biochem. J. 310, 225-233]. The isolated vinculin head binds much more strongly to alpha-actinin than intact vinculin. Using a proteolytic 81 kDa head fragment, we show that vinculin residues 1-107 are required for alpha-actinin binding. Antibodies directed against vinculin residues 808-850 inhibit the vinculin-alpha-actinin binding, suggesting that this sequence is directly involved in, or topographically related to, the alpha-actinin binding site.

Interaction of plant profilin with mammalian actin

The mode of interaction of birch and bovine profilins with actin was compared using a number of techniques. Birch profilin was purified from pollen or as a recombinant protein from Escherichia coli, using poly(L-proline) affinity chromatography and a monoclonal antibody for the identification of the isolated product. On two-dimensional gels, the genuine and recombinant proteins were identical in molecular mass and isoelectric point and revealed that birch profilin, in contrast to the basic profilins found in mammals, is an acidic protein, analogous to maize profilins. Bovine profilin was obtained from calf thymus. In viscometric assays, the birch protein was seen to modulate actin filament formation analogous to animal profilin. Birch profilin increased the critical concentration required for muscle and brain actin polymerization in a concentration-dependent manner, supporting the notion of the formation of a heterologous complex between the plant protein and animal actin. The effect was Mg(2+)-sensitive, as had been described for homologous complexes. The dissociation constants obtained for the plant/vertebrate and the vertebrate/vertebrate system were both in the micromolar range. The affinity of birch profilin for muscle actin was slightly lower than that for nonmuscle (brain) actin. A binary complex of birch profilin and skeletal muscle actin could be isolated by gel chromatography. Cross-linking experiments with actin, birch profilin, the G-actin binding peptide thymosin beta 4 and gelsolin segment 1, the N-terminal fragment of an actin capping protein, showed that profilin competed with thymosin beta 4, but had no effect on segment 1 binding to actin. These data indicate that the actin-binding domains in plant and animal profilins are functionally highly conserved, although the overall sequence similarity is less than 25%.

Exploitation of microfilament proteins by Listeria monocytogenes: microvillus-like composition of the comet tails and vectorial spreading in polarized epithelial sheets

Effective cell-to-cell spreading of the facultative intracellular pathogen Listeria monocytogenes requires the interaction between bacteria and the microfilament system of the host cell. By recruiting actin filaments into a ‘comet tail’ localized at one pole of the bacterial cell wall, Listeria become mobile and propel themselves through the cytoplasm. They create protrusions at the plasma membrane that can invaginate adjacent cells. In this work, we have analysed the structural composition of Listeria-recruited microfilaments in various epithelial cell lines by immunofluorescence microscopy. The microfilament-crosslinking proteins alpha-actinin, fimbrin and villin were localized around bacteria as soon as actin filaments could be detected on the bacterial surface. Surprisingly, the same was found for ezrin/radixin, proteins involved in linking microfilaments to the plasma membrane. We found that in a polarized cell line derived from brush border kidney epithelium (LLC-PK1), the actin filaments surrounding intracytoplasmic motile bacteria show the same immunoreactivity as the brush border-like microvilli, when analysed by a specific actin antibody. The successful invasion of polarized LLC-PK1 islets is vectorial, i.e. it progresses predominantly from the periphery of the islets towards the centre. Infection of the peripheral cells is sufficient for infiltration of the entire cellular islets, without any further contact with the extracellular milieu. This is in contrast to nonpolarized epithelial sheets, which can be invaded from the apical surface of any individual cell. The importance of active bacterial motility in this vectorial spreading is emphasized by our finding that an isogenic Listeria mutant that is unable to recruit actin filaments cannot colonize polarized epithelial layers but accumulates in the peripheral cells of the islets.

Characterization of an F-actin-binding domain in the cytoskeletal protein vinculin

Vinculin, a major structural component of vertebrate cell-cell and cell-matrix adherens junctions, has been found to interact with several other junctional components. In this report, we have identified and characterized a binding site for filamentous actin. These results included studies with gizzard vinculin, its proteolytic head and tail fragments, and recombinant proteins containing various gizzard vinculin sequences fused to the maltose binding protein (MBP) of Escherichia coli. In cosedimentation assays, only the vinculin tail sequence mediated a direct interaction with actin filaments. The binding was saturable, with a dissociation constant value in the micromolar range. Experiments with deletion clones localized the actin-binding domain to a region confined by residues 893-1016 in the 170-residue-long carboxyterminal segment, while the proline-rich hinge connecting the globular head to the rodlike tail was not required for this interaction. In fixed and permeabilized cells (cell models), as well as after microinjection, proteins containing the actin-binding domain specifically decorated stress fibers and the cortical network of fibroblasts and epithelial cells, as well as of brush border type microvilli. These results corroborated the sedimentation experiments. Our data support and extend previous work showing that vinculin binds directly to actin filaments. They are consistent with a model suggesting that in adhesive cells, the NH2-terminal head piece of vinculin directs this molecule to the focal contact sites, while its tail segment causes bundling of the actin filament ends into the characteristic spear tip-shaped structures.

Receptor-mediated endocytosis is sensitive to antibodies against the uncoating ATPase (hsc70)

We have investigated the functional role of the coated vesicle-uncoating ATPase (UA), a cognate heat shock protein (hsc70), in receptor-mediated endocytosis. A monoclonal antibody against bovine brain UA/hsc70 was generated that recognizes a 26 kDa proteolytic fragment harbouring the putative clathrin-binding site. In vitro, this antibody blocked the UA/hsc70-mediated release of clathrin from isolated coated vesicles (CVs). Upon microinjection into tissue culture cells, it specifically inhibited the heat shock-induced nuclear migration of UA/hsc70. This antibody also interfered with endocytosis of ligand-receptor complexes in injected cells. Two different systems were studied: the uptake of aggregated human IgG by BHK cells transfected with a human Fc receptor (FcRII), and the internalization of LDL by human fibroblasts. Injection of the monoclonal antibody in concentrations yielding approximately equal molar ratios of antibody to enzyme resulted in a reduction of endocytosis to 20–30% of control values, as seen by conventional light and confocal laser scanning microscopy, and by electron microscopy. In the transfected BHK cells, the endocytosed ligand remained associated with the labeling for clathrin and was not delivered to the endosomal compartment within the period expected from control serum- or non-injected cells. Thin sections revealed an accumulation of coated structures in the antibody-injected cells as compared to controls. Thus, our data show that UA is essential for normal receptor-mediated endocytosis, and is presumably involved in the uncoating of CVs preceding their fusion with endosomes.

Chicken antibodies to rabbit muscle actin with a restricted repertoire of F-actin recognition

This report describes a polyclonal antibody against actin with unexpected and unusual properties. The antibody was raised in chicken immunized with a complex of DNase I and rabbit skeletal muscle actin, and purified from egg yolk by affinity chromatography. In Western blots, it reacted with alpha, beta and gamma isoforms of actin. In immunofluorescence and dot blot assays, however, it recognized selectively actin filaments in myofibrils, microvilli of brush border-type epithelium and the "comet tails" of the intracellular parasite Listeria monocytogenes, while it did not react with stress fibers and peripheral belts of fibroblasts and epithelial cells, respectively. This reactivity pattern is reminiscent of that previously described for a monoclonal mouse antibody raised against smooth muscle actin (Sawtell et al., Cell Motil. Cytoskel. 11, 318, 1988). The data presented in this study are consistent with the hypothesis that the chicken antibody recognizes an actin epitope/actin epitopes either accessible in only a subpopulation of microfilaments, or expressed only in a particular conformation of F-actin.

Conformational states of pig brain myosin

The conventional, longtailed myosins expressed in smooth muscle and nonmuscle tissue are generally thought to exist in two discrete monomeric configurations which in vitro can be interconverted by changes in the ionic strength of the buffer. For nonmuscle myosin, only two species had been investigated so far. Here, we show that dephosphorylated pig brain myosin, consisting of at least two electrophoretic variants, can also adopt an extended and a folded configuration as seen in rotary shadowed molecules. Monoclonal antibodies were employed to analyze the possible role of specific tail domains in these changes. One of these antibodies (a-PBM9) which binds to an epitope in the first third of the tail, was found to induce and stabilize the extended form, as seen in ELISA with soluble myosin and in rotary-shadowed immune complexes, while another antibody (a-PBM4), binding close to the carboxy terminus of the tail, showed no such effect. Our data support the model that all nonmuscle myosins can shuttle between an extended, assembly-competent and a folded or collapsed form, and that specific regions within the tail play a crucial role in this interconversion.

Molecular shape of vinculin in aqueous solution

We have investigated the molecular structure of chicken gizzard vinculin in solution. The translational diffusion coefficient of the intact protein and its amino-terminal head fragment, as obtained by proteolytic digestion, was determined by photon correlation spectroscopy. The experimental data are compared with hydrodynamic calculations, where the anisotropic shape of the macromolecule is modeled by spherical subunits. Our results are in agreement with the concept of a "balloon on a string" for the molecular shape of native vinculin. The existence of dimer and oligomer structures in low ionic strength buffer can be excluded. The calculated dimensions of the head fragment were estimated to r = 3.3 nm for a spherical particle, but the diffusion coefficient suggests a slightly anisotropic shape. In solution, the rod-like tail exhibits some flexibility, which is probably located in the "neck region" of the protein, considering the known sequence data.