The profile of profilins

Profilins are small proteins involved in actin dynamics. In accordance with this function, they are found in all eukaryotes and are structurally highly conserved. However, their precise role in regulating actin-related functions is just beginning to emerge. This article recapitulates the wealth of information on structure, expression and functions accumulated on profilins from many different organisms in the 30 years after their discovery as actin-binding proteins. Emphasis is given to their interaction with a plethora of many different ligands in the cytoplasm as well as in the nucleus, which is considered the basis for their various activities and the significance of the tissue-specific expression of profilin isoforms.

Conformation-specific antibodies reveal distinct actin structures in the nucleus and the cytoplasm

For many years the existence of actin in the nucleus has been doubted because of the lack of phalloidin staining as well as the failure to document nuclear actin filaments by electron microscopy. More recent findings reveal actin to be a component of chromatin remodeling complexes and of the machinery involved in RNA synthesis and transport. With distinct functions for nuclear actin emerging, the quest for its conformation and oligomeric/polymeric structure in the nucleus has resumed importance. We used chemically cross-linked 'lower dimer' (LD) to generate mouse monoclonal antibodies specific for different actin conformations. One of the resulting antibodies, termed 1C7, recognizes an epitope that is buried in the F-actin filament, but is surface-exposed in G-actin as well as in the LD. In immunofluorescence studies with different cell lines, 1C7 selectively reacts with non-filamentous actin in the cytoplasm. In addition, it detects a discrete form of actin in the nucleus, which is different from the nuclear actin revealed by the previously described 2G2 [Gonsior, S.M., Platz, S., Buchmeier, S., Scheer, U., Jockusch, B.M., Hinssen, H., 1999. J. Cell Sci. 112, 797]. Upon latrunculin-induced disassembly of the filamentous cytoskeleton in Rat2 fibroblasts, we observed a perinuclear accumulation of the 1C7-reactive actin conformation. In addition, latrunculin treatment led to the assembly of phalloidin-staining actin structures in chromatin-free regions of the nucleus in these cells. Our results indicate that distinct actin conformations and/or structures are present in the nucleus and the cytoplasm of different cell types and that their distribution varies in response to external signals.

Raver1, a dual compartment protein, is a ligand for PTB/hnRNPI and microfilament attachment proteins

By screening a yeast two-hybrid library with COOH-terminal fragments of vinculin/metavinculin as the bait, we identified a new protein termed raver1. Raver1 is an 80-kD multidomain protein and widely expressed but to varying amounts in different cell lines. In situ and in vitro, raver1 forms complexes with the microfilament-associated proteins vinculin, metavinculin, and alpha-actinin and colocalizes with vinculin/metavinculin and alpha-actinin at microfilament attachment sites, such as cell-cell and cell matrix contacts of epithelial cells and fibroblasts, respectively, and in costameres of skeletal muscle. The NH2-terminal part of raver1 contains three RNA recognition motifs with homology to members of the heterogeneous nuclear RNP (hnRNP) family. Raver1 colocalizes with polypyrimidine tract binding protein (PTB)/hnRNPI, a protein involved in RNA splicing of microfilament proteins, in the perinucleolar compartment and forms complexes with PTB/hnRNPI. Hence, raver1 is a dual compartment protein, which is consistent with the presence of nuclear location signal and nuclear export sequence motifs in its sequence. During muscle differentiation, raver1 migrates from the nucleus to the costamere. We propose that raver1 may coordinate RNA processing and targeting as required for microfilament anchoring in specific adhesion sites.

Characterization of functional domains of mDia1, a link between the small GTPase Rho and the actin cytoskeleton

The widely expressed diaphanous proteins, a subclass of formins, comprise links between the Rho GTPases and the actin-based cytoskeleton. They contain several functional domains that are thought to be responsible for interaction with different ligands: the FH1 domain for binding the actin-associated protein profilin; the RBD for targeting activated Rho; and the C-terminal CIID module for autoregulation of the overall diaphanous activity. Using deletion constructs of the murine mDia1, we have analyzed the functional properties of these three domains separately in in vitro assays and in transiently and stably transfected cell lines. We show that the proline-rich FH1 domain effectively binds to profilins in vitro as well as in cells, that the RBD complexes with the CIID in a species-restricted manner and that overexpression of RBD causes spontaneous ruffling and loss of stress fibers, together with loss of directional motility. Supertransfection of cells stably expressing the RBD with dominant negative Rac effectively suppresses ruffling. Our data contribute to the understanding of the function of these domains in linking the actin cytoskeleton with the Rho-signaling cascade. Furthermore, they suggest that inactivation of Rho by exogenous RBD causes upregulation of Rac activity in the transfected cells.

3-Dimensional organization of the N-terminal vinculin head fragment

The cytoskeleton-associated protein vinculin is composed of a globular head and an elongated tail domain. The protein can be cleaved by V8 protease treatment into two fragments with apparent molecular masses of 90 and 29/27 kDa, respectively. So far, no high-resolution data on the tertiary structure of the N-terminal 90-kDa fragment are available. We analyzed the 90-kDa fragment in detail, using electron spectroscopic imaging in conjunction with modelling experiments. The front view projection of this fragment appears roughly rhomboidal, with 4 intensity maxima arranged at the vertices and a stain-filled region in the center. Based on a detailed examination of different particle projections, a 3-dimensional model was constructed which appears as a flattened tetrahedron. A comparison of the 90-kDa fragment with the intact protein allows for a correlation between the subdomain organization of the vinculin head and the biochemically defined V8 protease cleavage sites (aa 851 and 857).

The armadillo repeat region targets ARVCF to cadherin-based cellular junctions

The cytoplasmic domain of the transmembrane protein M-cadherin is involved in anchoring cytoskeletal elements to the plasma membrane at cell-cell contact sites. Several members of the armadillo repeat protein family mediate this linkage. We show here that ARVCF, a member of the p120 (ctn) subfamily, is a ligand for the cytoplasmic domain of M-cadherin, and characterize the regions involved in this interaction in detail. Complex formation in an in vivo environment was demonstrated in (1) yeast two-hybrid screens, using a cDNA library from differentiating skeletal muscle and part of the cytoplasmic M-cadherin tail as a bait, and (2) mammalian cells, using a novel experimental system, the MOM recruitment assay. Immunoprecipitation and in vitro binding assays confirmed this interaction. Ectopically expressed EGFP-ARVCF-C11, an N-terminal truncated fragment, targets to junctional structures in epithelial MCF7 cells and cardiomyocytes, where it colocalizes with the respective cadherins, beta-catenin and p120 (ctn). Hence, the N terminus of ARVCF is not required for junctional localization. In contrast, deletion of the four N-terminal armadillo repeats abolishes this ability in cardiomyocytes. Detailed mutational analysis revealed the armadillo repeat region of ARVCF as sufficient and necessary for interaction with the 55 membrane-proximal amino acids of the M-cadherin tail.

Phosphorylation of the vasodilator-stimulated phosphoprotein regulates its interaction with actin

The vasodilator-stimulated phosphoprotein (VASP) is a major substrate for cyclic nucleotide-dependent kinases in platelets and other cardiovascular cells. It promotes actin nucleation and binds to actin filaments in vitro and associates with stress fibers in cells. The VASP-actin interaction is salt-sensitive, arguing for electrostatic interactions. Hence, phosphorylation may significantly alter the actin binding properties of VASP. This hypothesis was investigated by analyzing complex formation of recombinant murine VASP with actin after phosphorylation with cAMP-dependent kinase in different assays. cAMP-dependent kinase phosphorylation had a negative effect on both actin nucleation and VASP interaction with actin filaments, with the actin nucleating capacity being more affected than actin filament binding and bundling. Replacing VASP residues known to be phosphorylated in vivo by acidic residues to mimic phosphorylation had similar although less dramatic effects on VASP-actin interactions. In contrast, phosphorylation had no significant effect on VASP oligomerization or its interaction with its known ligands profilin, vinculin, and zyxin. When overexpressing VASP mutants in eukaryotic cells, they all showed targeting to focal contacts and stress fibers. Our results imply that VASP phosphorylation may act as an immediate negative regulator of actin dynamics.

Functional characterization of green fluorescent protein-profilin fusion proteins

To clarify the role of profilins in cells, fusion proteins constructed with green fluorescent protein (GFP) should be extremely helpful. As profilins are considerably smaller than the GFP fusion partner (14-17 kDa compared with 27 kDa, respectively), we characterized the fusion proteins in vitro, to ascertain their biological function. We fused mouse profilin I and II to either the C-terminus or N-terminus of GFP. These fusion proteins were expressed in Escherichia coli and affinity-purified on polyproline-Sepharose. Interaction with vasodilator-stimulated phosphoprotein, a proline-rich ligand of profilin, was investigated by ELISA, as was binding to PtdIns(4,5)P2. The affinity for actin was quantitatively determined in polymerization assays. Our results show that fusion of GFP to the C-terminus of profilin I abolishes polyproline binding. In contrast, the other fusion proteins bound to polyproline-Sepharose and VASP. Binding to PtdIns(4,5)P2 was not significantly altered. Furthermore, fusion of either isoform with GFP did not decrease the affinity for actin. In localization studies with mammalian cells, all fusion proteins showed the localization expected for profilin in areas of high actin dynamics, such as leading lamellae and ruffles induced by epidermal growth factor. However, with regard to our in vitro data, we suspect that only a minor fraction of profilin I carrying the GFP at the C-terminus can target these sites. Therefore, other constructs should be preferred for further in vivo studies.

Distinct localizations of tropomyosin isoforms in LLC-PK1 epithelial cells suggests specialized function at cell-cell adhesions

At least eight nonmuscle, nonbrain tropomyosin isoforms have been described. We used antibodies, microinjection, and transfection to characterize their expression and localization in LLC-PK1 kidney epithelial cells and compared them with other cells. Similar to primary enterocytes, LLC-PK1 cells exhibited predominantly TM-1 and TM-3 of the high-molecular-weight (HMW) isoforms; TM-5 and TM-5b of the low-molecular-weight (LMW) isoforms. Neither TM-4 nor TM-5a was detectable in the LLC-PKI cells. Immunofluorescence studies revealed that HMW isoforms were localized only on stress fibers, not adhesion belts, whereas the adhesion belts were stained by LMW isoform antibodies. When exogenous proteins are introduced either by transfection or microinjection, the HMW isoforms do not incorporate into the adhesion belt, whereas the LMW isoforms can incorporate into the stress fibers, thus indicating there are different mechanisms at work for the selective localization. Temporal changes in the microfilament system of the LLC-PK1 cells were studied during differentiation in culture as defined by spectrin expression and F-actin architecture. Western blot analysis indicated that TM-5b is only expressed in the LLC-PK1 cells after a certain degree of maturation in culture, which suggests isoform switching after the cell-cell contacts are developed. Collectively these results demonstrate that epithelial cells express a complex pattern of TM isoforms, which exhibit differential localizations within the cells and different patterns of expression depending on their origin and stage of differentiation. The implication of differential localization of TM isoforms on their specific functions is discussed.

Monoclonal antibodies recognizing the N- and C-terminal regions of talin disrupt actin stress fibers when microinjected into human fibroblasts

We have characterized a panel of 6 monoclonal antibodies raised against human platelet talin by Western blotting, immune precipitation, and immunofluorescence, and shown that antibodies TA205 and TD77 disrupt actin stress fibers and focal adhesions, and inhibit cell motility when microinjected into human fibroblasts. Using a series of chick talin fusion proteins spanning the entire length of the molecule, we have mapped the epitopes recognized by these antibodies to the conserved N- and C-terminal regions of the protein. TA205 bound to an epitope contained within residues 139-433, a region which overlaps an F-actin binding site, and which shows homology with the ezrin/radixin/moesin family of cytoskeletal proteins. The epitope recognized by TD77 was located within the C-terminal region of the protein (residues 2269-2541) which also contains an F-actin binding site homologous to that in the yeast actin-binding protein SIa2p. To investigate the possibility that TD77 disrupts actin stress fibers by binding directly to the C-terminal actin binding site, additional talin fusion proteins were generated and analyzed for TD77 and actin binding. Fusion proteins containing residues 2269-2541, 2304-2541, and 2304-2463 all cosedimented with F-actin, whereas TD77 did not recognize the latter fusion protein. These results show that the C-terminal actin-binding site is distinct from the region recognized by the anti-functional antibody TD77, raising the possibility that it binds to a novel functionally important ligand-binding site in the talin molecule.

Differential colocalization of profilin with microfilaments in PtK2 cells

Profilins are thought to be involved in the control of actin dynamics in eukaryotic cells. In accordance with this concept, profilin was found to be colocalized with the cortical microfilament webs in leading lamellae of locomoting and spreading fibroblasts. However, so far, there is little information on the distribution of profilin in other cell types. In this study, we report on the colocalization of profilin with various microfilament suprastructures in the epithelial cell line PtK2. This cell line, which is derived from rat kangaroo, contains a profilin sharing an N-terminal epitope with bovine and human profilin I, as seen by immunoblotting with monoclonal antibodies. By using immunofluorescence in conjunction with conventional fluorescence microscopy and confocal laser-scanning microscopy, we found profilin in ruffling areas of the peripheral lamellae and nascent stress fibers of spreading cells, whereas the peripheral belts of stationary cells growing in epithelioid sheets lacked profilin staining. In these cells, profilin was primarily distributed in a fine reticular or vesicular network that was not related to the microfilament system. Conspicuously low levels of profilins was not related to the contractile ring of mitotic cells. This was found for different fixation protocols and antibodies of the IgG and IgM type, respectively, indicating that lack of staining of the cleavage furrow was not due to antibody penetration problems. Depending on the fixation protocol, the nuclear matrix appeared strongly positive or negative for profilin. Cells microinjected with birch pollen profilin and labeled with a birch profilin-specific monoclonal antibody corroborated the results obtained with the endogeneous protein: The injected profilin was targeted to the cortical web and to nascent stress fibers of spreading cells but not to the cleavage ring of mitotic cells. These results suggest that high concentrations of a profilin I homologue are preferentially located with those microfilament suprastructures in PtK2 cells that are subject to rapid modulation by external signals.

Accumulation of profilin II at the surface of Listeria is concomitant with the onset of motility and correlates with bacterial speed

The spatial and temporal activity of the actin cytoskeleton is precisely regulated during cell motility by several microfilament-associated proteins of which profilin plays an essential role. We have analysed the distribution of green fluorescent protein (GFP)-tagged profilins in cultured and in Listeria-infected cells. Among the different GFP-profilin fusion proteins studied, only the construct in which the GFP moiety was fused to the carboxy terminus of profilin II (profilin II-GFP) was recruited by intracellular Listeria. The in vitro ligand-binding properties of this construct, e.g. the binding to monomeric actin, poly-L-proline and phosphatidylinositol 4,5-bisphosphate (PIP2), were unaffected by GFP. Profilin II-GFP co-localised with vinculin and Mena to the focal adhesions in REF-52 fibroblasts and was distributed as a thin line at the front of protruding lamellipodia in B16-F1 mouse melanoma cells. In Listeria-infected cells, profilin II-GFP was recruited, in an asymmetric fashion, to the surface of Listeria at the onset of motility whereas it was not detectable on non-motile bacteria. In contrast to the vasodilator-stimulated phosphoprotein (VASP), profilin II-GFP localised at the bacterial surface only on motile Listeria. Moreover, the fluorescence intensity of profilin II-GFP directly correlated with the speed of the bacteria. Thus, the use of GFP-tagged profilin II provides new insights into the role of profilins in cellular motility.

A role for polyproline motifs in the spinal muscular atrophy protein SMN. Profilins bind to and colocalize with smn in nuclear gems

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by the loss of alpha-motoneurons in the spinal cord followed by atrophy of skeletal muscles. SMA-determining candidate genes, SMN1 and SMN2, have been identified on human chromosome 5q. The corresponding SMN protein is expressed ubiquitously. It is coded by seven exons and contains conspicuous proline-rich motifs in its COOH-terminal third (exons 4, 5, and 6). Such motifs are known to bind to profilins (PFNs), small proteins engaged in the control of actin dynamics. We tested whether profilins interact with SMN via its polyproline stretches. Using the yeast two-hybrid system we show that profilins bind to SMN and that this binding depends on its proline-rich motifs. These results were confirmed by coimmunoprecipitation and by in vitro binding studies. Two PFN isoforms, I and II, are known, of which II is characteristic for central nervous system tissue. We show by in situ hybridization that both PFNs are highly expressed in mouse spinal cord and that PFN II is expressed predominantly in neurons. In motoneurons, the primary target of neurodegeneration in SMA, profilins are highly concentrated and colocalize with SMN in the cytoplasm of the cell body and in nuclear gems. Likewise, SMN and PFN I colocalize in gems of HeLa cells. Although SMN interacts with both profilin isoforms, binding of PFN II was stronger than of PFN I in all assays employed. Because the SMN genes are expressed ubiquitously, our findings suggest that the interaction of PFN II with SMN may be involved in neuron-specific effects of SMN mutations.

Binding of the Shigella protein IpaA to vinculin induces F-actin depolymerization

Shigella flexneri, the causative agent of bacillary dysentery, enters into epithelial cells by a macropinocytic process. IpaA, a Shigella protein secreted upon cell contact, binds to the focal adhesion protein vinculin and is required for efficient bacterial uptake. IpaA was shown here to bind with high affinity to the N-terminal residues 1-265 of vinculin. Using co-sedimentation and solid-phase assays, we demonstrated that binding of IpaA to vinculin strongly increases the association of vinculin with F-actin. We also characterized a depolymerizing activity on actin filaments associated with the vinculin-IpaA complex both in vitro and in microinjected cells. We propose that the conformational change of vinculin induced by IpaA binding allows interaction of the vinculin-IpaA complex with F-actin and subsequent depolymerization of actin filaments.

Characterization of the actin binding properties of the vasodilator-stimulated phosphoprotein VASP

The vasodilator-stimulated phosphoprotein (VASP) colocalizes with the ends of stress fibers in cell-matrix and cell-cell contacts. We report here that bacterially expressed murine VASP directly interacts with skeletal muscle actin in several test systems including cosedimentation, viscometry and polymerization assays. It nucleates actin polymerization and tightly bundles actin filaments. The interaction with actin is salt-sensitive, indicating that the complex formation is primarily based on electrostatic interactions. Actin binding is confined to the C-terminal domain of VASP (EVH2). This domain, when expressed as a fusion protein with EGFP, associates with stress fibers in transiently transfected cells.

Role of proteins of the Ena/VASP family in actin-based motility of Listeria monocytogenes

Intracellular propulsion of Listeria monocytogenes is the best understood form of motility dependent on actin polymerization. We have used in vitro motility assays of Listeria in platelet and brain extracts to elucidate the function of the focal adhesion proteins of the Ena (Drosophila Enabled)/VASP (vasodilator-stimulated phosphoprotein) family in actin-based motility. Immunodepletion of VASP from platelet extracts and of Evl (Ena/VASP-like protein) from brain extracts of Mena knockout (-/-) mice combined with add-back of recombinant (bacterial or eukaryotic) VASP and Evl show that VASP, Mena, and Evl play interchangeable roles and are required to transform actin polymerization into active movement and propulsive force. The EVH1 (Ena/VASP homology 1) domain of VASP is in slow association-dissociation equilibrium high-affinity binding to the zyxin-homologous, proline-rich region of ActA. VASP also interacts with F-actin via its COOH-terminal EVH2 domain. Hence VASP/ Ena/Evl link the bacterium to the actin tail, which is required for movement. The affinity of VASP for F-actin is controlled by phosphorylation of serine 157 by cAMP-dependent protein kinase. Phospho-VASP binds with high affinity (0.5 x 10(8) M-1); dephospho-VASP binds 40-fold less tightly. We propose a molecular ratchet model for insertional polymerization of actin, within which frequent attachment-detachment of VASP to F-actin allows its sliding along the growing filament.

Conformational difference between nuclear and cytoplasmic actin as detected by a monoclonal antibody

Using a reconstituted complex of profilin and skeletal muscle actin as an antigen, we generated a monoclonal mouse antibody against actin, termed 2G2. As revealed by immunoblots of proteolytic actin fragments and by pepscan analysis, the antibody recognises a nonsequential epitope on actin which is located within three different regions of the sequence, consisting of aa131-139, aa155-169, and aa176-187. In the actin model derived from X-ray diffraction, these sequences lie spatially close together in the region of the nucleotide-binding cleft, but do not form a coherent patch. In immunoblots, 2G2 reacts with all SDS-denatured actin isoforms and with actins of many vertebrates. In contrast, its immunofluorescence reactivity is highly selective and fixation-dependent. In fibroblasts and myogenic cells, fixed and extracted by formaldehyde/detergent, stress fibres or myofibrils, respectively, remained unstained. Likewise, after microinjection into living cells, 2G2 did not bind to such microfilament bundles. Extraction of myosin and tropomyosin did not alter this pattern indicating that the lack in reactivity is probably not due to epitope-masking by actin-binding proteins. More likely, the reason for the lack of reactivity with filamentous actin is that its epitope is not accessible in F-actin. However, the antibody revealed a distinct pattern of nuclear dots in differentiated myogenic cells but not in myoblasts, and of fibrillar structures in nuclei of Xenopus oocytes. In contrast, after methanol treatment, a 2G2-specific staining of stress fibres and myofibrils was observed, but no nuclear dot staining. We conclude that 2G2, in addition to binding to SDS- and methanol-denatured actin, recognises a specific conformation of native actin which is present in the nucleus and specified by compaction of the antibody-reactive region into a coherent patch. This conformation is apparently present in differentiated myogenic cells and oocytes, but not in cytoplasmic actin filament bundles.

Effects of single amino acid substitutions in the actin-binding site on the biological activity of bovine profilin I

For a detailed analysis of the profilin-actin interaction, we designed several point mutations in bovine profilin I by computer modeling. The recombinant proteins were analyzed in vitro for their actin-binding properties. Mutant proteins with a putatively higher affinity for actin were produced by attempting to introduce an additional bond to actin. However, these mutants displayed a lower affinity for actin than wild-type profilin, suggesting that additional putative bonds created this way cannot increase profilin's affinity for actin. In contrast, mutants designed to have a reduced affinity for actin by eliminating profilin-actin bonds displayed the desired properties in viscosity assays, while their binding sites for poly(L)proline were still intact. The profilin mutant F59A, with an affinity for actin reduced by one order of magnitude as compared to wild-type profilin, was analyzed further in cells. When microinjected into fibroblasts, F59A colocalized with the endogenous profilin and actin in ruffling areas, suggesting that profilins are targeted to and tethered at these sites by ligands other than actin. Profilin null cells of Dictyostelium were transfected with bovine wild-type profilin I and F59A. Bovine profilin I, although expressed to only approximately 10% of the endogenous profilin level determined for wild-type Dictyostelium, caused a substantial rescue of the defects observed in profilin null amoebae, as seen by measuring the growth of colony surface areas and the percentage of polynucleated cells. The mutant protein was much less effective. These results emphasize the highly conserved biological function of profilins with low sequence homology, and correlate specifically their actin-binding capacity with cell motility and proliferation.

Differential actin organization by vinculin isoforms: implications for cell type-specific microfilament anchorage

Vinculin is found in all adherens junctions, while metavinculin, a larger splice variant, is coexpressed with vinculin only in smooth and cardiac muscle. To understand the significance of metavinculin expression, we compared ligand binding between turkey vinculin and metavinculin. Residues 1-258 were found essential for head-tail interactions in both proteins. The tail domains (VT and MVT, respectively) both bind to F-actin. However, while VT bundles F-actin, MVT generates highly viscous F-actin webs. In transfected PtK2 cells, VT causes F-actin needles or coils, while MVT-expressing cells display a diffuse F-actin distribution. Thus, the MVT-specific insert induces an F-actin supraorganization different from the VT-based form, suggesting that metavinculin has a specific role in muscle.

Vinculin is part of the cadherin-catenin junctional complex: complex formation between alpha-catenin and vinculin

In epithelial cells, alpha-, beta-, and gamma-catenin are involved in linking the peripheral microfilament belt to the transmembrane protein E-cadherin. alpha-Catenin exhibits sequence homologies over three regions to vinculin, another adherens junction protein. While vinculin is found in cell-matrix and cell-cell contacts, alpha-catenin is restricted to the latter. To elucidate, whether vinculin is part of the cell-cell junctional complex, we investigated complex formation and intracellular targeting of vinculin and alpha-catenin. We show that alpha-catenin colocalizes at cell-cell contacts with endogenous vinculin and also with the transfected vinculin head domain forming immunoprecipitable complexes. In vitro, the vinculin NH2-terminal head binds to alpha-catenin, as seen by immunoprecipitation, dot overlay, cosedimentation, and surface plasmon resonance measurements. The Kd of the complex was determined to 2-4 x 10(-7) M. As seen by overlays and affinity mass spectrometry, the COOH-terminal region of alpha-catenin is involved in this interaction. Complex formation of vinculin and alpha-catenin was challenged in transfected cells. In PtK2 cells, intact alpha-catenin and alpha-catenin1-670, harboring the beta-catenin- binding site, were directed to cell-cell contacts. In contrast, alpha-catenin697-906 fragments were recruited to cell-cell contacts, focal adhesions, and stress fibers. Our results imply that in vivo alpha-catenin, like vinculin, is tightly regulated in its ligand binding activity.

An alpha-actinin-profilin chimaera with two alternatively operating actin-binding sites

Studying the mode of interaction between actin and actin-binding proteins, we constructed a chimaeric protein consisting of the sequence for bovine profilin I (P), to which the sequence for the actin-binding domain of Dictyostelium discoideum alpha-actinin (alphaA1-2) was fused N-terminally. The resulting hybrid clone was expressed in Escherichia coli, and the chimaeric protein, alphaA1-2P, purified by affinity chromatography on poly-(L-proline) (PLP) columns and identified using specific antibodies. High resolution electron microscopy demonstrated that this protein consists of two discrete subdomains. In biochemical, viscometric and electron microscopic analyses, we showed that both modules in this molecule are biologically active. The chimaera binds to poly-(L-proline) and inhibits the polymerization of G-actin in KCl, which is consistent with the assumption that the profilin part is intact. Inhibition of actin polymerization in KCl was stronger than that of the parental profilin, and the Kd value of its interaction with rabbit skeletal muscle actin, as determined by falling ball viscometry, was smaller (mean value 0.5 x 10(-6) M, as compared to 1.9 x 10(-6) M for bovine profilin). In 2mM MgCl2, the actin polymerized rapidly, consistent with the interpretation that under these conditions the chimaera, like profilin, is less efficient as an actin-sequestering agent. In the presence of alphaA1-2P, the resulting filaments were decorated with particles projecting from the filament axis. We conclude that under these conditions the alphaA1-2 domain of alphaA1-2P is preferentially active, attaching the chimaeric particles laterally to the filaments. Hence, the parental modules combined in alphaA1-2P permit this molecule to switch from a G-actin- to an F-actin-binding form.

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.

Plant profilin induces actin polymerization from actin : beta-thymosin complexes and competes directly with beta-thymosins and with negative co-operativity with DNase I for binding to actin

Recombinant plant (birch) profilin was analyzed for its ability to promote actin polymerization from the actin:thymosin beta4 and beta9 complex. Depending on the nature of the divalent cation, recombinant plant (birch) profilin exhibited two different modes of interaction with actin, like mammalian profilin. In the presence of magnesium ions birch profilin promoted the polymerization of actin from A:Tbeta4. In contrast, in the presence of calcium but absence of magnesium ions birch profilin was unable to initiate the polymerization of actin from the complex with Tbeta4. However, under these conditions profilin formed a stable stoichiometric complex with skeletal muscle alpha-actin, as verified by its ability to increase the critical concentration of actin polymerization. Chemical cross-linking indicated that birch profilin competes with Tbeta4 for actin binding. Ternary complex formation of birch profilin with actin:DNase I complex was suggested by chemical cross-linking. However, the determination of the critical concentrations of actin polymerization in the simultaneous presence of birch profilin and DNase I indicated that profilin and DNase I did not form a ternary complex. These data indicated a negative co-operativity between the profilin and DNase I binding sites on actin.

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.