F-actin-binding proteins

Chaperonin-Containing TCP1 Subunit 5 Protects Against the Effect of Mer Receptor Tyrosine Kinase Knockdown in Retinal Pigment Epithelial Cells by Interacting With Filamentous Actin and Activating the LIM-Kinase 1/Cofilin Pathway

Retinitis pigmentosa (RP), characterized by the gradual loss of rod and cone photoreceptors that eventually leads to blindness, is the most common inherited retinal disorder, affecting more than 2.5 million people worldwide. However, the underlying pathogenesis of RP remains unclear and there is no effective cure for RP. Mutations in the Mer receptor tyrosine kinase (MERTK) gene induce the phagocytic dysfunction of retinal pigment epithelium (RPE) cells, leading to RP. Studies have indicated that filamentous actin (F-actin)—which is regulated by chaperonin-containing TCP1 subunit 5 (CCT5)—plays a vital role in phagocytosis in RPE cells. However, whether CCT5/F-actin signaling is involved in MERTK-associated RP remains largely unknown. In the present study, we specifically knocked down MERTK and CCT5 through siRNA transfection and examined the expression of CCT5 and F-actin in human primary RPE (HsRPE) cells. We found that MERTK downregulation inhibited cell proliferation, migration, and phagocytic function; significantly decreased the expression of F-actin; and disrupted the regular arrangement of F-actin. Importantly, our findings firstly indicate that CCT5 interacts with F-actin and is inhibited by MERTK siRNA in HsRPE cells. Upregulating CCT5 using CCT5-specific lentiviral vectors (CCT5-Le) rescued the cell proliferation, migration, and phagocytic function of HsRPE cells under the MERTK knockdown condition by increasing the expression of F-actin and restoring its regular arrangement via the LIMK1/cofilin, but not the SSH1/cofilin, pathway. In conclusion, CCT5 protects against the effect of MERTK knockdown in HsRPE cells and demonstrates the potential for effective treatment of MERTK-associated RP.

Alpha-actinin of the chlorarchiniophyte Bigelowiella natans

The genome of the chlorarchiniophyte Bigelowiella natans codes for a protein annotated as an α-actinin-like protein. Analysis of the primary sequence indicate that this protein has the same domain structure as other α-actinins, a N-terminal actin-binding domain and a C-terminal calmodulin-like domain. These two domains are connected by a short rod domain, albeit long enough to form a single spectrin repeat. To analyse the functional properties of this protein, the full-length protein as well as the separate domains were cloned and isolated. Characerisation showed that the protein is capable of cross-linking actin filaments into dense bundles, probably due to dimer formation. Similar to human α-actinin, calcium-binding occurs to the most N-terminal EF-hand motif in the calmodulin-like C-terminal domain. The results indicate that this Bigelowiella protein is a proper α-actinin, with all common characteristics of a typical α-actinin.

Characterisation of Schizosaccharomyces pombe α-actinin

The actin cytoskeleton plays a fundamental role in eukaryotic cells. Its reorganization is regulated by a plethora of actin-modulating proteins, such as a-actinin. In higher organisms, α-actinin is characterized by the presence of three distinct structural domains: an N-terminal actin-binding domain and a C-terminal region with EF-hand motif separated by a central rod domain with four spectrin repeats. Sequence analysis has revealed that the central rod domain of α-actinin from the fission yeast Schizosaccharomyces pombe consists of only two spectrin repeats. To obtain a firmer understanding of the structure and function of this unconventional α-actinin, we have cloned and characterized each structural domain. Our results show that this a-actinin isoform is capable of forming dimers and that the rod domain is required for this. However, its actin-binding and cross-linking activity appears less efficient compared to conventional α-actinins. The solved crystal structure of the actin-binding domain indicates that the closed state is stabilised by hydrogen bonds and a salt bridge not present in other α-actinins, which may reduce the affinity for actin.

Structure of the 34 kDa F-actin-bundling protein ABP34 from Dictyostelium discoideum

The crystal structure of the 34 kDa F-actin-bundling protein ABP34 from Dictyostelium discoideum was solved by Ca(2+)/S-SAD phasing and refined at 1.89 Å resolution. ABP34 is a calcium-regulated actin-binding protein that cross-links actin filaments into bundles. Its in vitro F-actin-binding and F-actin-bundling activities were confirmed by a co-sedimentation assay and transmission electron microscopy. The co-localization of ABP34 with actin in cells was also verified. ABP34 adopts a two-domain structure with an EF-hand-containing N-domain and an actin-binding C-domain, but has no reported overall structural homologues. The EF-hand is occupied by a calcium ion with a pentagonal bipyramidal coordination as in the canonical EF-hand. The C-domain structure resembles a three-helical bundle and superposes well onto the rod-shaped helical structures of some cytoskeletal proteins. Residues 216-244 in the C-domain form part of the strongest actin-binding sites (193-254) and exhibit a conserved sequence with the actin-binding region of α-actinin and ABP120. Furthermore, the second helical region of the C-domain is kinked by a proline break, offering a convex surface towards the solvent area which is implicated in actin binding. The F-actin-binding model suggests that ABP34 binds to the side of the actin filament and residues 216-244 fit into a pocket between actin subdomains -1 and -2 through hydrophobic interactions. These studies provide insights into the calcium coordination in the EF-hand and F-actin-binding site in the C-domain of ABP34, which are associated through interdomain interactions.

Microfabricated Systems and Assays for Studying the Cytoskeletal Organization, Micromechanics, and Motility Patterns of Cancerous Cells

Cell motions are driven by coordinated actions of the intracellular cytoskeleton - actin, microtubules (MTs) and substrate/focal adhesions (FAs). This coordination is altered in metastatic cancer cells resulting in deregulated and increased cellular motility. Microfabrication tools, including photolithography, micromolding, microcontact printing, wet stamping and microfluidic devices have emerged as a powerful set of experimental tools with which to probe and define the differences in cytoskeleton organization/dynamics and cell motility patterns in non-metastatic and metastatic cancer cells. In this review, we discuss four categories of microfabricated systems: (i) micropatterned substrates for studying of cell motility sub-processes (for example, MT targeting of FAs or cell polarization); (ii) systems for studying cell mechanical properties, (iii) systems for probing overall cell motility patterns within challenging geometric confines relevant to metastasis (for example, linear and ratchet geometries), and (iv) microfluidic devices that incorporate co-cultures of multiple cells types and chemical gradients to mimic in vivo intravasation/extravasation steps of metastasis. Together, these systems allow for creating controlled microenvironments that not only mimic complex soft tissues, but are also compatible with live cell high-resolution imaging and quantitative analysis of single cell behavior.

Calponin-like Chd64 is partly disordered

20-hydroxyecdysone (20E) and juvenile hormone (JH) signaling pathways interact to regulate insect development. Recently, two proteins, a calponin-like Chd64 and immunophilin FKBP39 have been found to play a pivotal role in the cross-talk between 20E and JH, although the molecular basis of interaction remains unknown. The aim of this work was to identify the structural features that would provide understanding of the role of Chd64 in multiple and dynamic complex that cross-links the signaling pathways. Here, we demonstrate the results of in silico and in vitro analyses of the structural organization of Chd64 from Drosophila melanogaster and its homologue from Tribolium castaneum. Computational analysis predicted the existence of disordered regions on the termini of both proteins, while the central region appeared to be globular, probably corresponding to the calponin homology (CH) domain. In vitro analyses of the hydrodynamic properties of the proteins from analytical size-exclusion chromatography and analytical ultracentrifugation revealed that DmChd64 and TcChd64 had an asymmetrical, elongated shape, which was further confirmed by small angle X-ray scattering (SAXS). The Kratky plot indicated disorderness in both Chd64 proteins, which could possibly be on the protein termini and which would give rise to specific hydrodynamic properties. Disordered tails are often involved in diverse interactions. Therefore, it is highly possible that there are intrinsically disordered regions (IDRs) on both termini of the Chd64 proteins that serve as platforms for multiple interaction with various partners and constitute the foundation for their regulatory function.

Mechanism of calponin stabilization of cross-linked actin networks

The actin-binding protein calponin has been previously implicated in actin cytoskeletal regulation and is thought to act as an actin stabilizer, but the mechanism of its function is poorly understood. To investigate this underlying physical mechanism, we studied an in vitro model system of cross-linked actin using bulk rheology. Networks with basic calponin exhibited a delayed onset of strain stiffening (10.0% without calponin, 14.9% with calponin) and were able to withstand a higher maximal strain before failing (35% without calponin, 56% with calponin). Using fluorescence microscopy to study the mechanics of single actin filaments, we found that calponin increased the flexibility of actin filaments, evident as a decrease in persistence length from 17.6 μm without to 7.7 μm with calponin. Our data are consistent with current models of affine strain behavior in semiflexible polymer networks, and suggest that calponin stabilization of actin networks can be explained purely by changes in single-filament mechanics. We propose a model in which calponin stabilizes actin networks against shear through a reduction of persistence length of individual filaments.

Computational prediction of actin-actin interaction

Actin is one of the most abundant proteins in eukaryotic cells, where it plays key roles in cell shape, motility, and regulation. Actin is found in globular (G) and filamentous (F) structure in the cell. The helix of actin occurs as a result of polymerization of monomeric G-actin molecules through sequential rowing, is called F-actin. Recently, the crystal structure of an actin dimer has been reported, which details molecular interface in F-actin. In this study, the computational prediction model of actin and actin complex has been constructed base on the atomic model structure of G-actin. To this end, a docking simulation was carried out using predictive docking tools to obtain modeled structures of the actin-actin complex. Following molecular dynamics refinement, hot spots interactions at the protein interface were identified, that were predicted to contribute substantially to the free energy of binding. These provided a detailed prediction of key amino acid interactions at the protein-protein interface. The obtained model can be used for future experimental and computational studies to draw biological and functional conclusions. Also, the identified interactions will be used for designing next studies to understand the occurrence of F-actin structure.

ALKBH4-dependent demethylation of actin regulates actomyosin dynamics

Regulation of actomyosin dynamics by post-transcriptional modifications in cytoplasmic actin is still poorly understood. Here we demonstrate that dioxygenase ALKBH4-mediated demethylation of a monomethylated site in actin (K84me1) regulates actin–myosin interaction and actomyosin-dependent processes such as cytokinesis and cell migration. ALKBH4-deficient cells display elevated K84me1 levels. Non-muscle myosin II only interacts with unmethylated actin and its proper recruitment to and interaction with actin depend on ALKBH4. ALKBH4 co-localizes with the actomyosin-based contractile ring and midbody via association with methylated actin. ALKBH4-mediated regulation of actomyosin dynamics is completely dependent on its catalytic activity. Disorganization of cleavage furrow components and multinucleation associated with ALKBH4 deficiency can all be restored by reconstitution with wild-type but not catalytically inactive ALKBH4. Similar to actin and myosin knock-out mice, homozygous Alkbh4 mutant mice display early embryonic lethality. These findings imply that ALKBH4-dependent actin demethylation regulates actomyosin function by promoting actin-non-muscle myosin II interaction.

The division of a single eukaryotic cell into two requires actomyosin-dependent contraction. Here the authors show that lysine methylation of actin inhibits contractility during cytokinesis by blocking its association with myosin, and this modification is reversed at the contractile ring by the demethylase ALKBH4.

ADF/cofilin regulates actomyosin assembly through competitive inhibition of myosin II binding to F-actin

The contractile actin cortex is important for diverse fundamental cell processes, but little is known about how the assembly of F-actin and myosin II motors is regulated. We report that depletion of actin depolymerizing factor (ADF)/cofilin proteins in human cells causes increased contractile cortical actomyosin assembly. Remarkably, our data reveal that the major cellular defects resulting from ADF/cofilin depletion, including cortical F-actin accumulation, were largely due to excessive myosin II activity. We identify that ADF/cofilins from unicellular organisms to humans share a conserved activity to inhibit myosin II binding to F-actin, indicating a mechanistic rationale for our cellular results. Our study establishes an essential requirement for ADF/cofilin proteins in the control of normal cortical contractility and in processes such as mitotic karyokinesis. We propose that ADF/cofilin proteins are necessary for controlling actomyosin assembly and intracellular contractile force generation, a function of equal physiological importance to their established roles in mediating F-actin turnover.

Actin filament curvature biases branching direction

Mechanical cues affect many important biological processes in metazoan cells, such as migration, proliferation, and differentiation. Such cues are thought to be detected by specialized mechanosensing molecules linked to the cytoskeleton, an intracellular network of protein filaments that provide mechanical rigidity to the cell and drive cellular shape change. The most abundant such filament, actin, forms branched networks nucleated by the actin-related protein (Arp) 2/3 complex that support or induce membrane protrusions and display adaptive behavior in response to compressive forces. Here we show that filamentous actin serves in a mechanosensitive capacity itself, by biasing the location of actin branch nucleation in response to filament bending. Using an in vitro assay to measure branching from curved sections of immobilized actin filaments, we observed preferential branch formation by the Arp2/3 complex on the convex face of the curved filament. To explain this behavior, we propose a fluctuation gating model in which filament binding or branch nucleation by Arp2/3 occur only when a sufficiently large, transient, local curvature fluctuation causes a favorable conformational change in the filament, and we show with Monte Carlo simulations that this model can quantitatively account for our experimental data. We also show how the branching bias can reinforce actin networks in response to compressive forces. These results demonstrate how filament curvature can alter the interaction of cytoskeletal filaments with regulatory proteins, suggesting that direct mechanotransduction by actin may serve as a general mechanism for organizing the cytoskeleton in response to force.

Effects of basic calponin on the flexural mechanics and stability of F-actin

The cellular actin cytoskeleton plays a central role in the ability of cells to properly sense, propagate, and respond to external stresses and other mechanical stimuli. Calponin, an actin-binding protein found both in muscle and non-muscle cells, has been implicated in actin cytoskeletal organization and regulation. In this work, we studied the mechanical and structural interaction of actin with basic calponin, a differentiation marker in smooth muscle cells, on a single filament level. We imaged fluorescently labeled thermally fluctuating actin filaments and found that at moderate calponin binding densities, actin filaments were more flexible, evident as a reduction in persistence length from 8.0 to 5.8 μm. When calponin-decorated actin filaments were subjected to shear, we observed a marked reduction of filament lengths after decoration with calponin, which we argue was due to shear-induced filament rupture rather than depolymerization. This increased shear susceptibility was exacerbated with calponin concentration. Cryo-electron microscopy results confirmed previously published negative stain electron microscopy results and suggested alterations in actin involving actin subdomain 2. A weakening of F-actin intermolecular association is discussed as the underlying cause of the observed mechanical perturbations.

Stretching actin filaments within cells enhances their affinity for the myosin II motor domain

To test the hypothesis that the myosin II motor domain (S1) preferentially binds to specific subsets of actin filaments in vivo, we expressed GFP-fused S1 with mutations that enhanced its affinity for actin in Dictyostelium cells. Consistent with the hypothesis, the GFP-S1 mutants were localized along specific portions of the cell cortex. Comparison with rhodamine-phalloidin staining in fixed cells demonstrated that the GFP-S1 probes preferentially bound to actin filaments in the rear cortex and cleavage furrows, where actin filaments are stretched by interaction with endogenous myosin II filaments. The GFP-S1 probes were similarly enriched in the cortex stretched passively by traction forces in the absence of myosin II or by external forces using a microcapillary. The preferential binding of GFP-S1 mutants to stretched actin filaments did not depend on cortexillin I or PTEN, two proteins previously implicated in the recruitment of myosin II filaments to stretched cortex. These results suggested that it is the stretching of the actin filaments itself that increases their affinity for the myosin II motor domain. In contrast, the GFP-fused myosin I motor domain did not localize to stretched actin filaments, which suggests different preferences of the motor domains for different structures of actin filaments play a role in distinct intracellular localizations of myosin I and II. We propose a scheme in which the stretching of actin filaments, the preferential binding of myosin II filaments to stretched actin filaments, and myosin II-dependent contraction form a positive feedback loop that contributes to the stabilization of cell polarity and to the responsiveness of the cells to external mechanical stimuli.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Identifying competitive protein antagonists for F-actin with reverse-phase high-performance liquid chromatography

F-actin binding constants are traditionally determined by centrifugal cosedimentation with actin microfilaments, where bound protein is separated from actin with SDS-PAGE and quantitated using densitometry. Here, we demonstrate that UV quantitation of reverse-phase HPLC-separated proteins provides increased accuracy and sensitivity, can be fully automated, and allows one to perform F-actin competition assays on similar sized proteins.

A finite element framework for computation of protein normal modes and mechanical response

A computational framework based on the Finite Element Method is presented to calculate the normal modes and mechanical response of proteins and their supramolecular assemblies. Motivated by elastic network models, proteins are treated as continuum elastic solids with molecular volume defined by their solvent-excluded surface. The discretized Finite Element representation is obtained using a surface simplification algorithm that facilitates the generation of models of arbitrary prescribed spatial resolution. The procedure is applied to a mutant of T4 phage lysozyme, G-actin, syntenin, cytochrome-c', beta-tubulin, and the supramolecular assembly filamentous actin (F-actin). Equilibrium thermal fluctuations of alpha-carbon atoms and their inter-residue correlations compare favorably with all-atom-based results, the Rotational-Translational Block procedure, and experiment. Additionally, the free vibration and compressive buckling responses of F-actin are in quantitative agreement with experiment. The proposed methodology is applicable to any protein or protein assembly and facilitates the incorporation of specific atomic-level interactions, including aqueous-electrolyte-mediated electrostatic effects and solvent damping. The procedure is equally applicable to proteins with known atomic coordinates as it is to electron density maps of proteins, protein complexes, and supramolecular assemblies of unknown atomic structure.

The structure of the C-terminal actin-binding domain of talin

Talin is a large dimeric protein that couples integrins to cytoskeletal actin. Here, we report the structure of the C-terminal actin-binding domain of talin, the core of which is a five-helix bundle linked to a C-terminal helix responsible for dimerisation. The NMR structure of the bundle reveals a conserved surface-exposed hydrophobic patch surrounded by positively charged groups. We have mapped the actin-binding site to this surface and shown that helix 1 on the opposite side of the bundle negatively regulates actin binding. The crystal structure of the dimerisation helix reveals an antiparallel coiled-coil with conserved residues clustered on the solvent-exposed face. Mutagenesis shows that dimerisation is essential for filamentous actin (F-actin) binding and indicates that the dimerisation helix itself contributes to binding. We have used these structures together with small angle X-ray scattering to derive a model of the entire domain. Electron microscopy provides direct evidence for binding of the dimer to F-actin and indicates that it binds to three monomers along the long-pitch helix of the actin filament.

Tropomyosin and ADF/cofilin as collaborators and competitors

Dynamics of actin filaments is pivotal to many fundamental cellular processes such as Dcytokinesis, motility, morphology, vesicle and organelle transport, gene transcription and senescence. In vivo kinetics of actin filament dynamics is far from the equilibrium in vitro and these profound differences are attributed to large number of regulatory proteins. In particular, proteins of the ADF/cofilin family greatly increase actin filament dynamics by severing filaments and enhancing depolymerization of ADP-actin monomers from their pointed ends. Cofilin binds cooperatively to a minor conformer of F-actin in which the subunits are slightly under rotated along the filament helical axis. At high stoichiometry of cofilin to actin subunits, cofilin actually stabilizes actin filaments. Many isoforms oftropomyosin appear to compete with ADF/cofilin proteins for binding to actin filaments. Tropomyosin isoforms studied to date prefer binding to the "untwisted" conformer of F-actin and through their protection and stabilization of F-actin, recruit myosin II and assemble different actin superstructures from the cofilin-actin filaments. However, some tropomyosin isoforms may synergize with ADF/cofilin to enhance filament dynamics, suggesting that the different isoforms of tropomyosins, many of which show developmental or tissue specific expression profiles, play major roles in the assembly and turnover of actin superstructures. Different actin superstructures can overlap both spatially and temporally within a cell, but can be differentiated from each other based upon their kinetic and kinematic properties. Furthermore, local regulation of ADF/cofilin activity through signal transduction pathways could be one mechanism to alter the dynamic balance in F-actin-binding of certain tropomyosin isoforms in subcellular domains.

UNC-87, a calponin-related protein in C. elegans, antagonizes ADF/cofilin-mediated actin filament dynamics

Stabilization of actin filaments is critical for supporting actomyosin-based contractility and for maintaining stable cellular structures. Tropomyosin is a well-characterized ubiquitous actin stabilizer that inhibits ADF/cofilin-dependent actin depolymerization. Here, we show that UNC-87, a calponin-related Caenorhabditis elegans protein with seven calponin-like repeats, competes with ADF/cofilin for binding to actin filaments and inhibits ADF/cofilin-dependent filament severing and depolymerization in vitro. Mutations in the unc-87 gene suppress the disorganized actin phenotype in an ADF/cofilin mutant in the C. elegans body wall muscle, supporting their antagonistic roles in regulating actin stability in vivo. UNC-87 and tropomyosin exhibit synergistic effects in stabilizing actin filaments against ADF/cofilin, and direct comparison reveals that UNC-87 effectively stabilizes actin filaments at much lower concentrations than tropomyosin. However, the in vivo functions of UNC-87 and tropomyosin appear different, suggesting their distinct roles in the regulation of actomyosin assembly and cellular contractility. Our results demonstrate that actin binding via calponin-like repeats competes with ADF/cofilin-driven cytoskeletal turnover, and is critical for providing the spatiotemporal regulation of actin filament stability.

Investigating cytoskeletal alterations as a potential marker of retinal and lens drug-related toxicity

Actin filaments play a critical role in the normal physiology of lenticular and retinal cells in the eye. Disruption of the actin cytoskeleton has been associated with retinal pathology and lens cataract formation. Ocular toxicity is an infrequent observation in drug safety studies, yet its impact to the drug development process is significant. Recognizing compounds through screening with a potential ocular safety liability is one way to prioritize development candidates while reducing development attrition. Lens epithelial cells from human, dog, and rat origins and retinal pigmented epithelium cells from human, monkey, and rat origins were cultured and investigated with immunocytochemical techniques. Cells were treated using noncytotoxic doses of the compound, fixed, stained for actin with rhodamine phalloidin, and counterstained for nuclei with TOTO-3, followed by confocal imaging. Tamoxifen and several experimental compounds known to be in vivo lens and retinal toxicants caused a reduction in F-actin fluorescence at noncytotoxic concentrations in all cells tested as observed by confocal microscopy. Developing an assay that predicts ocular toxicity helps the development process by prioritizing compounds for further investigation. Drug-induced cytoskeletal alterations may be useful as a potential safety-screening marker of retinal and lens toxicity. The knowledge of actin molecular biology and the application of other mechanistic screens to toxicology are discussed. Reducing this work to a high-throughput platform will enable chemists to select compounds with a reduced risk of ocular toxicity.

Dissecting the 3-D structure of vimentin intermediate filaments by cryo-electron tomography

Vimentin polymerizes via complex lateral interactions of coiled-coil dimers into long, flexible filaments referred to as intermediate filaments (IFs). Intermediate in diameter between microtubules and microfilaments, IFs constitute the third cytoskeletal filament system of metazoan cells. Here we investigated the molecular basis of the 3-D architecture of vimentin IFs by cryo-electron microscopy (cryo-EM) as well as cryo-electron tomography (Cryo-ET) 3-D reconstruction. We demonstrate that vimentin filaments in cross-section exhibit predominantly a four-stranded protofibrilar organization with a right-handed supertwist with a helical pitch of about 96 nm. Compact filaments imaged by cryo-EM appear surprisingly straight and hence appear very stiff. In addition, IFs exhibited an increased flexibility at sites of partial unraveling. This is in strong contrast to chemically fixed, negatively stained preparations of vimentin filaments that generally exhibit smooth bending without untwisting. At some point along the filament unraveling may be triggered and propagates in a cooperative manner so that long stretches of filaments appear to have unraveled rapidly in a coordinated fashion.

Cortactin binding to F-actin revealed by electron microscopy and 3D reconstruction

Cortactin and WASP activate Arp2/3-mediated actin filament nucleation and branching. However, different mechanisms underlie activation by the two proteins, which rely on distinct actin-binding modules and modes of binding to actin filaments. It is generally thought that cortactin binds to "mother" actin filaments, while WASP donates actin monomers to Arp2/3-generated "daughter" filament branches. Interestingly, cortactin also binds WASP in addition to F-actin and the Arp2/3 complex. However, the structural basis for the role of cortactin in filament branching remains unknown, making interpretation difficult. Here, electron microscopy and 3D reconstruction were carried out on F-actin decorated with the actin-binding repeating domain of cortactin, revealing conspicuous density on F-actin attributable to cortactin that is located on a consensus-binding site on subdomain-1 of actin subunits. Strikingly, the binding of cortactin widens the gap between the two long-pitch filament strands. Although other proteins have been found to alter the structure of the filament, the cortactin-induced conformational change appears unique. The results are consistent with a mechanism whereby alterations of the F-actin structure may facilitate recruitment of the Arp2/3 complex to the "mother" filament in the cortex of cells. In addition, cortactin may act as a structural adapter protein, stabilizing nascent filament branches while mediating the simultaneous recruitment of Arp2/3 and WASP.

Calcium-dependent protein kinase isoforms in Petunia have distinct functions in pollen tube growth, including regulating polarity

Calcium is a key regulator of pollen tube growth, but little is known concerning the downstream components of the signaling pathways involved. We identified two pollen-expressed calmodulin-like domain protein kinases from Petunia inflata, CALMODULIN-LIKE DOMAIN PROTEIN KINASE1 (Pi CDPK1) and Pi CDPK2. Transient overexpression or expression of catalytically modified Pi CDPK1 disrupted pollen tube growth polarity, whereas expression of Pi CDPK2 constructs inhibited tube growth but not polarity. Pi CDPK1 exhibited plasma membrane localization most likely mediated by acylation, and we present evidence that suggests this localization is critical to the biological function of this kinase. Pi CDPK2 substantially localized to as yet unidentified internal membrane compartments, and this localization was again, at least partially, mediated by acylation. In contrast with Pi CDPK1, altering the localization of Pi CDPK2 did not noticeably alter the effect of overexpressing this isoform on pollen tube growth. Ca(2+) requirements for Pi CDPK1 activation correlated closely with Ca(2+) concentrations measured in the growth zone at the pollen tube apex. Interestingly, loss of polarity associated with overexpression of Pi CDPK1 was associated with elevated cytosolic Ca(2+) throughout the bulging tube tip, suggesting that Pi CDPK1 may participate in maintaining Ca(2+) homeostasis. These results are discussed in relation to previous models for Ca(2+) regulation of pollen tube growth.

Actin-binding proteins--a unifying hypothesis

Actin participates in more protein-protein interactions than any other known protein, including the interaction of actin with itself to form the helical polymer F-actin. The vast majority of actin-binding proteins (ABPs) can be grouped into conserved families. Only a handful of structures of complexes of actin with ABPs have been determined so far. These structures are starting to reveal how certain ABPs, including gelsolin, vitamin D-binding protein and Wiskott-Aldrich syndrome protein (WASP)-homology domain-2-related proteins, share a common actin-binding motif. It is proposed here that other ABPs, including actin itself, might share this motif, providing a mechanism whereby ABPs and actin compete for a common binding site. Of particular interest is a hydrophobic pocket that mediates important interactions in five of the existing structures of actin complexes. As the pocket remains accessible in F-actin, it is proposed that this pocket represents a primary target for F-actin-binding proteins, such as calponin-homology-related proteins and myosin.

Phosphoglycerate kinase inhibits epithelial cell invasion by group B streptococci

Group B streptococci (GBS) are opportunistic human pathogens that cause infection and invasive disease in newborns, pregnant women and non-pregnant adults. The internalization of GBS into eukaryotic cells occurs in an actin-microfilament dependent process. The objective of our study was to understand what host cell and/or bacterial factors may be involved in this process. We focused on alpha-actinin, an actin binding protein closely associated with cytoplasmic F-actin in the eukaryotic cell, to determine if it is involved in actin recruitment upon GBS internalization. Initial work revealed that GBS does not recruit alpha-actinin. However, it was found that alpha-actinin antibodies bound to the surface of the GBS, suggesting GBS possess surface-exposed actin binding protein(s). Slide agglutination experiments revealed that when the bacteria were emulsified with F-actin, visible agglutination occurred, further suggesting the presence of an actin binding protein on the GBS cell. Western blot analysis found that anti-alpha-actinin antibodies bound to a 42 kDa protein; mass spectra analysis identified this protein as GBS phosphoglycerate kinase (PGK). Competitive binding assays suggest that the PGK-actin interaction is not a factor in the initial binding of GBS to epithelial cells, however, treating epithelial cells with PGK prior to performing an invasion assay inhibited GBS internalization. This occurred in a dose dependent manner with 10 microg/mL of PGK inhibiting invasion by over 70%, and 50 microg/mL PGK inhibits GBS invasion completely.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.

Structural Basis for Vertebrate Filamin Dimerization

Filamins are essential in cell motility and many developmental processes. They are large actin cross linking proteins that contain actin binding domains in their N termini and a long rod region constructed from 24 tandem Ig domains. Dimerization is crucial for the actin crosslinking function of filamins and requires the most C-terminal Ig domain. We describe here the crystal structure of this 24th Ig domain (Ig24) of human filamin C and show how it mediates dimerization. The dimer interface is novel and quite different to that seen in the Dictyostelium discoideum filamin analog. The sequence signature of the dimerization interface suggests that the C-terminal domains of all vertebrate filamins share the same dimerization mechanism. Furthermore, we show that point mutations in the dimerization interface disrupt the dimer and that the dissociation constant for recombinant Ig24 is in the micromolar range.

Characterization of an actin-binding site within the talin FERM domain

Talin is a large cytoskeletal protein that couples integrins to F-actin. Three actin-binding sites (ABS1-3) have been reported: one in the N-terminal head, and two in the C-terminal rod domain. Although the C-terminal ABS3 has been partially characterized, the presence and properties of ABS1 within the talin head are less well defined. We show here that the talin head binds F-actin in vitro and in vivo at a specific site within the actin filament. Thus, purified talin head liberated from gizzard talin by calpain cleavage cosediments with F-actin in a low salt buffer at pH 6.4 (conditions that are optimal for binding intact talin), and using recombinant polypeptides, we have mapped ABS1 to the FERM domain within the talin head. Both the F2 and F3 FERM subdomains contribute to binding, and EGFP-tagged FERM subdomains colocalize with actin stress fibers when expressed in COS cells. High-resolution electron microscopy of actin filaments decorated with F2F3 localizes binding to a site that is distinct from that recognized by members of the calponin-homology superfamily. Finally, we show that the FERM domain can couple F-actin to PIPkin, and by inference to integrins, since they bind to the same pocket in the F3 subdomain. This suggests that the talin FERM domain functions as a linker between PIPkin or integrins and F-actin at sites of cell-matrix adhesions.

Crystal Structure of Human Coactosin-like Protein

Human coactosin-like protein is an actin filament binding protein but does not bind to globular actin. It associates with 5-Lipoxygenase both in vivo and in vitro, playing important roles in modulating the activities of actin and 5-Lipoxygenase. Coactosin counteracts the capping activity of capping protein which inhibits the actin polymerization. We determined the crystal structures of human coactosin-like protein by multi-wavelength anomalous dispersion method. The structure showed a high level of similarity to ADF-H domain, although their amino acid sequences share low degree of homology. A few conserved hydrophobic residues that may contribute to the folding were identified. This structure suggests coactosin-like protein bind to F-actin in a different way from ADF/Cofilin family. Combined with the information from previous mutagenesis studies, the binding sites for F-actin and 5-Lipoxygenase were analyzed, respectively. These two sites are quite close, which might prevent F-actin and 5-Lipoxygenase from binding to coactosin simultaneously.

Crystal structure of human coactosin-like protein at 1.9 A resolution

Human coactosin-like protein (CLP) shares high homology with coactosin, a filamentous (F)-actin binding protein, and interacts with 5LO and F-actin. As a tumor antigen, CLP is overexpressed in tumor tissue cells or cell lines, and the encoded epitopes can be recognized by cellular and humoral immune systems. To gain a better understanding of its various functions and interactions with related proteins, the crystal structure of CLP expressed in Escherichia coli has been determined to 1.9 A resolution. The structure features a central beta-sheet surrounded by helices, with two very tight hydrophobic cores on each side of the sheet. CLP belongs to the actin depolymerizing protein superfamily, and is similar to yeast cofilin and actophilin. Based on our structural analysis, we observed that CLP forms a polymer along the crystallographic b axis with the exact same repeat distance as F-actin. A model for the CLP polymer and F-actin binding has therefore been proposed.

Molecular Structure of the Rod Domain of Dictyostelium Filamin

Dictyostelium discoideum filamin (ddFLN) is a two-chain F-actin crosslinking protein with an N-terminal actin-binding domain and a rod domain constructed from six tandem repeats of a 100 residue motif that has an immunoglobulin (Ig) fold. We report the 2.8 A resolution crystal structure of a homodimer of rod repeats 4, 5 and 6. The two chains are arranged in an antiparallel fashion and form an elongated element, which is shortened, however, compared to a fully extended, linear configuration because the long axis of each Ig domain is arranged at an angle to the long axis of the rod. Same arrangement of repeats should also be present in the rod domain of human FLNa, much longer than Dictyostelium FLN, which forms an extended structure able to crosslink F-actin chains over distances of more than 1000 A.

Structural Studies of Arthrin: Monoubiquitinated Actin

Here, we report on the structure and in situ location of arthrin (monoubiquitinated actin). Labelling of insect muscle thin filaments with a ubiquitin antibody reveals that every seventh subunit along the filament long-pitch helices is ubiquitinated. A three-dimensional reconstruction of frozen-hydrated arthrin filaments was produced. This was based on a novel algorithm that divides filament images into short segments that are used for single-particle image processing. Difference maps with an actin filament reconstruction locate ubiquitin at the side of actin sub-domain 1 opposite where myosin binds. Consistent with the reconstructions, peptide mapping places the ubiquitin linkage on lysine 118 in actin. Molecular modelling was used to generate arthrin monomers from ubiquitin and actin crystal structures. Filament models constructed from these monomers were compared with the arthrin reconstruction. The reconstruction suggests ubiquitin attached to Lys118 adopts one or a few conformers, stabilized by a small interface with actin. The function of actin ubiquitination is not known, but may involve regulation of muscle contractile activity.

Regulation of the neuronal actin cytoskeleton by ADF/cofilin

Actin and microtubules are major cytoskeletal elements of most cells including neurons. In order for a cell to move and change shape, its cytoskeleton must undergo rearrangements that involve breaking down and reforming filaments. Many recent reviews have focused on the signaling pathways emanating from receptors that ultimately affect axon growth and growth cone steering. This particular review will address changes in the actin cytoskeleton modulated by the family of actin dynamizing proteins known as actin depolymerizing factor (ADF)/cofilin or AC proteins. Though much is known about inactivation of AC proteins through phosphorylation at ser3 by LIM or TES kinases, new mechanisms of regulation of AC have recently emerged. A novel phosphatase, slingshot (SSH), and the 14-3-3 family of regulatory proteins have also been found to affect AC activity. The potential role of AC proteins in modulating the actin organizational changes that accompany neurite initiation, axonogenesis, growth cone guidance, and dendritic spine formation will be discussed.

Biochemical evidence for interaction between smoothelin and filamentous actin

The two major isoforms of smoothelin (A and B) contain a calponin homology (CH) domain, colocalize with alpha-smooth muscle actin (alpha-SMA) in stress fibers and are only expressed in contractile smooth muscle cells (SMCs). Based on these findings, we hypothesized that smoothelins are involved in smooth muscle cell contraction, presumably via interaction with actin. The interaction between smoothelins and three different actin isoforms (alpha- and gamma-smooth muscle and alpha-skeletal actin [alpha-SKA]) was investigated using several in vitro assays. Smoothelin-B co-immunoprecipitated with alpha-smooth muscle actin from pig aorta extracts. In rat embryonic fibroblasts, transfected smoothelins-A and -B associated with stress fibers. In vitro dot blot assays, in which immobilized actin was overlaid with radio-labeled smoothelin, showed binding of smoothelin-A to actin filaments, but not to monomeric G-actin. A truncated smoothelin, containing the calponin homology domain, associated with stress fibers when transfected and bound to actin filaments in overlay, but to a lesser extent. ELISA results showed that the binding of smoothelin to actin has no significant isoform specificity. Our results indicate an interaction between smoothelin and actin filaments. Moreover, the calponin homology domain and its surrounding sequences appear to be sufficient to accomplish this interaction, although the presence of other domains is apparently necessary to facilitate and/or strengthen the binding to actin.

An Arabidopsis ACT2 dominant-negative mutation, which disturbs F-actin polymerization, reveals its distinctive function in root development

Eight functional actin genes are present in Arabidopsis: The functional characterization of these genes in loss-of-function mutants is difficult, because highly conserved isovariants are generally expressed in the same tissue. We isolated a novel semi-dominant mutant allele (act2-2D) of an actin gene, ACT2, with a missense mutation which causes an amino acid substitution at the surface of the ACT2 protein. ACT2 promoter::ACT2-2D transgenic plants showed the same phenotype as act2-2D, indicating that act2-2D is a dominant-negative mutant. act2-2D exhibited defects in the initiation and elongation of root hairs, the elongation of root epidermal cells, and growth in aerial portions. Specifically, radial cell expansion was reduced and occasional cell death occurred in trichoblasts but not in atrichoblasts of the root epidermis. In contrast, cell division patterns in the root meristem were not affected. act2-3, a loss-of-function ACT2 mutant, did not develop most of these morphological abnormalities. Actin filament (F-actin) bundles in root epidermal cells of act2-2D were shorter than in the wild type and in the loss-of-function mutant. We conclude that defective F-actin polymerization caused the aberrant cell morphology in a dominant-negative manner, and that ACT2 functions in cell elongation and root hair formation.

Trisoxazole macrolide toxins mimic the binding of actin-capping proteins to actin

Marine macrolide toxins of trisoxazole family target actin with high affinity and specificity and have promising pharmacological properties. We present X-ray structures of actin in complex with two members of this family, kabiramide C and jaspisamide A, at a resolution of 1.45 and 1.6 A, respectively. The structures reveal the absolute stereochemistry of these toxins and demonstrate that their trisoxazole ring interacts with actin subdomain 1 while the aliphatic side chain is inserted into the hydrophobic cavity between actin subdomains 1 and 3. The binding site is essentially the same as the one occupied by the actin-capping domain of the gelsolin superfamily of proteins. The structural evidence suggests that actin filament severing and capping by these toxins is also analogous to that of gelsolin. Consequently, these macrolides may be viewed as small molecule biomimetics of an entire class of actin-binding proteins.

Effect of prenatal exposure to ethanol on hepatic elongation factor-2 and proteome in 21 d old rats: protective effect of folic acid

In this article, we study the effects of ethanol intake during pregnancy and lactation on hepatic and pancreatic elongation factor-2 (EF-2) of 21 d old progeny. At the same time, the effect of ethanol on the level of other relevant hepatic proteins was determined using proteomic analysis. The results show that ethanol not only produces a general increase of protein oxidation, but also produces an important depletion of EF-2 and several other proteins. Among the hepatic proteins affected by ethanol, the concomitant supplementation with folic acid to alcoholic mother rats prevented EF-2, RhoGDI-1, ER-60 protease, and gelsolin depletion. This protective effect of folic acid may be related to its antioxidant properties and suggests that this vitamin may be useful in minimizing the effect of ethanol in the uterus and lactation exposure of the progeny.

Contractile activity and smooth muscle alpha-actin organization in thrombin-induced human lung myofibroblasts

Activated fibroblasts, or myofibroblasts, are crucial players in tissue remodeling, wound healing, and various fibrotic disorders, including interstitial lung fibrosis associated with scleroderma. Here we characterize the signaling pathways in normal lung fibroblasts exposed to thrombin as they acquire two of the main features of myofibroblasts: smooth muscle (SM) alpha-actin organization and collagen gel contraction. Our results show that the small G protein Rho is involved in lung myofibroblast differentiation. Thrombin induces Rho-35S-labeled guanosine 5'-O-(3-thiotriphosphate) binding in a dose-dependent manner. It potently stimulates Rho activity in vivo and initiates protein kinase C (PKC)-epsilon-Rho complex formation. Toxin B, which inactivates Rho by ADP ribosylation, inhibits thrombin-induced SM alpha-actin organization, collagen gel contraction, and PKC-epsilon-SM alpha-actin and PKC-epsilon-RhoA coimmunoprecipitation. However, it has no effect on PKC-epsilon activation or translocation of PKC-epsilon to the membrane. Overexpression of constitutively active PKC-epsilon and constitutively active RhoA induces collagen gel contraction or SM alpha-actin organization, whereas, individually, they do not perform these functions. We therefore conclude that the contractile activity of myofibroblasts induced by thrombin is mediated via PKC-epsilon- and RhoA-dependent pathways and that activation of both of these molecules is required. We postulate that PKC-epsilon-RhoA complex formation is an early event in thrombin activation of lung fibroblasts, followed by PKC-epsilon-SM alpha-actin coimmunoprecipitation, which leads to the PKC-epsilon-RhoA-SM alpha-actin ternary complex formation.

Nitric oxide-dependent generation of reactive species in sickle cell disease. Actin tyrosine induces defective cytoskeletal polymerization

The intermittent vascular occlusion occurring in sickle cell disease (SCD) leads to ischemia-reperfusion injury and activation of inflammatory processes including enhanced production of reactive oxygen species and increased expression of inducible nitric-oxide synthase (NOS2). Appreciating that impaired nitric oxide-dependent vascular function and the concomitant formation of oxidizing and nitrating species occur in concert with increased rates of tissue reactive oxygen species production, liver and kidney NOS2 expression, tissue 3-nitrotyrosine (NO(2)Tyr) formation and apoptosis were evaluated in human SCD tissues and a murine model of SCD. Liver and kidney NOS2 expression and NO(2)Tyr immunoreactivity were significantly increased in SCD mice and humans, but not in nondiseased tissues. TdT-mediated nick end-label (TUNEL) staining showed apoptotic cells in regions expressing elevated levels of NOS2 and NO(2)Tyr in all SCD tissues. Gas chromatography mass spectrometry analysis revealed increased plasma protein NO(2)Tyr content and increased levels of hepatic and renal protein NO(2)Tyr derivatives in SCD (21.4 +/- 2.6 and 37.5 +/- 7.8 ng/mg) versus wild type mice (8.2 +/- 2.2 and 10 +/- 1.2 ng/mg), respectively. Western blot analysis and immunoprecipitation of SCD mouse liver and kidney proteins revealed one principal NO(2)Tyr-containing protein of 42 kDa, compared with controls. Enzymatic in-gel digestion and MALDI-TOF mass spectrometry identified this nitrated protein as actin. Electrospray ionization and fragment analysis by tandem mass spectrometry revealed that 3 of 15 actin tyrosine residues are nitrated (Tyr(91), Tyr(198), and Tyr(240)) at positions that significantly modify actin assembly. Confocal microscopy of SCD human and mouse tissues revealed that nitration led to morphologically distinct disorganization of filamentous actin. In aggregate, we have observed that the hemoglobin point mutation of sickle cell disease that mediates hemoglobin polymerization defects is translated, via inflammatory oxidant reactions, into defective cytoskeletal polymerization.

Structural insights into actin-binding, branching and bundling proteins

Structural advances in our understanding of the functions of the actin cytoskeleton have come from diverse sources. On the one hand, the determination of the structure of a bacterial actin-like protein MreB reveals the prokaryotic origins of the actin cytoskeleton, whereas on the other, cryo-electron microscopy and crystallography have yielded reconstructions of many actin crosslinking, regulatory and binding proteins in complex with F-actin. Not least, a high-resolution structure of the Arp2/3 complex and a reconstruction with F-actin provides considerable insight into the eukaryotic machinery, vital for the formation of new F-actin barbed ends, a prerequisite for rapid actin polymerisation involved in cell shape change and motility.

Thymosin beta 4 and a synthetic peptide containing its actin-binding domain promote dermal wound repair in db/db diabetic mice and in aged mice

Impaired wound healing is a problem for immobilized patients, diabetics, and the elderly. Thymosin beta 4 has previously been found to promote dermal and corneal repair in normal rats. Here we report that thymosin beta 4 was also active in accelerating wound repair in full-thickness dermal wounds in both db/db diabetic and aged mice. We found that thymosin beta 4 in either phosphate-buffered saline or a hydrogel formulation is active in promoting dermal wound repair in normal rats. In diabetic mice, where healing is delayed, we found that wound contracture and collagen deposition were significantly increased in the mice treated with thymosin beta 4 in either phosphate buffered saline solution or a hydrogel formulation. No difference was observed in keratinocyte migration, with all of the diabetic animals showing almost complete coverage of the wound at 8 days. Wound healing in 26-month-old (aged) animals was significantly delayed. Thymosin beta 4 accelerated wound healing in these aged mice, with increases in keratinocyte migration, wound contracture, and collagen deposition. The hydrogel formulation generally showed similar wound healing activity with thymosin beta 4 in PBS. The actin-binding domain of thymosin beta 4 duplicated in a seven-amino acid synthetic peptide, LKKTETQ, was able to promote repair in the aged animals comparable to that observed with the parent molecule. These studies show that thymosin beta 4 is active for wound repair in models of impaired healing and may have efficacy in chronic wounds in humans.

Tropomyosin and gelsolin cooperate in controlling the microfilament system

Tropomyosin has been shown to cause annealing of gelsolin-capped actin filaments. Here we show that tropomyosin is highly efficient in transforming even the smallest gelsolin-actin complexes into long actin filaments. At low concentrations of tropomyosin, the effect of tropomyosin depends on the length of the actin oligomer, and the cooperative nature of the process is a direct indication that tropomyosin induces a conformational change in the gelsolin-actin complexes, altering the structure at the actin (+) end such that capping by gelsolin is abolished. At increased concentrations of tropomyosin, heterodimers, trimers, and tetramers are converted to actin filaments. In addition, evidence is presented demonstrating that gelsolin, once removed from the (+) end of the actin, can reassociate with the newly formed tropomyosin-decorated actin filaments. Interestingly, the binding of gelsolin to the tropomyosin-actin filament complexes saturates at 2 gelsolin molecules per 14 actin and 2 tropomyosins, i.e. two gelsolins per tropomyosin-regulatory unit along the filament. These observations support the view that both tropomyosin and gelsolin are likely to have important functions in addition to those proposed earlier.

Steered molecular dynamics simulations on the "tail helix latch" hypothesis in the gelsolin activation process

The molecular basis of the "tail helix latch" hypothesis in the gelsolin activation process has been studied by using the steered molecular dynamics simulations. In the present nanosecond scale simulations, the tail helix of gelsolin was pulled away from the S2 binding surface, and the required forces were calculated, from which the properties of binding between the tail helix and S2 domain and their dynamic unbinding processes were obtained. The force profile provides a detailed rupture mechanism that includes six major unbinding steps. In particular, the hydrogen bonds formed between Arg-207 and Asp-744 and between Arg-221 and Leu-753 are of the most important interaction pairs. The two hydrogen bond "clamps" stabilize the complex. The subsequent simulation on Arg-207-Ala (R207A) mutation of gelsolin indicated that this mutation facilitates the unbinding of the tail helix and that the contribution of the hydrogen bond between Arg-207 and Asp-744 to the binding is more than 50%, which offers a new clue for further mutagenesis study on the activation mechanism of gelsolin. Surrounding water molecules enhance the stability of the tail helix and facilitate the rupture process. Additionally, temperature also has a significant effect on the conformation of the arginine and arginine-related interactions, which revealed the molecular basis of the temperature dependence in gelsolin activation.

Villin-type headpiece domains show a wide range of F-actin-binding affinities

The villin-type "headpiece" domain is a modular motif found at the extreme C-terminus of larger "core" domains in over 25 cytoskeletal proteins in plants and animals. Although headpiece is classified as an F-actin-binding domain, it has been suggested that some expressed fusion-proteins containing headpiece may lack F-actin-binding in vivo. To determine the intrinsic F-actin affinity of headpiece domains, we quantified the F-actin affinity of seven headpiece domains and three N-terminal truncations, under identical in vitro conditions. The constructs are folded and adopt the native headpiece structure. However, they show a wide range of affinities that can be grouped into high, low, and nonspecific-binding categories. Computer models of the structure and charged surface potential of these headpiece domains suggest features important for high F-actin affinity. We conclude that not all headpiece domains are intrinsically F-actin-binding motifs, and suggest that the surface charge distribution may be an important element for F-actin recognition.

Actin cytoskeleton and small heat shock proteins: how do they interact?

Actin and small heat shock proteins (sHsps) are ubiquitous and multifaceted proteins that exist in 2 reversible forms, monomers and multimers, ie, the microfilament of the cytoskeleton and oligomers of the sHsps, generally, supposed to be in a spherical and hollow form. Two situations are described in the literature, where the properties of actin are modulated by sHsps; the actin polymerization is inhibited in vitro by some sHsps acting as capping proteins, and the actin cytoskeleton is protected by some sHsps against the disruption induced by various stressful conditions. We propose that a direct actin-sHsp interaction occurs to inhibit actin polymerization and to participate in the in vivo regulation of actin filament dynamics. Protection of the actin cytoskeleton would result from an F-actin-sHsp interaction in which microfilaments would be coated by small oligomers of phosphorylated sHsps. Both proteins share common structural motives suggesting direct binding sites, but they remain to be demonstrated. Some sHsps would behave with the actin cytoskeleton as actin-binding proteins capable of either capping a microfilament when present as a nonphosphorylated monomer or stabilizing and protecting the microfilament when organized in small, phosphorylated oligomers.

Tropomyosin inhibits ADF/cofilin-dependent actin filament dynamics

Tropomyosin binds to actin filaments and is implicated in stabilization of actin cytoskeleton. We examined biochemical and cell biological properties of Caenorhabditis elegans tropomyosin (CeTM) and obtained evidence that CeTM is antagonistic to ADF/cofilin-dependent actin filament dynamics. We purified CeTM, actin, and UNC-60B (a muscle-specific ADF/cofilin isoform), all of which are derived from C. elegans, and showed that CeTM and UNC-60B bound to F-actin in a mutually exclusive manner. CeTM inhibited UNC-60B-induced actin depolymerization and enhancement of actin polymerization. Within isolated native thin filaments, actin and CeTM were detected as major components, whereas UNC-60B was present at a trace amount. Purified UNC-60B was unable to interact with the native thin filaments unless CeTM and other associated proteins were removed by high-salt extraction. Purified CeTM was sufficient to restore the resistance of the salt-extracted filaments from UNC-60B. In muscle cells, CeTM and UNC-60B were localized in different patterns. Suppression of CeTM by RNA interference resulted in disorganized actin filaments and paralyzed worms in wild-type background. However, in an ADF/cofilin mutant background, suppression of CeTM did not worsen actin organization and worm motility. These results suggest that tropomyosin is a physiological inhibitor of ADF/cofilin-dependent actin dynamics.

Caspase 8-mediated cleavage of plectin precedes F-actin breakdown in acinar cells during pancreatitis

Pancreatic acinar cells depend on the integrity of the cytoskeleton for regulated secretion. Stimulation of isolated rat pancreatic acini with the secretagogue CCK serves as a model for human acute edematous pancreatitis. It induces the breakdown of the actin filament system (F-actin) with the consecutive inhibition of secretion and premature activation of digestive enzymes. However, the mechanisms that regulate F-actin breakdown are largely unknown. Plectin is a versatile cytolinker protein regulating F-actin dynamics in fibroblasts. It was recently demonstrated that plectin is a substrate of caspase 8. In pancreatic acinar cells, plectin strongly colocalizes with apical and basolateral F-actin. Supramaximal secretory stimulation of acini with CCK leads to a rapid redistribution and activation of caspase 8, followed by degradation of plectin that in turn precedes the F-actin breakdown. Inhibition of caspase 8 before CCK hyperstimulation prevents plectin cleavage, stabilizes F-actin morphology, and reverses the inhibition of secretion. Thus we propose that the caspase 8-mediated degradation of plectin represents a critical biochemical event during CCK-induced secretory blockade and cell injury.