Essential functions and actin-binding surfaces of yeast cofilin revealed by systematic mutagenesis

Effect of destrin mutations on the gene expression profile in vivo

Remodeling of the actin cytoskeleton through actin dynamics (assembly and disassembly of filamentous actin) is known to be essential for numerous basic biological processes. In addition, recent studies have provided evidence that actin dynamics participate in the control of gene expression. A spontaneous mouse mutant, corneal disease 1 (corn1), is deficient for a regulator of actin dynamics, destrin (DSTN, also known as ADF), which causes epithelial hyperproliferation and neovascularization in the cornea. Dstn(corn1) mice exhibit an actin dynamics defect in the corneal epithelial cells, offering an in vivo model to investigate cellular mechanisms affected by the Dstn mutation and resultant actin dynamics abnormalities. To examine the effect of the Dstn(corn1) mutation on the gene expression profile, we performed a microarray analysis using the cornea from Dstn(corn1) and wild-type mice. A dramatic alteration of the gene expression profile was observed in the Dstn(corn1) cornea, with 1,226 annotated genes differentially expressed. Functional annotation of these genes revealed that the most significantly enriched functional categories are associated with actin and/or cytoskeleton. Among genes that belong to these categories, a considerable number of serum response factor target genes were found, indicating the possible existence of an actin-SRF pathway of transcriptional regulation in vivo. A comparative study using an allelic mutant strain with milder corneal phenotypes suggested that the level of filamentous actin may correlate with the level of gene expression changes. Our study shows that Dstn mutations and resultant actin dynamics abnormalities have a strong impact on the gene expression profile in vivo.

Mapping the cofilin binding site on yeast G-actin by chemical cross-linking

Cofilin is a major cytoskeletal protein that binds to both monomeric actin (G-actin) and polymeric actin (F-actin) and is involved in microfilament dynamics. Although an atomic structure of the G-actin-cofilin complex does not exist, models of the complex have been built using molecular dynamics simulations, structural homology considerations, and synchrotron radiolytic footprinting data. The hydrophobic cleft between actin subdomains 1 and 3 and, alternatively, the cleft between actin subdomains 1 and 2 have been proposed as possible high-affinity cofilin binding sites. In this study, the proposed binding of cofilin to the subdomain 1/subdomain 3 region on G-actin has been probed using site-directed mutagenesis, fluorescence labeling, and chemical cross-linking, with yeast actin mutants containing single reactive cysteines in the actin hydrophobic cleft and with cofilin mutants carrying reactive cysteines in the regions predicted to bind to G-actin. Mass spectrometry analysis of the cross-linked complex revealed that cysteine 345 in subdomain 1 of mutant G-actin was cross-linked to native cysteine 62 on cofilin. A cofilin mutant that carried a cysteine substitution in the alpha 3-helix (residue 95) formed a cross-link with residue 144 in actin subdomain 3. Distance constraints imposed by these cross-links provide experimental evidence for cofilin binding between actin subdomains 1 and 3 and fit a corresponding docking-based structure of the complex. The cross-linking of the N-terminal region of recombinant yeast cofilin to actin residues 346 and 374 with dithio-bis-maleimidoethane (12.4 A) and via disulfide bond formation was also documented. This set of cross-linking data confirms the important role of the N-terminal segment of cofilin in interactions with G-actin.

Characterization of the enzymatic activity of the actin cross-linking domain from the Vibrio cholerae MARTX Vc toxin

Vibrio cholerae is a Gram-negative bacterial pathogen that exports enterotoxins, which alter host cells through a number of mechanisms resulting in diarrheal disease. Among the secreted toxins is the multifunctional, autoprocessing RTX toxin (MARTX(Vc)), which disrupts actin cytoskeleton by covalently cross-linking actin monomers into oligomers. The region of the toxin responsible for cross-linking activity is the actin cross-linking domain (ACD). In this study, we demonstrate unambiguously that ACD utilizes G- and not F-actin as a substrate for the cross-linking reaction and hydrolyzes one molecule of ATP per cross-linking event. Furthermore, major actin-binding proteins that regulate actin cytoskeleton in vivo do not block the cross-linking reaction in vitro. Cofilin inhibits the cross-linking of G- and F-actin, at a high mole ratio to actin but accelerates F-actin cross-linking at low mole ratios. DNase I completely blocks the cross-linking of actin, likely due to steric hindrance with one of the cross-linking sites on actin. In the context of the holotoxin, the inhibition of Rho by the Rho-inactivating domain of MARTX(Vc) (Sheahan, K. L., and Satchell, K. J. F. (2007) Cell. Microbiol. 9, 1324-1335) would accelerate F-actin depolymerization and provide G-actin, alone or in complex with actin-binding proteins, for cross-linking by ACD, ultimately leading to the observed rapid cell rounding.

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.

Modeling of protein binary complexes using structural mass spectrometry data

In this article, we describe a general approach to modeling the structure of binary protein complexes using structural mass spectrometry data combined with molecular docking. In the first step, hydroxyl radical mediated oxidative protein footprinting is used to identify residues that experience conformational reorganization due to binding or participate in the binding interface. In the second step, a three-dimensional atomic structure of the complex is derived by computational modeling. Homology modeling approaches are used to define the structures of the individual proteins if footprinting detects significant conformational reorganization as a function of complex formation. A three-dimensional model of the complex is constructed from these binary partners using the ClusPro program, which is composed of docking, energy filtering, and clustering steps. Footprinting data are used to incorporate constraints-positive and/or negative-in the docking step and are also used to decide the type of energy filter-electrostatics or desolvation-in the successive energy-filtering step. By using this approach, we examine the structure of a number of binary complexes of monomeric actin and compare the results to crystallographic data. Based on docking alone, a number of competing models with widely varying structures are observed, one of which is likely to agree with crystallographic data. When the docking steps are guided by footprinting data, accurate models emerge as top scoring. We demonstrate this method with the actin/gelsolin segment-1 complex. We also provide a structural model for the actin/cofilin complex using this approach which does not have a crystal or NMR structure.

Actin hydrophobic loop 262-274 and filament nucleation and elongation

The importance of actin hydrophobic loop 262-274 dynamics to actin polymerization and filament stability has been shown recently with the use of the yeast mutant actin L180C/L269C/C374A, in which the hydrophobic loop could be locked in a "parked" conformation by a disulfide bond between C180 and C269. Such a cross-linked globular actin monomer does not form filaments, suggesting nucleation and/or elongation inhibition. To determine the role of loop dynamics in filament nucleation and/or elongation, we studied the polymerization of the cross-linked actin in the presence of cofilin, to assist with actin nucleation, and with phalloidin, to stabilize the elongating filament segments. We demonstrate here that together, but not individually, phalloidin and cofilin co-rescue the polymerization of cross-linked actin. The polymerization was also rescued by filament seeds added together with phalloidin but not with cofilin. Thus, loop immobilization via cross-linking inhibits both filament nucleation and elongation. Nevertheless, the conformational changes needed to catalyze ATP hydrolysis by actin occur in the cross-linked actin. When actin filaments are fully decorated by cofilin, the helical twist of filamentous actin (F-actin) changes by approximately 5 degrees per subunit. Electron microscopic analysis of filaments rescued by cofilin and phalloidin revealed a dense contact between opposite strands in F-actin and a change of twist by approximately 1 degrees per subunit, indicating either partial or disordered attachment of cofilin to F-actin and/or competition between cofilin and phalloidin to alter F-actin symmetry. Our findings show an importance of the hydrophobic loop conformational dynamics in both actin nucleation and elongation and reveal that the inhibition of these two steps in the cross-linked actin can be relieved by appropriate factors.

F-actin binding region of SPIN90 C-terminus is essential for actin polymerization and lamellipodia formation

We recently reported that SPIN90 is able to bind with several proteins involved in regulating actin cytoskeleton networks, including dynamin, WASP, beta PIX, and Nck. Based on these findings, we investigated how SPIN90 regulates the actin cytoskeleton and promotes actin assembly. This study demonstrated that aluminium fluoride-induced localization of SPIN90 to lamellipodia requires amino acids 582-722 at the SPIN90 C-terminus, which is also essential for F-actin binding and Arp2/3 complex mediated polymerization of actin into branched actin filaments. Furthermore, after deletion of the F-actin binding region (582-722 SPIN90) failed to localize at the membrane edge and was unable to promote lamellipodia formation, suggesting that the F-actin binding region in the SPIN90 C-terminus is essential for the formation of branched actin networks and regulation of the actin cytoskeleton at the leading edge of cells.

Cofilin recruitment and function during actin-mediated endocytosis dictated by actin nucleotide state

Cofilin is the major mediator of actin filament turnover in vivo. However, the molecular mechanism of cofilin recruitment to actin networks during dynamic actin-mediated processes in living cells and cofilin's precise in vivo functions have not been determined. In this study, we analyzed the dynamics of fluorescently tagged cofilin and the role of cofilin-mediated actin turnover during endocytosis in Saccharomyces cerevisiae. In living cells, cofilin is not necessary for actin assembly on endocytic membranes but is recruited to molecularly aged adenosine diphosphate actin filaments and is necessary for their rapid disassembly. Defects in cofilin function alter the morphology of actin networks in vivo and reduce the rate of actin flux through actin networks. The consequences of decreasing actin flux are manifested by decreased but not blocked endocytic internalization at the plasma membrane and defects in late steps of membrane trafficking to the vacuole. These results suggest that cofilin-mediated actin filament flux is required for the multiple steps of endocytic trafficking.

Identification of Arabidopsis cyclase-associated protein 1 as the first nucleotide exchange factor for plant actin

The actin cytoskeleton powers organelle movements, orchestrates responses to abiotic stresses, and generates an amazing array of cell shapes. Underpinning these diverse functions of the actin cytoskeleton are several dozen accessory proteins that coordinate actin filament dynamics and construct higher-order assemblies. Many actin-binding proteins from the plant kingdom have been characterized and their function is often surprisingly distinct from mammalian and fungal counterparts. The adenylyl cyclase-associated protein (CAP) has recently been shown to be an important regulator of actin dynamics in vivo and in vitro. The disruption of actin organization in cap mutant plants indicates defects in actin dynamics or the regulated assembly and disassembly of actin subunits into filaments. Current models for actin dynamics maintain that actin-depolymerizing factor (ADF)/cofilin removes ADP-actin subunits from filament ends and that profilin recharges these monomers with ATP by enhancing nucleotide exchange and delivery of subunits onto filament barbed ends. Plant profilins, however, lack the essential ability to stimulate nucleotide exchange on actin, suggesting that there might be a missing link yet to be discovered from plants. Here, we show that Arabidopsis thaliana CAP1 (AtCAP1) is an abundant cytoplasmic protein; it is present at a 1:3 M ratio with total actin in suspension cells. AtCAP1 has equivalent affinities for ADP- and ATP-monomeric actin (Kd approximately 1.3 microM). Binding of AtCAP1 to ATP-actin monomers inhibits polymerization, consistent with AtCAP1 being an actin sequestering protein. However, we demonstrate that AtCAP1 is the first plant protein to increase the rate of nucleotide exchange on actin. Even in the presence of ADF/cofilin, AtCAP1 can recharge actin monomers and presumably provide a polymerizable pool of subunits to profilin for addition onto filament ends. In turnover assays, plant profilin, ADF, and CAP act cooperatively to promote flux of subunits through actin filament barbed ends. Collectively, these results and our understanding of other actin-binding proteins implicate CAP1 as a central player in regulating the pool of unpolymerized ATP-actin.

The flare gene, which encodes the AIP1 protein of Drosophila, functions to regulate F-actin disassembly in pupal epidermal cells

Adult Drosophila are decorated with several types of polarized cuticular structures, such as hairs and bristles. The morphogenesis of these takes place in pupal cells and is mediated by the actin and microtubule cytoskeletons. Mutations in flare (flr) result in grossly abnormal epidermal hairs. We report here that flr encodes the Drosophila actin interacting protein 1 (AIP1). In other systems this protein has been found to promote cofilin-mediated F-actin disassembly. In Drosophila cofilin is encoded by twinstar (tsr). We show that flr mutations result in increased levels of F-actin accumulation and increased F-actin stability in vivo. Further, flr is essential for cell proliferation and viability and for the function of the frizzled planar cell polarity system. All of these phenotypes are similar to those seen for tsr mutations. This differs from the situation in yeast where cofilin is essential while aip1 mutations result in only subtle defects in the actin cytoskeleton. Surprisingly, we found that mutations in flr and tsr also result in greatly increased tubulin staining, suggesting a tight linkage between the actin and microtubule cytoskeleton in these cells.

Biochemical and genetic analyses provide insight into the structural and mechanistic properties of actin filament disassembly by the Aip1p cofilin complex in Saccharomyces cerevisiae

Explication of the Aip1p/cofilin/actin filament complex may lead to a more detailed understanding of the mechanisms by which Aip1p and cofilin collaborate to rapidly disassemble filaments. We further characterized the actin-Aip1p interface through a random mutagenic screen of ACT1, identifying a novel Aip1p interaction site on actin. This finding is consistent with our current ternary complex model and offers insights into how Aip1p may disturb intersubunit contacts within an actin filament. In addition, site-directed mutagenesis aimed at interfering with salt bridge interactions at the predicted Aip1p-cofilin interface revealed hyperactive alleles of cof1 and aip1 that support the ternary complex model and suggest that conformational changes in cofilin structure may be transmitted to actin filaments, causing increased destabilization. Furthermore, these data support an active role for Aip1p in promoting actin filament turnover.

Three-dimensional structure of cofilin bound to monomeric actin derived by structural mass spectrometry data

The cytoskeletal protein, actin, has its structure and function regulated by cofilin. In the absence of an atomic resolution structure for the actin/cofilin complex, the mechanism of cofilin regulation is poorly understood. Theoretical studies based on the similarities of cofilin and gelsolin segment 1 proposed the cleft between subdomains 1 and 3 in actin as the cofilin binding site. We used radiolytic protein footprinting with mass spectrometry and molecular modeling to provide an atomic model of how cofilin binds to monomeric actin. Footprinting data suggest that cofilin binds to the cleft between subdomains 1 and 2 in actin and that cofilin induces further closure of the actin nucleotide cleft. Site-specific fluorescence data confirm these results. The model identifies key ionic and hydrophobic interactions at the binding interface, including hydrogen-bonding between His-87 of actin to Ser-89 of cofilin that may control the charge dependence of cofilin binding. This model and its implications fill an especially important niche in the actin field, owing to the fact that ongoing crystallization efforts of the actin/cofilin complex have so far failed. This 3D binary complex structure is derived from a combination of solution footprinting data and computational approaches and outlines a general method for determining the structure of such complexes.

Missing-in-metastasis and IRSp53 deform PI(4,5)P2-rich membranes by an inverse BAR domain-like mechanism

The actin cytoskeleton plays a fundamental role in various motile and morphogenetic processes involving membrane dynamics. We show that actin-binding proteins MIM (missing-in-metastasis) and IRSp53 directly bind PI(4,5)P(2)-rich membranes and deform them into tubular structures. This activity resides in the N-terminal IRSp53/MIM domain (IMD) of these proteins, which is structurally related to membrane-tubulating BAR (Bin/amphiphysin/Rvs) domains. We found that because of a difference in the geometry of the PI(4,5)P(2)-binding site, IMDs induce a membrane curvature opposite that of BAR domains and deform membranes by binding to the interior of the tubule. This explains why IMD proteins induce plasma membrane protrusions rather than invaginations. We also provide evidence that the membrane-deforming activity of IMDs, instead of the previously proposed F-actin-bundling or GTPase-binding activities, is critical for the induction of the filopodia/microspikes in cultured mammalian cells. Together, these data reveal that interplay between actin dynamics and a novel membrane-deformation activity promotes cell motility and morphogenesis.

Mapping the interaction of cofilin with subdomain 2 on actin

Cofilin, a member of the actin-depolymerizing factor (ADF)/cofilin family of proteins, is a key regulator of actin dynamics. Cofilin binds to monomer (G-) and filamentous (F-) actin, severs the filaments, and increases their turnover rate. Electron microscopy studies suggested cofilin interactions with subdomains 2 and 1/3 on adjacent actin protomers in F-actin. To probe for the presence of a cryptic cofilin binding site in subdomain 2 in G-actin, we used transglutaminase-mediated cross-linking, which targets Gln41 in subdomain 2. The cross-linking proceeded with up to 85% efficiency with skeletal alpha-actin and WT yeast actin, yielding a single product corresponding to a 1:1 actin-cofilin complex but was strongly inhibited in Q41C yeast actin (in which Q41 was substituted with cysteine). LC-MS/MS analysis of the proteolytic fragments of this complex mapped the cross-linking to Gln41 on actin and Gly1 on recombinant yeast cofilin. The actin-cofilin (AC) heterodimer was purified on FPLC for analytical ultracentrifugation and electron microscopy analysis. Sedimentation equilibrium and velocity runs revealed oligomers of AC in G-actin buffer. In the presence of excess cofilin, the covalent AC heterodimer bound a second cofilin, forming a 2:1 cofilin/actin complex, as revealed by sedimentation results. Under polymerizing conditions the cross-linked AC formed mostly short filaments, which according to image reconstruction were similar to uncross-linked actin-cofilin filaments. Although a majority of the cross-linking occurs at Gln41, a small fraction of the AC cross-linked complex forms in the Q41C yeast actin mutant. This secondary cross-linking site was sequenced by MALDI-MS/MS as linking Gln360 in actin to Lys98 on cofilin. Overall, these results demonstrate that the region around Gln41 (subdomain 2) is involved in a weak binding of cofilin to G-actin.

Structural basis and evolutionary origin of actin filament capping by twinfilin

Dynamic reorganization of the actin cytoskeleton is essential for motile and morphological processes in all eukaryotic cells. One highly conserved protein that regulates actin dynamics is twinfilin, which both sequesters actin monomers and caps actin filament barbed ends. Twinfilin is composed of two ADF/cofilin-like domains, Twf-N and Twf-C. Here, we reveal by systematic domain-swapping/inactivation analysis that the two functional ADF-H domains of twinfilin are required for barbed-end capping and that Twf-C plays a critical role in this process. However, these domains are not functionally equivalent. NMR-structure and mutagenesis analyses, together with biochemical and motility assays showed that Twf-C, in addition to its binding to G-actin, interacts with the sides of actin filaments like ADF/cofilins, whereas Twf-N binds only G-actin. Our results indicate that during filament barbed-end capping, Twf-N interacts with the terminal actin subunit, whereas Twf-C binds between two adjacent subunits at the side of the filament. Thus, the domain requirement for actin filament capping by twinfilin is remarkably similar to that of gelsolin family proteins, suggesting the existence of a general barbed-end capping mechanism. Furthermore, we demonstrate that a synthetic protein consisting of duplicated ADF/cofilin domains caps actin filament barbed ends, providing evidence that the barbed-end capping activity of twinfilin arose through a duplication of an ancient ADF/cofilin-like domain.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

Mapping the ADF/cofilin binding site on monomeric actin by competitive cross-linking and peptide array: evidence for a second binding site on monomeric actin

The binding sites for actin depolymerising factor (ADF) and cofilin on G-actin have been mapped by competitive chemical cross-linking using deoxyribonuclease I (DNase I), gelsolin segment 1 (G1), thymosin beta4 (Tbeta4), and vitamin D-binding protein (DbP). To reduce ADF/cofilin induced actin oligomerisation we used ADP-ribosylated actin. Both vitamin D-binding protein and thymosin beta4 inhibit binding by ADF or cofilin, while cofilin or ADF and DNase I bind simultaneously. Competition was observed between ADF or cofilin and G1, supporting the hypothesis that cofilin preferentially binds in the cleft between sub-domains 1 and 3, similar to or overlapping the binding site of G1. Because the affinity of G1 is much higher than that of ADF or cofilin, even at a 20-fold excess of the latter, the complexes contained predominantly G1. Nevertheless, cross-linking studies using actin:G1 complexes and ADF or cofilin showed the presence of low concentrations of ternary complexes containing both ADF or cofilin and G1. Thus, even with monomeric actin, it is shown for the first time that binding sites for both G1 and ADF or cofilin can be occupied simultaneously, confirming the existence of two separate binding sites. Employing a peptide array with overlapping sequences of actin overlaid by cofilin, we have identified five sequence stretches of actin able to bind cofilin. These sequences are located within the regions of F-actin predicted to bind cofilin in the model derived from image reconstructions of electron microscopical images of cofilin-decorated filaments. Three of the peptides map to the cleft region between sub-domains 1 and 3 of the upper actin along the two-start long-pitch helix, while the other two are in the DNase I loop corresponding to the site of the lower actin in the helix. In the absence of any crystal structures of ADF or cofilin in complex with actin, these studies provide further information about the binding sites on F-actin for these important actin regulatory proteins.

Mechanism of actin filament turnover by severing and nucleation at different concentrations of ADF/cofilin

ADF/cofilins are key regulators of actin dynamics during cellular motility, yet their precise role and mechanism of action are shrouded in ambiguity. Direct observation of actin filaments by evanescent wave microscopy showed that cofilins from fission yeast and human do not increase the rate that pointed ends of actin filaments shorten beyond the rate for ADP-actin subunits, but both cofilins inhibit elongation and subunit dissociation at barbed ends. Direct observation also showed that cofilins from fission yeast, Acanthamoeba, and human sever actin filaments optimally at low-cofilin binding densities well below their K(d)s, but not at high binding densities. High concentrations of cofilin nucleate actin assembly. Thus, the action of cofilins in cells will depend on the local concentration of active cofilins: low concentrations favor severing, whereas high concentrations favor nucleation. These results establish a clear paradigm for actin turnover by cofilin in cells.

Mapping the Phosphoinositide-Binding Site on Chick Cofilin Explains How PIP2 Regulates the Cofilin-Actin Interaction

Cofilin plays a key role in the choreography of actin dynamics via its ability to sever actin filaments and increase the rate of monomer dissociation from pointed ends. The exact manner by which phosphoinositides bind to cofilin and inhibit its interaction with actin has proven difficult to ascertain. We determined the structure of chick cofilin and used NMR chemical shift mapping and structure-directed mutagenesis to unambiguously locate its recognition site for phosphoinositides (PIs). This structurally unique recognition site requires both the acyl chain and head group of the PI for a productive interaction, and it is not inhibited by phosphorylation of cofilin. We propose that the interaction of cofilin with membrane-bound PIs abrogates its binding to both actin and actin-interacting protein 1, and facilitates spatiotemporal regulation of cofilin activity.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

Regulation of apicomplexan microfilament dynamics by a minimal set of actin-binding proteins

Efficient and rapid host cell invasion is a prerequisite for an intracellular parasitic life style. Pathogens typically induce receptor-mediated endocytosis and hijack the force-transducing system of a host cell to gain access to a replication-competent niche. In striking contrast, apicomplexan parasites such as Plasmodium, the causative agent of malaria, and the human and animal pathogens Toxoplasma and Cryptosporidium employ their own actomyosin motor machinery to propel themselves into prospective host cells. Understanding the regulation and dynamics of actin-based motility of these parasites is therefore central to understanding their pathogenesis. The parasite genomes harbour surprisingly few potential actin-regulatory proteins indicating that a basic repertoire meets the requirements to regulate actin dynamics. In this article, we summarize our current knowledge of Plasmodium microfilament dynamics and describe its potential players.

Binding model of human coactosin-like protein with filament actin revealed by mutagenesis

Human coactosin-like protein (CLP) is a small (MW approximately 17 kDa) evolutionarily conserved actin-binding protein. It can bind to actin filaments but not globular actin and belongs to the fourth class of ADF-H-domain-containing proteins. Human CLP can also bind to 5LO, which plays an important role in cellular leukotriene synthesis. Although the structure of hCLP has been determined by both NMR and X-ray experiments, how hCLP binds to the actin filament is still a controversial question. To obtain insights into the structure of the complex, we studied the three-dimensional structure and backbone dynamics of hCLP using multidimensional NMR spectroscopy. Guided by the solution structure of the protein, a series of site-directed mutants were generated and their F-actin-binding activities were measured by high-speed cosedimentation assays. Furthermore, the structure model of the hCLP-F-actin complex was proposed using computational docking with the docking results filtered by the mutation data. Several previously untested residues (including T66, L89, R91, K102, D116 and E119) in hCLP were found important for the F-actin-binding activity. The extended region of beta4-beta5 of hCLP (residue 66-75) was found very flexible and very important for F-actin binding. The C-terminal residues of hCLP were not involved in F-actin binding, which was different from UNC-60B. Based on our hCLP-F-actin-binding model, different affinities of the four classes of ADF-H domain containing proteins for F-actin were explained.

Twinfilin is an actin-filament-severing protein and promotes rapid turnover of actin structures in vivo

Working in concert, multiple actin-binding proteins regulate the dynamic turnover of actin networks. Here, we define a novel function for the conserved actin-binding protein twinfilin, which until now was thought to function primarily as a monomer-sequestering protein. We show that purified budding yeast twinfilin (Twf1) binds to and severs actin filaments in vitro at pH below 6.0 in bulk kinetic and fluorescence microscopy assays. Further, we use total internal reflection fluorescence (TIRF) microscopy to demonstrate that Twf1 severs individual actin filaments in real time. It has been shown that capping protein directly binds to Twf1 and is required for Twf1 localization to cortical actin patches in vivo. We demonstrate that capping protein directly inhibits the severing activity of Twf1, the first biochemical function ascribed to this interaction. In addition, phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2] inhibits Twf1 filament-severing activity. Consistent with these biochemical activities, a twf1Δ mutation causes reduced rates of cortical actin patch turnover in living cells. Together, our data suggest that twinfilin coordinates filament severing and monomer sequestering at sites of rapid actin turnover and is controlled by multiple regulatory inputs.

Aip1 and cofilin promote rapid turnover of yeast actin patches and cables: a coordinated mechanism for severing and capping filaments

Rapid turnover of actin structures is required for dynamic remodeling of the cytoskeleton and cell morphogenesis, but the mechanisms driving actin disassembly are poorly defined. Cofilin plays a central role in promoting actin turnover by severing/depolymerizing filaments. Here, we analyze the in vivo function of a ubiquitous actin-interacting protein, Aip1, suggested to work with cofilin. We provide the first demonstration that Aip1 promotes actin turnover in living cells. Further, we reveal an unanticipated role for Aip1 and cofilin in promoting rapid turnover of yeast actin cables, dynamic structures that are decorated and stabilized by tropomyosin. Through systematic mutagenesis of Aip1 surfaces, we identify two well-separated F-actin-binding sites, one of which contributes to actin filament binding and disassembly specifically in the presence of cofilin. We also observe a close correlation between mutations disrupting capping of severed filaments in vitro and reducing rates of actin turnover in vivo. We propose a model for balanced regulation of actin cable turnover, in which Aip1 and cofilin function together to "prune" tropomyosin-decorated cables along their lengths. Consistent with this model, deletion of AIP1 rescues the temperature-sensitive growth and loss of actin cable defects of tpm1Delta mutants.

Twinstar, the Drosophila homolog of cofilin/ADF, is required for planar cell polarity patterning

Planar cell polarity (PCP) is a level of tissue organization in which cells adopt a uniform orientation within the plane of an epithelium. The process of tissue polarization is likely to be initiated by an extracellular gradient. Thus, determining how cells decode and convert this graded information into subcellular asymmetries is key to determining how cells direct the reorganization of the cytoskeleton to produce uniformly oriented structures. Twinstar (Tsr), the Drosophila homolog of Cofilin/ADF (actin depolymerization factor), is a component of the cytoskeleton that regulates actin dynamics. We show here that various alleles of tsr produce PCP defects in the wing, eye and several other epithelia. In wings mutant for tsr, Frizzled (Fz) and Flamingo (Fmi) proteins do not properly localize to the proximodistal boundaries of cells. The correct asymmetric localization of these proteins instructs the actin cytoskeleton to produce one actin-rich wing hair at the distal-most vertex of each cell. These results argue that actin remodeling is not only required in the manufacture of wing hairs, but also in the PCP read-out that directs where a wing hair will be secreted.

Cofilin cross-bridges adjacent actin protomers and replaces part of the longitudinal F-actin interface

ADF/cofilins are abundant actin binding proteins critical to the survival of eukaryotic cells. Most ADF/cofilins bind both G and F-actin, sever the filaments and accelerate their treadmilling. These effects are linked to rearrangements of interprotomer contacts, changes in the mean twist, and filament destabilization by ADF/cofilin. Paradoxically, it was reported that under certain in vitro and in vivo conditions cofilin may stabilize actin filaments and nucleate their formation. Here, we show that yeast cofilin and human muscle cofilin (cofilin-2) accelerate the nucleation and elongation of ADP-F-actin and stabilize such filaments. Moreover, cofilin rescues the polymerization of the assembly incompetent tethramethyl rhodamine (TMR)-actin and T203C/C374S yeast mutant actin. Filaments of cofilin-decorated TMR-actin and unlabeled actin are indistinguishable, as revealed by electron microscopy and three-dimensional reconstruction. Our data suggest that ADF/cofilins play an active role in establishing new interprotomer interfaces in F-actin that substitute for disrupted (as in TMR-actin and mutant actin) or weakened (as in ADP-actin) longitudinal contacts in filaments.

The two Caenorhabditis elegans actin-depolymerizing factor/cofilin proteins differently enhance actin filament severing and depolymerization

Actin-depolymerizing factor (ADF)/cofilin enhances the turnover of actin filaments by two separable activities: filament severing and pointed-end depolymerization. Multicellular organisms express multiple ADF/cofilin isoforms in a tissue-specific manner, and the vertebrate proteins are grouped into ADFs and cofilins on the basis of their biochemical activity. A recent comparative study has shown that ADF has greater severing and depolymerizing activities than cofilin [Chen, H., Bernstein, B. W., Sneider, J. M., Boyle, J. A., Minamide, L. S., and Bamburg, J. R. (2004) Biochemistry 43, 7127-7142]. Here, we show that the two Caenorhabditis elegans ADF/cofilin isoforms exhibit different activities for severing and depolymerizing actin filaments. The ADF-like non-muscle isoform UNC-60A had greater activities to cause net depolymerization and inhibit polymerization than the cofilin-like muscle isoform UNC-60B. Surprisingly, UNC-60B exhibited much stronger severing activity than UNC-60A, which was the opposite of what was observed for vertebrate counterparts. Moreover, UNC-60B induced much faster pointed-end depolymerization of rabbit muscle actin than UNC-60A, while UNC-60A caused slightly faster depolymerization of C. elegans actin than UNC-60B. These results suggest that cofilin-like UNC-60B is kinetically more efficient in enhancing actin turnover than ADF-like UNC-60A, while ADF-like UNC-60A is suitable for maintaining higher concentrations of monomeric actin. These functional differences might be specifically adapted for different actin dynamics in muscle and non-muscle cells.

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.

A Plasmodium actin-depolymerizing factor that binds exclusively to actin monomers

ADF/cofilins (AC) are essential F- and G-actin binding proteins that modulate microfilament turnover. The genome of Plasmodium falciparum, the parasite causing malaria, contains two members of the AC family. Interestingly, P. falciparum ADF1 lacks the F-actin binding residues of the AC consensus. Reverse genetics in the rodent malaria model system suggest that ADF1 performs vital functions during the pathogenic red blood cell stages, whereas ADF2 is not present in these stages. We show that recombinant PfADF1 interacts with monomeric actin but does not bind to actin polymers. Although other AC proteins inhibit nucleotide exchange on monomeric actin, the Plasmodium ortholog stimulates nucleotide exchange. Thus, PfADF1 differs in its biochemical properties from previously known AC proteins and seems to promote turnover exclusively by interaction with actin monomers. These findings provide important insights into the low cytosolic abundance and unique turnover characteristics of actin polymers in parasites of the phylum Apicomplexa.

Structural and functional dissection of the Abp1 ADFH actin-binding domain reveals versatile in vivo adapter functions

Abp1 is a multidomain protein that regulates the Arp2/3 complex and links proteins involved in endocytosis to the actin cytoskeleton. All of the proposed cellular functions of Abp1 involve actin filament binding, yet the actin binding site(s) on Abp1 have not been identified, nor has the importance of actin binding for Abp1 localization and function in vivo been tested. Here, we report the crystal structure of the Saccharomyces cerevisiae Abp1 actin-binding actin depolymerizing factor homology (ADFH) domain and dissect its activities by mutagenesis. Abp1-ADFH domain and ADF/cofilin structures are similar, and they use conserved surfaces to bind actin; however, there are also key differences that help explain their differential effects on actin dynamics. Using point mutations, we demonstrate that actin binding is required for localization of Abp1 in vivo, the lethality caused by Abp1 overexpression, and the ability of Abp1 to activate Arp2/3 complex. Furthermore, we genetically uncouple ABP1 functions that overlap with SAC6, SLA1, and SLA2, showing they require distinct combinations of activities and interactions. Together, our data provide the first structural and functional view of the Abp1-actin interaction and show that Abp1 has distinct cellular roles as an adapter, linking different sets of ligands for each function.

Actin Dynamics: Growth from Dendritic Branches

The dendritic nucleation model was devised to explain the cycle of actin dynamics resulting in actin filament network assembly and disassembly in two contexts--at the leading edge of motile cells and in the actin comet tails of intracellular pathogenic bacteria and viruses. Due to the detailed nature of its biochemical predictions, the model has provided an excellent focus for subsequent experimentation. This review summarizes recent work on actin dynamics in the context of the dendritic nucleation model. One outcome of this research is the possibility that additional proteins, as well as the six proteins included in the original model, might increase the efficiency of dendritic nucleation or modify the resulting actin network. In addition, actin dynamics at the leading edge might be influenced by a second actin filament network, independent of dendritic nucleation.

Solution structure of coactosin reveals structural homology to ADF/cofilin family proteins

Coactosin is a small (MW approximately 15 kDa) evolutionarily conserved actin filament binding protein. It displays remote sequence homology to ADF/cofilin proteins and to the ADF-H domains of twinfilin and Abp1/drebrin. However, biochemical analyses have demonstrated that coactosin has a very different role in actin dynamics from the ones of ADF/cofilin, twinfilin or Abp1/drebrin. To elucidate the molecular mechanism of coactosin/actin interaction, we determined the three-dimensional structure of mouse coactosin by multidimensional NMR spectroscopy. We find that the coactosin structure is homologous to ADF/cofilin and to the ADF-H domains of twinfilin. Furthermore, the regions that have been shown to be important for actin filament interactions in ADF/cofilins are structurally conserved in coactosin suggesting that these two proteins interact with F-actin through a conserved interface. Our analysis also identifies key structural differences between these proteins that may account for the differences in biochemical activities and cellular roles of these proteins.

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.

A high-affinity interaction with ADP-actin monomers underlies the mechanism and in vivo function of Srv2/cyclase-associated protein

Cyclase-associated protein (CAP), also called Srv2 in Saccharomyces cerevisiae, is a conserved actin monomer-binding protein that promotes cofilin-dependent actin turnover in vitro and in vivo. However, little is known about the mechanism underlying this function. Here, we show that S. cerevisiae CAP binds with strong preference to ADP-G-actin (Kd 0.02 microM) compared with ATP-G-actin (Kd 1.9 microM) and competes directly with cofilin for binding ADP-G-actin. Further, CAP blocks actin monomer addition specifically to barbed ends of filaments, in contrast to profilin, which blocks monomer addition to pointed ends of filaments. The actin-binding domain of CAP is more extensive than previously suggested and includes a recently solved beta-sheet structure in the C-terminus of CAP and adjacent sequences. Using site-directed mutagenesis, we define evolutionarily conserved residues that mediate binding to ADP-G-actin and demonstrate that these activities are required for CAP function in vivo in directing actin organization and polarized cell growth. Together, our data suggest that in vivo CAP competes with cofilin for binding ADP-actin monomers, allows rapid nucleotide exchange to occur on actin, and then because of its 100-fold weaker binding affinity for ATP-actin compared with ADP-actin, allows other cellular factors such as profilin to take the handoff of ATP-actin and facilitate barbed end assembly.

In Vitro Activity Differences between Proteins of the ADF/Cofilin Family Define Two Distinct Subgroups†

The actin depolymerizing factor (ADF)/cofilins are an essential group of proteins that are important regulators of actin filament turnover in vivo. Although protists and yeasts express only a single member of this family, metazoans express two or more members in many cell types. In cells expressing both ADF and cofilin, differences have been reported in the regulation of their expression, their pH sensitivity, and their intracellular distribution. Each member has qualitatively similar interactions with actin, but quantitative differences have been noted. Here we compared quantitative differences between chick ADF and chick cofilin using several assays that measure G-actin binding, actin filament length distribution, and assembly/disassembly dynamics. Quantitative differences were measured in the critical concentrations of the complexes required for assembly, in the effects of nucleotide and divalent metal on actin monomer binding, in pH-dependent severing, in enhancement of filament minus end off-rates, and in steady-state filament length distributions generated in similar mixtures. Some of these assays were used to compare the activities of several ADF/cofilins from across phylogeny, most of which fall into one of two groups based upon their behavior. The ADF-like group has higher affinities for Mg(2+)-ATP-G-actin than the cofilin-like group and a greater pH-dependent depolymerizing activity.

Identification of functional residues on Caenorhabditis elegans actin-interacting protein 1 (UNC-78) for disassembly of actin depolymerizing factor/cofilin-bound actin filaments

Actin-interacting protein 1 (AIP1) is a WD40 repeat protein that enhances actin filament disassembly in the presence of actin-depolymerizing factor (ADF)/cofilin. AIP1 also caps the barbed end of ADF/cofilin-bound actin filament. However, the mechanism by which AIP1 interacts with ADF/cofilin and actin is not clearly understood. We determined the crystal structure of Caenorhabditis elegans AIP1 (UNC-78), which revealed 14 WD40 modules arranged in two seven-bladed beta-propeller domains. The structure allowed for the mapping of conserved surface residues, and mutagenesis studies identified five residues that affected the ADF/cofilin-dependent actin filament disassembly activity. Mutations of these residues, which reside in blades 3 and 4 in the N-terminal propeller domain, had significant effects on the disassembly activity but did not alter the barbed end capping activity. These data support a model in which this conserved surface of AIP1 plays a direct role in enhancing fragmentation/depolymerization of ADF/cofilin-bound actin filaments but not in barbed end capping.

Synchrotron radiolysis and mass spectrometry: a new approach to research on the actin cytoskeleton

Hydroxyl radicals generated from millisecond exposure of aqueous solutions to synchrotron X-rays react with proteins to yield stable oxidative modifications of solvent-accessible amino acid side chains. Following proteolysis, HPLC/MS analysis is performed to quantitate the side chain reactivities, and MS/MS analysis is used to identify the modification site(s). Side chain reactivity is shown to be correlated with solvent accessibility; thus the method provides detailed site-specific information about protein structure. The application of these techniques to the study of the actin cytoskeleton is described in detail, including defining the binding sites of monomeric actin with gelsolin segment-1, the actin monomer binding surface on cofilin, the divalent cation-dependent structure changes of monomeric actin, and the conformational changes associated with actin filamentous assembly.

Coordinated regulation of actin filament turnover by a high-molecular-weight Srv2/CAP complex, cofilin, profilin, and Aip1

BACKGROUND

Dynamic remodeling of the actin cytoskeleton requires rapid turnover of actin filaments, which is regulated in part by the actin filament severing/depolymerization factor cofilin/ADF. Two factors that cooperate with cofilin are Srv2/CAP and Aip1. Human CAP enhances cofilin-mediated actin turnover in vitro, but its biophysical properties have not been defined, and there has been no in vivo evidence reported for its role in turnover. Xenopus Aip1 forms a cofilin-dependent cap at filament barbed ends. It has been unclear how these diverse activities are coordinated in vivo.

RESULTS

Purified native yeast Srv2/CAP forms a high molecular weight structure comprised solely of actin and Srv2. The complex is linked to actin filaments via the SH3 domain of Abp1. Srv2 complex catalytically accelerates cofilin-dependent actin turnover by releasing cofilin from ADP-actin monomers and enhances the ability of profilin to stimulate nucleotide exchange on ADP-actin. Yeast Aip1 forms a cofilin-dependent filament barbed end cap, disrupted by the cof1-19 mutant. Genetic analyses show that specific combinations of activities mediated by cofilin, Srv2, Aip1, and capping protein are required in vivo.

CONCLUSIONS

We define two genetically and biochemically separable functions for cofilin in actin turnover. One is formation of an Aip1-cofilin cap at filament barbed ends. The other is cofilin-mediated severing/depolymerization of filaments, accelerated indirectly by Srv2 complex. We show that the Srv2 complex is a large multimeric structure and functions as an intermediate in actin monomer processing, converting cofilin bound ADP-actin monomers to profilin bound ATP-actin monomers and recycling cofilin for new rounds of filament depolymerization.

Cofilin (ADF) Affects Lateral Contacts in F-actin

The effect of yeast cofilin on lateral contacts between protomers of yeast and skeletal muscle actin filaments was examined in solution. These contacts are presumably stabilized by the interactions of loop 262-274 of one protomer with two other protomers on the opposite strand in F-actin. Cofilin inhibited several-fold the rate of interstrand disulfide cross-linking between Cys265 and Cys374 in yeast S265C mutant F-actin, but enhanced excimer formation between pyrene probes attached to these cysteine residues. The possibility that these effects are due to a translocation of the C terminus of actin by cofilin was ruled out by measurements of fluorescence resonance energy transfer (FRET) from tryptophan residues and ATP to acceptor probes at Cys374. Such measurements did not reveal cofilin-induced changes in FRET efficiency, suggesting that changes in Cys265-Cys374 cross-linking and excimer formation stem from the perturbation of loop 262-274 by cofilin. Changes in lateral interactions in F-actin were indicated also by the cofilin-induced partial release of rhodamine phalloidin. Disulfide cross-linking of S265C yeast F-actin inhibited strongly and reversibly the release of rhodamine phalloidin by cofilin. Overall, this study provides solution evidence for the weakening of lateral interactions in F-actin by cofilin.

Cofilin peptide homologs interfere with immunological synapse formation and T cell activation

The formation of supramolecular activation clusters within the immunological synapse, crucial for sustained signaling and T lymphocyte activation, requires costimulation-dependent reorganization of the actin cytoskeleton. Here we have identified the actin-remodeling protein cofilin as a key player in this process. Cell-permeable peptides that block costimulation-induced cofilin/F-actin interactions in untransformed human T lymphocytes impair receptor capping and immunological synapse formation at the interface between T cells and antigen-presenting cells. As a consequence, T cell activation, as measured by cytokine production and proliferation, is inhibited.

Regulation of the Actin Cytoskeleton by PI(4,5)P2 and PI(3,4,5)P3

The actin cytoskeleton is fundamental for various motile and morphogenetic processes in cells. The structure and dynamics of the actin cytoskeleton are regulated by a wide array of actin-binding proteins, whose activities are controlled by various signal transduction pathways. Recent studies have shown that certain membrane phospholipids, especially PI(4,5)P2 and PI(3,4,5)P3, regulate actin filament assembly in cells and in cell extracts. PI(4,5)P2 appears to be a general regulator of actin polymerization at the plasma membrane or at membrane microdomains, whereas PI(3,4,5)P3 promotes the assembly of specialized actin filament structures in response to some growth factors. Biochemical studies have demonstrated that the activities of many proteins promoting actin assembly are upregulated by PI(4,5)P2, whereas proteins that inhibit actin assembly or promote filament disassembly are down-regulated by PI(4,5)P2. PI(3,4,5)P3 promotes its effects on the actin cytoskeleton mainly through activation of the Rho family of small GTPases. In addition to their effects on actin dynamics, both PI(4,5)P2 and PI(3,4,5)P3 promote the formation of specific actin filament structures through activation/inactivation of actin filament cross-linking proteins and proteins that mediate cytoskeleton-plasma membrane interactions.

A conserved mechanism for Bni1- and mDia1-induced actin assembly and dual regulation of Bni1 by Bud6 and profilin

Formins have conserved roles in cell polarity and cytokinesis and directly nucleate actin filament assembly through their FH2 domain. Here, we define the active region of the yeast formin Bni1 FH2 domain and show that it dimerizes. Mutations that disrupt dimerization abolish actin assembly activity, suggesting that dimers are the active state of FH2 domains. The Bni1 FH2 domain protects growing barbed ends of actin filaments from vast excesses of capping protein, suggesting that the dimer maintains a persistent association during elongation. This is not a species-specific mechanism, as the activities of purified mammalian formin mDia1 are identical to those of Bni1. Further, mDia1 partially complements BNI1 function in vivo, and expression of a dominant active mDia1 construct in yeast causes similar phenotypes to dominant active Bni1 constructs. In addition, we purified the Bni1-interacting half of the cell polarity factor Bud6 and found that it binds specifically to actin monomers and, like profilin, promotes rapid nucleotide exchange on actin. Bud6 and profilin show additive stimulatory effects on Bni1 activity and have a synthetic lethal genetic interaction in vivo. From these results, we propose a model in which Bni1 FH2 dimers nucleate and processively cap the elongating barbed end of the actin filament, and Bud6 and profilin generate a local flux of ATP-actin monomers to promote actin assembly.

Solution structure of human cofilin: actin binding, pH sensitivity, and relationship to actin-depolymerizing factor

Human actin-depolymerizing factor (ADF) and cofilin are pH-sensitive, actin-depolymerizing proteins. Although 72% identical in sequence, ADF has a much higher depolymerizing activity than cofilin at pH 8. To understand this, we solved the structure of human cofilin using nuclear magnetic resonance and compared it with human ADF. Important sequence differences between vertebrate ADF/cofilins were correlated with unique structural determinants in the F-actin-binding site to account for differences in biochemical activities of the two proteins. Cofilin has a short beta-strand at the C terminus, not found in ADF, which packs against strands beta3/beta4, changing the environment around Lys96, a residue essential for F-actin binding. A salt bridge involving His133 and Asp98 (Glu98 in ADF) may explain the pH sensitivity of human cofilin and ADF; these two residues are fully conserved in vertebrate ADF/cofilins. Chemical shift perturbations identified residues that (i) differ in their chemical environments between wild type cofilin and mutants S3D, which has greatly reduced G-actin binding, and K96Q, which does not bind F-actin; (ii) are affected when G-actin binds cofilin; and (iii) are affected by pH change from 6 to 8. Many residues affected by G-actin binding also show perturbation in the mutants or in response to pH. Our evidence suggests the involvement of residues 133-138 of strand beta5 in all of the activities examined. Because residues in beta5 are perturbed by mutations that affect both G-actin and F-actin binding, this strand forms a "boundary" or "bridge" between the proposed F- and G-actin-binding sites.

Regulation of Actin Filament Dynamics by Actin Depolymerizing Factor/Cofilin and Actin-Interacting Protein 1: New Blades for Twisted Filaments

Actin depolymerizing factor (ADF)/cofilin enhances turnover of actin filaments by severing and depolymerizing filaments. A number of proteins functionally interact with ADF/cofilin to modulate the dynamics of actin filaments. Actin-interacting protein 1 (AIP1) has emerged as a conserved WD-repeat protein that specifically enhances ADF/cofilin-induced actin dynamics. Interaction of AIP1 with actin was originally characterized by a yeast two-hybrid system. However, biochemical studies revealed its unique activity on ADF/cofilin-bound actin filaments. AIP1 alone has negligible effects on actin filament dynamics, whereas in the presence of ADF/cofilin, AIP1 enhances filament fragmentation by capping ends of severed filaments. Studies in model organisms demonstrated that AIP1 genetically interacts with ADF/cofilin and participates in several actin-dependent cellular events. The crystal structure of AIP1 revealed its unique structure with two seven-bladed beta-propeller domains. Thus, AIP1 is a new class of actin regulatory proteins that selectively enhances ADF/cofilin-dependent actin filament dynamics.

Actin filament disassembling activity of Caenorhabditis elegans actin-interacting protein 1 (UNC-78) is dependent on filament binding by a specific ADF/cofilin isoform

Actin-interacting protein 1 (AIP1) is a conserved WD-repeat protein that enhances actin filament disassembly only in the presence of actin depolymerizing factor (ADF)/cofilin. In the nematode Caenorhabditis elegans, an AIP1 ortholog is encoded by the unc-78 gene that is required for organized assembly of muscle actin filaments. We produced bacterially expressed UNC-78 protein and found that it enhances actin filament disassembly preferentially in the presence of a specific ADF/cofilin isoform. Extensive and rapid filament disassembly by UNC-78 was observed in the presence of UNC-60B, a muscle-specific C. elegans ADF/cofilin isoform. UNC-78 also reduced the rate of spontaneous polymerization and enhanced subunit dissociation from filaments in the presence of UNC-60B. However, in the presence of UNC-60A, a non-muscle C. elegans ADF/cofilin isoform, UNC-78 only slightly enhanced filament disassembly. Interestingly, UNC-78 failed to enhance disassembly by mouse muscle-type cofilin. Using mutant forms of UNC-60B, we demonstrated that the F-actin-specific binding site of UNC-60B at the C terminus is required for filament disassembly by UNC-78. UNC-78 was expressed in body wall muscle and co-localized with actin where UNC-60B was also present. Surprisingly, UNC-78 was co-localized with actin in unc-60B null mutants, suggesting that the AIP1-actin interaction is not dependent on ADF/cofilin in muscle. These results suggest that UNC-78 closely collaborates with UNC-60B to regulate actin dynamics in muscle cells.

Specific requirement for two ADF/cofilin isoforms in distinct actin-dependent processes in Caenorhabditis elegans

Actin depolymerizing factor (ADF)/cofilin is an essential enhancer of actin turnover. Multicellular organisms express multiple ADF/cofilin isoforms in different patterns of tissue distribution. However, the functional significance of different ADF/cofilin isoforms is not understood. The Caenorhabditis elegans unc-60 gene generates two ADF/cofilins, UNC-60A and UNC-60B, by alternative splicing. These two ADF/cofilin proteins have different effects on actin dynamics in vitro, but their functional difference in vivo remains unclear. Here, we demonstrate that the two isoforms are expressed in different tissues and are required for distinct morphogenetic processes. UNC-60A was ubiquitously expressed in most embryonic cells and enriched in adult gonads, intestine and oocytes. In contrast, UNC-60B was specifically expressed in the body wall muscle, vulva and spermatheca. RNA interference of UNC-60A caused embryonic lethality with variable defects in cytokinesis and developmental patterning. In severely affected embryos, a cleavage furrow was formed and progressed but reversed before completion of the cleavage. Also, in some affected embryos, positioning of the blastomeres became abnormal, which resulted in embryonic arrest. In contrast, an unc-60B-null mutant was homozygous viable, underwent normal early embryogenesis and caused disorganization of actin filaments specifically in body wall muscle. These results suggest that the ADF/cofilin isoforms play distinct roles in specific aspects of actin reorganization in vivo.

Latrunculin B or ATP depletion induces cofilin-dependent translocation of actin into nuclei of mast cells

Increasing cellular G-actin, using latrunculin B, in either intact or permeabilized rat peritoneal mast cells, caused translocation of both actin and an actin regulatory protein, cofilin, into the nuclei. The effect was not associated with an increase in the proportion of apoptotic cells. The major part of the nuclear actin was not stained by rhodamine-phalloidin but could be visualized with an actin antibody, indicating its monomeric or a conformationally distinct state, e.g. cofilin-decorated filaments. Introduction of anti-cofilin into permeabilized cells inhibited nuclear actin accumulation, implying that an active, cofilin-dependent, import exists in this system. Nuclear actin was localized outside the ethidium bromide-stained region, in the extrachromosomal nuclear domain. In permeabilized cells, the appearance of nuclear actin and cofilin was not significantly affected by increasing [Ca(2+)] and/or adding guanosine 5'-O-(3-thiotriphosphate), but was greatly promoted when ATP was withdrawn. Similarly, ATP depletion in intact cells also induced nuclear actin accumulation. In contrast to the effects of latrunculin B, ATP depletion was associated with an increase in cortical F-actin. Our results suggest that the presence of actin in the nucleus may be required for certain stress-induced responses and that cofilin is essential for the nuclear import of actin.

Actin binding proteins: regulation of cytoskeletal microfilaments

The actin cytoskeleton is a complex structure that performs a wide range of cellular functions. In 2001, significant advances were made to our understanding of the structure and function of actin monomers. Many of these are likely to help us understand and distinguish between the structural models of actin microfilaments. In particular, 1) the structure of actin was resolved from crystals in the absence of cocrystallized actin binding proteins (ABPs), 2) the prokaryotic ancestral gene of actin was crystallized and its function as a bacterial cytoskeleton was revealed, and 3) the structure of the Arp2/3 complex was described for the first time. In this review we selected several ABPs (ADF/cofilin, profilin, gelsolin, thymosin beta4, DNase I, CapZ, tropomodulin, and Arp2/3) that regulate actin-driven assembly, i.e., movement that is independent of motor proteins. They were chosen because 1) they represent a family of related proteins, 2) they are widely distributed in nature, 3) an atomic structure (or at least a plausible model) is available for each of them, and 4) each is expressed in significant quantities in cells. These ABPs perform the following cellular functions: 1) they maintain the population of unassembled but assembly-ready actin monomers (profilin), 2) they regulate the state of polymerization of filaments (ADF/cofilin, profilin), 3) they bind to and block the growing ends of actin filaments (gelsolin), 4) they nucleate actin assembly (gelsolin, Arp2/3, cofilin), 5) they sever actin filaments (gelsolin, ADF/cofilin), 6) they bind to the sides of actin filaments (gelsolin, Arp2/3), and 7) they cross-link actin filaments (Arp2/3). Some of these ABPs are essential, whereas others may form regulatory ternary complexes. Some play crucial roles in human disorders, and for all of them, there are good reasons why investigations into their structures and functions should continue.