Actophorin preferentially binds monomeric ADP-actin over ATP-bound actin: consequences for cell locomotion

Distinct functional constraints driving conservation of the cofilin N-terminal regulatory tail

Cofilin family proteins have essential roles in remodeling the cytoskeleton through filamentous actin depolymerization and severing. The short, unstructured N-terminal region of cofilin is critical for actin binding and harbors the major site of inhibitory phosphorylation. Atypically for a disordered sequence, the N-terminal region is highly conserved, but specific aspects driving this conservation are unclear. Here, we screen a library of 16,000 human cofilin N-terminal sequence variants for their capacity to support growth in S. cerevisiae in the presence or absence of the upstream regulator LIM kinase. Results from the screen and biochemical analysis of individual variants reveal distinct sequence requirements for actin binding and regulation by LIM kinase. LIM kinase recognition only partly explains sequence constraints on phosphoregulation, which are instead driven to a large extent by the capacity for phosphorylation to inactivate cofilin. We find loose sequence requirements for actin binding and phosphoinhibition, but collectively they restrict the N-terminus to sequences found in natural cofilins. Our results illustrate how a phosphorylation site can balance potentially competing sequence requirements for function and regulation.

Actin's C-terminus coordinates actin structural changes and functions

Actin is essential to eukaryotic cellular processes. Actin's C-terminus appears to play a direct role in modulating actin's structure and properties, facilitating the binding and function of actin-binding proteins (ABPs). The structural and functional characterization of filamentous actin's C-terminus has been impeded by its inherent flexibility, as well as actin's resistance to crystallization for x-ray diffraction and the historical resolution constraints associated with electron microscopy. Many biochemical studies have established that actin's C-terminus must retain its flexibility and structural integrity to modulate actin's structure and functions. For example, C-terminal structural changes are known to affect nucleotide binding and exchange, as well as propagate actin structural changes throughout extensive allosteric networks, facilitating the binding and function of ABPs. Advances in electron microscopy have resulted in high-resolution structures of filamentous actin, providing insights into subtle structural changes that are mediated by actin's C-terminus. Here, we review existing knowledge establishing the importance of actin's C-terminus within actin structural changes and functions and discuss how modern structural characterization techniques provide the tools to understand the role of actin's C-terminus in cellular processes.

Distinct functional constraints driving conservation of the cofilin N-terminal regulatory tail

Cofilin family proteins have essential roles in remodeling the cytoskeleton through filamentous actin depolymerization and severing. The short unstructured N-terminal region of cofilin is critical for actin binding and harbors the major site of inhibitory phosphorylation. Atypically for a disordered sequence, the N-terminal region is highly conserved, but the aspects of cofilin functionality driving this conservation are not clear. Here, we screened a library of 16,000 human cofilin N-terminal sequence variants for their capacity to support growth in S. cerevisiae in the presence or absence of the upstream regulator LIM kinase. Results from the screen and subsequent biochemical analysis of individual variants revealed distinct sequence requirements for actin binding and regulation by LIM kinase. While the presence of a serine, rather than threonine, phosphoacceptor residue was essential for phosphorylation by LIM kinase, the native cofilin N-terminus was otherwise a suboptimal LIM kinase substrate. This circumstance was not due to sequence requirements for actin binding and severing, but rather appeared primarily to maintain the capacity for phosphorylation to inactivate cofilin. Overall, the individual sequence requirements for cofilin function and regulation were remarkably loose when examined separately, but collectively restricted the N-terminus to sequences found in natural cofilins. Our results illustrate how a regulatory phosphorylation site can balance potentially competing sequence requirements for function and regulation.

Unravelling the Biology of EhActo as the First Cofilin From Entamoeba histolytica

Actin-depolymerising factors (ADF) are a known family of proteins that regulate actin dynamics. Actin regulation is critical for primitive eukaryotes since it drives their key cellular processes. Entamoeba histolytica, a protist human pathogen harbours eleven proteins within this family, however, with no actin depolymerising protein reported to date. We present here the NMR model of EhActo, the first Cofilin from E. histolytica that severs actin filaments and also participates in cellular events like phagocytosis and pseudopod formation. The model typically represents the ADF-homology domain compared to other cofilins. Uniquely, EhActo lacks the critical Serine3 residue present in all known actophorins mediating its phospho-regulation. The second mode of regulation that cofilin’s are subjected to is via their interaction with 14-3-3 proteins through the phosphorylated Serine residue and a consensus binding motif. We found a unique interaction between EhActo and 14-3-3 without the presence of the consensus motif or the phosphorylated Serine. These interesting results present unexplored newer mechanisms functional in this pathogen to regulate actophorin. Through our structural and biochemical studies we have deciphered the mechanism of action of EhActo, implicating its role in amoebic biology.

Actin(g) on mitochondria - a role for cofilin1 in neuronal cell death pathways

Actin dynamics, the coordinated assembly and disassembly of actin filaments (F-actin), are essential for fundamental cellular processes, including cell shaping and motility, cell division or organelle transport. Recent studies highlighted a novel role for actin dynamics in the regulation of mitochondrial morphology and function, for example, through mitochondrial recruitment of dynamin-related protein 1 (Drp1), a key factor in the mitochondrial fission machinery. Mitochondria are dynamic organelles, and permanent fission and fusion is essential to maintain their function in energy metabolism, calcium homeostasis and regulation of reactive oxygen species (ROS). Here, we summarize recent insights into the emerging role of cofilin1, a key regulator of actin dynamics, for mitochondrial shape and function under physiological conditions and during cellular stress, respectively. This is of peculiar importance in neurons, which are particularly prone to changes in actin regulation and mitochondrial integrity and function. In neurons, cofilin1 may contribute to degenerative processes through formation of cofilin-actin rods, and through enhanced mitochondrial fission, mitochondrial membrane permeabilization, and the release of cytochrome c. Overall, mitochondrial impairment induced by dysfunction of actin-regulating proteins such as cofilin1 emerge as important mechanisms of neuronal death with relevance to acute brain injury and neurodegenerative diseases, such as Parkinson's or Alzheimer's disease.

Towards a structural understanding of the remodeling of the actin cytoskeleton

Actin filaments (F-actin) are a key component of eukaryotic cells. Whether serving as a scaffold for myosin or using their polymerization to push onto cellular components, their function is always related to force generation. To control and fine-tune force production, cells have a large array of actin-binding proteins (ABPs) dedicated to control every aspect of actin polymerization, filament localization, and their overall mechanical properties. Although great advances have been made in our biochemical understanding of the remodeling of the actin cytoskeleton, the structural basis of this process is still being deciphered. In this review, we summarize our current understanding of this process. We outline how ABPs control the nucleation and disassembly, and how these processes are affected by the nucleotide state of the filaments. In addition, we highlight recent advances in the understanding of actomyosin force generation, and describe recent advances brought forward by the developments of electron cryomicroscopy.

Functional characterisation of the actin-depolymerising factor from the apicomplexan Neospora caninum (NcADF)

Neospora caninum is an apicomplexan parasite that causes infectious abortion in cows. As an obligate intracellular parasite, N. caninum requires a host cell environment to survive and replicate. The locomotion and invasion mechanisms of apicomplexan parasites are centred on the actin-myosin system to propel the parasite forwards and into the host cell. The functions of actin, an intrinsically dynamic protein, are modulated by actin-binding proteins (ABPs). Actin-depolymerising factor (ADF) is a ubiquitous ABP responsible for accelerating actin turnover in eukaryotic cells and is one of the few known conserved ABPs from apicomplexan parasites. Apicomplexan ADFs have nonconventional properties compared with ADF/cofilins from higher eukaryotes. In the present paper, we characterised the ADF from N. caninum (NcADF) using computational and in vitro biochemical approaches to investigate its function in rabbit muscle actin dynamics. Our predicted computational tertiary structure of NcADF demonstrated a conserved structure and phylogeny with respect to other ADF/cofilins, although certain differences in filamentous actin (F-actin) binding sites were present. The activity of recombinant NcADF on heterologous actin was regulated in part by pH and the presence of inorganic phosphate. In addition, our data suggest a comparatively weak disassembly of F-actin by NcADF. Taken together, the data presented herein represent a contribution to the field towards the understanding of the role of ADF in N. caninum and a comparative analysis of ABPs in the phylum Apicomplexa.

Functions of actin-interacting protein 1 (AIP1)/WD repeat protein 1 (WDR1) in actin filament dynamics and cytoskeletal regulation

Actin-depolymerizing factor (ADF)/cofilin and actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), are conserved among eukaryotes and play critical roles in dynamic reorganization of the actin cytoskeleton. AIP1 preferentially promotes disassembly of ADF/cofilin-decorated actin filaments but exhibits minimal effects on bare actin filaments. Therefore, AIP1 has been often considered to be an ancillary co-factor of ADF/cofilin that merely boosts ADF/cofilin activity level. However, genetic and cell biological studies show that AIP1 deficiency often causes lethality or severe abnormalities in multiple tissues and organs including muscle, epithelia, and blood, suggesting that AIP1 is a major regulator of many biological processes that depend on actin dynamics. This review summarizes recent progress in studies on the biochemical mechanism of actin filament severing by AIP1 and in vivo functions of AIP1 in model organisms and human diseases.

ADF/Cofilin Accelerates Actin Dynamics by Severing Filaments and Promoting Their Depolymerization at Both Ends

Actin-depolymerizing factor (ADF)/cofilins contribute to cytoskeletal dynamics by promoting rapid actin filament disassembly. In the classical view, ADF/cofilin sever filaments, and capping proteins block filament barbed ends whereas pointed ends depolymerize, at a rate that is still debated. Here, by monitoring the activity of the three mammalian ADF/cofilin isoforms on individual skeletal muscle and cytoplasmic actin filaments, we directly quantify the reactions underpinning filament severing and depolymerization from both ends. We find that, in the absence of monomeric actin, soluble ADF/cofilin can associate with bare filament barbed ends to accelerate their depolymerization. Compared to bare filaments, ADF/cofilin-saturated filaments depolymerize faster from their pointed ends and slower from their barbed ends, resulting in similar depolymerization rates at both ends. This effect is isoform specific because depolymerization is faster for ADF- than for cofilin-saturated filaments. We also show that, unexpectedly, ADF/cofilin-saturated filaments qualitatively differ from bare filaments: their barbed ends are very difficult to cap or elongate, and consequently undergo depolymerization even in the presence of capping protein and actin monomers. Such depolymerizing ADF/cofilin-decorated barbed ends are produced during 17% of severing events. They are also the dominant fate of filament barbed ends in the presence of capping protein, because capping allows growing ADF/cofilin domains to reach the barbed ends, thereby promoting their uncapping and subsequent depolymerization. Our experiments thus reveal how ADF/cofilin, together with capping protein, control the dynamics of actin filament barbed and pointed ends. Strikingly, our results propose that significant barbed-end depolymerization may take place in cells.

The effect of mouse twinfilin-1 on the structure and dynamics of monomeric actin

The effect of twinfilin-1 on the structure and dynamics of monomeric actin was investigated with fluorescence spectroscopy and differential scanning calorimetry experiments. Fluorescence anisotropy measurements proved that G-actin and twinfilin-1 could form a complex. Due to the formation of the complexes the dissociation of the nucleotide slowed down from the nucleotide-binding pocket of actin. Fluorescence quenching experiments showed that the accessibility of the actin bound ε-ATP decreased in the presence of twinfilin-1. Temperature dependent fluorescence resonance energy transfer and differential scanning calorimetry experiments revealed that the protein matrix of actin becomes more rigid and more heat resistant in the presence of twinfilin-1. The results suggest that the nucleotide binding cleft shifted into a more closed and stable conformational state of actin in the presence of twinfilin-1.

Actin dynamics and cofilin-actin rods in alzheimer disease

Cytoskeletal abnormalities and synaptic loss, typical of both familial and sporadic Alzheimer disease (AD), are induced by diverse stresses such as neuroinflammation, oxidative stress, and energetic stress, each of which may be initiated or enhanced by proinflammatory cytokines or amyloid-β (Aβ) peptides. Extracellular Aβ-containing plaques and intracellular phospho-tau-containing neurofibrillary tangles are postmortem pathologies required to confirm AD and have been the focus of most studies. However, AD brain, but not normal brain, also have increased levels of cytoplasmic rod-shaped bundles of filaments composed of ADF/cofilin-actin in a 1:1 complex (rods). Cofilin, the major ADF/cofilin isoform in mammalian neurons, severs actin filaments at low cofilin/actin ratios and stabilizes filaments at high cofilin/actin ratios. It binds cooperatively to ADP-actin subunits in F-actin. Cofilin is activated by dephosphorylation and may be oxidized in stressed neurons to form disulfide-linked dimers, required for bundling cofilin-actin filaments into stable rods. Rods form within neurites causing synaptic dysfunction by sequestering cofilin, disrupting normal actin dynamics, blocking transport, and exacerbating mitochondrial membrane potential loss. Aβ and proinflammatory cytokines induce rods through a cellular prion protein-dependent activation of NADPH oxidase and production of reactive oxygen species. Here we review recent advances in our understanding of cofilin biochemistry, rod formation, and the development of cognitive deficits. We will then discuss rod formation as a molecular pathway for synapse loss that may be common between all three prominent current AD hypotheses, thus making rods an attractive therapeutic target. © 2016 Wiley Periodicals, Inc.

Specification of Architecture and Function of Actin Structures by Actin Nucleation Factors

The actin cytoskeleton is essential for diverse processes in mammalian cells; these processes range from establishing cell polarity to powering cell migration to driving cytokinesis to positioning intracellular organelles. How these many functions are carried out in a spatiotemporally regulated manner in a single cytoplasm has been the subject of much study in the cytoskeleton field. Recent work has identified a host of actin nucleation factors that can build architecturally diverse actin structures. The biochemical properties of these factors, coupled with their cellular location, likely define the functional properties of actin structures. In this article, we describe how recent advances in cell biology and biochemistry have begun to elucidate the role of individual actin nucleation factors in generating distinct cellular structures. We also consider how the localization and orientation of actin nucleation factors, in addition to their kinetic properties, are critical to their ability to build a functional actin cytoskeleton.

The other side of the coin: functional and structural versatility of ADF/cofilins

Several cellular processes rely on the fine tuning of actin cytoskeleton. A central component in the regulation of this cellular machinery is the ADF-H domain proteins. Despite sharing the same domain, ADF-H domain proteins produce a diverse functional landscape in the regulation of the actin cytoskeleton. Recent findings emphasize that the functional and structural features of these proteins can differ not only between ADF-H families but even within the same family. The structural and evolutional background of this functional diversity is poorly understood. This review focuses on the specific functional characteristics of ADF-H domain proteins and how these features can be linked to structural differences in the ADF-H domain and also to different conformational transitions in actin. In the light of recent discoveries we pay special attention to the ADF/cofilin proteins to find tendencies along which the functional and structural diversification is governed through the evolution.

ATP-dependent regulation of actin monomer-filament equilibrium by cyclase-associated protein and ADF/cofilin

CAP (cyclase-associated protein) is a conserved regulator of actin filament dynamics. In the nematode Caenorhabditis elegans, CAS-1 is an isoform of CAP that is expressed in striated muscle and regulates sarcomeric actin assembly. In the present study, we report that CAS-2, a second CAP isoform in C. elegans, attenuates the actin-monomer-sequestering effect of ADF (actin depolymerizing factor)/cofilin to increase the steady-state levels of actin filaments in an ATP-dependent manner. CAS-2 binds to actin monomers without a strong preference for either ATP- or ADP-actin. CAS-2 strongly enhances the exchange of actin-bound nucleotides even in the presence of UNC-60A, a C. elegans ADF/cofilin that inhibits nucleotide exchange. UNC-60A induces the depolymerization of actin filaments and sequesters actin monomers, whereas CAS-2 reverses the monomer-sequestering effect of UNC-60A in the presence of ATP, but not in the presence of only ADP or the absence of ATP or ADP. A 1:100 molar ratio of CAS-2 to UNC-60A is sufficient to increase actin filaments. CAS-2 has two independent actin-binding sites in its N- and C-terminal halves, and the C-terminal half is necessary and sufficient for the observed activities of the full-length CAS-2. These results suggest that CAS-2 (CAP) and UNC-60A (ADF/cofilin) are important in the ATP-dependent regulation of the actin monomer-filament equilibrium.

Glia maturation factor (GMF) interacts with Arp2/3 complex in a nucleotide state-dependent manner

Glia maturation factor (GMF) is a member of the actin-depolymerizing factor (ADF)/cofilin family. ADF/cofilin promotes disassembly of aged actin filaments, whereas GMF interacts specifically with Arp2/3 complex at branch junctions and promotes debranching. A distinguishing feature of ADF/cofilin is that it binds tighter to ADP-bound than to ATP-bound monomeric or filamentous actin. The interaction is also regulated by phosphorylation at Ser-3 of mammalian cofilin, which inhibits binding to actin. However, it is unknown whether these two factors play a role in the interaction of GMF with Arp2/3 complex. Here we show using isothermal titration calorimetry that mammalian GMF has very low affinity for ATP-bound Arp2/3 complex but binds ADP-bound Arp2/3 complex with 0.7 μM affinity. The phosphomimetic mutation S2E in GMF inhibits this interaction. GMF does not bind monomeric ATP- or ADP-actin, confirming its specificity for Arp2/3 complex. We further show that mammalian Arp2/3 complex nucleation activated by the WCA region of the nucleation-promoting factor N-WASP is not affected by GMF, whereas nucleation activated by the WCA region of WAVE2 is slightly inhibited at high GMF concentrations. Together, the results suggest that GMF functions by a mechanism similar to that of other ADF/cofilin family members, displaying a preference for ADP-Arp2/3 complex and undergoing inhibition by phosphorylation of a serine residue near the N terminus. Arp2/3 complex nucleation occurs in the ATP state, and nucleotide hydrolysis promotes debranching, suggesting that the higher affinity of GMF for ADP-Arp2/3 complex plays a physiological role by promoting debranching of aged branch junctions without interfering with Arp2/3 complex nucleation.

Effects of actin-binding proteins on the thermal stability of monomeric actin

Differential scanning calorimetry (DSC) was applied to investigate the thermal unfolding of rabbit skeletal muscle G-actin in its complexes with actin-binding proteins, cofilin, twinfilin, and profilin. The results show that the effects of these proteins on the thermal stability of G-actin depend on the nucleotide, ATP or ADP, bound in the nucleotide-binding cleft between actin subdomains 2 and 4. Interestingly, cofilin binding stabilizes both ATP-G-actin and ADP-G-actin, whereas twinfilin increases the thermal stability of the ADP-G-actin but not that of the ATP-G-actin. By contrast, profilin strongly decreases the thermal stability of the ATP-G-actin but has no appreciable effect on the ADP-G-actin. Comparison of these DSC results with literature data reveals a relationship between the effects of actin-binding proteins on the thermal unfolding of G-actin, stabilization or destabilization, and their effects on the rate of nucleotide exchange in the nucleotide-binding cleft, decrease or increase. These results suggest that the thermal stability of G-actin depends, at least partially, on the conformation of the nucleotide-binding cleft: the actin molecule is more stable when the cleft is closed, while an opening of the cleft leads to significant destabilization of G-actin. Thus, DSC studies of the thermal unfolding of G-actin can provide new valuable information about the conformational changes induced by actin-binding proteins in the actin molecule.

Rapid nucleotide exchange renders Asp-11 mutant actins resistant to depolymerizing activity of cofilin, leading to dominant toxicity in vivo

Conserved Asp-11 of actin is a part of the nucleotide binding pocket, and its mutation to Gln is dominant lethal in yeast, whereas the mutation to Asn in human α-actin dominantly causes congenital myopathy. To elucidate the molecular mechanism of those dominant negative effects, we prepared Dictyostelium versions of D11N and D11Q mutant actins and characterized them in vitro. D11N and D11Q actins underwent salt-dependent reversible polymerization, although the resultant polymerization products contained small anomalous structures in addition to filaments of normal appearance. Both monomeric and polymeric D11Q actin released bound nucleotides more rapidly than the wild type, and intriguingly, both monomeric and polymeric D11Q actins hardly bound cofilin. The deficiency in cofilin binding can be explained by rapid exchange of bound nucleotide with ATP in solution, because cofilin does not bind ATP-bound actin. Copolymers of D11Q and wild type actins bound cofilin, but cofilin-induced depolymerization of the copolymers was slower than that of wild type filaments, which may presumably be the primary reason why this mutant actin is dominantly toxic in vivo. Purified D11N actin was unstable, which made its quantitative biochemical characterization difficult. However, monomeric D11N actin released nucleotides even faster than D11Q, and we speculate that D11N actin also exerts its toxic effects in vivo through a defective interaction with cofilin. We have recently found that two other dominant negative actin mutants are also defective in cofilin binding, and we propose that the defective cofilin binder is a major class of dominant negative actin mutants.

Cyclase-associated protein (CAP) acts directly on F-actin to accelerate cofilin-mediated actin severing across the range of physiological pH

Fast actin depolymerization is necessary for cells to rapidly reorganize actin filament networks. Utilizing a Listeria fluorescent actin comet tail assay to monitor actin disassembly rates, we observed that although a mixture of actin disassembly factors (cofilin, coronin, and actin-interacting protein 1 is sufficient to disassemble actin comet tails in the presence of physiological G-actin concentrations this mixture was insufficient to disassemble actin comet tails in the presence of physiological F-actin concentrations. Using biochemical complementation, we purified cyclase-associated protein (CAP) from thymus extracts as a factor that protects against the inhibition of excess F-actin. CAP has been shown to participate in actin dynamics but has been thought to act by liberating cofilin from ADP·G-actin monomers to restore cofilin activity. However, we found that CAP augments cofilin-mediated disassembly by accelerating the rate of cofilin-mediated severing. We also demonstrated that CAP acts directly on F-actin and severs actin filaments at acidic, but not neutral, pH. At the neutral pH characteristic of cytosol in most mammalian cells, we demonstrated that neither CAP nor cofilin are capable of severing actin filaments. However, the combination of CAP and cofilin rapidly severed actin at all pH values across the physiological range. Therefore, our results reveal a new function for CAP in accelerating cofilin-mediated actin filament severing and provide a mechanism through which cells can maintain high actin turnover rates without having to alkalinize cytosol, which would affect many biochemical reactions beyond actin depolymerization.

Actin-depolymerizing factor homology domain: a conserved fold performing diverse roles in cytoskeletal dynamics

Actin filaments form contractile and protrusive structures that play central roles in many processes such as cell migration, morphogenesis, endocytosis, and cytokinesis. During these processes, the dynamics of the actin filaments are precisely regulated by a large array of actin-binding proteins. The actin-depolymerizing factor homology (ADF-H) domain is a structurally conserved protein motif, which promotes cytoskeletal dynamics by interacting with monomeric and/or filamentous actin, and with the Arp2/3 complex. Despite their structural homology, the five classes of ADF-H domain proteins display distinct biochemical activities and cellular roles, only parts of which are currently understood. ADF/cofilin promotes disassembly of aged actin filaments, whereas twinfilin inhibits actin filament assembly via sequestering actin monomers and interacting with filament barbed ends. GMF does not interact with actin, but instead binds Arp2/3 complex and promotes dissociation of Arp2/3-mediated filament branches. Abp1 and drebrin are multidomain proteins that interact with actin filaments and regulate the activities of other proteins during various actin-dependent processes. The exact function of coactosin is currently incompletely understood. In this review article, we discuss the biochemical functions, cellular roles, and regulation of the five groups of ADF-H domain proteins.

Conformational dynamics of actin: effectors and implications for biological function

Actin is a protein abundant in many cell types. Decades of investigations have provided evidence that it has many functions in living cells. The diverse morphology and dynamics of actin structures adapted to versatile cellular functions is established by a large repertoire of actin-binding proteins. The proper interactions with these proteins assume effective molecular adaptations from actin, in which its conformational transitions play essential role. This review attempts to summarise our current knowledge regarding the coupling between the conformational states of actin and its biological function.

ADF/cofilin binds phosphoinositides in a multivalent manner to act as a PIP(2)-density sensor

Actin-depolymerizing-factor (ADF)/cofilins have emerged as key regulators of cytoskeletal dynamics in cell motility, morphogenesis, endocytosis, and cytokinesis. The activities of ADF/cofilins are regulated by membrane phospholipid PI(4,5)P(2) in vitro and in cells, but the mechanism of the ADF/cofilin-PI(4,5)P(2) interaction has remained controversial. Recent studies suggested that ADF/cofilins interact with PI(4,5)P(2) through a specific binding pocket, and that this interaction is dependent on pH. Here, we combined systematic mutagenesis with biochemical and spectroscopic methods to elucidate the phosphoinositide-binding mechanism of ADF/cofilins. Our analysis revealed that cofilin does not harbor a specific PI(4,5)P(2)-binding pocket, but instead interacts with PI(4,5)P(2) through a large, positively charged surface of the molecule. Cofilin interacts simultaneously with multiple PI(4,5)P(2) headgroups in a cooperative manner. Consequently, interactions of cofilin with membranes and actin exhibit sharp sensitivity to PI(4,5)P(2) density. Finally, we show that cofilin binding to PI(4,5)P(2) is not sensitive to changes in the pH at physiological salt concentration, although the PI(4,5)P(2)-clustering activity of cofilin is moderately inhibited at elevated pH. Collectively, our data demonstrate that ADF/cofilins bind PI(4,5)P(2) headgroups through a multivalent, cooperative mechanism, and suggest that the actin filament disassembly activity of ADF/cofilin can be accurately regulated by small changes in the PI(4,5)P(2) density at cellular membranes.

A central role for the WH2 domain of Srv2/CAP in recharging actin monomers to drive actin turnover in vitro and in vivo

Cellular processes propelled by actin polymerization require rapid disassembly of filaments, and then efficient recycling of ADF/cofilin-bound ADP-actin monomers back to an assembly-competent ATP-bound state. How monomer recharging is regulated in vivo is still not well understood, but recent work suggests the involvement of the ubiquitous actin-monomer binding protein Srv2/CAP. To better understand Srv2/CAP mechanism, we explored the contribution of its WH2 domain, the function of which has remained highly elusive. We found that the WH2 domain binds to actin monomers and, unlike most other WH2 domains, exhibits similar binding affinity for ATP-actin and ADP-actin (K(d) approximately 1.5 microM). Mutations in the WH2 domain that impair actin binding disrupt the ability of purified full-length Srv2/CAP to catalyze nucleotide exchange on ADF/cofilin-bound actin monomers and accelerate actin turnover in vitro. The same mutations impair Srv2/CAP function in vivo in regulating actin organization, cell growth, and cell morphogenesis. Thus, normal cell growth and organization depend on the ability of Srv2/CAP to recharge actin monomers, and the WH2 domain plays a central role in this process. Our data also reveal that while most isolated WH2 domains inhibit nucleotide exchange on actin, WH2 domains in the context of intact proteins can help promote nucleotide exchange.

The effects of ADF/cofilin and profilin on the conformation of the ATP-binding cleft of monomeric actin

Actin depolymerizing factor (ADF)/cofilin and profilin are small actin-binding proteins, which have central roles in cytoskeletal dynamics in all eukaryotes. When bound to an actin monomer, ADF/cofilins inhibit the nucleotide exchange, whereas most profilins accelerate the nucleotide exchange on actin monomers. In this study the effects of ADF/cofilin and profilin on the accessibility of the actin monomer's ATP-binding pocket was investigated by a fluorescence spectroscopic method. The fluorescence of the actin bound epsilon-ATP was quenched with a neutral quencher (acrylamide) in steady-state and time dependent experiments, and the data were analyzed with a complex form of the Stern-Volmer equation. The experiments revealed that in the presence of ADF/cofilin the accessibility of the bound epsilon-ATP decreased, indicating a closed and more compact ATP-binding pocket induced by the binding of ADF/cofilin. In the presence of profilin the accessibility of the bound epsilon-ATP increased, indicating a more open and approachable protein matrix around the ATP-binding pocket. The results of the fluorescence quenching experiments support a structural mechanism regarding the regulation of the nucleotide exchange on actin monomers by ADF/cofilin and profilin.

Allele-specific effects of human deafness gamma-actin mutations (DFNA20/26) on the actin/cofilin interaction

Auditory hair cell function requires proper assembly and regulation of the nonmuscle gamma isoactin-rich cytoskeleton, and six point mutations in this isoactin cause a type of delayed onset autosomal dominant nonsyndromic progressive hearing loss, DFNA20/26. The molecular basis underlying this actin-dependent hearing loss is unknown. To address this problem, the mutations have been introduced into yeast actin, and their effects on actin function were assessed in vivo and in vitro. Because we previously showed that polymerization was unaffected in five of the six mutants, we have focused on proteins that regulate actin, in particular cofilin, which severs F-actin and sequesters actin monomers. The mutations do not affect the interaction of cofilin with G-actin. However, T89I and V370A mutant F-actins are much more susceptible to cofilin disassembly than WT filaments in vitro. Conversely, P332A filaments demonstrate enhanced resistance. Wild type actin solutions containing T89I, K118M, or P332A mutant actins at mole fractions similar to those found in the hair cell respond in vitro toward cofilin in a manner proportional to the level of the mutant present. Finally, depression of cofilin action in vivo by elimination of the cofilin-activating protein, Aip1p, rescues the inability to grow on glycerol caused by K118M, T278I, P332A, and V370A. These results suggest that a filament instability caused by these mutations can be balanced by decreasing a system in vivo that promotes increased filament turnover. Such mutant-dependent filament destabilization could easily result in hair cell malfunction leading to the late-onset hearing loss observed in these patients.

Two biochemically distinct and tissue-specific twinfilin isoforms are generated from the mouse Twf2 gene by alternative promoter usage

Twf (twinfilin) is an evolutionarily conserved regulator of actin dynamics composed of two ADF-H (actin-depolymerizing factor homology) domains. Twf binds actin monomers and heterodimeric capping protein with high affinity. Previous studies have demonstrated that mammals express two Twf isoforms, Twf1 and Twf2, of which at least Twf1 also regulates cytoskeletal dynamics by capping actin filament barbed-ends. In the present study, we show that alternative promoter usage of the mouse Twf2 gene generates two isoforms, which differ from each other only at their very N-terminal region. Of these isoforms, Twf2a is predominantly expressed in non-muscle tissues, whereas expression of Twf2b is restricted to heart and skeletal muscle. Both proteins bind actin monomers and capping protein, as well as efficiently capping actin filament barbed-ends. However, the N-terminal ADF-H domain of Twf2b interacts with ADP-G-actin with a 5-fold higher affinity than with ATP-G-actin, whereas the corresponding domain of Twf2a binds ADP-G-actin and ATP-G-actin with equal affinities. Taken together, these results show that, like Twf1, mouse Twf2 is a filament barbed-end capping protein, and that two tissue-specific and biochemically distinct isoforms are generated from the Twf2 gene through alternative promoter usage.

An integrative simulation model linking major biochemical reactions of actin-polymerization to structural properties of actin filaments

We report on an advanced universal Monte Carlo simulation model of actin polymerization processes offering a broad application panel. The model integrates major actin-related reactions, such as assembly of actin nuclei, association/dissociation of monomers to filament ends, ATP-hydrolysis via ADP-Pi formation and ADP-ATP exchange, filament branching, fragmentation and annealing or the effects of regulatory proteins. Importantly, these reactions are linked to information on the nucleotide state of actin subunits in filaments (ATP hydrolysis) and the distribution of actin filament lengths. The developed stochastic simulation modelling schemes were validated on: i) synthetic theoretical data generated by a deterministic model and ii) sets of our and published experimental data obtained from fluorescence pyrene-actin experiments. Build on an open-architecture principle, the designed model can be extended for predictive evaluation of the activities of other actin-interacting proteins and can be applied for the analysis of experimental pyrene actin-based or fluorescence microscopy data. We provide a user-friendly, free software package ActinSimChem that integrates the implemented simulation algorithms and that is made available to the scientific community for modelling in silico any specific actin-polymerization system.

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.

Polymerization kinetics of ADP- and ADP-Pi-actin determined by fluorescence microscopy

We used fluorescence microscopy to determine how polymerization of Mg-ADP-actin depends on the concentration of phosphate. From the dependence of the elongation rate on the actin concentration and direct observations of depolymerizing filaments, we measured the polymerization rate constants of ADP-actin and ADP-P(i)-actin. Saturating phosphate reduces the critical concentration for polymerization of Mg-ADP-actin from 1.8 to 0.06 microM almost entirely by reducing the dissociation rate constants at both ends. Saturating phosphate increases the barbed end association rate constant of Mg-ADP-actin 15%, but this value is still threefold less than that of ATP-actin. Thus, ATP hydrolysis without phosphate dissociation must change the conformation of polymerized actin. Analysis of depolymerization experiments in the presence of phosphate suggests that phosphate dissociation near the terminal subunits is much faster than in the interior. Remarkably, 10 times more phosphate is required to slow the depolymerization of the pointed end than the barbed end, suggesting a weak affinity of phosphate near the pointed end. Our observations of single actin filaments provide clues about the origins of the difference in the critical concentration at the two ends of actin filaments in the presence of ATP.

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.

Dual effects of staurosporine on A431 and NRK cells: microfilament disassembly and uncoordinated lamellipodial activity followed by cell death

The general protein kinase inhibitor staurosporine (STS) has dual effects on human epidermoid cancer cells (A431) and normal rat kidney fibroblasts (NRK). It almost immediately stimulated increased lamellipodial activity of both cell lines and after 2 h induced typical signs of apoptosis, including cytoplasmic condensation, nuclear fragmentation, caspase-3 activation and DNA degradation. In the early phase we observed disruption of actin-containing stress fibres and accumulation of monomeric actin in the perinuclear region and cell nucleus. Increased lamellipodial-like extensions were observed particularly in A431 cells as demonstrated by co-localisation of actin and Arp2/3 complex, whereas NRK cells shrunk and exhibited numerous thin long extensions. These extensions exhibited uncoordinated centrifugal motile activity that appeared to tear the cells apart. Both cofilin and ADF were translocated from perinuclear regions to the cell cortex and, as expected in the presence of a kinase inhibitor, all the cofilin was dephosphorylated. Myosin II was absent from the extensions, and a reduction of phosphorylated myosin light chains was observed within the cytoplasm indicating myosin inactivation. Microtubules and intermediate filaments retained their characteristic filamentous organisation after STS exposure even when the cells became rounded and disorganised. Simultaneous treatment of NRK cells with STS and the caspase inhibitor zVAD did not inhibit the morphological and cytoskeletal changes. However, the cells underwent cell death as verified by positive annexin-V-staining. Thus it seems likely that cell death induced by STS may not only be a consequence of the activation of caspase, instead the disruption of the many motile processes involving the actin cytoskeleton may by itself suffice to induce caspase-independent cell death.

Inorganic phosphate regulates the binding of cofilin to actin filaments

Inorganic phosphate (Pi) and cofilin/actin depolymerizing factor proteins have opposite effects on actin filament structure and dynamics. Pi stabilizes the subdomain 2 in F-actin and decreases the critical concentration for actin polymerization. Conversely, cofilin enhances disorder in subdomain 2, increases the critical concentration, and accelerates actin treadmilling. Here, we report that Pi inhibits the rate, but not the extent of cofilin binding to actin filaments. This inhibition is also significant at physiological concentrations of Pi, and more pronounced at low pH. Cofilin prevents conformational changes in F-actin induced by Pi, even at high Pi concentrations, probably because allosteric changes in the nucleotide cleft decrease the affinity of Pi to F-actin. Cofilin induced allosteric changes in the nucleotide cleft of F-actin are also indicated by an increase in fluorescence emission and a decrease in the accessibility of etheno-ADP to collisional quenchers. These changes transform the nucleotide cleft of F-actin to G-actin-like. Pi regulation of cofilin binding and the cofilin regulation of Pi binding to F-actin can be important aspects of actin based cell motility.

The motility of glioblastoma tumour cells is modulated by intracellular cofilin expression in a concentration-dependent manner

The invasive behaviour of tumour cells has been attributed in part to dysregulated cell motility. Members of the ADF/Cofilin family of actin-binding proteins are known to increase microfilament dynamics by increasing the rate at which actin monomers leave the pointed end of the filament and by a filament-severing activity. As depolymerisation is a rate-limiting step in actin dynamics, ADF/Cofilins are suspected to facilitate the motility of cells. To test this, we investigated the influence of cofilin on tumour motility by transient and stably overexpressing cofilin in the human glioblastoma cell line, U373 MG. Several different methods were used to ascertain the level of cofilin in overexpressing clones and this was correlated with their rate of random locomotion. A biphasic relationship between cofilin level and locomotory rate was found. Clones that displayed a moderate amount of overproduction of cofilin were found to have increased rates of locomotion approximately linear to the overproduction of cofilin up to an optimal cofilin level of about 4.5 times that of wild type cells at which the cells were almost twice as fast. However, clones producing more than this optimal amount were found to locomote at progressively reduced speeds. Cells that overexpress cofilin have reduced stress fibres compared to control cells showing that the excess cofilin affects the actin cytoskeleton. We conclude that overexpression of cofilin enhances the motility of glioblastoma tumour cells in a concentration-dependent fashion, which is likely to contribute to their invasiveness.

An intermediate form of ADP-F-actin

With yeast actin, contrary to other actins, filament formation, ATP hydrolysis, and Pi release are concurrent at low actin concentrations, the condition usually employed to assess actin polymerization. This observation leads to a question concerning the conformation of the filament barbed end that might be recognized by specific actin-binding proteins. To try to detect possible new actin polymer conformations that might be intermediate in the pathway leading to mature F-actin, we monitored the change in intrinsic tryptophan fluorescence of yeast and muscle actins polymerized at pH 6 to accelerate the rate of filament formation. This allowed temporal resolution of the Pi release process from the slower process of polymerization. With both actins, we detected a biphasic instead of the usual monophasic fluorescence change, a rapid decrease that tracks with filament formation followed by a slower rebound (the second phase). This second phase postpolymerization conformational change requires Pi release and occurs nearly coincident with its release. The addition of Pi causes this second phase response to disappear, and the inclusion of Pi during polymerization prevents its appearance. At pH 7.5, with higher yeast actin concentrations to accelerate polymerization, a two-phase fluorescence change is also observed. In this case, the second phase change lags substantially behind Pi release. Pi release could also be resolved from polymer formation. V159N yeast actin, hypothesized previously as remaining in a postpolymerization ATP-like state, exhibits the same two-phase intrinsic tryptophan fluorescence behavior as wild-type yeast actin. Together, these observations demonstrate the presence of an intermediate filament state between ADP-Pi and mature ADP-F-actin.

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.

Regulation of cytoskeletal dynamics by actin-monomer-binding proteins

The actin cytoskeleton is a vital component of several key cellular and developmental processes in eukaryotes. Many proteins that interact with filamentous and/or monomeric actin regulate the structure and dynamics of the actin cytoskeleton. Actin-filament-binding proteins control the nucleation, assembly, disassembly and crosslinking of actin filaments, whereas actin-monomer-binding proteins regulate the size, localization and dynamics of the large pool of unpolymerized actin in cells. In this article, we focus on recent advances in understanding how the six evolutionarily conserved actin-monomer-binding proteins - profilin, ADF/cofilin, twinfilin, Srv2/CAP, WASP/WAVE and verprolin/WIP - interact with actin monomers and regulate their incorporation into filament ends. We also present a model of how, together, these ubiquitous actin-monomer-binding proteins contribute to cytoskeletal dynamics and actin-dependent cellular processes.

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.

Selective estrogen receptor modulator regulated proteins in endometrial cancer cells

Tamoxifen is the primary hormonal therapy for breast cancer and is also used as a breast cancer chemopreventative agent. A major problem with tamoxifen therapy is undesirable endometrial proliferation. To identify proteins associated with the growth stimulatory effects of tamoxifen in an ER-positive model, the present study profiled total cellular and secreted proteins regulated by estradiol and selective estrogen receptor modifiers (SERMs) in the Ishikawa endometrial adenocarcinoma cell line using two-dimensional gel electrophoresis. Following 24 h incubation with 10(-8) M estradiol, 10(-7) M 4-hydroxytamoxifen, or 10(-7) M EM-652 (Acolbifene), nine proteins exhibited significant increase in expression. The proteins identified were heat shock protein 90-alpha, and -beta, heterogeneous nuclear ribonucleoprotein F, RNA polymerase II-mediating protein, cytoskeletal keratin 8, cytoskeletal keratin 18, ubiquitin-conjugating enzyme E2-18 kDa and nucleoside diphosphate kinase B. These protein profiles may serve as novel indices of SERM response and may also provide insight into novel mechanisms of SERM-mediated growth.

Capping protein binding to S100B: implications for the tentacle model for capping the actin filament barbed end

S100B binds tightly to a 12-amino acid peptide derived from heterodimeric capping protein. In native intact capping protein, this sequence is in the C terminus of the alpha-subunit, which is important for capping the actin filament. This C-terminal region is proposed to act as a flexible "tentacle," extending away from the body of capping protein in order to bind actin. To this hypothesis, we analyzed the interaction between S100B and capping protein in solution. The C-terminal 28 amino acids of the alpha-subunit, the proposed tentacle, bound to S100B as a free synthetic peptide or a glutathione S-transferase fusion (K(d) approximately 0.4-1 microm). In contrast, S100B did not bind to whole native capping protein or functionally affect its capping activity. S100B does not bind, with any significant affinity, to the proposed alpha-tentacle sequence of whole native capping protein in solution. In the NMR structure of S100B complexed with the alpha-subunit-derived 12-amino acid peptide, the hydrophobic side of a short alpha-helix in the peptide, containing an important tryptophan residue, contacts S100B. In the x-ray structure of native capping protein, the corresponding sequence of the alpha-subunit C terminus, including Trp(271), interacts closely with the body of the protein. Therefore, our results suggest the alpha-subunit C terminus is not mobile as predicted by the tentacle model. Addition of non-ionic detergent allowed whole capping protein to bind weakly to S100B, indicating that the alpha-subunit C terminus can be mobilized from the surface of the capping protein molecule, presumably by weakening the hydrophobic binding at the contact site.

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.

Renal ischemia induces tropomyosin dissociation-destabilizing microvilli microfilaments

Ischemic-induced cell injury results in rapid duration-dependent actin-depolymerizing factor (ADF)/cofilin-mediated disruption of the apical microvilli microfilament cores. Because intestinal microvillar microfilaments are bound and stabilized in the terminal web by the actin-binding protein tropomyosin, we questioned whether a protective effect of tropomyosin localization to the terminal web of the proximal tubule microfilament cores is disrupted during ischemic injury. With tropomyosin-specific antibodies, we examined rat cortical sections under physiological conditions and following ischemic injury by confocal microscopy. In addition, Western blot analysis of cortical extracts and urine was undertaken. Our studies demonstrated the presence of tropomyosin isoforms in the proximal tubule microvillar terminal web under physiological conditions and their dissociation in response to 25 min of ischemic injury. This correlated with the excretion of tropomyosin-containing plasma membrane vesicles in urine from ischemic rats. In addition, we noted increased tropomyosin Triton X-100 solubility following ischemia in cortical extracts. These studies suggest tropomyosin binds to and stabilizes the microvillar microfilament core in the terminal web under physiological conditions. With the onset of ischemic injury, we propose that tropomyosin dissociates from the microfilament core providing access to microfilaments in the terminal web for F-actin binding, severing and depolymerizing actions of ADF/cofilin proteins.

ADF/cofilin use an intrinsic mode of F-actin instability to disrupt actin filaments

Proteins in the ADF/cofilin (AC) family are essential for rapid rearrangements of cellular actin structures. They have been shown to be active in both the severing and depolymerization of actin filaments in vitro, but the detailed mechanism of action is not known. Under in vitro conditions, subunits in the actin filament can treadmill; with the hydrolysis of ATP driving the addition of subunits at one end of the filament and loss of subunits from the opposite end. We have used electron microscopy and image analysis to show that AC molecules effectively disrupt one of the longitudinal contacts between protomers within one helical strand of F-actin. We show that in the absence of any AC proteins, this same longitudinal contact between actin protomers is disrupted at the depolymerizing (pointed) end of actin filaments but is prominent at the polymerizing (barbed) end. We suggest that AC proteins use an intrinsic mechanism of F-actin's internal instability to depolymerize/sever actin filaments in the cell.

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.

Mammals have two twinfilin isoforms whose subcellular localizations and tissue distributions are differentially regulated

Twinfilin is a highly conserved actin monomer-binding protein that regulates cytoskeletal dynamics in organisms from yeast to mammals. In addition to the previously characterized mammalian twinfilin-1, a second protein with approximately 65% sequence identity to twinfilin-1 exists in mouse and humans. However, previous studies failed to identify any actin binding activity in this protein (Rohwer, A., Kittstein, W., Marks, F., and Gschwendt, M. (1999) Eur. J. Biochem. 263, 518-525). Here we show that this protein, which we named twinfilin-2, is indeed an actin monomer-binding protein. Similar to twinfilin-1, mouse twinfilin-2 binds ADP-G-actin with a higher affinity (KD = 0.12 microM) than ATP-G-actin (KD = 1.96 microM) and efficiently inhibits actin filament assembly in vitro. Both mouse twinfilins inhibit the nucleotide exchange on actin monomers and directly interact with capping protein. Furthermore, the actin interactions of mouse twinfilin-1 and twinfilin-2 are inhibited by phosphatidylinositol (4,5)-bisphosphate. Although biochemically very similar, our Northern blots and in situ hybridizations show that these two proteins display distinct expression patterns. Twinfilin-1 is the major isoform in embryos and in most adult mouse non-muscle cell-types, whereas twinfilin-2 is the predominant isoform of adult heart and skeletal muscles. Studies with isoform-specific antibodies demonstrated that although the two proteins show similar localizations in unstimulated cells, they are regulated by different mechanisms. The small GTPases Rac1 and Cdc42 induce the redistribution of twinfilin-1 to membrane ruffles and cell-cell contacts, respectively, but do not affect the localization of twinfilin-2. Taken together, these data show that mammals have two twinfilin isoforms, which are differentially expressed and regulated through distinct cellular signaling pathways.

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.

ADF/cofilin mediates actin cytoskeletal alterations in LLC-PK cells during ATP depletion

Ischemic injury induces actin cytoskeleton disruption and aggregation, but mechanisms affecting these changes remain unclear. To determine the role of actin-depolymerizing factor (ADF)/ cofilin participation in ischemic-induced actin cytoskeletal breakdown, we utilized porcine kidney cultured cells, LLC-PK(A4.8), and adenovirus containing wild-type (wt), constitutively active, and inactive Xenopus ADF/cofilin linked to green fluorescence protein [XAC(wt)-GFP] in an ATP depletion model. High adenoviral infectivity (70%) in LLC-PK(A4.8) cells resulted in linearly increasing XAC(wt)-GFP and phosphorylated (p)XAC(wt)-GFP (inactive) expression. ATP depletion rapidly induced dephosphorylation, and, therefore, activation, of endogenous pcofilin as well as pXAC(wt)-GFP in conjunction with the formation of fluorescent XAC(wt)-GFP/actin aggregates and rods. No significant actin cytoskeletal alterations occurred with short-term ATP depletion of LLC-PK(A4.8) cells expressing GFP or the constitutively inactive mutant XAC(S3E)-GFP, but cells expressing the constitutively active mutant demonstrated nearly instantaneous actin disruption with aggregate and rod formation. Confocal image three-dimensional volume reconstructions of normal and ATP-depleted LLC-PK(A4.8) cells demonstrated that 25 min of ATP depletion induced a rapid increase in XAC(wt)-GFP apical and basal signal in addition to XAC-GFP/actin aggregate formation. These data demonstrate XAC(wt)-GFP participates in ischemia-induced actin cytoskeletal alterations and determines the rate and extent of these ATP depletion-induced cellular alterations.

ADF/cofilin and actin dynamics in disease

ADF/cofilins are key regulators of actin dynamics in normal cells. Recent findings suggest that, under cellular stress, the wild-type proteins might form complexes with actin that can alter cell function. Owing to their rapid formation, these complexes might initiate or aid in the progression of diseases as diverse as Alzheimer's disease and ischemic kidney disease. Although evidence for their involvement in diseases other than Alzheimer's and ischemic kidney disease is tenuous, recent studies suggest that altered production, regulation or localization of these proteins might lead to cognitive impairment, inflammation, infertility, immune deficiencies and other pathophysiological defects.

Structural conservation between the actin monomer-binding sites of twinfilin and actin-depolymerizing factor (ADF)/cofilin

Twinfilin is an evolutionarily conserved actin monomer-binding protein that regulates cytoskeletal dynamics in organisms from yeast to mammals. It is composed of two actin-depolymerization factor homology (ADF-H) domains that show approximately 20% sequence identity to ADF/cofilin proteins. In contrast to ADF/cofilins, which bind both G-actin and F-actin and promote filament depolymerization, twinfilin interacts only with G-actin. To elucidate the molecular mechanisms of twinfilin-actin monomer interaction, we determined the crystal structure of the N-terminal ADF-H domain of twinfilin and mapped its actin-binding site by site-directed mutagenesis. This domain has similar overall structure to ADF/cofilins, and the regions important for actin monomer binding in ADF/cofilins are especially well conserved in twinfilin. Mutagenesis studies show that the N-terminal ADF-H domain of twinfilin and ADF/cofilins also interact with actin monomers through similar interfaces, although the binding surface is slightly extended in twinfilin. In contrast, the regions important for actin-filament interactions in ADF/cofilins are structurally different in twinfilin. This explains the differences in actin-interactions (monomer versus filament binding) between twinfilin and ADF/cofilins. Taken together, our data show that the ADF-H domain is a structurally conserved actin-binding motif and that relatively small structural differences at the actin interfaces of this domain are responsible for the functional variation between the different classes of ADF-H domain proteins.

The two ADF-H domains of twinfilin play functionally distinct roles in interactions with actin monomers

Twinfilin is a ubiquitous and abundant actin monomer-binding protein that is composed of two ADF-H domains. To elucidate the role of twinfilin in actin dynamics, we examined the interactions of mouse twinfilin and its isolated ADF-H domains with G-actin. Wild-type twinfilin binds ADP-G-actin with higher affinity (K(D) = 0.05 microM) than ATP-G-actin (K(D) = 0.47 microM) under physiological ionic conditions and forms a relatively stable (k(off) = 1.8 s(-1)) complex with ADP-G-actin. Data from native PAGE and size exclusion chromatography coupled with light scattering suggest that twinfilin competes with ADF/cofilin for the high-affinity binding site on actin monomers, although at higher concentrations, twinfilin, cofilin, and actin may also form a ternary complex. By systematic deletion analysis, we show that the actin-binding activity is located entirely in the two ADF-H domains of twinfilin. Individually, these domains compete for the same binding site on actin, but the C-terminal ADF-H domain, which has >10-fold higher affinity for ADP-G-actin, is almost entirely responsible for the ability of twinfilin to increase the amount of monomeric actin in cosedimentation assays. Isolated ADF-H domains associate with ADP-G-actin with rapid second-order kinetics, whereas the association of wild-type twinfilin with G-actin exhibits kinetics consistent with a two-step binding process. These data suggest that the association with an actin monomer induces a first-order conformational change within the twinfilin molecule. On the basis of these results, we propose a kinetic model for the role of twinfilin in actin dynamics and its possible function in cells.

Twinfilin, a molecular mailman for actin monomers

Twinfilin is a ubiquitous actin-monomer-binding protein that is composed of two ADF-homology domains. It forms a 1:1 complex with ADP-actin-monomers,inhibits nucleotide exchange on actin monomers and prevents assembly of the monomer into filaments. The two ADF-H domains in twinfilin probably have 3D structures similar to those of the ADF/cofilin proteins and overlapping actin-binding sites. Twinfilin also interacts with PtdIns(4,5)P2, which inhibits its actin-monomer-sequestering activity in vitro. Mutations in the twinfilin gene result in defects in the bipolar budding pattern in S. cerevisiae and in a rough eye phenotype and aberrant bristle morphology in Drosophila melanogaster. These phenotypes are caused by the uncontrolled polymerization of actin filaments in the absence of twinfilin. Studies on budding yeast suggest that twinfilin contributes to actin filament turnover by localizing actin monomers, in their `inactive'ADP-form, to the sites of rapid filament assembly. This is mediated through direct interactions between twinfilin and capping protein. Therefore,twinfilin might serve as a link between rapid actin filament depolymerization and assembly in cells.