Inhibition of the interactions of cofilin, destrin, and deoxyribonuclease I with actin by phosphoinositides

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.

A role for cofilin in the activation of store-operated calcium entry by de novo conformational coupling in human platelets

Store-operated Ca2+ entry (SOCE) is a major mechanism for Ca2+ influx in platelets and other cells. De novo conformational coupling between elements in the plasma membrane and Ca2+ stores, where the actin cytoskeleton plays an important regulatory role, has been proposed as the most likely mechanism to activate SOCE in platelets. Here we have examined for the first time changes in platelet F-actin levels on a subsecond time scale. Using stopped-flow fluorimetry and a quenched-flow approach, we provide evidence for the involvement of cofilin in actin filament reorganization and SOCE in platelets. Thrombin (0.1 U/mL) evoked an initial decrease in F-actin that commenced within 0.1 second and reached a minimum 0.9 second after stimulation, prior to the activation of SOCE. F-actin then increased, exceeding basal levels approximately 2.5 seconds after stimulation. Thrombin also induced cofilin dephosphorylation and activation, which paralleled the changes observed in F-actin, and rapid Btk activation. Inhibition of cofilin dephosphorylation by LFM-A13 resulted in the loss of net actin depolymerization and an increased delay in SOCE initiation. These results suggest that cofilin is important for the rapid actin remodeling necessary for the activation of SOCE in platelets through de novo conformational coupling.

Cofilin, actin and their complex observed in vivo using fluorescence resonance energy transfer

Actin is the principal component of microfilaments. Its assembly/disassembly is essential for cell motility, cytokinesis, and a range of other functions. Recent evidence suggests that actin is present in the nucleus where it may be involved in the regulation of gene expression and that cofilin binds actin and can translocate into the nucleus during times of stress. In this report, we combine fluorescence resonance energy transfer and confocal microscopy to analyze the interactions of cofilin and G-actin within the nucleus and cytoplasm. By measuring the rate of photobleaching of fluorescein-labeled actin in the presence and absence of Cy5-labeled cofilin, we determined that almost all G-actin in the nucleus is bound to cofilin, whereas approximately (1/2) is bound in the cytoplasm. Using fluorescence resonance energy transfer imaging techniques we observed that a significant proportion of fluorescein-labeled cofilin in both the nucleus and cytoplasm binds added tetramethylrhodamine-labeled G-actin. Our data suggest there is significantly more cofilin-G-actin complex and less free cofilin in the nucleus than in the cytoplasm.

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.

Cofilin takes the lead

Cofilin has emerged as a key regulator of actin dynamics at the leading edge of motile cells. Through its actin-severing activity, it creates new actin barbed ends for polymerization and also depolymerizes old actin filaments. Its function is tightly regulated in the cell. Spatially, its activity is restricted by other actin-binding proteins, such as tropomyosin, which compete for accessibility of actin filament populations in different regions of the cell. At the molecular level, it is regulated by phosphorylation, pH and phosphatidylinositol (4,5)-bisphosphate binding downstream of signaling cascades. In addition, it also appears to be regulated by interactions with 14-3-3zeta and cyclase-associated protein. In vivo, cofilin acts synergistically with the Arp2/3 complex to amplify local actin polymerization responses upon cell stimulation, which gives it a central role in setting the direction of motility in crawling cells.

Role of synaptojanin 2 in glioma cell migration and invasion

The small GTPase Rac1 is thought to play an important role in cell migration and invasion. We have previously identified synaptojanin 2, a phosphoinositide phosphatase, as an effector of Rac1. Here, we show that small interfering RNA-mediated depletion of either Rac1 or synaptojanin 2 inhibits invasion of SNB19 and U87MG glioblastoma cells through Matrigel and rat brain slices. Depletion of Rac1 or synaptojanin 2 also inhibits migration of SNB19 and U87MG cells on glioma-derived extracellular matrix. In addition, we found that depletion of Rac1 or synaptojanin 2 inhibits the formation of lamellipodia and invadopodia, specialized membrane structures that are thought to be involved in extracellular matrix degradation. These results suggest that synaptojanin 2 contributes to the role of Rac1 in cell invasion and migration by regulating the formation of invadopodia and lamellipodia. This study also identifies synaptojanin 2 as a novel potential target for therapeutic intervention in malignant tumors.

Phospholipase C and cofilin are required for carcinoma cell directionality in response to EGF stimulation

The epidermal growth factor (EGF)–induced increase in free barbed ends, resulting in actin polymerization at the leading edge of the lamellipodium in carcinoma cells, occurs as two transients: an early one at 1 min and a late one at 3 min. Our results reveal that phospholipase (PLC) is required for triggering the early barbed end transient. Phosphoinositide-3 kinase selectively regulates the late barbed end transient. Inhibition of PLC inhibits cofilin activity in cells during the early transient, delays the initiation of protrusions, and inhibits the ability of cells to sense a gradient of EGF. Suppression of cofilin, using either small interfering RNA silencing or function-blocking antibodies, selectively inhibits the early transient. Therefore, our results demonstrate that the early PLC and cofilin-dependent barbed end transient is required for the initiation of protrusions and is involved in setting the direction of cell movement in response to EGF.

The co-workers of actin filaments: from cell structures to signals

Cells have various surface architectures, which allow them to carry out different specialized functions. Actin microfilaments that are associated with the plasma membrane are important for generating these cell-surface specializations, and also provide the driving force for remodelling cell morphology and triggering new cell behaviour when the environment is modified. This phenomenon is achieved through a tight coupling between cell structure and signal transduction, a process that is modulated by the regulation of actin-binding proteins.

Efficient Salmonella entry requires activity cycles of host ADF and cofilin

Entry of Salmonella into mammalian cells is strictly dependent on the reorganization of actin cytoskeleton induced by a panel of Salmonella type III secreted proteins. Although several factors have been identified to be responsible for inducing the actin polymerization and stability, little is known about how the actin depolymerization contributes to Salmonella-induced actin rearrangements. We report here that activity cycles of host actin depolymerizing factor (ADF and cofilin) are modulated by Salmonella during bacterial entry. Efficient Salmonella internalization involves an initial dephosphorylation of ADF and cofilin followed by phosphorylation, suggesting that ADF and cofilin activities are increased briefly. Expression of a kinase dead form of an ADF/cofilin kinase (LIM kinase 1) or a catalytically inactive ADF/cofilin phosphatase (Slingshot), but not constitutively active LIM kinase 1 or wild-type Slingshot, resulted in decreased invasion. These data suggest that ADF/cofilin activities play a key role in the actin polymerization/depolymerization process induced by Salmonella. The activation of ADF/cofilin is brief and has to be reversed to facilitate efficient bacterial entry. Surprisingly, co-expression of constitutive active ADF and cofilin prevented efficient Salmonella entry, whereas expression of either one alone had no effect. We propose that ADF and cofilin actin-dynamizing activities and their activity cycling via phosphorylation are required for efficient Salmonella internalization.

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.

A Ratiometric Expressible FRET Sensor for Phosphoinositides Displays a Signal Change in Highly Dynamic Membrane Structures in Fibroblasts

Phosphoinositides are important signal transduction intermediates in cell growth, survival, and motility. We have invented a fluorescence sensor for polyphosphorylated phosphoinositides based on a peptide derived from the Listeria protein ActA that undergoes a random coil to helix transition upon lipid binding. The sensor, termed CAY, is a fusion protein of cyan and yellow fluorescent proteins flanking the peptide at its N- and C-termini, respectively. CAY displays fluorescence resonance energy transfer in vitro in the absence of phosphorylated phosphoinositides, and this energy transfer is lost upon interaction with these phospholipids. These results demonstrate that a short peptide undergoing a coil to helix transition can be sufficient for the engineering of a FRET-based biosensor. CAY is predominantly localized to the cytoplasm in fibroblasts expressing the sensor but shows loss of fluorescence resonance energy transfer in regions of active actin dynamics such as ruffles that have previously been demonstrated to contain high levels of phosphoinositides.

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.

Association of villin with phosphatidylinositol 4,5-bisphosphate regulates the actin cytoskeleton

Villin, an epithelial cell actin-binding protein, severs actin in vitro and in vivo. Previous studies report that phosphatidylinositol 4,5-bisphosphate (PIP(2)) regulates actin severing by villin, presumably by interaction with villin. However, direct association of villin with PIP(2) has never been characterized. In this report, we presented mutational analysis to identify the PIP(2)-binding sites in villin. Villin (human) binds PIP(2) with a K(d) of 39.5 microm, a stoichiometry of 3.3, and a Hill coefficient of 1. We generated deletion mutants of villin lacking putative PIP(2)-binding sites and examined the impact of these mutations on PIP(2) binding and actin dynamics. Our analysis revealed the presence of three PIP(2)-binding sites, two in the amino-terminal core and one in the carboxyl-terminal headpiece of human villin. Synthetic peptides analogous with these sites confirmed the binding domains. Circular dichroism and quenching of intrinsic tryptophan fluorescence revealed a significant conformational change in these peptides ensuing in their association with PIP(2). By using site-directed mutagenesis (arginine 138 to alanine), we demonstrated the presence of an identical F-actin and PIP(2)-binding site in the capping and severing domain of villin. In contrast, the mutants lysine 822 and 824 to alanine demonstrated the presence of an overlapping F-actin and PIP(2)-binding site in the actin cross-linking domain of villin. Consistent with this observation, association of villin with PIP(2) inhibited the actin capping and severing functions of villin and enhanced the actin bundling function of villin. Our studies revealed that structural changes induced by association with PIP(2) could regulate the actin-modifying functions of villin. This study provided biochemical proof of the functional significance of villin association with PIP(2) and identified the molecular mechanisms involved in the regulation of actin dynamics by villin and PIP(2).

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.

The structure of Aip1p, a WD repeat protein that regulates Cofilin-mediated actin depolymerization

Actin-interacting protein 1 (Aip1p) is a 67-kDa WD repeat protein known to regulate the depolymerization of actin filaments by cofilin and is conserved in organisms ranging from yeast to mammals. The crystal structure of Aip1p from Saccharomyces cerevisiae was determined to a 2.3-A resolution and a final crystallographic R-factor of 0.204. The structure reveals that the overall fold is formed by two connected seven-bladed beta-propellers and has important implications for the structure of Aip1 from other organisms and WD repeat-containing proteins in general. These results were unexpected because a maximum of 10 WD repeats had been reported in the literature for this protein using sequence data. The surfaces of the beta-propellers formed by the D-A and B-C loops are positioned adjacent to one another, giving Aip1p a shape that resembles an open "clamshell." The mapping of conserved residues to the structure of Aip1p reveals dense patches of conserved residues on the surface of one beta-propeller and at the interface of the two beta-propellers. These two patches of conserved residues suggest a potential binding site for F-actin on Aip1p and that the orientation of the beta-propellers with respect to one another plays a role in binding an actin-cofilin complex. In addition, the conserved interface between the domains is mediated by a number of interactions that appear to impart rigidity between the two domains of Aip1p and may make a large substrate-induced conformational change difficult.

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.

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.

Products of phosphoinositide specific phospholipase C can trigger dephosphorylation of cofilin in chemoattractant stimulated neutrophils

The signal transduction pathways that trigger dephosphorylation of cofilin in neutrophils stimulated with the chemoattractant fMet-Leu-Phe (fMLP) were investigated with a phospho-specific antibody that recognized cofilin only when this protein was phosphorylated on ser-3. Unlike earlier studies that monitored changes in (32)P-labeled cofilin, this Ab allowed us to monitor changes in the total mass of phosphorylated cofilin during neutrophil stimulation. Neutrophils stimulated with fMLP (1.0 microM) for 1.0 min exhibited a massive loss (> 85%) of phosphate from cofilin, which was blocked by an antagonist of phosphoinositide-specific phospholipase C (PI-PLC) (1.0 microM U73122). Products of PI-PLC, sn-1,2-diglyceride and inositol (1,4,5)-trisphosphate, are known to activate protein kinase C (PKC) and increase intracellular Ca(2+), respectively. Treatment of neutrophils with agents that selectively activate PKC [4beta-phorbol 12-myristate 13-acetate (PMA) ] or cellular Ca(2+) (ionophore A23187) also triggered dephosphorylation of cofilin. Both a nonspecific (100 nM staurosporine) and a highly selective antagonist of PKC (200 nM bisindolylmaleimide I) blocked dephosphorylation of cofilin in neutrophils stimulated with PMA but not with fMLP or ionophore A23187. The calmodulin (CaM) antagonists trifluoperazine (15 microM) and W-7 (50 microM) blocked dephosphorylation of cofilin in stimulated neutrophils whereas inactive/less-active analogs of these inhibitors (15 microM promethazine, 50 microM W-5) were substantially less effective. Calyculin A (40 nM), an antagonist of type 1 and 2A protein phosphatases, also triggered a massive dephosphorylation of cofilin in unstimulated neutrophils through a pathway that was insensitive to inhibitors of type 2B phosphatases. These data suggest that both PKC-dependent and independent pathways can trigger dephosphorylation of cofilin in neutrophils with the latter pathway predominating in fMLP-stimulated cells. These pathways may also contain CaM and a type 2C and/or novel phosphatase (e.g., slingshot).

Actin cytoskeletal dynamics in T lymphocyte activation and migration

Dynamic rearrangements of the actin cytoskeleton are crucial for the function of numerous cellular elements including T lymphocytes. They are required for migration of T lymphocytes through the body to scan for the presence of antigens, as well as for the formation and stabilization of the immunological synapse at the interface between antigen-presenting cells and T lymphocytes. Supramolecular activation clusters within the immunological synapse play an important role for the initiation of T cell responses and for the execution of T cell effector functions. In addition to the T cell receptor/CD3 induced actin nucleation via Wasp/Arp2/3-activation, signals through accessory receptors of the T cell (i.e., costimulation) regulate actin cytoskeletal dynamics. In this regard, the actin-binding proteins cofilin and L-plastin represent prominent candidates linking accessory receptor stimulation to the rearrangement of the actin cytoskeleton. Cofilin enhances actin polymerization via its actin-severing activity, and as a long-lasting effect, cofilin generates novel actin monomers through F-actin depolymerization. L-plastin stabilizes actin filament structures by means of its actin-bundling activity.

Leishmania EF-1alpha activates the Src homology 2 domain containing tyrosine phosphatase SHP-1 leading to macrophage deactivation

The human leishmaniasis are persistent infections of macrophages caused by protozoa of the genus Leishmania. The chronic nature of these infections is in part related to induction of macrophage deactivation, linked to activation of the Src homology 2 domain containing tyrosine phosphatase-1 (SHP-1) in infected cells. To investigate the mechanism of SHP-1 activation, lysates of Leishmania donovani promastigotes were subjected to SHP-1 affinity chromatography and proteins bound to the matrix were sequenced by mass spectrometry. This resulted in the identification of Leishmania elongation factor-1alpha (EF-1alpha) as a SHP-1-binding protein. Purified Leishmania EF-1alpha, but not host cell EF-1alpha, bound directly to SHP-1 in vitro leading to its activation. Three independent lines of evidence indicated that Leishmania EF-1alpha may be exported from the phagosome thereby enabling targeting of host SHP-1. First, cytosolic fractions prepared from macrophages infected with [(35)S]methionine-labeled organisms contained Leishmania EF-1alpha. Second, confocal, fluorescence microscopy using Leishmania-specific antisera detected Leishmania EF-1alpha in the cytosol of infected cells. Third, co-immunoprecipitation showed that Leishmania EF-1alpha was associated with SHP-1 in vivo in infected cells. Finally, introduction of purified Leishmania EF-1alpha, but not the corresponding host protein into macrophages activated SHP-1 and blocked the induction of inducible nitric-oxide synthase expression in response to interferon-gamma. Thus, Leishmania EF-1alpha is identified as a novel SHP-1-binding and activating protein that recapitulates the deactivated phenotype of infected macrophages.

A structural basis for the pH-dependence of cofilin. F-actin interactions

A marked pH-dependent interaction with F-actin is an important property of typical members of the actin depolymerizing factor (ADF)/cofilin family of abundant actin-binding proteins. ADF/cofilins tend to bind to F-actin with a ratio of 1 : 1 at pH values around 6.5, and to G-actin at pH 8.0. We have investigated the mechanism for the pH-sensitivity. We found no evidence for pH-dependent changes in the structure of cofilin itself, nor for the interaction of cofilin with G-actin. None of the actin-derived, cofilin-binding peptides that we had previously identified [Renoult, C., Ternent, D., Maciver, S.K., Fattoum, A., Astier, C., Benyamin, Y. & Roustan, C. (1999) J. Biol. Chem. 274, 28893-28899] bound cofilin in a pH-sensitive manner. However, we have detected a conformational change in region 75-105 in the actin subdomain 1 by the use of a peptide-directed antibody. A pH-dependent conformational change has also been detected spectroscopically in a similar peptide (84-103) on binding to cofilin. These results are consistent with a model in which pH-dependent motion of subdomain 1 relative to subdomain 2 (through region 75-105) of actin reveals a second cofilin binding site on actin (centered around region 112-125) that allows ADF/cofilin association with the actin filament. This motion requires salt in addition to low pH.

The distribution of cofilin and DNase I in vivo

Actin is the principal component of the cytoskeleton, a structure that can be disassembled and reassembled in a matter of seconds in vivo. The state of assembly of actin in vivo is primarily regulated by one or more actin binding proteins (ABPs). Typically, the actions of ABPs have been studied one by one, however, we propose that multiple ABPs, acting cooperatively, may be involved in the control of actin filament length. Cofilin and DNase I are two ABPs that have previously been demonstrated to form a ternary complex with actin in vitro. This is the first report to demonstrate their co-localisation in vivo, and differences in their distributions. Our observations strongly suggest a physiological role for higher order complexes of actin in regulation of cytoskeletal assembly during processes such as cell division.

Phosphoinositide-binding domains: Functional units for temporal and spatial regulation of intracellular signalling

Inositol phospholipid (phosphoinositide) is a versatile lipid characterized by its isomer-specific localization, as well as its molecular diversity attributable to phosphorylation events. Phosphoinositides act as signal mediators in a spatially and temporally controlled manner. Information about the timing and location of their production is received by phosphoinositide-binding proteins and transmitted to multiple lines of intracellular events such as signal transduction, cytoskeletal rearrangement, and membrane trafficking. Among those proteins, a significant portion possess globular structural units, called domains, which are specialized for phosphoinositide binding. The pleckstrin homology (PH) domain was the first phosphoinositide-binding domain identified. It contains the largest number of members and is associated with the formation of signalling complexes on the plasma membrane. Recent studies identified other novel phosphoinositide-binding domains (Fab1p, YOTB, Vps27p, EEA1 (FYVE), Phox homology (PX), and epsin N-terminal homology (ENTH)), thus extending our knowledge of how the functional versatility of phosphoinositides is achieved.

Distribution of gelsolin and phosphoinositol 4,5-bisphosphate in lamellipodia during EGF-induced motility

During induced cell motility the actin cytoskeleton at the leading edge must undergo constant reorganization. Recently, phosphoinositides have been shown to be central to cytoskeleton-membrane linkages and actin organization and turnover. Epidermal growth factor (EGF) receptor (EGFR)-mediated cell motility requires phospholipase C-gamma (PLCgamma), hydrolysis of phosphoinsotide 4,5-bisphosphate (PIP(2)) and subsequent release of gelsolin. We hypothesized this led to the mobilization of PIP(2)-binding proteins which modify the actin cytoskeleton and thus sought to determine whether the leading edge was a site of active PIP(2) hydrolysis and gelsolin redistribution to cytoskeleton. Herein, we report that during EGF-induced motility, the leading edge's submembranous region constitutes a distinct subcellular locale. The relevant phosphoinositide composition of this space was determined by probing with an antibody to PIP(2) and a green fluorescence protein (GFP)-tagged pleckstrin homology (PH) domain of PLCdelta (GFP-PH) that recognizes both PIP(2) and inositol 1,4,5-trisphosphate (IP(3)). PIP(2) was absent from leading lamellipodia despite an increase in IP(3) generation, suggesting an increase in PIP(2) hydrolysis at the leading edge. Visualized with immunofluorescence, gelsolin preferentially concentrated near the leading edge in a punctate fashion. Examining the Triton X-insoluble actin cytoskeleton fractions, we observe a PLCgamma-dependent increase of gelsolin incorporation upon EGF stimulation. At a molecular level, field emission scanning electron microscopy (FE-SEM) shows that gelsolin incorporates preferentially into the submembranous actin arcs at the leading edge of the lamellipodia. Together these data suggest a model of PIP(2) hydrolysis at the leading edge causing a localized release of PIP(2)-binding proteins-particularly gelsolin-that drives cytoskeletal rearrangement and protrusion.

Mapping the G-actin binding surface of cofilin using synchrotron protein footprinting

Cofilin is an actin regulatory protein that binds to both monomeric and filamentous actin, and has filament severing activity. Although crystal structures for the monomeric forms of both G-actin and cofilin have been described, the structure of the binary cofilin-G-actin complex is not available. Synchrotron protein footprinting is used to identify specific side chain residues on the cofilin surface that are buried in the formation of the cofilin-G-actin binary complex. Exposure to synchrotron X-rays results in stable oxidative modifications of aromatic, aliphatic, and sulfur-containing side chains, with the rate of modification for a particular residue being dependent on its intrinsic reactivity and solvent accessibility. The rates of modification were monitored for a number of peptides generated by digestion of oxidized cofilin, both in isolation and in its binary complex with G-actin. After binding to G-actin takes place, a significant decrease in modification rates, indicating protection of side chain groups, is seen for cofilin peptides corresponding to residues 4-20, 10-17, 83-96, 91-105, and 106-117. A number of other peptides show no change in reactivity, and are presumed to represent regions distal to the binding site. Tandem mass spectrometry demonstrates that residues Leu 13, Pro 94, Met 99, and Leu 108 and 112 directly participate in the binding interface. These results are generally consistent with, and complementary to, the results of previous site-directed mutagenesis studies and extend our understanding of the G-actin binding surface of cofilin.

Phosphoinositides regulate membrane-dependent actin assembly by latex bead phagosomes

Actin assembly on membrane surfaces is an elusive process in which several phosphoinositides (PIPs) have been implicated. We have reconstituted actin assembly using a defined membrane surface, the latex bead phagosome (LBP), and shown that the PI(4,5)P(2)-binding proteins ezrin and/or moesin were essential for this process (). Here, we provide several lines of evidence that both preexisting and newly synthesized PI(4,5)P(2), and probably PI(4)P, are essential for phagosomal actin assembly; only these PIPs were routinely synthesized from ATP during in vitro actin assembly. Treatment of LBP with phospholipase C or with adenosine, an inhibitor of type II PI 4-kinase, as well as preincubation with anti-PI(4)P or anti-PI(4,5)P(2) antibodies all inhibited this process. Incorporation of extra PI(4)P or PI(4,5)P(2) into the LBP membrane led to a fivefold increase in the number of phagosomes that assemble actin. An ezrin mutant mutated in the PI(4,5)P(2)-binding sites was less efficient in binding to LBPs and in reconstituting actin assembly than wild-type ezrin. Our data show that PI 4- and PI 5-kinase, and under some conditions also PI 3-kinase, activities are present on LBPs and can be activated by ATP, even in the absence of GTP or cytosolic components. However, PI 3-kinase activity is not required for actin assembly, because the process was not affected by PI 3-kinase inhibitors. We suggest that the ezrin-dependent actin assembly on the LBP membrane may require active turnover of D4 and D5 PIPs on the organelle membrane.

Tropomyosin inhibits ADF/cofilin-dependent actin filament dynamics

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

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.

Determining the differences in actin binding by human ADF and cofilin

The actin-depolymerizing factor (ADF)/cofilin family of proteins play an essential role in actin dynamics and cytoskeletal re-organization. Human tissues express two isoforms in the same cells, ADF and cofilin, and these two proteins are more than 70% identical in amino acid sequence. We show that ADF is a much more potent actin-depolymerizing agent than cofilin: the maximum level of depolymerization at pH 8 by ADF is about 20 microM compared to 5 microM for cofilin, but little depolymerization occurs at pH 6.5 with either protein. However, we find little difference between the two proteins in their binding to filaments, their severing activities or their activation of subunit release from the pointed ends of filaments. Likewise, they show no significant differences in their affinities for monomeric actin: both bind 15-fold more tightly to actin.ADP than to actin.ATP. Complexes between actin.ADP and ADF or cofilin associate with both barbed and pointed ends of filaments at similar rates (close to those of actin.ATP and much higher than those of actin.ADP). This explains why high concentrations of both proteins reverse the activation of subunit release at pointed ends. The major difference between the two proteins is that the nucleating activity of cofilin-actin.ADP complexes is twice that of ADF-actin.ADP complexes and this, in turn, is twice that of actin.ATP alone. It is this weaker nucleating potential of ADF-actin.ADP that accounts for the much higher steady-state depolymerizing activity. The pH-sensitivity is due to the nucleating activity of complexes being greater at pH 6.5 than at pH 8. Sequence analysis of mammalian and avian isoforms shows a consistent pattern of charge differences in regions of the protein associated with F-actin-binding that may account for the differences in activity between ADF and cofilin.

Human cofilin forms oligomers exhibiting actin bundling activity

Human cofilin possesses the tendency for self-association, as indicated by the rapid formation of dimers and oligomers when reacted with water-soluble carbodiimide, Ellman's reagent, or glutathione disulfide. Intermolecular disulfide bonds involve Cys(39) and probably Cys(147) of two adjacent cofilin units. The disulfide-linked dimers and oligomers exhibit a biological activity distinct from the monomer. While monomeric cofilin decreased viscosity and light-scattering of F-actin solutions, dimers and oligomers caused an increase in viscosity and light scattering. Electron microscopy revealed that cofilin oligomers induce the formation of highly ordered actin bundles with occasionally blunt ends similar to actin-cofilin rods observed in cells under oxidative stress. Bundling activity of the disulfide-linked oligomers could be completely reversed into severing activity by dithiothreitol. Formation of cofilin oligomers occurred also in the presence of actin at pH 8, but not at pH 6.6, and was significantly enhanced in the presence of phosphatidylinositol 4,5-bisphosphate. Our data are consistent with the idea that cofilin exists in two forms in vivo also: as monomers exhibiting the known severing activity and as oligomers exhibiting actin bundling activity. However, stabilization of cofilin oligomers in cytoplasm is probably achieved not by disulfide bonds but by a local increase in cofilin concentration and/or binding of regulatory proteins.

Identification of yeast cofilin residues specific for actin monomer and PIP2 binding

Cofilin/ADF is a ubiquitous actin-binding protein that is important for rapid actin dynamics in vivo. The long alpha-helix (helix 3 in yeast cofilin) forms the most highly conserved region in cofilin/ADF proteins, and residues in the NH2-terminal half of this alpha-helix have been shown to be essential for actin binding in cofilin/ADF. Recent studies also suggested that the basic residues in the COOH-terminal half of this alpha-helix would play an important role in F-actin binding. In contrast to these studies, we show here that the charged residues in the COOH-terminal half of helix 3 are not important for actin filament binding in yeast cofilin. Mutations in these residues, however, result in a small defect in actin monomer interactions. We also show that yeast cofilin can differentiate between various phosphatidylinositides, and mapped the PI(4,5)P2 binding site by using a collection of cofilin mutants. The PI(4,5)P2 binding site of yeast cofilin is a large positively charged surface that consists of residues in helix 3 as well as residues in other parts of the cofilin molecule. This suggests that cofilin/ADF proteins probably interact simultaneously with more than one PI(4,5)P2 molecule. The PI(4,5)P2-binding site overlaps with areas that are important for F-actin binding, explaining why the actin-related activities of cofilin/ADF are inhibited by PI(4,5)P2. The biological roles of actin and PI(4,5)P2 interactions of cofilin are discussed in light of phenotypes of specific yeast strains carrying mutations in residues that are important for actin and PI(4,5)P2 binding.

The second ADF/cofilin actin-binding site exists in F-actin, the cofilin-G-actin complex, but not in G-actin

ADF/cofilins are actin binding proteins that bind actin close to both the N- and C-termini (site 1), and we have found a second cofilin binding site (site 2) centered around helix 112-125 [Renoult, C., Ternent, D., Maciver, S.K., Fattoum, A., Astier, C., Benyamin, Y. & Roustan, C. (1999) J. Biol. Chem. 274, 28893-28899]. We proposed a model in which ADF/cofilin intercalated between subdomains 1 and 2 of two longitudinally associated actin monomers within the actin:cofilin cofilament, explaining the change in twist that ADF/cofilins induce in the filament [McGough, A. Pope, B., Chiu, W. & Weeds, A. (1998) J. Cell Biol. 138, 771-781]. Here, we have determined the fuller extent of the cofilin footprint on site 1 of actin. Site 1 is primarily the G-actin binding site. Experiments with both peptide mimetics and fluorescently labeled cofilin suggest that site 2 only becomes available for cofilin binding within the filament, possibly due to motion between subdomains 1 and 2 within an actin monomer. We have detected motion between subdomains 1 and 2 of G-actin by FRET induced by cofilin, to reveal the second cofilin-binding site. This motion may also explain how cofilins inhibit the nucleotide exchange of actin, and why the actin:cofilin complex is polymerizable without dissociation.

Ischemic injury induces ADF relocalization to the apical domain of rat proximal tubule cells

Breakdown of proximal tubule cell apical membrane microvilli is an early-occurring hallmark of ischemic acute renal failure. Intracellular mechanisms responsible for these apical membrane changes remain unknown, but it is known that actin cytoskeleton alterations play a critical role in this cellular process. Our laboratory previously demonstrated that ischemia-induced cell injury resulted in dephosphorylation and activation of the actin-binding protein, actin depolymerizing factor [(ADF); Schwartz, N, Hosford M, Sandoval RM, Wagner MC, Atkinson SJ, Bamburg J, and Molitoris BA. Am J Physiol Renal Fluid Electrolyte Physiol 276: F544-F551, 1999]. Therefore, we postulated that ischemia-induced ADF relocalization from the cytoplasm to the apical microvillar microfilament core was an early event occurring before F-actin alterations. To directly investigate this hypothesis, we examined the intracellular localization of ADF in ischemic rat cortical tissues by immunofluorescence and quantified the concentration of ADF in brush-border membrane vesicles prepared from ischemic rat kidneys by using Western blot techniques. Within 5 min of the induction of ischemia, ADF relocalized to the apical membrane region. The length of ischemia correlated with the time-related increase in ADF in isolated brush-border membrane vesicles. Finally, depolymerization of microvillar F-actin to G-actin was documented by using colocalization studies for G- and F-actin. Collectively, these data indicate that ischemia induces ADF activation and relocalization to the apical domain before microvillar destruction. These data further suggest that ADF plays a critical role in microvillar microfilament destruction and apical membrane damage during ischemia.

Phosphorylation of cofilin by LIM-kinase is necessary for semaphorin 3A-induced growth cone collapse

Semaphorin 3A is a chemorepulsive axonal guidance molecule that depolymerizes the actin cytoskeleton and collapses growth cones of dorsal root ganglia neurons. Here we investigate the role of LIM-kinase 1, which phosphorylates an actin-depolymerizing protein, cofilin, in semaphorin 3A-induced growth cone collapse. Semaphorin 3A induced phosphorylation and dephosphorylation of cofilin at growth cones sequentially. A synthetic cell-permeable peptide containing a cofilin phosphorylation site inhibited LIM-kinase in vitro and in vivo, and essentially suppressed semaphorin 3A-induced growth cone collapse. A dominant-negative LIM kinase, which could not be activated by PAK or ROCK, suppressed the collapsing activity of semaphorin 3A. Phosphorylation of cofilin by LIM-kinase may be a critical signaling event in growth cone collapse by semaphorin 3A.

Phosphatidylinositol 4,5-bisphosphate induces actin stress-fiber formation and inhibits membrane ruffling in CV1 cells

Phosphatidylinositol 4,5 bisphosphate (PIP(2)) is widely implicated in cytoskeleton regulation, but the mechanisms by which PIP(2) effect cytoskeletal changes are not defined. We used recombinant adenovirus to infect CV1 cells with the mouse type I phosphatidylinositol phosphate 5-kinase alpha (PIP5KI), and identified the players that modulate the cytoskeleton in response to PIP(2) signaling. PIP5KI overexpression increased PIP(2) and reduced phosphatidylinositol 4 phosphate (PI4P) levels. It promoted robust stress-fiber formation in CV1 cells and blocked PDGF-induced membrane ruffling and nucleated actin assembly. Y-27632, a Rho-dependent serine/threonine protein kinase (ROCK) inhibitor, blocked stress-fiber formation and inhibited PIP(2) and PI4P synthesis in cells. However, Y-27632 had no effect on PIP(2) synthesis in lysates, although it inhibited PI4P synthesis. Thus, ROCK may regulate PIP(2) synthesis by controlling PI4P availability. PIP5KI overexpression decreased gelsolin, profilin, and capping protein binding to actin and increased that of ezrin. These changes can potentially account for the increased stress fiber and nonruffling phenotype. Our results establish the physiological role of PIP(2) in cytoskeletal regulation, clarify the relation between Rho, ROCK, and PIP(2) in the activation of stress-fiber formation, and identify the key players that modulate the actin cytoskeleton in response to PIP(2).

Regulation of a wheat actin-depolymerizing factor during cold acclimation

We have previously shown that the wheat (Triticum aestivum) TaADF gene expression level is correlated with the plants capacity to tolerate freezing. Sequence analysis revealed that this gene encodes a protein homologous to members of the actin-depolymerizing factor (ADF)/cofilin family. We report here on the characterization of the recombinant TaADF protein. Assays for ADF activity showed that TaADF is capable of sequestering actin, preventing nucleotide exchange, and inducing actin depolymerization. In vitro phosphorylation studies showed that TaADF is a substrate for a wheat 52-kD kinase. The activity of this kinase is modulated by low temperature during the acclimation period. Western-blot analyses revealed that TaADF is expressed only in cold-acclimated Gramineae species and that the accumulation level is much higher in the freezing-tolerant wheat cultivars compared with the less tolerant ones. This accumulation was found to be regulated by a factor(s) encoded by a gene(s) located on chromosome 5A, the chromosome most often found to be associated with cold hardiness. The induction of an active ADF during cold acclimation and the correlation with an increased freezing tolerance suggest that the protein may be required for the cytoskeletal rearrangements that may occur upon low temperature exposure. These remodelings might be important for the enhancement of freezing tolerance.

Intracellular pH modulation of ADF/cofilin proteins

The ADF/cofilin (AC) proteins are necessary for the high rates of actin filament turnover seen in vivo. Their regulation is complex enough to underlie the precision in filament dynamics needed by stimulated cells. Disassembly of actin by AC proteins is inhibited in vitro by phosphorylation of ser3 and pH<7.1. This study of Swiss 3T3 cells demonstrates that pH also affects AC behavior in vivo: (1) Wounded cells show pH-dependent AC translocation to alkaline-induced ruffling membrane; (2) The Triton extractable (soluble) ADF from Swiss 3T3 cells decreases from 42+/-4% to 23+/-4% when the intracellular pH (pH(i)) is reduced from 7.4 to 6.6; (3) Covariance and colocalization analyses of immunostained endogenous proteins show that ADF partitions more with monomeric actin and less with polymeric actin when pH(i) increases. However, the distribution of cofilin, a less pH-sensitive AC in vitro, does not change with pH; (4) Only the unphosphorylatable AC mutant (A3), when overexpressed as a GFP chimera, uniquely produces aberrant cellular phenotypes and only if the pH is shifted from 7.1 to 6.6 or 7.4. A mechanism is proposed that explains why AC(A3)-GFP and AC(wt)-GFP chimeras generate different phenotypes in response to pH changes. Phospho-AC levels increase with cell density, and in motile cells, phospho-AC increases with alkalization, suggesting a homeostatic mechanism that compensates for increased AC activity and filament turnover. These results show that the behavior of AC proteins with pH-sensitivity in vitro is affected by pH in vivo.

The actin cytoskeleton and plasma membrane connection: PtdIns(4,5)P(2) influences cytoskeletal protein activity at the plasma membrane

The co-ordination of rearrangements of the actin cytoskeleton depends on its tight connection to the plasma membrane. Phosphatidylinositol 4,5-bisphosphate is thought to transmit signals originating at the plasma membrane to the underlying actin cytoskeleton. This lipid binds to, and influences the activity of, several actin-associated proteins in vitro that regulate the architecture of the actin cytoskeleton. Signalling intermediates in this process include focal adhesion molecules such as vinculin and members of two families of proteins, ERM and WASP. These proteins interact with phosphatidylinositol 4,5-bisphosphate and appear to be regulated by interplay between small GTPases and phosphatidylinositol 4,5-bisphosphate metabolism, and thus link the plasma membrane with cytoskeletal remodelling.

The competitive interaction of actin and PIP2 with actophorin is based on overlapping target sites: design of a gain-of-function mutant

We studied the effect of mutations in an alpha-helical region of actophorin (residues 91-108) on F-actin and PIP(2) binding. As in cofilin, residues in the NH(2)-terminal half of this region are involved in F-actin binding. We show here also that basic residues in the COOH-terminal half of the region participate in this interaction whereby we extend the previously defined actin binding interface [Lappalainen, P., et al. (1997) EMBO J. 16, 5520-5530]. In addition, we demonstrate that some of the lysines in this alpha-helical region in actophorin are implicated in PIP(2) binding. This indicates that the binding sites of F-actin and PIP(2) on actophorin overlap, explaining the mutually exclusive binding of these ligands. The Ca(2+)-dependent F-actin binding of a number of actophorin mutants (carrying a lysine to glutamic acid substitution at the COOH-terminal positions of the actin binding helical region) may mimic the behavior of members of the gelsolin family. In addition, we show that PIP(2) binding, but not actin binding, of actophorin is strongly enhanced by a point mutation that leads to a reinforcement of the positive electrostatic potential of the studied alpha-helical region.

Uncoupling actin filament fragmentation by cofilin from increased subunit turnover

The actin depolymerizing factor (ADF)/cofilin family of proteins interact with actin monomers and filaments in a pH-sensitive manner. When ADF/cofilin binds F-actin it induces a change in the helical twist and fragmentation; it also accelerates the dissociation of subunits from the pointed ends of filaments, thereby increasing treadmilling or depolymerization. Using site-directed mutagenesis we characterized the two actin-binding sites on human cofilin. One target site was chosen because we previously showed that the villin head piece competes with ADF for binding to F-actin. Limited sequence homology between ADF/cofilin and the part of the villin headpiece essential for actin binding suggested an actin-binding site on cofilin involving a structural loop at the opposite end of the molecule to the alpha-helix already implicated in actin binding. Binding through the alpha-helix is primarily to monomeric actin, whereas the loop region is specifically involved in filament association. We have characterized the actin binding properties of each site independently of the other. Mutation of a single lysine residue in the loop region abolishes binding to filaments, but not to monomers. Using the mutation analogous to the phosphorylated form of cofilin (S3D), we show that filament binding is inhibited at physiological ionic strength but not under low salt conditions. At low ionic strength, this mutant induces both the twist change and fragmentation characteristic of wild-type cofilin, but does not activate subunit dissociation. The results suggest a two-site binding to filaments, initiated by association through the loop site, followed by interaction with the adjacent subunit through the "helix" site at the opposite end of the molecule. Together, these interactions induce twist and fragmentation of filaments, but the twist change itself is not responsible for the enhanced rate of actin subunit release from filaments.

A protein kinase from neutrophils that specifically recognizes Ser-3 in cofilin

Cofilin promotes the depolymerization of actin filaments, which is required for a variety of cellular responses such as the formation of lamellipodia and chemotaxis. Phosphorylation of cofilin on serine residue 3 is known to block these activities. We now report that neutrophils contain a protein kinase that selectively catalyzes the phosphorylation of cofilin on serine 3 (>/=70%) and a nonspecific kinase that recognizes multiple sites in this protein. The selective serine 3 cofilin kinase binds to a deoxyribonuclease I affinity column, whereas the nonspecific cofilin kinase does not. Deoxyribonuclease I forms a very tight complex with actin, and deoxyribonuclease affinity columns have been utilized to identify a variety of proteins that interact with the cytoskeleton. The serine 3 cofilin kinase did not react with antibodies to LIM kinase 1 or 2, which can catalyze the phosphorylation of cofilin in other cell types. The activity of the serine 3 cofilin kinase was insensitive to a variety of selective antagonists of protein kinases but was blocked by staurosporine. This pattern of inhibition is similar to that observed for the kinase that is active with cofilin in intact neutrophils. Thus, neutrophils contain a protein kinase distinct from LIM kinase-1/2 that selectively recognizes serine 3 in cofilin.

Phosphatidylinositol 4,5-bisphosphate functions as a second messenger that regulates cytoskeleton-plasma membrane adhesion

Binding interactions between the plasma membrane and the cytoskeleton define cell functions such as cell shape, formation of cell processes, cell movement, and endocytosis. Here we use optical tweezers tether force measurements and show that plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2) acts as a second messenger that regulates the adhesion energy between the cytoskeleton and the plasma membrane. Receptor stimuli that hydrolyze PIP2 lowered adhesion energy, a process that could be mimicked by expressing PH domains that sequester PIP2 or by targeting a 5'-PIP2-phosphatase to the plasma membrane to selectively lower plasma membrane PIP2 concentration. Our study suggests that plasma membrane PIP2 controls dynamic membrane functions and cell shape by locally increasing and decreasing the adhesion between the actin-based cortical cytoskeleton and the plasma membrane.

Proteins of the ADF/cofilin family: essential regulators of actin dynamics

Ubiquitous among eukaryotes, the ADF/cofilins are essential proteins responsible for the high turnover rates of actin filaments in vivo. In vertebrates, ADF and cofilin are products of different genes. Both bind to F-actin cooperatively and induce a twist in the actin filament that results in the loss of the phalloidin-binding site. This conformational change may be responsible for the enhancement of the off rate of subunits at the minus end of ADF/cofilin-decorated filaments and for the weak filament-severing activity. Binding of ADF/cofilin is competitive with tropomyosin. Other regulatory mechanisms in animal cells include binding of phosphoinositides, phosphorylation by LIM kinases on a single serine, and changes in pH. Although vertebrate ADF/cofilins contain a nuclear localization sequence, they are usually concentrated in regions containing dynamic actin pools, such as the leading edge of migrating cells and neuronal growth cones. ADF/cofilins are essential for cytokinesis, phagocytosis, fluid phase endocytosis, and other cellular processes dependent upon actin dynamics.

Cofilin phosphorylation and actin cytoskeletal dynamics regulated by rho- and Cdc42-activated LIM-kinase 2

The rapid turnover of actin filaments and the tertiary meshwork formation are regulated by a variety of actin-binding proteins. Protein phosphorylation of cofilin, an actin-binding protein that depolymerizes actin filaments, suppresses its function. Thus, cofilin is a terminal effector of signaling cascades that evokes actin cytoskeletal rearrangement. When wild-type LIMK2 and kinase-dead LIMK2 (LIMK2/KD) were respectively expressed in cells, LIMK2, but not LIMK2/KD, phosphorylated cofilin and induced formation of stress fibers and focal complexes. LIMK2 activity toward cofilin phosphorylation was stimulated by coexpression of activated Rho and Cdc42, but not Rac. Importantly, expression of activated Rho and Cdc42, respectively, induced stress fibers and filopodia, whereas both Rho- induced stress fibers and Cdc42-induced filopodia were abrogated by the coexpression of LIMK2/KD. In contrast, the coexpression of LIMK2/KD with the activated Rac did not affect Rac-induced lamellipodia formation. These results indicate that LIMK2 plays a crucial role both in Rho- and Cdc42-induced actin cytoskeletal reorganization, at least in part by inhibiting the functions of cofilin. Together with recent findings that LIMK1 participates in Rac-induced lamellipodia formation, LIMK1 and LIMK2 function under control of distinct Rho subfamily GTPases and are essential regulators in the Rho subfamilies-induced actin cytoskeletal reorganization.