An actin-interacting heptapeptide in the cofilin sequence

Expression and Functional Analysis of cofilin1-like in Craniofacial Development in Zebrafish

Pharyngeal pouches, a series of outgrowths of the pharyngeal endoderm, are a key epithelial structure governing facial skeleton development in vertebrates. Pouch formation is achieved through collective cell migration and rearrangement of pouch-forming cells controlled by actin cytoskeleton dynamics. While essential transcription factors and signaling molecules have been identified in pouch formation, regulators of actin cytoskeleton dynamics have not been reported yet in any vertebrates. Cofilin1-like (Cfl1l) is a fish-specific member of the Actin-depolymerizing factor (ADF)/Cofilin family, a critical regulator of actin cytoskeleton dynamics in eukaryotic cells. Here, we report the expression and function of cfl1l in pouch development in zebrafish. We first showed that fish cfl1l might be an ortholog of vertebrate adf, based on phylogenetic analysis of vertebrate adf and cfl genes. During pouch formation, cfl1l was expressed sequentially in the developing pouches but not in the posterior cell mass in which future pouch-forming cells are present. However, pouches, as well as facial cartilages whose development is dependent upon pouch formation, were unaffected by loss-of-function mutations in cfl1l. Although it could not be completely ruled out a possibility of a genetic redundancy of Cfl1l with other Cfls, our results suggest that the cfl1l expression in the developing pouches might be dispensable for regulating actin cytoskeleton dynamics in pouch-forming cells.

The remodelling of actin composition as a hallmark of cancer

Actin is a key structural protein that makes up the cytoskeleton of cells, and plays a role in functions such as division, migration, and vesicle trafficking. It comprises six different cell-type specific isoforms: ACTA1, ACTA2, ACTB, ACTC1, ACTG1, and ACTG2. Abnormal actin isoform expression has been reported in many cancers, which led us to hypothesize that it may serve as an early biomarker of cancer. We show an overview of the different actin isoforms and highlight mechanisms by which they may contribute to tumorigenicity. Furthermore, we suggest how the aberrant expression of actin subunits can confer cells with greater proliferation ability, increased migratory capability, and chemoresistance through incorporation into the normal cellular F-actin network and altered actin binding protein interaction. Studying this fundamental change that takes place within cancer cells can further our understanding of neoplastic transformation in multiple tissue types, which can ultimately aid in the early-detection, diagnosis and treatment of cancer.

STIP1/HOP Regulates the Actin Cytoskeleton through Interactions with Actin and Changes in Actin-Binding Proteins Cofilin and Profilin

Cell migration plays a vital role in both health and disease. It is driven by reorganization of the actin cytoskeleton, which is regulated by actin-binding proteins cofilin and profilin. Stress-inducible phosphoprotein 1 (STIP1) is a well-described co-chaperone of the Hsp90 chaperone system, and our findings identify a potential regulatory role of STIP1 in actin dynamics. We show that STIP1 can be isolated in complex with actin and Hsp90 from HEK293T cells and directly interacts with actin in vitro via the C-terminal TPR2AB-DP2 domain of STIP1, potentially due to a region spanning two putative actin-binding motifs. We found that STIP1 could stimulate the in vitro ATPase activity of actin, suggesting a potential role in the modulation of F-actin formation. Interestingly, while STIP1 depletion in HEK293T cells had no major effect on total actin levels, it led to increased nuclear accumulation of actin, disorganization of F-actin structures, and an increase and decrease in cofilin and profilin levels, respectively. This study suggests that STIP1 regulates the cytoskeleton by interacting with actin, or via regulating the ratio of proteins known to affect actin dynamics.

Regulation of the Plasma Membrane Calcium ATPases by the actin cytoskeleton

Associations between the cortical cytoskeleton and the components of the plasma membrane are no longer considered to be merely of structural and mechanical nature but are nowadays recognized as dynamic interactions that modulate a plethora of cellular responses. Reorganization of actin filaments upon diverse stimuli - among which is the rise in cytosolic Ca²⁺ - is involved in cell motility and adhesion, phagocytosis, cytokinesis, and secretion. Actin dynamics also participates in the regulation of ion transport across the membranes where it not only plays a key role in the delivery and stabilization of channels and transporters in the plasma membrane but also in the regulation of their activity. The recently described functional interaction between actin and the Plasma Membrane Ca²⁺-ATPase (PMCA) represents a novel regulatory mechanism of the pump at the time that unveils a new pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca²⁺ homeostasis. In this review, we summarize the current knowledge on the interaction between the cortical actin cytoskeleton and the PMCA and discuss the possible mechanisms that may explain the pump's modulation.

Interleukin-13 increases pendrin abundance to the cell surface in bronchial NCI-H292 cells via Rho/actin signaling

Interleukin-13 (IL13) is a major player in the development of airway hyperresponsiveness in several respiratory disorders. Emerging data suggest that an increased expression of pendrin in airway epithelia is associated with elevated airway hyperreactivity in asthma. Here, we investigate the effect of IL13 on pendrin localization and function using bronchiolar NCI-H292 cells. The data obtained revealed that IL13 increases the cell surface expression of pendrin. This effect was paralleled by a significant increase in the intracellular pH, possibly via indirect stimulation of NHE. IL13 effect on pendrin localization and intracellular pH was reversed by theophylline, a bronchodilator compound used to treat asthma. IL13 upregulated RhoA activity, a crucial protein controlling actin dynamics, via G-alpha-13. Specifically, IL13 stabilized actin cytoskeleton and promoted co-localization and a direct molecular interaction between pendrin and F-actin in the plasma membrane region. These effects were reversed following exposure of cells to theophylline. Selective inhibition of Rho kinase, a downstream effector of Rho, reduced the IL13-dependent cell surface expression of pendrin. Together, these data indicate that IL13 increases pendrin abundance to the cell surface via Rho/actin signaling, an effect reversed by theophylline.

Plasma membrane calcium ATPase activity is regulated by actin oligomers through direct interaction

As recently described by our group, plasma membrane calcium ATPase (PMCA) activity can be regulated by the actin cytoskeleton. In this study, we characterize the interaction of purified G-actin with isolated PMCA and examine the effect of G-actin during the first polymerization steps. As measured by surface plasmon resonance, G-actin directly interacts with PMCA with an apparent 1:1 stoichiometry in the presence of Ca(2+) with an apparent affinity in the micromolar range. As assessed by the photoactivatable probe 1-O-hexadecanoyl-2-O-[9-[[[2-[(125)I]iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, the association of PMCA to actin produced a shift in the distribution of the conformers of the pump toward a calmodulin-activated conformation. G-actin stimulates Ca(2+)-ATPase activity of the enzyme when incubated under polymerizing conditions, displaying a cooperative behavior. The increase in the Ca(2+)-ATPase activity was related to an increase in the apparent affinity for Ca(2+) and an increase in the phosphoenzyme levels at steady state. Although surface plasmon resonance experiments revealed only one binding site for G-actin, results clearly indicate that more than one molecule of G-actin was needed for a regulatory effect on the pump. Polymerization studies showed that the experimental conditions are compatible with the presence of actin in the first stages of assembly. Altogether, these observations suggest that the stimulatory effect is exerted by short oligomers of actin. The functional interaction between actin oligomers and PMCA represents a novel regulatory pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca(2+) homeostasis.

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.

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.

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 actin-depolymerizing factor destrin has an actin-stabilizing domain

Destrin is a 19 kDa actin-depolymerizing protein of the ADF–cofilin family. Destrin was digested with trypsin to a structurally stable 9.2 kDa fragment that contains the actin-binding sequence. The purified 9.2 kDa fragment has an actin filament stabilizing activity, rather than an actin filament depolymerizing activity. The deleted region is probably essential for the actin filament depolymerizing activity of intact destrin. Surprisingly, the 9.2 kDa fragment also has an assembly-promoting activity in the absence of ATP.Key words: actin, destrin, cofilin, ADF.

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.

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.

Espin contains an additional actin-binding site in its N terminus and is a major actin-bundling protein of the Sertoli cell-spermatid ectoplasmic specialization junctional plaque

The espins are actin-binding and -bundling proteins localized to parallel actin bundles. The 837-amino-acid "espin" of Sertoli cell-spermatid junctions (ectoplasmic specializations) and the 253-amino-acid "small espin" of brush border microvilli are splice isoforms that share a C-terminal 116-amino-acid actin-bundling module but contain different N termini. To investigate the roles of espin and its extended N terminus, we examined the actin-binding and -bundling properties of espin constructs and the stoichiometry and developmental accumulation of espin within the ectoplasmic specialization. An espin construct bound to F-actin with an approximately threefold higher affinity (K(d) = approximately 70 nM) than small espin and was approximately 2.5 times more efficient at forming bundles. The increased affinity appeared to be due to an additional actin-binding site in the N terminus of espin. This additional actin-binding site bound to F-actin with a K(d) of approximately 1 microM, decorated actin stress fiber-like structures in transfected cells, and was mapped to a peptide between the two proline-rich peptides in the N terminus of espin. Espin was detected at approximately 4-5 x 10(6) copies per ectoplasmic specialization, or approximately 1 espin per 20 actin monomers and accumulated there coincident with the formation of parallel actin bundles during spermiogenesis. These results suggest that espin is a major actin-bundling protein of the Sertoli cell-spermatid ectoplasmic specialization.

Structural features of LIM kinase that control effects on the actin cytoskeleton

LIM kinase phosphorylates and inactivates the actin binding/depolymerizing factor cofilin and induces actin cytoskeletal changes. Several unique structural features within LIM kinase were investigated for their roles in regulation of LIM kinase activity. Disruption of the second LIM domain or the PDZ domain or deletion of the entire amino terminus increased activity in vivo measured as increasing aggregation of the actin cytoskeleton. A kinase-deleted alternate splice product was identified and characterized. This alternate splice product and a kinase inactive mutant inhibited LIM kinase in vivo, indicating that the amino terminus suppresses activity of the kinase domain. Mutation of threonine 508 in the activation loop to valine abolished activity whereas replacement with 2 glutamic acid residues resulted in a fully active enzyme. Dephosphorylation of LIM kinase inhibited cofilin phosphorylation. Mutation of the basic insert in the activation loop inhibited activity in vivo, but not in vitro. These results indicate phosphorylation is an essential regulatory feature of LIM kinase and indicate that threonine 508 and the adjacent basic insert sequences of the activation loop are required for this process. A combination of structural features are thus involved in receiving upstream signals that regulate LIM kinase-induced actin cytoskeletal reorganization.

Structural modules in actin-binding proteins: towards a new classification

The number of actin binding proteins for which (part of) the three-dimensional structure is known, is steadily increasing. This has led to a picture in which defined structural modules with actin binding capacity are shared between different actin binding proteins. A classification of these based on their common three-dimensional modules appears a logical future step and in this review we provide an initial list starting from the currently known structures. The discussed cases illustrate that a comparison of the similarities and variations within the common structural actin binding unit of different members of a particular class may ultimately provide shortcuts for defining their actin target site and for understanding their effect on actin dynamics. Within this concept, the multitude of possible interactions by an extensive, and still increasing, list of actin binding proteins becomes manageable because they can be presented as variations upon a limited number of structural themes. We discuss the possible evolutionary routes that may have produced the present array of actin binding modules.

Cofilin and gelsolin segment-1: molecular dynamics simulation and biochemical analysis predict a similar actin binding mode

An understanding of the actin-depolymerizing function attributed to members of the ADF/cofilin/destrin superfamily requires a structural model of these proteins in complex with actin. As a step toward defining actin-cofilin interactions, the complex of yeast cofilin with monomeric actin was predicted, starting with the actin-gelsolin segment-1 binding mode recently suggested for the actin-destrin complex. After refinement by molecular dynamics simulation, the structure of cofilin converged in a new binding mode that required only minimal changes induced in the actin-cofilin interface. The predicted complex exhibits strong interactions between the N termini of actin and cofilin, mediated by a salt bridge of cofilin Arg3 with actin Asp1. The forming of this salt bridge could be prevented by the phosphorylation of cofilin Ser4, which is believed to inhibit cofilin depolymerization activity. Recent mutagenesis studies, crosslinking experiments and peptide binding studies are consistent with the predicted model of the actin-cofilin complex. The structural homology between cofilin and gelsolin segment-1 binding to actin was confirmed experimentally by two types of competitive binding assays.

Interaction of actin monomers with Acanthamoeba actophorin (ADF/cofilin) and profilin

Acanthamoeba actophorin is a member of ADF/cofilin family that binds both actin monomers and filaments. We used fluorescence anisotropy to study the interaction of actin monomers with recombinant actophorin labeled with rhodamine on a cysteine substituted for Serine-88. Labeled actophorin retains its affinity for actin and ability to reduce the low shear viscosity of actin filaments. At physiological ionic strength, actophorin binds Mg-ADP-actin monomers (Kd = 0.1 microM) 40 times stronger than Mg-ATP-actin monomers. When bound to actin monomers, actophorin has no effect on elongation at either end of actin filaments by Mg-ATP-actin and slightly increases the rate of elongation at both ends by Mg-ADP-actin. Thus actophorin does not sequester actin monomers. Sedimentation equilibrium ultracentrifugation shows that actophorin and profilin compete for binding actin monomers. Actophorin and profilin have opposite effects on the rate of exchange of nucleotide bound to actin monomers. Despite the high affinity of actophorin for ADP-actin, physiological concentrations of profilin overcome the inhibition of ADP exchange by actophorin. Profilin rapidly recycles ADP-actin back to the profilin-ATP-actin pool ready for elongation of actin filaments.

Capping and dynamic relation between domains 1 and 2 of gelsolin

Gelsolin is a protein that severs and caps actin filaments. The two activities are located in the N-terminal half of the gelsolin molecules. Severing and subsequent capping requires the binding of domains 2 and 3 (S2-3) to the side of the filaments to position the N-terminal domain 1 (S1) at the barbed end of actin (actin subdomains 1 and 3). The results provide a structural basis for the gelsolin capping mechanism. The effects of a synthetic peptide derived from the sequence of a binding site located in gelsolin S2 on actin properties have been studied. CD and IR spectra indicate that this peptide presented a secondary structure in solution which would be similar to that expected for the native full length gelsolin molecule. The binding of the synthetic peptide induces conformational changes in actin subdomain 1 and actin oligomerization. An increase in the polymerization rate was observed, which could be attributed to a nucleation kinetics effect. The combined effects of two gelsolin fragments, the synthetic peptide derived from an S2 sequence and the purified segment 1 (S1), were also investigated as a molecule model. The two fragments induced nucleation enhancement and inhibited actin depolymerization, two characteristic properties of capping. In conclusion, for the first time it is reported that the binding of a small synthetic fragment is sufficient to promote efficient capping by S1 at the barbed end of actin filaments.

Analogous F-actin binding by cofilin and gelsolin segment 2 substantiates their structural relationship

Cofilin is representative for a family of low molecular weight actin filament binding and depolymerizing proteins. Recently the three-dimensional structure of yeast cofilin and of the cofilin homologs destrin and actophorin were resolved, and a striking similarity to segments of gelsolin and related proteins was observed (Hatanaka, H., Ogura, K., Moriyama, K., Ichikawa, S., Yahara, I., and Inagaka, F. (1996) Cell 85, 1047-1055; Fedorov, A. A., Lappalainen, P., Fedorov, E. V., Drubin, D. G., and Almo, S. C. (1997) Nat. Struct. Biol. 4, 366-369; Leonard, S. A., Gittis, A. G., Petrella, E. C., Pollard, T. D., and Lattman, E. E. (1997) Nat. Struct. Biol. 4, 369-373). Using peptide mimetics, we show that the actin binding site stretches over the entire cofilin alpha-helix 112-128. In addition, we demonstrate that cofilin and its actin binding peptide compete with gelsolin segments 2-3 for binding to actin filaments. Based on these competition data, we propose that cofilin and segment 2 of gelsolin use a common structural topology to bind to actin and probably share a similar target site on the filament. This adds a functional dimension to their reported structural homology, and this F-actin binding mode provides a basis to further enlighten the effect of members of the cofilin family on actin filament dynamics.

Glycogenin, the primer of glycogen synthesis, binds to actin

We have studied the intracellular localization of glycogenin by fusing green fluorescent protein (GFP) to the N-terminus of rabbit muscle glycogenin and expressing the chimeric protein in C2C12, COS-1 and rat hepatic cells. The fusion protein showed a nuclear and cytosolic distribution and partially co-localized with actin in the cytosol. Disruption of the actin cytoskeleton with cytochalasin D led to a change in the pattern of green fluorescence, which coincided with that observed for the remaining non-depolymerized actin. The distribution of the single point mutant K324A was completely uniform and was not affected by this drug. These findings indicate that rabbit muscle glycogenin binds to actin through the heptapeptide 321DNIKKKL327, a common motif found in other actin-binding proteins, which is located at the C-terminal end of this protein, and suggest that the actin cytoskeleton plays an important role in glycogen metabolism.

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

Cofilin stimulates actin filament turnover in vivo. The phenotypes of twenty yeast cofilin mutants generated by systematic mutagenesis were determined. Ten grew as well as the wild type and showed no cytoskeleton defects, seven were recessive-lethal and three were conditional-lethal and caused severe actin organization defects. Biochemical characterization of interactions between nine mutant yeast cofilins and yeast actin provided evidence that F-actin binding and depolymerization are essential cofilin functions. Locating the mutated residues on the yeast cofilin molecular structure allowed several important conclusions to be drawn. First, residues required for actin monomer binding are proximal to each other. Secondly, additional residues are required for interactions with actin filaments; these residues might bind an adjacent subunit in the actin filament. Thirdly, despite striking structural similarity, cofilin interacts with actin in a different manner from gelsolin segment-1. Fourthly, a previously unrecognized cofilin function or interaction is suggested by identification of spatially proximal residues important for cofilin function in vivo, but not for actin interactions in vitro. Finally, mutation of the cofilin N-terminus suggests that its sequence is conserved because of its critical role in actin interactions, not because it is sometimes a target for protein kinases.

F-actin and G-actin binding are uncoupled by mutation of conserved tyrosine residues in maize actin depolymerizing factor (ZmADF)

Actin depolymerizing factors (ADF) are stimulus responsive actin cytoskeleton modulating proteins. They bind both monomeric actin (G-actin) and filamentous actin (F-actin) and, under certain conditions, F-actin binding is followed by filament severing. In this paper, using mutant maize ADF3 proteins, we demonstrate that the maize ADF3 binding of F-actin can be spatially distinguished from that of G-actin. One mutant, zmadf3-1, in which Tyr-103 and Ala-104 (equivalent to destrin Tyr-117 and Ala-118) have been replaced by phenylalanine and glycine, respectively, binds more weakly to both G-actin and F-actin compared with maize ADF3. A second mutant, zmadf3-2, in which both Tyr-67 and Tyr-70 are replaced by phenylalanine, shows an affinity for G-actin similar to maize ADF3, but F-actin binding is abolished. The two tyrosines, Tyr-67 and Tyr-70, are in the equivalent position to Tyr-82 and Tyr-85 of destrin, respectively. Using the tertiary structure of destrin, yeast cofilin, and Acanthamoeba actophorin, we discuss the implications of removing the aromatic hydroxyls of Tyr-82 and Tyr-85 (i.e., the effect of substituting phenylalanine for tyrosine) and conclude that Tyr-82 plays a critical role in stabilizing the tertiary structure that is essential for F-actin binding. We propose that this tertiary structure is maintained as a result of a hydrogen bond between the hydroxyl of Tyr-82 and the carbonyl of Tyr-117, which is located in the long alpha-helix; amino acid components of this helix (Leu-111 to Phe-128) have been implicated in G-actin and F-actin binding. The structures of human destrin and yeast cofilin indicate a hydrogen distance of 2.61 and 2.77 A, respectively, with corresponding bond angles of 99.5 degrees and 113 degrees, close to the optimum for a strong hydrogen bond.

Crystal structure of the actin-binding protein actophorin from Acanthamoeba

Actophorin is a member of the actin-depolymerizing factor/cofilin family. It severs actin filaments and sequesters actin monomers. The crystal structure of actophorin will help to elucidate actin-ADF/cofilin interactions.

Tertiary structure of destrin and structural similarity between two actin-regulating protein families

Destrin is an isoprotein of cofilin that regulates actin cytoskeleton in various eukaryotes. We determined the tertiary structure of destrin by triple-resonance multidimensional nuclear magnetic resonance. In spite of there being no significant amino acid sequence homology, we found that the folding of destrin was strikingly similar to that of repeated segments in the gelsolin family, which resulted in a new protein fold group. Sequential dissimilarity of the actin-binding helix of destrin to that of gelsolin explains the Ca2+-independent actin-binding of destrin. Possible mechanisms of phosphorylation-sensitive phosphoinositide-competitive actin binding, of pH-dependent filament severing, and of nuclear translocation with actin in response to stresses, are discussed on the basis of the tertiary structure.

Mutations in twinstar, a Drosophila gene encoding a cofilin/ADF homologue, result in defects in centrosome migration and cytokinesis

We describe the phenotypic and molecular characterization of twinstar (tsr), an essential gene in Drosophila melanogaster. Two P-element induced alleles of tsr (tsr1 and tsr2) result in late larval or pupal lethality. Cytological examination of actively dividing tissues in these mutants reveals defects in cytokinesis in both mitotic (larval neuroblast) and meiotic (larval testis) cells. In addition, mutant spermatocytes show defects in aster migration and separation during prophase/prometaphase of both meiotic divisions. We have cloned the gene affected by these mutations and shown that it codes for a 17-kD protein in the cofilin/ADF family of small actin severing proteins. A cDNA for this gene has previously been described by Edwards et al. (1994). Northern analysis shows that the tsr gene is expressed throughout development, and that the tsr1 and tsr2 alleles are hypomorphs that accumulate decreased levels of tsr mRNA. These findings prompted us to examine actin behavior during male meiosis to visualize the effects of decreased twinstar protein activity on actin dynamics in vivo. Strikingly, both mutants exhibit abnormal accumulations of F-actin. Large actin aggregates are seen in association with centrosomes in mature primary spermatocytes. Later, during ana/telophase of both meiotic divisions, aberrantly large and misshaped structures appear at the site of contractile ring formation and fail to disassemble at the end of telophase, in contrast with wild-type. We discuss these results in terms of possible roles of the actin-based cytoskeleton in centrosome movement and in cytokinesis.

Reactivation of phosphorylated actin depolymerizing factor and identification of the regulatory site

Actin depolymerizing factor (ADF) occurs naturally in two forms, one of which contains a phosphorylated Ser and does not bind G-actin or depolymerize F-actin. Removal of this phosphate in vitro by alkaline phosphatase restores full F-actin depolymerizing activity. To identify the phosphorylation site, [32P]pADF was purified and digested with endoproteinase Lys-C. The digest contained only one 32P-labeled peptide. Further digestion with endoproteinase Asp-N and mass spectrometric analysis showed that this peptide came from the N terminus of ADF. Alkaline phosphatase treatment of one Asp-N peptide (mass 753) converted it to a peptide of mass 673, demonstrating that this peptide contains the phosphate group. Tandem mass spectrometric sequence analysis of this peptide identified the phosphorylated Ser as the encoded Ser3 (Ser2 in the processed protein). HeLa cells, transfected with either chick wild-type ADF cDNA or a cDNA mutated to code for Ala in place of Ser24 or Thr25, express and phosphorylate the exogenous ADF. Cells also expressed high levels of mutant ADF when Ser3 was deleted or converted to either Ala or Glu. However, none of these mutants was phosphorylated, confirming that Ser3 in the encoded ADF is the single in vivo regulatory site.

Actin monomer binding proteins

Small actin monomer binding proteins are essential components of the actin polymerization machinery. Originally thought of as passive buffers that prevent polymerization of actin monomers, recent discoveries elucidate how some actin monomer binding proteins can promote as well as inhibit polymerization, and how they cooperate to regulate actin assembly.

A mutation in the alpha tropomyosin gene TPM3 associated with autosomal dominant nemaline myopathy

Nemaline myopathies are diseases characterized by the presence in muscle fibres of pathognomonic rod bodies. These are composed largely of alpha-actinin and actin. We have identified a missense mutation in the alpha-tropomyosin gene, TPM3, which segregates completely with the disease in a family whose autosomal dominant nemaline myopathy we had previously localized to chromosome 1p13-q25. The mutation substitutes an arginine residue for a highly conserved methionine in a putative actin-binding site near the N terminus of the alpha-tropomyosin. The mutation may strengthen tropomyosin - actin binding, leading to rod body formation, by adding a further basic residue to the postulated actin-binding motif.

Identification of two distinct synucleins from human brain

Two abundant proteins of 140 and 134 amino acids were purified and sequenced from human brain. They were identified through their reactivity on immunoblots with a partially characterised monoclonal antibody that recognises tau protein in a phosphorylation-dependent manner. The 140 amino acid protein is identical with the precursor of the non-A beta component of Alzheimer's disease amyloid which in turn is highly homologous to synuclein from Torpedo electroplaques and rat brain. The 134 amino acid protein is the human homologue of bovine phosphoneuroprotein 14; it is 61% identical in sequence to the 140 amino acid protein. The previously unrecognised homology between these two proteins defines a family of human brain synucleins. We refer to the 140 and 134 amino acid proteins as alpha-synuclein and beta-synuclein, respectively. Both synucleins are expressed predominantly in brain, where they are concentrated in presynaptic nerve terminals.

Costimulatory signals for human T-cell activation induce nuclear translocation of pp19/cofilin

Resting T lymphocytes that have recognized antigen bound to a major histocompatibility complex molecule with the T-cell receptor require costimulatory signals through accessory receptors, including CD2, CD4, CD8, and CD28, for their clonal growth and expression of their functional repertoires. Absence of costimulation, in contrast, can induce clonal anergy in vitro and selective tolerance in vivo. Here we have defined a potential intracellular messenger for T-cell activation which is strictly regulated by costimulatory signals mediated through accessory receptors: pp19/cofilin, a small actin-binding protein, undergoes dephosphorylation and subsequent translocation from the cytosol into the nucleus. In untransformed T cells this process correlates with functional responses essential for the induction of T-cell proliferation (i.e., production of interleukin 2). Moreover, spontaneous dephosphorylation as well as nuclear translocation of pp19/cofilin occur in the autonomously proliferating T-lymphoma cell line Jurkat.

Identification of a novel microtubule-binding domain in microtubule-associated protein 1A (MAP1A)

Several microtubule-associated proteins (MAPs) have been shown to bind to microtubules via short sequences with repeated amino acids motifs. A microtubule-binding domain has hitherto not been defined for the adult brain microtubule-associated protein 1A (MAP1A). We have searched for a microtubule-binding domain by expressing different protein regions of MAP1A in cultured cell lines using cDNA constructs. One construct included an area with homology to the microtubule-binding domain of MAP1B (Noble et al. (1989) J. Cell Biol. 109, 437–448), but this did not bind to microtubules in transfected cells. Further investigation of other areas of MAP1A revealed a protein domain, capable of autonomously binding to microtubules, which bears no homology to any previously described microtubule-binding sequence. This MAP1A domain is rich in charged amino acids, as are other mammalian microtubule-binding domains, but unlike them has no identifiable sequence repeats. Whereas all previously described mammalian microtubule-binding domains are basic, this novel microtubule-binding domain of MAP1A is acidic. The expression of this polypeptide in cultured cell lines led to a rearrangement of the microtubules in a pattern distinct from that produced by MAP2 or tau, and increased their resistance to treatment with the microtubule depolymerising agent nocodazole. When the MAP1A microtubule-binding domain was co-expressed in cultured cell lines together with MAP2c, the MAP1A microtubule-binding domain was able to bind to the MAP2c-induced microtubule bundles. These results suggest that different microtubule-binding sequences have a common ability to stabilise microtubules but differ in their influence on microtubule arrangement in the cell. This may be significant in neurons, where microtubule-associated proteins with different microtubule-binding sequences are expressed in different cell compartments and at different times during development.

Characterisation of the F-actin binding domains of villin: classification of F-actin binding proteins into two groups according to their binding sites on actin

The F-actin binding properties of chicken villin, its headpiece and domains 2-3 (V2-3) have been analysed to identify sites involved in bundle formation. Headpiece and V2-3 bind actin with Kd values of approximately 7 microM and approximately 0.3 microM, respectively, at low ionic strength. V2-3 binding, like that of villin, is weakened with increasing salt concentration; headpiece binding is not. Competition experiments show that headpiece and V2-3 bind to different sites on actin, forming the two cross-linking sites of villin. Headpiece does not compete with the F-actin binding domains of gelsolin or alpha-actinin, but it dissociates actin depolymerizing factor. We suggest that the F-actin binding domains of actin severing, crosslinking and capping proteins can be organized into two classes.

Cofilin is an essential component of the yeast cortical cytoskeleton

We have biochemically identified the Saccharomyces cerevisiae homologue of the mammalian actin binding protein cofilin. Cofilin and related proteins isolated from diverse organisms are low molecular weight proteins (15-20 kD) that possess several activities in vitro. All bind to monomeric actin and sever filaments, and some can stably associate with filaments. In this study, we demonstrate using viscosity, sedimentation, and actin assembly rate assays that yeast cofilin (16 kD) possesses all of these properties. Cloning and sequencing of the S. cerevisiae cofilin gene (COF1) revealed that yeast cofilin is 41% identical in amino acid sequence to mammalian cofilin and, surprisingly, has homology to a protein outside the family of cofilin-like proteins. The NH2-terminal 16kD of Abp1p, a 65-kD yeast protein identified by its ability to bind to actin filaments, is 23% identical to yeast cofilin. Immunofluorescence experiments showed that, like Abp1p, cofilin is associated with the membrane actin cytoskeleton. A complete disruption of the COF1 gene was created in diploid cells. Sporulation and tetrad analysis revealed that yeast cofilin has an essential function in vivo. Although Abp1p shares sequence similarity with cofilin and has the same distribution as cofilin in the cell, multiple copies of the ABP1 gene cannot compensate for the loss of cofilin. Thus, cofilin and Abp1p are structurally related but functionally distinct components of the yeast membrane cytoskeleton.

Coactosin, a 17 kDa F-actin binding protein from Dictyostelium discoideum

A 17 kDa protein, designated as coactosin, has been purified from an actin-myosin complex reconstituted in vitro from a soluble fraction of Dictyostelium discoideum cells. The protein binds to F-actin in vitro without significantly altering its viscosity. Immunoblots labeled with monoclonal antibodies indicate that part of the protein is associated with the detergent-insoluble cytoskeleton. cDNA clones comprising the entire coding region of coactosin have been isolated from an expression library. The cDNA-derived amino-acid sequence reveals similarities of coactosin to the drebrins identified in neurons and to actin-binding proteins from other organisms, including yeast ABP1p, and yeast and vertebrate cofilins.

An actin-binding site containing a conserved motif of charged amino acid residues is essential for the morphogenic effect of villin

The actin-binding protein villin induces microvillus growth and reorganization of the cytoskeleton in cells that do not normally produce this protein. Transfection of mutagenized villin cDNAs into CV-1 cells was used to show that a conserved, COOH-terminally located cluster of charged amino acid residues (KKEK) is crucial for the morphogenic activity of villin in vivo. In vitro experiments with a 22 amino acid synthetic peptide corresponding to this region of villin provide evidence that this motif is part of an F-actin-binding site that induces G-actin to polymerize. Chemical cross-linking of actin to this peptide, the effects of amino acid substitutions in peptides, and the behavior of villin variants further corroborate the participation of the KKEK sequence in actin contacts.

Thymosin beta 4 (Fx peptide) is a potent regulator of actin polymerization in living cells

Thymosin beta 4 (beta 4) is a 5-kDa polypeptide originally identified in calf thymus. Although numerous activities have been attributed to beta 4, its physiological role remains elusive. Recently, beta 4 was found to bind actin in human platelet extracts and to inhibit actin polymerization in vitro, raising the possibility that it may be a physiological regulator of actin assembly. To examine this potential function, we have increased the cellular beta 4 concentration by microinjecting synthetic beta 4 into living epithelial cells and fibroblasts. The injection induced a diminution of stress fibers and a dose-dependent depolymerization of actin filaments as indicated by quantitative image analysis of phalloidin binding. Our results show that beta 4 is a potent regulator of actin assembly in living cells.

Definition of a Ca2(+)-sensitive interface in the plasma gelsolin-actin complex

Gelsolin is a Ca2(+)-dependent protein which severs actin filaments, caps their fast-growing ends and promotes nucleation. We report here results that delimit one of the interfaces between serum gelsolin and actin monomer. An actin-derived synthetic peptide (amino acids 305-326 of actin) coupled to a hydrophilic resin was tested for its possible interaction with gelsolin. We selected this sequence because it corresponds to a region implicated in the gelsolin-actin complex in a previous work [Boyer, Feinberg, Hue, Capony, Benyamin & Roustan (1987) Biochem. J. 248, 359-364]. We showed that this actin sequence is located at the surface of the actin molecule and observed a Ca2(+)-sensitive binding of gelsolin to this actin-derived peptide. In addition, by using a chymotryptic digest of gelsolin, we reported that only the C-terminal half of gelsolin interacts with the actin-(305-326)-peptide. These results that the Ca2(+)-sensitive interface includes both amino acids 305-326 of actin and probably amino acids 660-738 of gelsolin.

Actin-binding proteins

Much new information on the sequence, structure, and function of filament crosslinking, capping, and severing proteins is now known. Other significant findings include identification of a new abundant monomer-sequestering protein in platelets, and evidence that many actin-binding proteins interact with phosphoinositides and that this interaction may have metabolic consequences.