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

Increase in neurite outgrowth mediated by overexpression of actin depolymerizing factor

Growth cone motility is regulated by changes in actin dynamics. Actin depolymerizing factor (ADF) is an important regulator of actin dynamics, and extracellular signal-induced changes in ADF activity may influence growth cone motility and neurite extension. To determine this directly, we overexpressed ADF in primary neurons and analyzed neurite lengths. Recombinant adenoviruses were constructed that express wild-type Xenopus ADF/cofilin [XAC(wt)], as well as two mutant forms of XAC, the active but nonphosphorylatable XAC(A3) and the less active, pseudophosphorylated XAC(E3). XAC expression was detectable on Western blots 24 hr after infection and peaked at 3 d in cultured rat cortical neurons. Peak expression was approximately 75% that of endogenous ADF. XAC(wt) expression caused a slight increase in growth cone area and filopodia but decreased filopodia numbers on neurite shafts. At maximal XAC levels, neurite lengths increased >50% compared with controls infected with a green fluorescent protein-expressing adenovirus. Increased neurite extension was directly related to the expression of active XAC. Expression of the XAC(E3) mutant did not increase neurite extension, whereas expression of the XAC(A3) mutant increased neurite extension but to a lesser extent than XAC(wt), which was partially phosphorylated. XAC expression had minimal, if any, impact on F-actin levels and did not result in compensatory changes in the expression of endogenous ADF or actin. However, F-actin turnover appeared to increase based on F-actin loss after treatment with drugs that block actin polymerization. These results provide direct evidence that increased ADF activity promotes process extension and neurite outgrowth.

Relationships between the central spindle and the contractile ring during cytokinesis in animal cells

During late anaphase and telophase, animal cells develop a bundle of antiparallel, interdigitating microtubules between the two daughter nuclei. Recent data indicate that this structure, called the central spindle, plays an essential role during cytokinesis. Studies in Drosophila and on vertebrate cells strongly suggest that the molecular signals for cytokinesis specifically emanate from the central spindle midzone. Moreover, the analysis of Drosophila mutants defective in cytokinesis has revealed a cooperative interaction between the central spindle microtubules and the contractile ring: when either of these structures is perturbed, the proper assembly of the other is disrupted. Based on these results we propose a model for the role of the central spindle during cytokinesis. We suggest that the interaction between central spindle microtubules and cortical actin filaments leads to two early events crucial for cytokinesis: the positioning of the contractile ring, and the stabilization of the plus ends of the interdigitating microtubules that comprise the central spindle. The latter event would provide the cell with a specialized microtubule scaffold that could mediate the translocation of plus-end-directed molecular motors to the cell's equator. Among the cargoes transported by these motors could be proteins involved in the regulation and execution of cytokinesis.

Increase in neurite outgrowth mediated by overexpression of actin depolymerizing factor

Growth cone motility is regulated by changes in actin dynamics. Actin depolymerizing factor (ADF) is an important regulator of actin dynamics, and extracellular signal-induced changes in ADF activity may influence growth cone motility and neurite extension. To determine this directly, we overexpressed ADF in primary neurons and analyzed neurite lengths. Recombinant adenoviruses were constructed that express wild-type Xenopus ADF/cofilin [XAC(wt)], as well as two mutant forms of XAC, the active but nonphosphorylatable XAC(A3) and the less active, pseudophosphorylated XAC(E3). XAC expression was detectable on Western blots 24 hr after infection and peaked at 3 d in cultured rat cortical neurons. Peak expression was approximately 75% that of endogenous ADF. XAC(wt) expression caused a slight increase in growth cone area and filopodia but decreased filopodia numbers on neurite shafts. At maximal XAC levels, neurite lengths increased >50% compared with controls infected with a green fluorescent protein-expressing adenovirus. Increased neurite extension was directly related to the expression of active XAC. Expression of the XAC(E3) mutant did not increase neurite extension, whereas expression of the XAC(A3) mutant increased neurite extension but to a lesser extent than XAC(wt), which was partially phosphorylated. XAC expression had minimal, if any, impact on F-actin levels and did not result in compensatory changes in the expression of endogenous ADF or actin. However, F-actin turnover appeared to increase based on F-actin loss after treatment with drugs that block actin polymerization. These results provide direct evidence that increased ADF activity promotes process extension and neurite outgrowth.

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.

The actin cytoskeleton is required to elaborate and maintain spatial patterning during trichome cell morphogenesis in Arabidopsis thaliana

Arabidopsis thaliana trichomes provide an attractive model system to dissect molecular processes involved in the generation of shape and form in single cell morphogenesis in plants. We have used transgenic Arabidopsis plants carrying a GFP-talin chimeric gene to analyze the role of the actin cytoskeleton in trichome cell morphogenesis. We found that during trichome cell development the actin microfilaments assumed an increasing degree of complexity from fine filaments to thick, longitudinally stretched cables. Disruption of the F-actin cytoskeleton by actin antagonists produced distorted but branched trichomes which phenocopied trichomes of mutants belonging to the ‘distorted’ class. Subsequent analysis of the actin cytoskeleton in trichomes of the distorted mutants, alien, crooked, distorted1, gnarled, klunker and wurm uncovered actin organization defects in each case. Treatments of wild-type seedlings with microtubule-interacting drugs elicited a radically different trichome phenotype characterized by isotropic growth and a severe inhibition of branch formation; these trichomes did not show defects in actin cytoskeleton organization. A normal actin cytoskeleton was also observed in trichomes of the zwichel mutant which have reduced branching. ZWICHEL, which was previously shown to encode a kinesin-like protein is thought to be involved in microtubule-linked processes. Based on our results we propose that microtubules establish the spatial patterning of trichome branches whilst actin microfilaments elaborate and maintain the overall trichome pattern during development.

Cytoskeleton in plant development

The plant cytoskeleton has crucial functions in a number of cellular processes that are essential for cell morphogenesis, organogenesis and development. These functions have been intensively investigated using single cell model systems. With the recent characterization of plant mutants that show aberrant organogenesis resulting from primary defects in cytoskeletal organization, an integrated understanding of the importance of the cytoskeleton for plant development has begun to emerge. Newly established techniques that allow the non-destructive visualization of microtubules or actin filaments in living plant cells and organs will further advance this understanding.

Putting a new twist on actin: ADF/cofilins modulate actin dynamics

The actin-depolymerizing factor (ADF)/cofilins are a family of essential actin regulatory proteins, ubiquitous among eukaryotes, that enhance the turnover of actin by regulating the rate constants of polymerization and depolymerization at filament ends, changing the twist of the filament and severing actin filaments. Genetic and cell-biological studies have shown that an ADF/cofilin is required to drive the high turnover of the actin cytoskeleton observed in vivo. The activity of ADF/cofilin is regulated by a variety of mechanisms, including specific phosphorylation and dephosphorylation. This review addresses aspects of ADF/cofilin structure, dynamics, regulation and function.

A putative exchange factor for Rho1 GTPase is required for initiation of cytokinesis in Drosophila

Cytokinesis ensures the successful completion of the cell cycle and distribution of chromosomes, organelles, and cytoplasm between daughter cells. It is accomplished by formation and constriction of an actomyosin contractile ring that drives the progression of a cleavage furrow. Microinjection experiments and in vitro transfection assays have suggested a requirement for small GTPases of the Rho family in cytokinesis. Yet, the identity of proteins regulating Rho signaling pathways during cytokinesis remains unknown. Here we show that in Drosophila, Pebble (Pbl), a putative exchange factor for Rho GTPases (RhoGEF), is required for the formation of the contractile ring and initiation of cytokinesis. The dynamics of Pbl expression and its distribution during mitosis, as well as structure-function analysis, indicate that it is a key regulatory component of the pathway. pbl interacts genetically with Rho1, but not with Rac1 or Cdc42, and Pbl and Rho1 proteins interact in vivo in yeast. Similar to mutations in pbl, loss of Rho1 or expression of a dominant-negative Rho1 blocks cytokinesis. Our results identify Pbl as a RhoGEF specifically required for cytokinesis and linked through Rho1 activity to the reorganization of the actin cytoskeleton at the cleavage furrow.

BimC motor protein KLP61F cycles between mitotic spindles and fusomes in Drosophila germ cells

KLP61F in Drosophila is a member of the BimC family of kinesins and, as for other family members [1], is required for spindle assembly [2] [3]. KLP61F is a bipolar homotetramer that cross-links spindle microtubules [4]. It is not known, however, whether the function of KLP61F is dedicated to mitosis or whether KLP61F interacts exclusively with microtubules. Previous work suggested that KLP61F functions during interphase in proliferating germ cells [3]. Cytokinesis is incomplete in germ cells and a branched cortical structure known as a fusome extrudes through intercellular bridges called ring canals. Here I show that, in germ cells, KLP61F cycles between spindles during mitosis and fusomes during interphase. Inspection of fusome-deficient hu-li tai shao (hts) mutants indicated that KLP61F gains fusome-dependent interactions near telophase that mediate its incorporation into these structures. KLP61F proved to be maintained in fusomes by microtubule-independent, detergent-resistant interactions. Inspection of KLP61F mutants indicated that KLP61F is required to recruit fusome material to spindle midbodies near telophase and for normal fusome organization. These observations suggest that KLP61F is bifunctional in germ cells, with microtubule-dependent functions in spindle assembly and microtubule-independent functions in fusome organization. Cytological analyses with antibodies against phosphorylated Eg5 peptide [4] suggest that cycling of KLP61F might reflect phosphorylation.

Control of actin filament length and turnover by actin depolymerizing factor (ADF/cofilin) in the presence of capping proteins and ARP2/3 complex

The effect of Arabidopsis thaliana ADF1 and human ADF on the number of filaments in F-actin solutions has been examined using a seeded polymerization assay. ADF did not sever filaments in a catalytic fashion, but decreased the steady-state length distribution of actin filaments in correlation with its effect on actin dynamics. The increase in filament number was modest as compared with the large increase in filament turnover. ADF did not decrease the length of filaments shorter than 1 micrometer. ADF promoted the rapid turnover of gelsolin-capped filaments in a manner dependent on the number of pointed ends. To explain these results, we propose that, as a consequence of the cooperative binding of ADF to F-actin, two populations of energetically different filaments coexist in solution pending a flux of subunits from one to the other. The ADF-decorated filaments depolymerize rapidly from their pointed ends, while undecorated filaments polymerize. ADF also promotes rapid turnover of gelsolin-capped filaments in the presence of the pointed end capper Arp2/3 complex. It is shown that the Arp2/3 complex steadily generates new barbed ends in solutions of gelsolin-capped filaments, which represents an important aspect of its function in actin-based motility.

The role of anillin in meiotic cytokinesis of Drosophila males

Anillin is a 190 kDa actin-binding protein that concentrates in the leading edges of furrow canals during Drosophila cellularization and in the cleavage furrow of both somatic and meiotic cells. We analyzed anillin behavior during D. melanogaster spermatogenesis, and focused on the relationships between this protein and the F-actin enriched structures. In meiotic anaphases anillin concentrates in a narrow band around the cell equator. Cytological analysis of wild-type meiosis and examination of mutants defective in contractile ring assembly (chickadee and KLP3A), revealed that the formation of the anillin cortical band occurs before, and does not require the assembly of the F-actin based contractile ring. However, once the acto-myosin ring is assembled, the anillin band precisely colocalizes with this cytokinetic structure, accompanying its contraction throughout anaphase and telophase. In chickadee and KLP3A mutant ana-telophases the cortical anillin band fails to constrict, indicating that its contraction is normally driven by the cytokinetic ring. These findings, coupled with the analysis of anillin behavior in twinstar mutants, suggested a model on the role of anillin during cytokinesis. During anaphase anillin would concentrate in the cleavage furrow before the assembly of the contractile ring, binding the equatorial cortex, perhaps through its carboxy-terminal pleckstrin homology (PH) domain. Anillin would then interact with the actin filaments of the acto-myosin ring through its actin-binding domain, anchoring the contractile ring to the plasma membrane throughout cytokinesis.

Hyperosmotic stress-induced reorganization of actin bundles in Dictyostelium cells over-expressing cofilin

BACKGROUND

Cofilin is a low-molecular weight actin-modulating protein, which binds to, severs, and depolymerizes actin filaments in vitro. Aip1, an actin-interacting protein, was recently identified as a product of a gene on a multicopy plasmid which suppresses the temperature-sensitive phenotype of a cofilin mutant in Saccharomyces cerevisiae. Actin cytoskeleton plays an essential role in resistance to hyperosmotic stress in Dictyostelium discoideum. The roles of cofilin and Aip1 in this resistance are not known.

RESULTS

In response to hyperosmotic stress, D. discoideum cells round up. This stress-induced morphological change involves the redistribution of cofilin, together with actin filaments, into cortical contractile portions of the cells, followed by their contraction. Over-expression of cofilin increases and thickens cortical actin bundles in cells. The bundles become tight and are reorganized into a ring-shaped structure in response to hyperosmotic stress. The ring structure of actin bundles had two characteristic bands across them; bright and dark bands, heavily stained and not stained with phalloidin. In the bundles, straight filaments with a diameter of 5.3-nm were aligned parallel by cross-bridge structures. In cells lacking the myosin-II heavy chain, the bundles, which were induced by an over-expression of cofilin, shortened and became straight following hyperosmotic stress, forming a polygonal structure. D. discoideum Aip1/Wrp2 enhanced the severing of actin filaments by cofilin in vitro and colocalized with cofilin in cells, including those that were over-expressing cofilin before and after exposure to hyperosmotic stress.

CONCLUSIONS

Cofilin plays a pivotal role in concert with Aip1/Wrp2 in the reorganization of actin architectures into bundles that contract in a myosin-II-independent manner, in response to hyperosmotic stress.

XAIP1: a Xenopus homologue of yeast actin interacting protein 1 (AIP1), which induces disassembly of actin filaments cooperatively with ADF/cofilin family proteins

We carried out affinity column chromatography using Xenopus ADF/cofilin (XAC), identified several polypeptides in oocytes specifically bound to this column with actin, and isolated a full-length cDNA clone for a 65 kDa protein in this fraction. The predicted amino acid sequence revealed that the 65 kDa protein has seven obvious WD repeats and exhibits striking homology with yeast actin interacting protein 1 (AIP1). Thus, we designated this protein Xenopus AIP1 (XAIP1). We purified XAIP1 from Xenopus oocytes, and its interaction with actin was characterized by a pelleting assay, photometrical analysis and electron microscopy. Although XAIP1 itself cosedimented with F-actin and increased unsedimented actin to some extent, it induced a rapid, drastic disassembly of actin filaments associated with XAC. Electron microscopic observation revealed that XAIP1 severs actin filaments in the presence of XAC. To elucidate the in vivo effects of XAIP1, the purified protein was injected into blastomeres at the two-cell stage. Although the localization of XAIP1 was similar to that of XAC, at the cortical cytoskeleton and diffusely in the cytoplasm, injection of a large amount of XAIP1 arrested development and abolished the strong cortical staining of both actin and XAC. From these results, we concluded that XAIP1 regulates the dynamics of the cortical actin cytoskeleton cooperatively with XAC in eggs.

UNC-60B, an ADF/cofilin family protein, is required for proper assembly of actin into myofibrils in Caenorhabditis elegans body wall muscle

The Caenorhabditis elegans unc-60 gene encodes two functionally distinct isoforms of ADF/cofilin that are implicated in myofibril assembly. Here, we show that one of the gene products, UNC-60B, is specifically required for proper assembly of actin into myofibrils. We found that all homozygous viable unc-60 mutations resided in the unc-60B coding region, indicating that UNC-60B is responsible for the Unc-60 phenotype. Wild-type UNC-60B had F-actin binding, partial actin depolymerizing, and weak F-actin severing activities in vitro. However, mutations in UNC-60B caused various alterations in these activities. Three missense mutations resulted in weaker F-actin binding and actin depolymerizing activities and complete loss of severing activity. The r398 mutation truncated three residues from the COOH terminus and resulted in the loss of severing activity and greater actin depolymerizing activity. The s1307 mutation in a putative actin-binding helix caused greater activity in actin-depolymerizing and severing. Using a specific antibody for UNC-60B, we found varying protein levels of UNC-60B in mutant animals, and that UNC-60B was expressed in embryonic muscles. Regardless of these various molecular phenotypes, actin was not properly assembled into embryonic myofibrils in all unc-60 mutants to similar extents. We conclude that precise control of actin filament dynamics by UNC-60B is required for proper integration of actin into myofibrils.

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.

Molecular genetic approaches to understanding the actin cytoskeleton

New tools in molecular genetics, such as genetic interaction screens and conditional gene targeting, have advanced the study of actin dynamics in a number of model systems. Yeast, Dictyostelium, Caenorhabditis elegans, Drosophila, and mice have contributed much in recent years to a better understanding of both the numerous functions and modes of regulation of the actin cytoskeleton.

Cooperation of two actin-binding proteins, cofilin and Aip1, in Saccharomyces cerevisiae

BACKGROUND

Cofilin is a low-molecular weight actin-modulating protein, and is structurally and functionally conserved among eukaryotes. Cofilin is encoded by COF1 in Saccharomyces cerevisiae, and is essential for cell viability. Cofilin binds to and severs actin filaments in vitro, and also enhances their depolymerization. A partner protein that cooperates with cofilin in vivo has not been identified.

RESULTS

When COF1 was over-expressed in yeast cells under the GAL1 promoter in a medium containing galactose as a sole carbon source, the cells did not survive. These results indicate that cells can grow only when the expression of cofilin is appropriately regulated. Several temperature sensitive (ts-) mutants were independently created by the random mutagenesis of COF1 with hydroxylamine. Mutated amino acids in ts-mutants were mapped in the sequences that were presumed to be involved in actin binding. A gene on a multicopy plasmid which suppresses the ts-phenotype of cof1-101, a typical ts-cofilin mutant, was isolated. The suppressor gene, SCF1, was found to be identical to AIP1, a gene encoding an actin-interacting protein. Although SCF1/AIP1 is not essential for cell viability, a combination of cof1-101 and Deltascf1/aip1 is synthetic lethal. Immunofluorescence staining of a wild-type strain using anti-Aip1 antibodies revealed that Aip1 was distributed in cortical actin patches where cofilin was also co-localized. Thick and long fibres stained with anti-cofilin antibody were detected in Deltascf1/aip1 cells, but not in SCF1/AIP1 cells.

CONCLUSIONS

These results suggest the cooperative modulation of the actin cytoskeleton by cofilin and Aip1.

Detection of a sequence involved in actin-binding and phosphoinositide-binding in the N-terminal side of cofilin

Cofilin is an actin-binding protein of low molecular weight which is widely distributed in eukaryotes and is deeply involved in the dynamics of actin assembly in the cytoplasm. The actin-binding ability of cofilin is inhibited by inositol phosphates (PIP2), and the PIP2- and actin-binding site(s) has been localized in residues W104-M115 of the cofilin primary sequence (Yonezawa et al. 1991 ). In the present study, in order to further clarify the functional domains in cofilin molecule, we constructed expression vectors containing cDNAs of different size with deletion at the 3'-region of the open reading frame. The truncated cofilin molecules produced in E. coli were purified and examined for their actin-binding and PIP2-binding ability. We found that the truncated cofilin molecule without C-terminal residues #100-#166 including the previously-described actin-binding site could be cross-linked with actin by EDC, a zero-length cross-linker. In addition, these truncated peptides as well as synthetic peptides corresponding to the N-terminal sequence of cofilin suppressed the inhibitory action of PIP2 on actin-cofilin interaction. These results strongly suggest that additional actin- and PIP2-binding sites exist in the N-terminal region of cofilin.

Spindle self-organization and cytokinesis during male meiosis in asterless mutants of Drosophila melanogaster

While Drosophila female meiosis is anastral, both meiotic divisions in Drosophila males exhibit prominent asters. We have identified a gene we call asterless (asl) that is required for aster formation during male meiosis. Ultrastructural analysis showed that asl mutants have morphologically normal centrioles. However, immunostaining with antibodies directed either to gamma tubulin or centrosomin revealed that these proteins do not accumulate in the centrosomes, as occurs in wild-type. Thus, asl appears to specify a function required for the assembly of centrosomal material around the centrioles. Despite the absence of asters, meiotic cells of asl mutants manage to develop an anastral spindle. Microtubules grow from multiple sites around the chromosomes, and then focus into a peculiar bipolar spindle that mediates chromosome segregation, although in a highly irregular way. Surprisingly, asl spermatocytes eventually form a morphologically normal ana-telophase central spindle that has full ability to stimulate cytokinesis. These findings challenge the classical view on central spindle assembly, arguing for a self-organization of this structure from either preexisting or newly formed microtubules. In addition, these findings strongly suggest that the asters are not required for signaling cytokinesis.

In vitro model system for the identification and characterization of proteins involved in inflammatory processes

An in vitro model featuring important inflammatory cellular states was established, based on the murine monocyte/macrophage cell line RAW 264.7. Macrophages are key players in chronic inflammation, and major parts of the biochemical reactions taking place in vivo, e.g., the production of proinflammatory cytokines, can be triggered in vitro by stimulation of the cells with bacterial lipopolysaccharide (LPS). A mastergel, representing a synthetic image of the expressed basic set of cellular proteins, was designed by a computer-assisted overlay of a statistically significant number of two-dimensional electrophoresis (2-DE) gels of unstimulated RAW 264.7 cells. This image served as a reference for qualitative and quantitative changes in the protein pattern induced by stimulation of the macrophages with LPS. The optimal conditions for LPS stimulation were evaluated by monitoring the expression and secretion of the proinflammatory cytokine tumor necrosis factor-alpha(TNF-alpha). The comparison of the mastergel with the 2-DE gels of LPS-stimulated cells revealed several changes in the protein pattern. In order to prove the relevance of the presented model system, we focused on two low molecular weight proteins, which showed significant changes in the apparent concentration in a 2-DE pattern. These proteins were further characterized by microsequencing of internal peptides. A comparison of the obtained sequences with protein databases identified them as cofilin and keratinocyte lipid-binding protein.

Regulation of actin dynamics through phosphorylation of cofilin by LIM-kinase

Cell division, cell motility and the formation and maintenance of specialized structures in differentiated cells depend directly on the regulated dynamics of the actin cytoskeleton. To understand the mechanisms of these basic cellular processes, the signalling pathways that link external signals to the regulation of the actin cytoskeleton need to be characterized. Here we identify a pathway for the regulation of cofilin, a ubiquitous actin-binding protein that is essential for effective depolymerization of actin filaments. LIM-kinase 1, also known as KIZ, is a protein kinase with two amino-terminal LIM motifs that induces stabilization of F-actin structures in transfected cells. Dominant-negative LIM-kinasel inhibits the accumulation of the F-actin. Phosphorylation experiments in vivo and in vitro provide evidence that cofilin is a physiological substrate of LIM-kinase 1. Phosphorylation by LIM-kinase 1 inactivates cofilin, leading to accumulation of actin filaments. Constitutively active Rac augmented cofilin phosphorylation and LIM-kinase 1 autophosphorylation whereas phorbol ester inhibited these processes. Our results define a mechanism for the regulation of cofilin and hence of actin dynamics in vivo. By modulating the stability of actin cytoskeletal structures, this pathway should play a central role in regulating cell motility and morphogenesis.

pavarotti encodes a kinesin-like protein required to organize the central spindle and contractile ring for cytokinesis

Mutations in the Drosophila gene pavarotti result in the formation of abnormally large cells in the embryonic nervous system. In mitotic cycle 16, cells of pav mutant embryos undergo normal anaphase but then develop an abnormal telophase spindle and fail to undertake cytokinesis. We show that the septin Peanut, actin, and the actin-associated protein Anillin, do not become correctly localized in pav mutants. pav encodes a kinesin-like protein, PAV-KLP, related to the mammalian MKLP-1. In cellularized embryos, the protein is localized to centrosomes early in mitosis, and to the midbody region of the spindle in late anaphase and telophase. We show that Polo kinase associates with PAV-KLP with which it shows an overlapping pattern of subcellular localization during the mitotic cycle and this distribution is disrupted in pav mutants. We suggest that PAV-KLP is required both to establish the structure of the telophase spindle to provide a framework for the assembly of the contractile ring, and to mobilize mitotic regulator proteins.

Two Caenorhabditis elegans actin depolymerizing factor/cofilin proteins, encoded by the unc-60 gene, differentially regulate actin filament dynamics

The Caenorhabditis elegans unc-60 gene encodes two actin depolymerizing factor/cofilin proteins which are implicated in the regulation of actin filament assembly in body wall muscle. We examined the interaction of recombinant UNC-60A and B proteins with actin and found that they differentially regulate actin filament dynamics. Co-pelleting assays with F-actin showed that UNC-60A depolymerized but did not remain bound to F-actin, whereas UNC-60B bound to but did not depolymerize F-actin. In the pH range of 6.8-8.0, the apparent activities of UNC-60A and B did not change although UNC-60A showed greater actin-depolymerizing activity at higher pH. These activities were further confirmed by a light scattering assay and electron microscopy. The effects of these proteins on actin polymerization were quite different. UNC-60A inhibited polymerization in a concentration-dependent manner. On the other hand, UNC-60B strongly inhibited the nucleation process but accelerated the following elongation step. However, an excess amount of UNC-60B increased the amount of unpolymerized actin. These results indicate that UNC-60A depolymerizes actin filaments and inhibits actin polymerization, whereas UNC-60B strongly binds to F-actin without depolymerizing it and, through binding to G-actin, changes the rate of actin polymerization depending on the UNC-60B:actin ratio. These data suggest that the two UNC-60 isoforms play differential roles in regulating actin filament dynamics in vivo.

Cooperative interactions between the central spindle and the contractile ring during Drosophila cytokinesis

We analyzed male meiosis in mutants of the chickadee (chic) locus, a Drosophila melanogaster gene that encodes profilin, a low molecular weight actin-binding protein that modulates F-actin polymerization. These mutants are severely defective in meiotic cytokinesis. During ana-telophase of both meiotic divisions, they exhibit a central spindle less dense than wild type; certain chic allelic combinations cause almost complete disappearance of the central spindle. Moreover, chic mutant spermatocytes fail to form an actomyosin contractile ring. To further investigate the relationships between the central spindle and the contractile ring, we examined meiosis in the cytokinesis-defective mutants KLP3A and diaphanous and in testes treated with cytochalasin B. In all cases, we found that the central spindle and the contractile ring in meiotic ana-telophases were simultaneously absent. Together, these results suggest a cooperative interaction between elements of the actin-based contractile ring and the central spindle microtubules: When one of these structures is disrupted, the proper assembly of the other is also affected. In addition to effects on the central spindle and the cytokinetic apparatus, we observed another consequence of chic mutations: A large fraction of chic spermatocytes exhibit abnormal positioning and delayed migration of asters to the cell poles. A similar phenotype was seen in testes treated with cytochalasin B and has been noted previously in mutants at the twinstar locus, a gene that encodes a Drosophila member of the cofilin/ADF family of actin-severing proteins. These observations all indicate that proper actin assembly is necessary for centrosome separation and migration.

In vivo functions of actin-binding proteins

Actin and actin-binding proteins have been identified in eukaryotic organisms across the evolutionary spectrum. Although many actin-binding proteins have been purified and studied in vitro, our understanding of the in vivo functions of these proteins has, until recently, lagged behind. In the past year, in vivo studies, especially those using genetic approaches, have led to significant advances in our understanding of how actin-binding proteins function in a cellular environment.

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.

The mechanism and control of cytokinesis

Cytokinesis is under active investigation in each of the dominant experimental model systems. During 1996 and 1997, several developments necessitated the reassessment of the prevailing model for cytokinesis. In addition, the inventory of proteins required for cytokinesis has grown considerably. However, a molecular understanding of cytokinesis still remains elusive.

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.

Cofilin changes the twist of F-actin: implications for actin filament dynamics and cellular function

Cofilin is an actin depolymerizing protein found widely distributed in animals and plants. We have used electron cryomicroscopy and helical reconstruction to identify its binding site on actin filaments. Cofilin binds filamentous (F)-actin cooperatively by bridging two longitudinally associated actin subunits. The binding site is centered axially at subdomain 2 of the lower actin subunit and radially at the cleft between subdomains 1 and 3 of the upper actin subunit. Our work has revealed a totally unexpected (and unique) property of cofilin, namely, its ability to change filament twist. As a consequence of this change in twist, filaments decorated with cofilin have much shorter 'actin crossovers' ( approximately 75% of those normally observed in F-actin structures). Although their binding sites are distinct, cofilin and phalloidin do not bind simultaneously to F-actin. This is the first demonstration of a protein that excludes another actin-binding molecule by changing filament twist. Alteration of F-actin structure by cofilin/ADF appears to be a novel mechanism through which the actin cytoskeleton may be regulated or remodeled.

Cofilin promotes rapid actin filament turnover in vivo

The ability of actin filaments to function in cell morphogenesis and motility is coupled to their capacity for rapid assembly and disassembly. Because disassembly in vitro is much slower than in vivo, cellular factors that stimulate disassembly have long been assumed to exist. Although numerous proteins can affect actin dynamics in vitro, demonstration of in vivo relevance of these effects has not been achieved. We have used genetics and an actin-inhibitor in yeast to demonstrate that rapid cycles of actin assembly and disassembly depend on the small actin-binding protein cofilin, and that cofilin stimulates filament disassembly. These results may explain why cofilin is ubiquitous in eukaryotes and is essential for viability in every organism in which its function has been tested genetically. Magnitudes of disassembly defects in cofilin mutants in vivo were found to be correlated closely with the magnitudes of disassembly defects observed in vitro, supporting our conclusions. Furthermore, these cofilin mutants provided an opportunity to distinguish in living cells those actin functions that depend specifically on filament turnover (endocytosis) from those that do not (cortical actin patch motility).

Control of actin dynamics in cell motility

Actin polymerization plays a major role in cell movement. The controls of actin sequestration/desequestration and of filament turnover are two important features of cell motility. Actin binding proteins use properties derived from the steady-state monomer-polymer cycle of actin in the presence of ATP, to control the F-actin/G-actin ratio and the turnover rate of actin filaments. Capping proteins and profilin regulate the size of the pools of F-actin and unassembled actin by affecting the steady-state concentration of ATP-G-actin. At steady state, the treadmilling cycle of actin filaments is fed by their disassembly from the pointed ends. It is regulated in two different ways by capping proteins and ADF, as follows. Capping proteins, in decreasing the number of growing barbed ends, increase their individual rate of growth and create a "funneled" treadmilling process. ADF/cofilin, in increasing the rate of pointed-end disassembly, increases the rate of filament turnover, hence the rate of barbed-end growth. In conclusion, capping proteins and ADF cooperate to increase the rate of actin assembly up to values that support the rates of actin-based motility processes.

Structure determination of yeast cofilin

Cofilin, a ubiquitous 15,000 M(r) protein, plays a central role in regulating cytoskeletal dynamics. Cofilin binds to actin monomers and filaments, and has a pH-dependent actin severing activity. The structure will allow for a detailed analysis of cofilin function.

Mutation of a gene for a Drosophila kinesin-like protein, Klp38B, leads to failure of cytokinesis

Mutations in a gene (Klp38B) encoding a novel kinesin-like protein in Drosophila melanogaster lead to the formation of polyploid cells in the larval central nervous system and in the follicle cells of adult egg chambers. Some homozygous mutants survive to adulthood and also exhibit morphological defects indicative of abnormal cell cycle progression, including rough eyes, missing bristles, and abnormal abdominal cuticles. In larval brains, there is no accumulation of mitotic cells and the frequency of anaphase figures is comparable to wild type, suggesting that nuclear division is not affected. Such brains contain polyploid cells with metaphase and anaphase chromosomes associated with bipolar spindles. Such spindles have a number of unseparated centrosomes at their poles reflecting the degree of polyploidy of the cell. Follicle cells frequently contain two nuclei of roughly equal size. Taken together, we conclude that these Klp38B mutations lead to a failure of cytokinesis resulting in polyploidy, and discuss whether or not this is a direct effect of the mutation.

Xenopus actin depolymerizing factor/cofilin (XAC) is responsible for the turnover of actin filaments in Listeria monocytogenes tails

In contrast to the slow rate of depolymerization of pure actin in vitro, populations of actin filaments in vivo turn over rapidly. Therefore, the rate of actin depolymerization must be accelerated by one or more factors in the cell. Since the actin dynamics in Listeria monocytogenes tails bear many similarities to those in the lamellipodia of moving cells, we have used Listeria as a model system to isolate factors required for regulating the rapid actin filament turnover involved in cell migration. Using a cell-free Xenopus egg extract system to reproduce the Listeria movement seen in a cell, we depleted candidate depolymerizing proteins and analyzed the effect that their removal had on the morphology of Listeria tails. Immunodepletion of Xenopus actin depolymerizing factor (ADF)/cofilin (XAC) from Xenopus egg extracts resulted in Listeria tails that were approximately five times longer than the tails from undepleted extracts. Depletion of XAC did not affect the tail assembly rate, suggesting that the increased tail length was caused by an inhibition of actin filament depolymerization. Immunodepletion of Xenopus gelsolin had no effect on either tail length or assembly rate. Addition of recombinant wild-type XAC or chick ADF protein to XAC-depleted extracts restored the tail length to that of control extracts, while addition of mutant ADF S3E that mimics the phosphorylated, inactive form of ADF did not reduce the tail length. Addition of excess wild-type XAC to Xenopus egg extracts reduced the length of Listeria tails to a limited extent. These observations show that XAC but not gelsolin is essential for depolymerizing actin filaments that rapidly turn over in Xenopus extracts. We also show that while the depolymerizing activities of XAC and Xenopus extract are effective at depolymerizing normal filaments containing ADP, they are unable to completely depolymerize actin filaments containing AMPPNP, a slowly hydrolyzible ATP analog. This observation suggests that the substrate for XAC is the ADP-bound subunit of actin and that the lifetime of a filament is controlled by its nucleotide content.

Microtubule-microfilament synergy in the cytoskeleton

This review describes examples of structural and functional synergy of the microtubule and actin filament cytoskeleton. An analysis of basal body (centriole)-associated fibrillar networks includes studies of ciliated epithelium, neurosensory epithelium, centrosomes, and ciliated protozoa. Microtubule and actin filament interactions in cell division and development are illustrated by centrosome motility, cleavage furrow positioning, centriole migration, nuclear migration, dynamics in the phragmoplast, growth cone motility, syncytial organization, and ring canals. Model systems currently used for studies on organelle transport are described in relation to mitochondrial transport in axons and vesicular transport in polarized epithelium. Evidence that both anterograde and retrograde motors are associated with one organelle is also discussed. The final section reviews proteins that bind both microtubules and actin filaments and are possible regulators of microtubule-microfilament interactions. Regulatory roles for posttranslational modifications, microtubule and microfilament dynamics, and multisubunit complexes are considered.

Bipolar spindle attachments affect redistributions of ZW10, a Drosophila centromere/kinetochore component required for accurate chromosome segregation

Previous efforts have shown that mutations in the Drosophila ZW10 gene cause massive chromosome missegregation during mitotic divisions in several tissues. Here we demonstrate that mutations in ZW10 also disrupt chromosome behavior in male meiosis I and meiosis II, indicating that ZW10 function is common to both equational and reductional divisions. Divisions are apparently normal before anaphase onset, but ZW10 mutants exhibit lagging chromosomes and irregular chromosome segregation at anaphase. Chromosome missegregation during meiosis I of these mutants is not caused by precocious separation of sister chromatids, but rather the nondisjunction of homologs. ZW10 is first visible during prometaphase, where it localizes to the kinetochores of the bivalent chromosomes (during meiosis I) or to the sister kinetochores of dyads (during meiosis II). During metaphase of both divisions, ZW10 appears to move from the kinetochores and to spread toward the poles along what appear to be kinetochore microtubules. Redistributions of ZW10 at metaphase require bipolar attachments of individual chromosomes or paired bivalents to the spindle. At the onset of anaphase I or anaphase II, ZW10 rapidly relocalizes to the kinetochore regions of the separating chromosomes. In other mutant backgrounds in which chromosomes lag during anaphase, the presence or absence of ZW10 at a particular kinetochore predicts whether or not the chromosome moves appropriately to the spindle poles. We propose that ZW10 acts as part of, or immediately downstream of, a tension-sensing mechanism that regulates chromosome separation or movement at anaphase onset.

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.

Xenopus laevis actin-depolymerizing factor/cofilin: a phosphorylation-regulated protein essential for development

Two cDNAs, isolated from a Xenopus laevis embryonic library, encode proteins of 168 amino acids, both of which are 77% identical to chick cofilin and 66% identical to chick actin-depolymerizing factor (ADF), two structurally and functionally related proteins. These Xenopus ADF/cofilins (XADs) differ from each other in 12 residues spread throughout the sequence but do not differ in charge. Purified GST-fusion proteins have pH-dependent actin-depolymerizing and F-actin-binding activities similar to chick ADF and cofilin. Similarities in the developmental and tissue specific expression, embryonic localization, and in the cDNA sequence of the noncoding regions, suggest that the two XACs arise from allelic variants of the pseudotetraploid X. laevis. Immunofluorescence localization of XAC in oocyte sections with an XAC-specific monoclonal antibody shows it to be diffuse in the cortical cytoplasm. After fertilization, increased immunostaining is observed in two regions: along the membrane, particularly that of the vegetal hemisphere, and at the interface between the cortical and animal hemisphere cytoplasm. The cleavage furrow and the mid-body structure are stained at the end of first cleavage. Neuroectoderm derived tissues, notochord, somites, and epidermis stain heavily either continuously or transiently from stages 18-34. A phosphorylated form of XAC (pXAC) was identified by 2D Western blotting, and it is the only species found in oocytes. Dephosphorylation of >60% of the pXAC occurs within 30 min after fertilization. Injection of one blastomere at the 2 cell stage, either with constitutively active XAC or with an XAC inhibitory antibody, blocked cleavage of only the injected blastomere in a concentration-dependent manner without inhibiting nuclear division. The cleavage furrow of eggs injected with constitutively active XAC completely regressed. Blastomeres injected with neutralized antibody developed normally. These results suggest that XAC is necessary for cytokinesis and that its activity must be properly regulated for cleavage to occur.

Site-directed mutagenesis of the phosphorylation site of cofilin: its role in cofilin-actin interaction and cytoplasmic localization

It has been demonstrated that the activity of ADF and cofilin, which constitute a functionally related protein family, is markedly altered by phosphorylation, and that the phosphorylation site is Ser 3 in their amino acid sequences [Agnew et al., 1995: J. Biol. Chem. 270:17582-17587; Moriyama et al., 1996: Genes Cells 1:73-86]. In order to clarify the function of the phosphorylated and unphosphorylated forms of cofilin in living cells especially in the process of cytokinesis, we generated analogs of the unphosphorylated form (A3-cofilin) and phosphorylated form (D3-cofilin) by converting the phosphorylation site (Ser 3) of cofilin to Ala and Asp, respectively. The mutated proteins were produced in an Escherichia coli expression system, and conjugated with fluorescent dyes. In in vitro functional assay, labeled A3-cofilin retained the authentic ability to bind to and sever F-actin, while labeled D3-cofilin failed to interact with actin. They were then injected into living cells to examine their cellular distribution. They exhibited distinct localization patterns in the cytoplasm; A3-cofilin was highly concentrated at the membrane ruffles and cleavage furrow, where endogenous cofilin is also known to be enriched. In contrast, D3-cofilin showed only diffuse distribution both in the cytoplasm and nucleus. These results suggest that the subcellular distribution of cofilin as well as its interacting with actin in vivo is regulated by its phosphorylation and dephosphorylation.