Destrin, a mammalian actin-depolymerizing protein, is closely related to cofilin. Cloning and expression of porcine brain destrin cDNA

Cofilin overactivation improves hippocampus-dependent short-term memory

Many living organisms of the animal kingdom have the fundamental ability to form and retrieve memories. Most information is initially stored as short-term memory, which is then converted to a more stable long-term memory through a process called memory consolidation. At the neuronal level, synaptic plasticity is crucial for memory storage. It includes the formation of new spines, as well as the modification of existing spines, thereby tuning and shaping synaptic efficacy. Cofilin critically contributes to memory processes as upon activation, it regulates the shape of dendritic spines by targeting actin filaments. We previously found that prolonged activation of cofilin in hippocampal neurons attenuated the formation of long-term object-location memories. Because the modification of spine shape and structure is also essential for short-term memory formation, we determined whether overactivation of hippocampal cofilin also influences the formation of short-term memories. To this end, mice were either injected with an adeno-associated virus expressing catalytically active cofilin, or an eGFP control, in the hippocampus. We show for the first time that cofilin overactivation improves short-term memory formation in the object-location memory task, without affecting anxiety-like behavior. Surprisingly, we found no effect of cofilin overactivation on AMPA receptor expression levels. Altogether, while cofilin overactivation might negatively impact the formation of long-lasting memories, it may benefit short-term plasticity.

The actin depolymerizing factor destrin serves as a negative feedback inhibitor of smooth muscle cell differentiation

We have previously shown that several components of the RhoA signaling pathway control smooth muscle cell (SMC) phenotype by altering serum response factor (SRF)-dependent gene expression. Because our genome-wide analyses of chromatin structure and transcription factor binding suggested that the actin depolymerizing factor, destrin (DSTN), was regulated in a SMC-selective fashion, the goals of the current study were to identify the transcription mechanisms that control DSTN expression in SMC and to test whether it regulates SMC function. Immunohistochemical analyses revealed strong and at least partially SMC-selective expression of DSTN in many mouse tissues, a result consistent with human data from the genotype-tissue expression (GTEx) consortium. We identified several regulatory regions that control DSTN expression including a SMC-selective enhancer that was activated by myocardin-related transcription factor-A (MRTF-A), recombination signal binding protein for immunoglobulin κ-J region (RBPJ), and the SMAD transcription factors. Indeed, enhancer activity and endogenous DSTN expression were upregulated by RhoA and transforming growth factor-β (TGF-β) signaling and downregulated by inhibition of Notch cleavage. We also showed that DSTN expression was decreased in vivo by carotid artery injury and in cultured SMC cells by platelet-derived growth factor-BB (PDGF-BB) treatment. siRNA-mediated depletion of DSTN significantly enhanced MRTF-A nuclear localization and SMC differentiation marker gene expression, decreased SMC migration in scratch wound assays, and decreased SMC proliferation, as measured by cell number and cyclin-E expression. Taken together our data indicate that DSTN is a negative feedback inhibitor of RhoA/SRF-dependent gene expression in SMC that coordinately promotes SMC phenotypic modulation. Interventions that target DSTN expression or activity could serve as potential therapies for atherosclerosis and restenosis.NEW & NOTEWORTHY First, DSTN is selectively expressed in SMC in RhoA/SRF-dependent manner. Second, a SMC-selective enhancer just upstream of DSTN TSS harbors functional SRF, SMAD, and Notch/RBPJ binding elements. Third, DSTN depletion increased SRF-dependent SMC marker gene expression while inhibiting SMC migration and proliferation. Taken together, our data suggest that DSTN is a critical negative feedback inhibitor of SMC differentiation.

The actin remodeling protein cofilin is crucial for thymic αβ but not γδ T-cell development

Cofilin is an essential actin remodeling protein promoting depolymerization and severing of actin filaments. To address the relevance of cofilin for the development and function of T cells in vivo, we generated knock-in mice in which T-cell-specific nonfunctional (nf) cofilin was expressed instead of wild-type (WT) cofilin. Nf cofilin mice lacked peripheral αβ T cells and showed a severe thymus atrophy. This was caused by an early developmental arrest of thymocytes at the double negative (DN) stage. Importantly, even though DN thymocytes expressed the TCRβ chain intracellularly, they completely lacked TCRβ surface expression. In contrast, nf cofilin mice possessed normal numbers of γδ T cells. Their functionality was confirmed in the γδ T-cell-driven, imiquimod (IMQ)-induced, psoriasis-like murine model. Overall, this study not only highlights the importance of cofilin for early αβ T-cell development but also shows for the first time that an actin-binding protein is differentially involved in αβ versus γδ T-cell development.

AD-Related N-Terminal Truncated Tau Is Sufficient to Recapitulate In Vivo the Early Perturbations of Human Neuropathology: Implications for Immunotherapy

The NH₂tau 26-44 aa (i.e., NH₂htau) is the minimal biologically active moiety of longer 20-22-kDa NH₂-truncated form of human tau-a neurotoxic fragment mapping between 26 and 230 amino acids of full-length protein (htau40)-which is detectable in presynaptic terminals and peripheral CSF from patients suffering from AD and other non-AD neurodegenerative diseases. Nevertheless, whether its exogenous administration in healthy nontransgenic mice is able to elicit a neuropathological phenotype resembling human tauopathies has not been yet investigated. We explored the in vivo effects evoked by subchronic intracerebroventricular (i.c.v.) infusion of NH₂htau or its reverse counterpart into two lines of young (2-month-old) wild-type mice (C57BL/6 and B6SJL). Six days after its accumulation into hippocampal parenchyma, significant impairment in memory/learning performance was detected in NH₂htau-treated group in association with reduced synaptic connectivity and neuroinflammatory response. Compromised short-term plasticity in paired-pulse facilitation paradigm (PPF) was detected in the CA3/CA1 synapses from NH₂htau-impaired animals along with downregulation in calcineurin (CaN)-stimulated pCREB/c-Fos pathway(s). Importantly, these behavioral, synaptotoxic, and neuropathological effects were independent from the genetic background, occurred prior to frank neuronal loss, and were specific because no alterations were detected in the control group infused with its reverse counterpart. Finally, a 2.0-kDa peptide which biochemically and immunologically resembles the injected NH₂htau was endogenously detected in vivo, being present in hippocampal synaptosomal preparations from AD subjects. Given that the identification of the neurotoxic tau species is mandatory to develop a more effective tau-based immunological approach, our evidence can have important translational implications for cure of human tauopathies.

New Insights into the Disease Progression Control Mechanisms by Comparing Long-Term-Nonprogressors versus Normal-Progressors among HIV-1-Positive Patients Using an Ion Current-Based MS1 Proteomic Profiling

For decades, epidemiological studies have found significant differences in the susceptibility to disease progression among HIV-carrying patients. One unique group of HIV-1-positive patients, the long-term-nonprogressors (LTNP), exhibits far superior ability in virus control compared with normal-progressors (NP), which proceed to Acquired Immune Deficiency Syndrome (AIDS) much more rapidly. Nonetheless, elucidation of the underlying mechanisms of virus control in LTNP is highly valuable in disease management and treatment but remains poorly understood. Peripheral blood mononuclear cells (PBMC) have been known to play important roles in innate immune responses and thereby would be of great interest for the investigation of the mechanisms of virus defense in LTNP. Here, we described the first comparative proteome analysis of PBMC from LTNP (n = 10) and NP (n = 10) patients using a reproducible ion-current-based MS1 approach, which includes efficient and reproducible sample preparation and chromatographic separation followed by an optimized pipeline for protein identification and quantification. This strategy enables analysis of many biological samples in one set with high quantitative precision and extremely low missing data. In total, 925 unique proteins were quantified under stringent criteria without missing value in any of the 20 subjects, and 87 proteins showed altered expressions between the two patient groups. These proteins are implicated in key processes such as cytoskeleton organization, defense response, apoptosis regulation, intracellular transport, etc., which provided novel insights into the control of disease progressions in LTNP versus NP, and the expression and phosphorylation states of key regulators were further validated by immunoassay. For instance, (1) SAMH1, a potent and "hot" molecule facilitating HIV-1 defense, was for the first time found elevated in LTNP compared with NP or healthy controls; elevated proteins from IFN-α response pathway may also contribute to viral control in LTNP; (2) decreased proapoptotic protein ASC along with the elevation of antiapoptotic proteins may contribute to the less apoptotic profile in PBMC of LTNP; and (3) elevated actin polymerization and less microtubule assembly that impede viral protein transport were first observed in LTNP. These results not only enhanced the understanding of the mechanisms for nonprogression of LTNP, but also may afford highly valuable clues to direct therapeutic efforts. Moreover, this work also demonstrated the ion-current-based MS1 approach as a reliable tool for large-scale clinical research.

FAM123A binds to microtubules and inhibits the guanine nucleotide exchange factor ARHGEF2 to decrease actomyosin contractility

The FAM123 gene family comprises three members: FAM123A, the tumor suppressor WTX (also known as FAM123B), and FAM123C. WTX is required for normal development and causally contributes to human disease, in part through its regulation of β-catenin-dependent WNT signaling. The roles of FAM123A and FAM123C in signaling, cell behavior, and human disease remain less understood. We defined and compared the protein-protein interaction networks for each member of the FAM123 family by affinity purification and mass spectrometry. Protein localization and functional studies suggest that the FAM123 family members have conserved and divergent cellular roles. In contrast to WTX and FAM123C, we found that microtubule-associated proteins were enriched in the FAM123A protein interaction network. FAM123A interacted with and tracked with the plus end of dynamic microtubules. Domain interaction experiments revealed a "SKIP" amino acid motif in FAM123A that mediated interaction with the microtubule tip tracking proteins end-binding protein 1 (EB1) and EB3--and therefore with microtubules. Cells depleted of FAM123A showed compartment-specific effects on microtubule dynamics, increased actomyosin contractility, larger focal adhesions, and decreased cell migration. These effects required binding of FAM123A to and inhibition of the guanine nucleotide exchange factor ARHGEF2, a microtubule-associated activator of RhoA. Together, these data suggest that the SKIP motif enables FAM123A, but not the other FAM123 family members, to bind to EB proteins, localize to microtubules, and coordinate microtubule dynamics and actomyosin contractility.

The actin binding protein destrin is associated with growth and perineural invasion of pancreatic cancer

BACKGROUND/OBJECTIVES

The small actin-binding protein destrin is one of the key regulators involved in remodeling of the actin cytoskeleton, a process crucial for cytokinesis, cell migration and polarized cell growth as well as for cancer cell migration and invasion.

METHODS

A novel ex vivo nerve invasion model mirroring perineural cancer cell invasion as a key feature of pancreatic ductal adenocarcinoma has been previously established. Using this model, highly nerve-invasive clones of human pancreatic cancer cell lines have been obtained. Genome-wide transcriptional analyses of these cells revealed up-regulation of destrin in highly versus lowly nerve-invasive pancreatic cancer cells.

RESULTS

Increased expression of destrin in these nerve-invasive cells was validated using quantitative RT-PCR and immunoblotting; concomitant changes in cell morphology were demonstrated using immunofluorescence analysis. Silencing of destrin by two specific siRNA oligonucleotides in Panc-1 pancreatic cancer cells decreased invasiveness and migration, and reduced proliferation of these cells.

CONCLUSIONS

Destrin is upregulated in nerve-invasive pancreatic cancer cells and its expression might be related to perineural invasiveness.

Cofilin is required for organization of sarcomeric actin filaments in chicken skeletal muscle cells

Cofilin is an actin regulatory protein that plays a critical role in actin filament dynamics in a variety of cells. We have previously demonstrated that excess cofilin in skeletal muscle cells leads to disruption of actin filaments, followed by actin-cofilin rod formation in the cytoplasm. In this study, to further clarify the role of cofilin in actin assembly during myofibrillogenesis, cofilin expression was suppressed in cultured chicken skeletal muscle cells. First, we confirmed that turnover of cofilin in myotubes was much higher than that of actin, and that the cofilin level could be decreased drastically within 2 days when cofilin de novo synthesis was suppressed. Next, cofilin expression in individual myotubes was suppressed by introducing antisense morpholino oligonucleotides into the cells by microinjection. Cofilin depletion at the early phase of myofibrillogenesis caused abnormal actin aggregates in myotubes and impaired actin organization into cross-striated myofibril structures. However, when cofilin expression was suppressed in developed myotubes, actin localization in striated myofibrils was scarcely affected. These results indicate that cofilin plays a critical role in the regulation of actin assembly at the early process of myofibrillogenesis.

Dynamic regulation of sarcomeric actin filaments in striated muscle

In striated muscle, the actin cytoskeleton is differentiated into myofibrils. Actin and myosin filaments are organized in sarcomeres and specialized for producing contractile forces. Regular arrangement of actin filaments with uniform length and polarity is critical for the contractile function. However, the mechanisms of assembly and maintenance of sarcomeric actin filaments in striated muscle are not completely understood. Live imaging of actin in striated muscle has revealed that actin subunits within sarcomeric actin filaments are dynamically exchanged without altering overall sarcomeric structures. A number of regulators for actin dynamics have been identified, and malfunction of these regulators often result in disorganization of myofibril structures or muscle diseases. Therefore, proper regulation of actin dynamics in striated muscle is critical for assembly and maintenance of functional myofibrils. Recent studies have suggested that both enhancers of actin dynamics and stabilizers of actin filaments are important for sarcomeric actin organization. Further investigation of the regulatory mechanism of actin dynamics in striated muscle should be a key to understanding how myofibrils develop and operate.

Differential involvement of destrin and cofilin-1 in the control of invasive properties of Isreco1 human colon cancer cells

Actin depolymerizing factor (ADF)/cofilin family proteins are key regulators of actin filament turnover and cytoskeleton reorganization. The role of cofilin-1 in cell motility has been demonstrated in several cell types but remained poorly documented in the case of colon cancer. In addition, the putative function of destrin (also known as ADF) had not been explored in this context despite the fact that it is expressed in all colon cancer cell lines examined. We were therefore prompted to evaluate the respective contributions of these proteins to the invasive properties of the human colon cancer Isreco1 cell line, which expresses a comparatively high destrin/cofilin ratio. Reduction of cofilin-1 or destrin expression in Isreco1 cells using RNA interference led to an increase of the number of multinucleated cells and altered polarized lamellipodium protrusion and distribution of paxillin-containing adhesions. Both cofilin-1 and destrin silencing enhanced cell adhesion to extracellular matrix components. However, only destrin appeared to be required for cell migration on collagen I and for cell invasion through Matrigel in response to the proinvasive neuroendocrine peptide bombesin. This differential functional involvement was supported by a destrin-dependent, cofilin-independent phosphorylation of p130Crk-associated substrate (p130Cas) upon cell adhesion to collagen I or Matrigel. Taken together, our results suggest that destrin is a significant regulator of various processes important for invasive phenotype of human colon cancer Isreco1 cells whereas cofilin-1 may be involved in only a subset of them.

Protein expression profiles in the epidermis of cyclooxygenase-2 transgenic mice by 2-dimensional gel electrophoresis and mass spectrometry

Exposure of murine skin to tumor-promoting agents such as 12-O-tetradecanoyl-phorbol-13-acetate (TPA) causes up-regulation of cyclooxygenase-2 (COX-2) and increased prostaglandin (PG) synthesis. Pharmacological inhibition of COX-2 significantly reduces skin tumor development. However, we previously demonstrated that K14.COX-2 transgenic (TG) mice that overexpressed COX-2 in the epidermis were unexpectedly resistant to tumor development under the classical 7,12-dimethylbenz[a]anthracene-TPA protocol. In the present study, we employed a proteomic approach of 2-dimensional gel electrophoresis (2-DE) and mass spectrometry to profile differentially expressed proteins in the epidermis of K14.COX-2 TG and wild-type control mice. Various 2-DE approaches were used to identify the maximum number of differentially expressed proteins: 20 for untreated samples, 3 for acetone-treated samples, and 22 for TPA-treated samples. These proteins include 14-3-3 sigma, numerous actin fragments, actin filament related proteins cofilin-1 and destrin, galectin-3, galectin-7, prohibitin, S100A6, S100A9, and many others. The differential expression of galectin-3, galectin-7, S100A9 was validated by Western blot analysis and/or immunohistochemical analysis. The current data suggest that some of the differentially expressed proteins might increase apoptosis and cell cycle arrest, which, in turn, may provide insight into the role of COX-2 in skin tumorigenesis.

Actin deficiency induces cofilin phosphorylation: proteome analysis of HeLa cells after beta-actin gene silencing

Actin-binding proteins regulate the dynamic structure and function of actin filaments in the cell. Much is known about how manipulation of the actin-binding proteins affects the structure and function of actin filaments; however, little is known about how manipulation of actin in the cell affects actin-binding proteins. We addressed this question by utilizing two technologies: RNA interference and 2-dimensional gel electrophoresis. We knocked down beta-actin expression in HeLa cells using short interfering RNA and applied 2-DGE to examine alterations in the HeLa cell proteome. We revealed a 2-5 fold increases of four protein spots on 2-D gels and identified these proteins by mass spectrometry. Three of the four proteins were actin-binding proteins, including cofilin, which promotes both disassembly and assembly of actin filaments but becomes inactivated when phosphorylated. Further examination revealed that the cofilin total protein level barely increased, but the phosphorylated cofilin level increased dramatically in HeLa cells after beta-actin siRNA treatment. These results suggest that in response to siRNA-induced beta-actin deficiency HeLa cells inactivate cofilin by phosphorylation rather than down-regulate its protein expression level. This study also demonstrates that the combination of RNA interference and 2-dimensional gel electrophoresis technologies provides a valuable method to study protein interactions in a specific cellular pathway.

Association between CFL1 gene polymorphisms and spina bifida risk in a California population

BACKGROUND

CFL1 encodes human non-muscle cofilin (n-cofilin), which is an actin-depolymerizing factor and is essential in cytokinesis, endocytosis, and in the development of all embryonic tissues. Cfl1 knockout mice exhibit failure of neural tube closure at E10.5 and die in utero. We hypothesized that genetic variation within the human CFL1 gene may alter the protein's function and result in defective actin depolymerizing and cellular activity during neural tube closure. Such alterations may be associated with an increased risk for neural tube defects (NTDs).

METHODS

Having re-sequenced the human CFL1 gene and identified five common single nucleotide polymorphisms (SNPs) in our target population, we investigated whether there existed a possible association between the genetic variations of the CFL1 gene and risk of spina bifida. Samples were obtained from a large population-based case-control study in California. Allele association, genotype association and haplotype association were evaluated in two different ethnicity groups, non-Hispanic white and Hispanic white.

RESULTS

Homozygosity for the minor alleles of the SNPs studied (rs652021, rs665306, rs667555, rs4621 and rs11227332) appeared to produce an increased risk for spina bifida. Subjects with the haplotype composed of all minor alleles (CCGGT) appeared to have increased spina bifida risk (OR = 1.6, 95% CI: 0.9~2.9), however, this finding is not statistically significant likely due to limited sample size.

CONCLUSION

The sequence variation of human CFL1 gene is a genetic modifier for spina bifida risk in this California population.

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.

The actin depolymerizing factor n-cofilin is essential for neural tube morphogenesis and neural crest cell migration

Cofilin/ADF proteins are a ubiquitously expressed family of F-actin depolymerizing factors found in eukaryotic cells including plants. In vitro, cofilin/ADF activity has been shown to be essential for actin driven motility, by accelerating actin filament turnover. Three actin depolymerizing factors (n-cofilin, m-cofilin, ADF) can be found in mouse and human. Here we show that in mouse the non-muscle-specific gene-n-cofilin-is essential for migration of neural crest cells as well as other cell types in the paraxial mesoderm. The main defects observed in n-cofilin mutant embryos are an impaired delamination and migration of neural crest cells, affecting the development of neural crest derived tissues. Neural crest cells lacking n-cofilin do not polarize, and F-actin bundles or fibers are not detectable. In addition, n-cofilin is required for neuronal precursor cell proliferation and scattering. These defects result in a complete lack of neural tube closure in n-cofilin mutant embryos. Although ADF is overexpressed in mutant embryos, this cannot compensate the lack of n-cofilin, suggesting that they might have a different function in embryonic development. Our data suggest that in mammalian development, regulation of the actin cytoskeleton by the F-actin depolymerizing factor n-cofilin is critical for epithelial-mesenchymal type of cell shape changes as well as cell proliferation.

The RNA-binding protein SUP-12 controls muscle-specific splicing of the ADF/cofilin pre-mRNA in C. elegans

Tissue-specific alternative pre-mRNA splicing is essential for increasing diversity of functionally different gene products. In Caenorhabditis elegans, UNC-60A and UNC-60B, nonmuscle and muscle isoforms of actin depolymerizing factor (ADF)/cofilin, are expressed by alternative splicing of unc-60 and regulate distinct actin-dependent developmental processes. We report that SUP-12, a member of a new family of RNA recognition motif (RRM) proteins, including SEB-4, regulates muscle-specific splicing of unc-60. In sup-12 mutants, expression of UNC-60B is decreased, whereas UNC-60A is up-regulated in muscle. sup-12 mutations strongly suppress muscle defects in unc-60B mutants by allowing expression of UNC-60A in muscle that can substitute for UNC-60B, thus unmasking their functional redundancy. SUP-12 is expressed in muscle and localized to the nuclei in a speckled pattern. The RRM domain of SUP-12 binds to several sites of the unc-60 pre-mRNA including the UG repeats near the 3′-splice site in the first intron. Our results suggest that SUP-12 is a novel tissue-specific splicing factor and regulates functional redundancy among ADF/cofilin isoforms.

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

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

Detection and characterization of a rat parotid gland protein kinase that catalyzes phosphorylation of matured destrin at Ser-2

Destrin, an actin-binding protein, is partly phosphorylated at Ser-2 (numbering on the matured form) in the resting rat parotid gland, and beta-adrenergic or cholinergic stimulation of this gland induces its dephosphorylation. In this study, we searched for the protein kinase involved in phosphorylation of destrin. We developed an assay method for the kinase, using an antibody specific to destrin phosphorylated at Ser-2, and detected the kinase in the rat parotid homogenate. This enzyme was predominantly (93%) present in the soluble fraction, and the enzyme in this fraction was characterized. It had an optimum pH at 6.8 and required 3-5 mM Mg2+ for its maximum activity. Ca2+ (1 mM) had no effect whereas Mn2+ (5 mM) inhibited the enzyme activity by 75%. The apparent Km values for destrin and ATP were 92 microg/ml and 170 microM, respectively. GTP was an inefficient phosphate donor, and cAMP had no effect. Heat-denatured destrin was poorly phosphorylated. Two-dimensional PAGE analysis of destrin phosphorylated with the soluble fraction indicated that it was exclusively phosphorylated at Ser-2. None of the protein kinase inhibitors tested here was specific to this enzyme. At 1 mM, ML-7, Y-27632, KN-93, HA-1077, H-7, and H-8 inhibited the activity by 88, 61, 58, 49, 46, and 42%, respectively. Staurosporine (2 microM) and H-89 (50 microM) inhibited the activity by 48 and 33%, respectively. Heparin (30 microg/ml) had no effect. These results suggest that the rat parotid gland contains a novel, constitutively active, soluble protein kinase catalyzing specific phosphorylation of destrin at Ser-2.

Actin binding proteins: regulation of cytoskeletal microfilaments

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

Identification of the cofilin-binding sites in the large cytoplasmic domain of Na,K-ATPase

Na,K-ATPase, an alpha, beta heterodimer, is found in the plasma membrane of all animal cells. The alpha chain is believed to have 10 transmembrane regions and a large cytoplasmic domain between the 4th and 5th transmembrane regions (H4-H5). In our previous report, the large (3rd) cytoplasmic domains of the alpha1 and alpha2 isoform were found to interact with cofilin, an actin-modulating protein, by the yeast two-hybrid system. Here we show that cofilin interacts only with the 3rd cytoplasmic domain of the alpha2 subunit but not with the 2nd, 4th, and 5th cytoplasmic domains or the cytoplasmic region of the beta subunit of Na,K-ATPase. We also demonstrate that cofilin interacts with the large cytoplasmic domains of the alpha1, alpha2 and alpha3 isoforms of Na,K-ATPase, but not with those of glucose transporter 1, glucose transporter 4, cystic fibrosis transmembrane conductance regulator and plasma membrane Ca-ATPase. We introduced 10 mutations into the 3rd cytoplasmic domain of Na,K-ATPase to identify the binding sites with cofilin. Eight of these mutants were single amino acid substitutions (R417Q, K470Q, K654G, D672A, K691A, R700G, R700A and D710G) and two were double mutant (K654GR700G and K719AK720A). Analysis of the activity of the reporter gene of these mutants shows that residues D672 and R700 of the 3rd cytoplasmic domain of Na,K-ATPase are involved in the interaction with cofilin.

The three mouse actin-depolymerizing factor/cofilins evolved to fulfill cell-type-specific requirements for actin dynamics

Actin-depolymerizing factor (ADF)/cofilins are essential regulators of actin filament turnover. Several ADF/cofilin isoforms are found in multicellular organisms, but their biological differences have remained unclear. Herein, we show that three ADF/cofilins exist in mouse and most likely in all other mammalian species. Northern blot and in situ hybridization analyses demonstrate that cofilin-1 is expressed in most cell types of embryos and adult mice. Cofilin-2 is expressed in muscle cells and ADF is restricted to epithelia and endothelia. Although the three mouse ADF/cofilins do not show actin isoform specificity, they all depolymerize platelet actin filaments more efficiently than muscle actin. Furthermore, these ADF/cofilins are biochemically different. The epithelial-specific ADF is the most efficient in turning over actin filaments and promotes a stronger pH-dependent actin filament disassembly than the two other isoforms. The muscle-specific cofilin-2 has a weaker actin filament depolymerization activity and displays a 5-10-fold higher affinity for ATP-actin monomers than cofilin-1 and ADF. In steady-state assays, cofilin-2 also promotes filament assembly rather than disassembly. Taken together, these data suggest that the three biochemically distinct mammalian ADF/cofilin isoforms evolved to fulfill specific requirements for actin filament dynamics in different cell types.

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.

Major role for neuronal NO synthase in curtailing choroidal blood flow autoregulation in newborn pig

We examined whether nitric oxide (NO) generated from neuronal NO synthase (nNOS) contributes to the reduced ability of the newborn to autoregulate retinal blood flow (RBF) and choroidal blood flow (ChBF) during acute rises in perfusion pressure. In newborn pigs (1-2 days old), RBF (measured by microsphere) is autoregulated over a narrow range of perfusion pressure, whereas ChBF is not autoregulated. N(G)-nitro-L-arginine methyl ester (L-NAME) or specific nNOS inhibitors 7-nitroindazole, 3-bromo-7-nitroindazole, and 1-(2-trifluoromethyl-phenyl)imidazole as well as ganglionic blocker hexamethonium, unveiled a ChBF autoregulation as observed in juvenile (4- to 6-wk old) animals, whereas autoregulation of RBF in the newborn was only enhanced by L-NAME. All NOS inhibitors and hexamethonium prevented the hypertension-induced increase in NO mediator cGMP in the choroid. nNOS mRNA expression and activity were three- to fourfold higher in the choroid of newborn pigs than in tissues of juvenile pigs. It is concluded that increased production of NO from nNOS curtails ChBF autoregulation in the newborn and suggests a role for the autonomic nervous system in this important hemodynamic function, whereas, for RBF autoregulation, endothelial NOS seems to exert a more important contribution in limiting autoregulation.

Two closely related cDNAs encoding actin-depolymerizing factors of petunia are mainly expressed in vegetative tissues

Actin-depolymerizing factor (ADF) is one of the small actin-binding proteins that regulate actin dynamics in cells. We have isolated two cDNA clones, PhADF1 and PhADF2, encoding ADF from cDNA libraries constructed from petal protoplast cultures and flowers of Petunia hybrida. PhADF1 and PhADF2 encode polypeptides of 139 and 143 amino acids with a calculated molecular mass of 16.04 and 16.51kDa, respectively. Co-sedimentation assay showed that the recombinant PhADF1 protein produced in Escherichia coli binds to F-actin at pH7. 0 and preferentially depolymerizes it at pH8.0. Gene tree analysis indicates that the plant ADF family can be grouped into four classes, and PhADFs are included in class I. Southern blot analyses revealed that one or two copies of PhADF genes are present in petunia genome, and several other related isoforms also exist. Northern blot analyses indicated that PhADF1 and PhADF2 are closely related and abundantly expressed in every plant organ except pollen. In addition, they are highly accumulated in mature vegetative tissue (petal, leaf, and stem). Our results indicate that the transcription of petunia ADF genes is differentially regulated by developmental signals.

Searching for a function for nuclear actin

The abundant cytoskeletal protein actin has numerous cytoplasmic roles. Although there are many reports of the presence of actin in the nucleus, in general they have been discounted as artifactual. However, recent work has begun to provide evidence for important roles for actin in nuclear processes ranging from chromatin remodelling to splicing. In addition, several regulators of actin polymerization are localized to the nucleus or translocate to the nucleus in a regulated manner, suggesting that there is some function of actin in the nucleus that is subject to regulation. This review discusses the evidence for actin in the nucleus and summarizes recent work suggesting that actin or actin-related proteins are involved in the regulation of nuclear processes such as chromatin remodelling.

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.

Mechanism of interaction of Acanthamoeba actophorin (ADF/Cofilin) with actin filaments

We characterized the interaction of Acanthamoeba actophorin, a member of ADF/cofilin family, with filaments of amoeba and rabbit skeletal muscle actin. The affinity is about 10 times higher for muscle actin filaments (Kd = 0.5 microM) than amoeba actin filaments (Kd = 5 microM) even though the affinity for muscle and amoeba Mg-ADP-actin monomers (Kd = 0.1 microM) is the same (Blanchoin, L., and Pollard, T. D. (1998) J. Biol. Chem. 273, 25106-25111). Actophorin binds slowly (k+ = 0.03 microM-1 s-1) to and dissociates from amoeba actin filaments in a simple bimolecular reaction, but binding to muscle actin filaments is cooperative. Actophorin severs filaments in a concentration-dependent fashion. Phosphate or BeF3 bound to ADP-actin filaments inhibit actophorin binding. Actophorin increases the rate of phosphate release from actin filaments more than 10-fold. The time course of the interaction of actophorin with filaments measured by quenching of the fluorescence of pyrenyl-actin or fluorescence anisotropy of rhodamine-actophorin is complicated, because severing, depolymerization, and repolymerization follows binding. The 50-fold higher affinity of actophorin for Mg-ADP-actin monomers (Kd = 0.1 microM) than ADP-actin filaments provides the thermodynamic basis for driving disassembly of filaments that have hydrolyzed ATP and dissociated gamma-phosphate.

Suppression of an actin-binding protein, drebrin, by antisense transfection attenuates neurite outgrowth in neuroblastoma B104 cells

Drebrins, actin-binding proteins, are dominantly expressed during embryogenesis and accumulated in neurite processes of postmigratory neurons. While the cytoskeletal proteins are the important factors for regulating neurite outgrowth, the cellular mechanism in neurons is still unclear. To address the role of drebrins in the neurite outgrowth, we have studied the effect of suppression of drebrin on a rat neuroblastoma B104 cell line, which constitutively expresses drebrin. Deprivation of serum or addition of gangliosides in the culture medium induced remarkable neurite outgrowth of B104 cells. We transfected B104 cells with an antisense construct of human drebrin E cDNA and found that the drebrin expression was significantly reduced in the stable antisense cell lines. In response to serum deprivation and gangliosides treatment, their ability to extend neurite processes was significantly attenuated. In contrast, the cell proliferation of the antisense transfectants was arrested by serum deprivation similar to control B104 cells. These data suggest that the drebrins are required for neurite outgrowth in neuronal cells.

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.

A quantitative analysis of G-actin binding proteins and the G-actin pool in developing chick brain

The large G-actin pool in individual actively motile cells has been shown to be maintained primarily by the actin sequestering protein thymosin beta four (Tbeta4). It is not clear whether Tbeta4 or an isoform also plays a primary role in neural tissue containing highly motile axonal growth cones. To address this question we have made a definitive analysis of the relative contributions of all the known G-actin sequestering proteins: Tbeta4, Tbeta10, profilin, and phosphorylated (inactive) and unphosphorylated (potentially active) forms of both ADF and cofilin, in relation to the G-actin pool in developing chick brain at embryonic days 13 and 17. From our measurements we estimate the intracellular concentration of G-actin as 30-37 microM and of Tbeta4 as 50-60 microM in an 'average' brain cell in embryonic chick brain. No other beta thymosin isoforms were detected in these brain extracts. The ratio of soluble, unphosphorylated ADF to Tbeta4 is only 1:7 at 13 embryonic days, but increases to 1:4 at 17 days. Profilin and cofilin concentrations are an order of magnitude lower than Tbeta4. Combining the contributions of Tbeta4, unphosphorylated ADF and unphosphorylated cofilin, we estimate a mean G-actin critical concentration of approximately 0.45 microM and approximately 0.2 microM, respectively, in day 13 and day 17 embryonic brain extracts, suggesting a significant developmental decrease. We conclude that (a) Tbeta4 is the major actin sequestering protein in embryonic chick brain and the only beta thymosin isoform present; (b) ADF may play a significant developmental role, as its concentration changes significantly with age; (c) the known G-actin binding proteins can adequately account for the G-actin pool in embryonic chick brain.

Complete amino acid sequences and phosphorylation sites, determined by Edman degradation and mass spectrometry, of rat parotid destrin- and cofilin-like proteins

Beta-adrenergic or cholinergic stimulation of the rat parotid gland was earlier shown to induce dephosphorylation of endogenous destrin- and cofilin-like proteins, which are phosphorylated in resting cells at Ser residues probably present near the N-terminals. The primary structures and phosphorylation sites were determined here. The rat destrin-like protein had a sequence 95% identical to the cDNA-derived sequence of porcine destrin. The rat cofilin-like protein was 98% identical to that of porcine cofilin. Each protein lacked the initiator Met and began with an acetylalanine residue followed by a Ser residue. The N-terminal peptides generated with endoproteinase Asp-N were isolated; they were each phosphorylated at Ser-2. Earlier work had shown that partial cleavage of the phosphorylated destrin- and cofilin-like proteins with cyanogen bromide provides unphosphorylated 16.7- and 18.3-kDa fragments, respectively. It was here confirmed that they contained all the Ser residues other than those present in the N-terminal peptides. From these observations, it was now concluded that the destrin- and cofilin-like proteins are rat parotid destrin and cofilin (non-muscle type), respectively, and that each protein is phosphorylated exclusively at Ser-2 in resting cells and dephosphorylated at this site in response to beta-adrenergic or cholinergic stimulation.

PGE2, via EP3 receptors, regulates brain nitric oxide synthase in the perinatal period

We tested the hypothesis that high prostaglandin levels during the perinatal period might regulate brain nitric oxide synthase (nNOS) expression. nNOS and cyclooxygenase (COX)-2 mRNAs were higher in brain cortex and the periventricular area of newborn rats and pigs compared with adult brain. Nitric oxide synthase activity was also 2. 5- to 4-fold higher in newborn than in adult brain. Administration of nonselective COX inhibitor ibuprofen or COX-2 inhibitor nimesulide every 8 h for 24 h to newborn rats and pigs reduced prostaglandin levels and caused comparable reductions in nNOS mRNA, protein, and activity to levels of adults; COX inhibitor-induced changes were prevented by cotreatment with PGE2 analog, 16, 16-dimethyl-PGE2, and agonist for the EP3 receptor of PGE2, sulprostone, but not by PGI2 analog carbaprostacyclin, PGD2, EP1 receptor agonist 17-phenyl trinor-PGE2, and EP2 agonist butaprost. Concordant observations were made in vitro and revealed that nNOS expression (detected by NADPH diaphorase reactivity) mostly present in neurons of the deeper cortical layers was reduced by COX inhibitor, and this effect was prevented by EP3 agonist. In conclusion, high levels of PGE2 in neonatal brain contribute to the increased expression of nNOS by acting on EP3 receptors; this positive interaction between PGE2 and nNOS might be required physiologically for normal brain development.

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.

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.

A novel gene (drad-1) expressed in hematopoiesis-supporting stromal cell lines, ST2, PA6 and A54 preadipocytes: use of mRNA differential display

We established a differentiation-inducible preadipocyte cell line, designated A54 preadipocytes, from C3H10T1/2 (10T1/2) mouse embryo fibroblasts. A54 preadipocytes had marked hematopoiesis-supporting ability in vitro but this ability was lost after terminal differentiation to adipocytes. In this study, to identify molecules that contribute to the hematopoiesis-supporting ability of A54 preadipocytes, we screened genes that were differentially expressed in A54 preadipocytes and isolated seven novel genes by reverse transcriptase polymerase chain reaction mRNA differential display. An RNase protection assay confirmed that one of these genes was expressed at high levels in parent 10T1/2 cells and A54 preadipocytes but to a much lesser extent in fully differentiated A54 adipocytes. This gene was defined as a gene that was downregulated during adipocyte differentiation-1 (drad-1). The size of drad-1 mRNA was 8.2 kb, and the gene was expressed in other mouse preadipocytes, namely, ST2 and PA6 cells, that have hematopoiesis-supporting ability. Moreover, drad-1 was also found to be expressed in bone marrow in vivo. The function of the protein encoded by drad-1 is unknown, but the expression of the gene may be useful as a molecular marker of adipocyte differentiation.

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.

Differential regulation of actin depolymerizing factor and cofilin in response to alterations in the actin monomer pool

Myoblasts, transfected with a human gene encoding a beta-actin point mutation, down-regulate expression of actin depolymerizing factor (ADF) and its mRNA. Regulation is posttranscriptional. Expression of cofilin, a structurally similar protein, and profilin, CapG, and tropomodulin is not altered with increasing mutant beta-actin expression. Myoblasts expressing either human gamma-actin or the mutant beta-actin down-regulate the endogenous mouse actin genes to keep a constant level of actin mRNA, whereas the gamma-actin transfectants do not down-regulate ADF. Thus, ADF expression is regulated differently from actin expression. The mutant beta-actin binds to ADF with about the same affinity as normal actin; however, it does not assemble into normal actin filaments. The decrease in ADF expression correlates with an increase in the unassembled actin pool. When the actin monomer pool in untransfected myoblasts is increased 70% by treatment with latrunculin A, synthesis of ADF and actin are down-regulated compared with cofilin and 19 other proteins selected at random. Increasing the actin monomer pool also results in nearly complete phosphorylation of both ADF and cofilin. Thus, ADF and cofilin are coordinately regulated by posttranslational modification, but their expression is differentially regulated. Furthermore, expression of ADF is responsive to the utilization of actin by the cell.

Actin depolymerizing factor (ADF/cofilin) enhances the rate of filament turnover: implication in actin-based motility

Actin-binding proteins of the actin depolymerizing factor (ADF)/cofilin family are thought to control actin-based motile processes. ADF1 from Arabidopsis thaliana appears to be a good model that is functionally similar to other members of the family. The function of ADF in actin dynamics has been examined using a combination of physical-chemical methods and actin-based motility assays, under physiological ionic conditions and at pH 7.8. ADF binds the ADP-bound forms of G- or F-actin with an affinity two orders of magnitude higher than the ATP- or ADP-Pi-bound forms. A major property of ADF is its ability to enhance the in vitro turnover rate (treadmilling) of actin filaments to a value comparable to that observed in vivo in motile lamellipodia. ADF increases the rate of propulsion of Listeria monocytogenes in highly diluted, ADF-limited platelet extracts and shortens the actin tails. These effects are mediated by the participation of ADF in actin filament assembly, which results in a change in the kinetic parameters at the two ends of the actin filament. The kinetic effects of ADF are end specific and cannot be accounted for by filament severing. The main functionally relevant effect is a 25-fold increase in the rate of actin dissociation from the pointed ends, while the rate of dissociation from the barbed ends is unchanged. This large increase in the rate-limiting step of the monomer-polymer cycle at steady state is responsible for the increase in the rate of actin-based motile processes. In conclusion, the function of ADF is not to sequester G-actin. ADF uses ATP hydrolysis in actin assembly to enhance filament dynamics.

Large-scale sequencing in human chromosome 12p13: experimental and computational gene structure determination

The detailed genomic organization of a gene-dense region at human chromosome 12p13, spanning 223 kb of contiguous sequence, was determined. This region is composed of 20 genes and several other expressed sequences. Experimental tools including RT-PCR and cDNA sequencing, combined with gene prediction programs, were utilized in the analysis of the sequence. Various computer software programs were employed for sequence similarity searches and functional predictions. The high number of genes with diverse functions and complex transcriptional patterns make this region ideal for addressing challenges of gene discovery and genomic characterization amenable to large-scale sequence analysis.

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.

Axonal transport of actin and actin-binding proteins in the rat sciatic nerve

Actin is one of the major cytoskeletal proteins carried in slow axonal transport. Since more than 50% of actin in the axon was recovered in the high-speed supernatant, we looked for G-actin-binding proteins in slow axonal transport. Two weeks after injection of L-[35S]methionine into the rat spinal cord (L3-L5), labeled proteins in the sciatic nerve were extracted and those with potential abilities to interact with G-actin were detected by two independent methods: (A) DNAase I affinity chromatography and (B) blot overlay with biotinylated actin. By method (A), a 68 kDa Ca(2+)-dependent binding protein and a 45 kDa Ca(2+)-independent binding protein were detected. The 68 kDa protein was also a major protein binding to actin in method (B). The 68 kDa protein was identified with the Ca(2+)-dependent phospholipid binding protein annexin VI by two-dimensional electrophoresis and Western blotting. As annexin VI is a component of slow axonal transport, it does not seem to be bound to membranous organelles in the axon. Our results suggest that annexin VI may play a role in the control of actin assembly and membrane-microfilament interaction.

The Caenorhabditis elegans unc-60 gene encodes proteins homologous to a family of actin-binding proteins

Mutations in the unc-60 gene of the nematode Caenorhabditis elegans result in paralysis. The thin filaments of the muscle cells are severely disorganized and not bundled with myosin into functional contractile units. Here we report the cloning and sequence of unc-60. Two unc-60 transcripts, 1.3 and 0.7 kb in size, were detected. The transcripts share a single exon encoding only the initial methionine, yet encode proteins with homologous sequences. The predicted protein products are 165 and 152 amino acids in length and their sequences are 38% identical. Both proteins are homologous to a family of actin depolymerizing proteins identified in vertebrate, plant and protozoan systems. We propose that the unc-60 locus encodes proteins that depolymerize growing actin filaments in muscle cells, and that these proteins are required for the assembly of actin filaments into the contractile myofilament lattice of C. elegans muscle. unc-60 has an essential function in development, since one unc-60 allele, s1586, has a recessive lethal phenotype. Our characterization of s1586 has shown that it is a small deletion which disrupts both coding regions.

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.

Human actin depolymerizing factor mediates a pH-sensitive destruction of actin filaments

ADF (actin depolymerizing factor) is an M(r) 19,000 actin-binding protein present in many vertebrate tissues and particularly abundant in neuronal cells. We have cloned human ADF and here show it to be identical in sequence to porcine destrin. Human ADF expressed in Escherichia coli behaves like native ADF from porcine brain. It binds to G-actin at pH 8 with a 1:1 stoichiometry and Kd approximately 0.2 microM, thereby sequestering monomers and preventing polymerization. It does not cosediment with F-actin at this pH, but severs actin filaments in a calcium-insensitive manner. The severing activity is only about 0.1% efficient. By contrast, at pH values below 7, ADF binds to actin filaments in a highly cooperative manner and at a 1:1 ratio to filament subunits. When the pH is raised to 8.0, the decorated filaments are rapidly severed and depolymerized.

Analysis of the interactions of actin depolymerizing factor with G- and F-actin

Chick actin depolymerizing factor (ADF) is an actin binding protein previously shown to rapidly depolymerize actin filaments in vitro, yielding a 1:1 complex of ADF and actin monomer. Here we show that ADF protects actin monomer from denaturation by EDTA by inhibiting the exchange of actin-bound nucleotide. Under low ionic strength conditions, the approximate dissociation constant (KD) for the ADF-actin complex determined from exchange of nucleotide (1,N6-etheno-ATP) is about 150 and is calcium-independent. Addition of ADF to monomeric actin inhibits actin assembly as well as the ATP hydrolysis that normally accompanies assembly. Complex formation is demonstrated between ADF and actin containing either ATP, ADP, or AMPPNP as the bound nucleotide. A KD of 0.1-0.2 microM was calculated for both the ADF-ATP-actin and ADF-AMPPNP-actin complexes, whereas the KD for the ADF-ADP-actin complex is about 1.3 microM. ADF can either depolymerize or cosediment with F-actin in a stoichiometric fashion, but these reciprocal activities are pH-dependent. At pHs between 6.5 and 7.1, ADF cosediments with F-actin and demonstrates only weak depolymerizing activity. ADF binding is cooperative and saturates at a 1:1 ADF:actin molar ratio. At pHs between 7.1 and 7.7, ADF shows increasing depolymerizing activity and less F-actin binding. At pH 8.0, ADF depolymerizes F-actin in a stoichiometric manner. Both the F-actin binding and the depolymerizing activities of ADF are inhibited by phalloidin.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of the apple fruit ethylene-forming enzyme

The enzyme that oxidatively converts 1-aminocyclopropanecarboxylic acid (ACC) to ethylene, a key plant growth hormone, has been classified, on the basis of a comparison of homologous protein sequences (derived from the cDNA sequences), as a member of a family of non-heme iron proteins that includes plant and bacterial oxidative enzymes. This knowledge has facilitated the purification of the relatively abundant ethylene-forming enzyme to homogeneity from apple tissue. The properties of the enzyme are consistent with two other recent reports that describes its purification by different protocols, lending credence to the assertion that the key protein has been isolated. New characterizations of the protein have been conducted. Electrospray mass spectrometry shows that its molecular weight (35 331.8 +/- 5 amu) is approximately 50 amu higher than that predicted from the cDNA sequence, identifying the blocking group at the N-terminus as acetyl. The enzyme is activated by bicarbonate at low concentration but is inhibited at high concentration, with the maximum activation occurring at 5 mM. The iron concentration leading to half-maximal activity is 1 microM. The enzyme self-inactivates during turnover. The availability of the purified enzyme will permit definitive studies of the mechanism by which ethylene is produced and provide opportunities to discover molecules that inhibit the process.

Colocalization of ADF and cofilin in intranuclear actin rods of cultured muscle cells

Immunofluorescence microscopy revealed that two actin-binding proteins of low molecular weight with different functional activity, ADF and cofilin, are transported into nuclei of cultured myogenic cells to form rod structures there together with actin, when the cells were incubated in medium containing dimethylsulfoxide. In most cases, ADF and cofilin colocalized in the same nuclear actin rods, but ADF appeared to predominate in mononucleated cells, while cofilin was present in multinucleated myotubes. In some mononucleated cells, the nuclear actin rods were composed of ADF and actin but devoid of cofilin. An ADF homologue in mammals, destrin, was also translocated into nuclear actin rods under similar conditions. As a nuclear transport signal sequence exists in cofilin and ADF but not in actin, ADF and/or cofilin may be responsible for the nuclear import of actin in myogenic cells under certain conditions.

Isolation of a yeast essential gene, COF1, that encodes a homologue of mammalian cofilin, a low-M(r) actin-binding and depolymerizing protein

We have cloned a Saccharomyces cerevisiae gene (COF1) encoding a low-M(r) actin-binding protein of 143 amino acid (aa) residues (yeast cofilin; Cof); its aa sequence is 35% identical to porcine Cof. The yeast recombinant Cof produced in Escherichia coli exhibited in vitro activities on actin filaments similar to those of mammalian and avian Cof. Gene disruption and tetrad analysis showed that gene COF1 is essential for yeast cell growth. Expression of the cDNA of porcine Cof or destrin (Des), the latter a Cof-related protein, complemented the cof1 null allele in yeast cells.