Beta-centractin: characterization and distribution of a new member of the centractin family of actin-related proteins

Upregulation of Brain's Calcium Binding Proteins in Mitragynine Dependence: A Potential Cellular Mechanism to Addiction

Background: Mitragyna speciosa Korth or Kratom is widely used traditionally for its medicinal values. The major alkaloid content of kratom leaves is mitragynine, which binds to opioid receptors to give opioid-like effects. This study aimed to analyse the brain proteome of animals that displayed addictive behaviors. Design and Methods: Six groups (n=6-8) of rats made up of negative control, positive control using morphine (10 mg/kg), and treatment groups at low (1mg/kg) and high doses of mitragynine (30 mg/kg) for 1 and 4 days. The rats' behaviors were evaluated and subsequently the rats' brains were harvested for proteomic analysis that was performed by using 2D gel electrophoresis and LC/MS/MS. Results: The rats developed physical dependence only on day 4 following morphine and mitragynine (1 and 30mg/kg) treatments. Among the proteins that were up-regulated in treatment groups were four calcium-binding proteins, namely calretinin, F-actin, annexin A3 and beta-centractin. Conclusions: Upregulation of calretinin acted as low Ca²⁺ buffering upon the blockage of Ca²⁺ ion channel by mitragynine in the brain, which subsequently caused a reduction of GABA released and inversely increased the dopamine secretions that contributed to dependence indicators.

An RNAi Screen in a Novel Model of Oriented Divisions Identifies the Actin-Capping Protein Z β as an Essential Regulator of Spindle Orientation

Oriented cell divisions are controlled by a conserved molecular cascade involving Gαi, LGN, and NuMA. We developed a new cellular model of oriented cell divisions combining micropatterning and localized recruitment of Gαi and performed an RNAi screen for regulators acting downstream of Gαi. Remarkably, this screen revealed a unique subset of dynein regulators as being essential for spindle orientation, shedding light on a core regulatory aspect of oriented divisions. We further analyze the involvement of one novel regulator, the actin-capping protein CAPZB. Mechanistically, we show that CAPZB controls spindle orientation independently of its classical role in the actin cytoskeleton by regulating the assembly, stability, and motor activity of the dynein/dynactin complex at the cell cortex, as well as the dynamics of mitotic microtubules. Finally, we show that CAPZB controls planar divisions in vivo in the developing neuroepithelium. This demonstrates the power of this in cellulo model of oriented cell divisions to uncover new genes required in spindle orientation in vertebrates.

The novel, actin-like protein Tact3 is expressed in rodent testicular haploid germ cells

Mouse testis actin-like proteins 1 and 2 (mTact1 and mTact2), which are expressed in murine haploid germ cells, have been described previously. Here, we report the cloning and characterization of a third actin-like protein from rat, rat testis actin-like protein 3 (rTact3). The complete cDNA of the rTact3 gene was approximately 3.7 kb in length, and its corresponding amino acid sequence consisted of 1219 amino acids. The rTact3 gene lacks introns, similar to mTact1 and mTact2. The 356 C-terminal amino acids of rTact3 showed 43% homology with mTact1, whereas the 863 N-terminal amino acids did not show any significant homology. Northern blot analysis revealed that rTact3 mRNA was expressed only in adult rat testes and not during the prepubescent stage. In situ hybridization revealed that rTact3 was expressed exclusively during round and elongated spermatids maturation stages in rat testes. Immunohistochemical experiments using antibodies raised against a synthetic peptide showed that the expression of the rTact3 protein was also restricted in round and elongated spermatids, specifically in the head and acrosome of mature rat sperm. The 5′-flanking region of the mTact3 gene was found to contain a TATA-box motif as well as two putative CREB/c-Jun and five C/EBP motifs. mTact3 promoter activity was enhanced in a dose-dependent manner by the transfection of CREB, c-Jun, or C/EBP in NIH3T3 cells. These results suggest that Tact3 proteins might play an important role in rodent germ-cell development.

Differential gene expression in rainbow trout (Oncorhynchus mykiss) liver and ovary after exposure to zearalenone

Zearalenone (ZEA) is a mycotoxin of worldwide occurrence, and it has been shown to produce numerous adverse effects in both laboratory and domestic animals. However, regardless of recent achievements, the molecular mechanisms underlying ZEA toxicity remain elusive, and little is known about transcriptome changes of fish cells in response to ZEA occurrence. In the present study, differential display PCR was used to generate a unique cDNA fingerprint of differentially expressed transcripts in the liver and ovary of juvenile rainbow trout after either 24, 72, or 168 h of intraperitoneal exposure to ZEA (10 mg/kg of body mass). From a total of 59 isolated cDNA bands (ESTs), 5 could be confirmed with Real-Time qPCR and their nucleotide sequences were identified as mRNAs of: acty (β-centractin), the cytoskeleton structural element; bccip, responsible for DNA repair and cell cycle control; enoa (α-enolase), encoding enzyme of the glycolysis process; proc (protein C), that takes part in the blood coagulation process; and frih, encoding the heavy chain of ferritin, the protein complex important for iron storage. Further qPCR analysis of the confirmed ESTs expression profiles revealed significant mRNA level alterations in both tissues of exposed fish during the 168 h study. The results revealed a complex network of genes associated with different biological processes that may be engaged in the cellular response to ZEA exposure, i.e. blood coagulation or iron-storage processes.

Evolution of the eukaryotic dynactin complex, the activator of cytoplasmic dynein

Background

Dynactin is a large multisubunit protein complex that enhances the processivity of cytoplasmic dynein and acts as an adapter between dynein and the cargo. It is composed of eleven different polypeptides of which eight are unique to this complex, namely dynactin1 (p150^Glued), dynactin2 (p50 or dynamitin), dynactin3 (p24), dynactin4 (p62), dynactin5 (p25), dynactin6 (p27), and the actin-related proteins Arp1 and Arp10 (Arp11).

Results

To reveal the evolution of dynactin across the eukaryotic tree the presence or absence of all dynactin subunits was determined in most of the available eukaryotic genome assemblies. Altogether, 3061 dynactin sequences from 478 organisms have been annotated. Phylogenetic trees of the various subunit sequences were used to reveal sub-family relationships and to reconstruct gene duplication events. Especially in the metazoan lineage, several of the dynactin subunits were duplicated independently in different branches. The largest subunit repertoire is found in vertebrates. Dynactin diversity in vertebrates is further increased by alternative splicing of several subunits. The most prominent example is the dynactin1 gene, which may code for up to 36 different isoforms due to three different transcription start sites and four exons that are spliced as differentially included exons.

Conclusions

The dynactin complex is a very ancient complex that most likely included all subunits in the last common ancestor of extant eukaryotes. The absence of dynactin in certain species coincides with that of the cytoplasmic dynein heavy chain: Organisms that do not encode cytoplasmic dynein like plants and diplomonads also do not encode the unique dynactin subunits. The conserved core of dynactin consists of dynactin1, dynactin2, dynactin4, dynactin5, Arp1, and the heterodimeric actin capping protein. The evolution of the remaining subunits dynactin3, dynactin6, and Arp10 is characterized by many branch- and species-specific gene loss events.

Actin-related proteins localized in the nucleus: from discovery to novel roles in nuclear organization

The actin family consists of conventional actin and actin-related proteins (ARPs), and the members show moderate similarity and share the same basal structure. Following the finding of various ARPs in the cytoplasm in the 1990s, multiple subfamilies that are localized predominantly in the nucleus were identified. Consistent with these cytological observations, subsequent biochemical analyses revealed the involvement of the nuclear ARPs in ATP-dependent chromatin-remodeling and histone acetyltransferase complexes. In addition to their contribution to chromatin remodeling, recent studies have shown that nuclear ARPs have roles in the organization of the nucleus that are independent of the activity of the above-mentioned complexes. Therefore, nuclear ARPs are recognized as novel key regulators of genome function, and affect not only the remodeling of chromatin but also the spatial arrangement and dynamics of chromatin within the nucleus.

Identification of novel downstream targets of platelet glycoprotein VI activation by differential proteome analysis: implications for thrombus formation

Platelets play a key role in hemostasis and various diseases including arterial thrombosis. Glycoprotein VI (GPVI) mediates adhesion to collagen structures exposed at sites of vascular injury and subsequent platelet activation. We determined the effects of specific activation of GPVI on the human platelet proteome. Isolated human platelets were stimulated with an activating monoclonal antibody specific for GPVI. Platelet proteins were analyzed by 2-dimensional difference gel electrophoresis (2D-DIGE) and mass spectrometry. We identified 8 differentially abundant proteins associated with cell signaling, metabolism, organization and rearrangement of the cytoskeleton, and membrane trafficking. Differentially abundant proteins included aldose reductase (AR), beta-centractin, charged multivesicular body protein 3, Src substrate cortactin, ERp57, and pleckstrin. Importantly, GPVI-modulated protein abundance was functionally relevant. Correspondingly, AR enzyme activity significantly increased upon GPVI activation and inhibition of AR resulted in reduced platelet aggregation. Furthermore, ERp57 was released upon ligation of platelet GPVI and increased the activity of tissue factor, a major initiator of blood coagulation. In summary, GPVI activation results in differential changes in abundance of platelet proteins, including AR and ERp57, which support platelet aggregation and platelet-dependent coagulation. These results provide further insight into the mechanisms that underlie platelet activation through the GPVI receptor and may help to identify novel pharmacologic targets.

Dynactin is essential for growth cone advance

Dynactin is a multi-subunit complex that serves as a critical cofactor of the microtubule motor cytoplasmic dynein. We previously identified dynactin in the nerve growth cone. However, the function of dynactin in the growth cone is still unclear. Here we show that dynactin in the growth cone is required for constant forward movement of the growth cone. Chromophore-assisted laser inactivation (CALI) of dynamitin, a dynactin subunit, within the growth cone markedly decreases the rate of growth cone advance. CALI of dynamitin in vitro dissociates another dynactin subunit, p150(Glued), from dynamitin. These results indicate that dynactin, especially the interaction between dynamitin and p150(Glued), plays an essential role in growth cone advance.

Down-regulation of beta-centractin might be involved in dendritic cells dysfunction and subsequent hepatocellular carcinoma immune escape: a proteomic study

AIMS

Proteomic study was used to clarify the mechanism of hepatocellular carcinoma (HCC) immune escape concerning Dendritic cells (DCs') dysfunction and their association with HCC invasion.

METHODS

Human peripheral blood mononuclear cells (PBMCs) derived DCs from healthy donors were pulsed with soluble cell lysates prepared from different metastatic potential human HCC cell lines. The total protein of these DCs was analyzed by two-dimensional electrophoresis and Electro-Spray Mass Spectrometry. The allostimulatoy capacity and phenotype of these DCs were also evaluated. The clinical significance of beta-centractin, one of the largest quantitative changed spot, down-regulation in DCs was further evaluated in autologous PBMCs derived DCs pulsed with auto-tumor lysates in 26 HCC patients.

RESULTS

The expression of beta-centractin was found to be considerably lower either in DCs pulsed with HCCLM6 (high metastatic potential HCC cell line) lysates, accompanied by down-regulation of CD86 molecule and impaired allostimulatory capacity, than those of DCs pulsed with lysates from HCC cell lines with low or without metastatic potential or in DCs pulsed with lysates from HCC with invasiveness than those without invasiveness.

CONCLUSIONS

The down-regulation of beta-centractin in DCs pulsed with high metastatic potential HCC lysates might associate with DCs dysfunction and HCC invasiveness.

Actin' like actin?

The most biologically significant property of actin is its ability to self-associate and form two-stranded polymeric microfilaments. In living cells, these micro filaments form the actin cytoskeleton, essential for maintenance of the shape, passive mechanical properties and active motility of eukaryotic cells. Recently discovered actin-related proteins (ARPs) appear to share a common ancestor with conventional actin. At present, six classes of ARPs have been discovered, three of which have representatives in diverse species across eukaryotic phyla and may share functional characteristics with conventional actin. The three most ubiquitous ARPs are predicted to share a common core structure with actin and contain all the residues required for ATP binding. Surface residues involved in protein protein interactions, however, have diverged. Models of these proteins based on the atomic structure of actin provide some clues about how ARPs interact with each other, with conventional actin and with conventional actin-binding proteins.

Analysis of gene expression patterns and chromosomal changes associated with aging

Age is the largest single risk factor for the development of cancer in mammals. Age-associated chromosomal changes, such as aneuploidy and telomere erosion, may be vitally involved in the initial steps of tumorigenesis. However, changes in gene expression specific for increased aneuploidy with age have not yet been characterized. Here, we address these questions by using a panel of fibroblast cell lines and lymphocyte cultures from young and old age groups. Oligonucleotide microarrays were used to characterize the expression of 14,500 genes. We measured telomere length and analyzed chromosome copy number changes and structural rearrangements by multicolor interphase fluorescence in situ hybridization and 7-fluorochrome multiplex fluorescence in situ hybridization, and we tried to show a relationship between gene expression patterns and chromosomal changes. These analyses revealed a number of genes involved in both the cell cycle and proliferation that are differently expressed in aged cells. More importantly, our data show an association between age-related aneuploidy and the gene expression level of genes involved in centromere and kinetochore function and in the microtubule and spindle assembly apparatus. To verify that some of these genes may also be involved in tumorigenesis, we compared the expression of these genes in chromosomally stable microsatellite instability and chromosomally unstable chromosomal instability colorectal tumor cell lines. Three genes (Notch2, H2AFY2, and CDC5L) showed similar expression differences between microsatellite instability and chromosomal instability cell lines as observed between the young and old cell cultures suggesting that they may play a role in tumorigenesis.

Plant actin-related proteins

Actin-related proteins (ARPs) constitute a family of divergent and evolutionarily ancient eukaryotic proteins whose primary sequences display homology to conventional actins. Whereas actins play well-characterized cytoskeletal roles, the ARPs are implicated in various cellular functions in both the cytoplasm and in the nucleus. Cytoplasmic ARPs, for example, are known to participate in the assembly of branched actin filaments and dynein-mediated movement of vesicles in many eukaryotes. Nuclear ARPs, by contrast, are enigmatic components of various chromatin-modifying complexes involved in transcriptional regulation. Here, we review homologs to several known classes of ARPs and two distinct ARP classes in plants, and summarize recent work elucidating the biological functions of ARPs in eukaryotes.

beta III spectrin binds to the Arp1 subunit of dynactin

Cytoplasmic dynein is an intracellular motor responsible for endoplasmic reticulum-to-Golgi vesicle trafficking and retrograde axonal transport. The accessory protein dynactin has been proposed to mediate the association of dynein with vesicular cargo. Dynactin contains a 37-nm filament made up of the actin-related protein, Arp1, which may interact with a vesicle-associated spectrin network. Here, we demonstrate that Arp1 binds directly to the Golgi-associated betaIII spectrin isoform. We identify two Arp1-binding sites in betaIII spectrin, one of which overlaps with the actin-binding site conserved among spectrins. Although conventional actin binds weakly to betaIII spectrin, Arp1 binds robustly in the presence of excess F-actin. Dynein, dynactin, and betaIII spectrin co-purify on vesicles isolated from rat brain, and betaIII spectrin co-immunoprecipitates with dynactin from rat brain cytosol. In interphase cells, betaIII spectrin and dynactin both localize to cytoplasmic vesicles, co-localizing most significantly in the perinuclear region of the cell. In dividing cells, betaIII spectrin and dynactin co-localize to the developing cleavage furrow and mitotic spindle, a novel localization for betaIII spectrin. We hypothesize that the interaction between betaIII spectrin and Arp1 recruits dynein and dynactin to intracellular membranes and provides a direct link between the microtubule motor complex and its membrane-bounded cargo.

Genomic organization and embryonic expression of Suppressor of Fused, a candidate gene for the split-hand/split-foot malformation type 3

The genes for human and mouse Suppressor of Fused (SU(FU)/Su(Fu)) in the Hedgehog signaling pathway were characterized and found to contain 12 exons. Human SU(FU) localized on chromosome 10q24-25 between the markers D10S192 and AFM183XB12. We detected three additional SU(FU) isoforms, two of which have lost their ability to interact with the transcription factor GLI1. Expression analysis using whole mount in situ hybridization revealed strong expression of Su(Fu) in various mouse embryonic tissues. SU(FU) was considered a candidate gene for the split-hand/split-foot malformation type 3 (SHFM3). However, no alterations in the SU(FU) gene were found in SHFM3 patients.

ARP3beta, the gene encoding a new human actin-related protein, is alternatively spliced and predominantly expressed in brain neuronal cells

A cDNA encoding a new human actin-related protein (ARP) was cloned. The corresponding protein is highly conserved with the previously described ARP3 protein, suggesting that it represents a second isoform of the human ARP3 subfamily. This new actin-related protein was subsequently named ARP3beta and represents the second example of multiple isoforms of an actin-related protein in a single organism. The ARP3beta gene was mapped to chromosome band 7q34, centromeric to Sonic Hedgehog. Gene structure analysis revealed that at least part of the observed ARP3beta mRNA heterogeneity is caused by alternative splicing resulting in exon skipping. Transcripts produced after exon 2 skipping are predicted to encode truncated products, whose functionality is still unclear. An ARP3beta pseudogene was detected on chromosome 2p11 by database searching. Several ARP3beta mRNA species were detected by Northern blotting and their abundance varied importantly among tissues: the highest expression levels were detected in fetal and adult brain, whereas lower levels were observed in liver, muscle and pancreas. In contrast, ARP3 mRNAs were detected in all tissues tested. Using in situ hybridization, the expression of ARP3beta in brain was shown to be restricted to neurons and epithelial cells from choroid plexus. This suggests a specific function for ARP3beta in the physiology of the development and/or maintenance of distinct subsets of nerve cells.

Actin-related proteins

Actin-related proteins (Arps) participate in a diverse array of cellular processes. They modulate assembly of conventional actin, contribute to microtubule-based motility catalyzed by dynein, and serve as integral components of large protein complexes required for gene expression. We highlight here recent work aimed at understanding the roles played by Arps in each of these processes.

Overexpression of normal and mutant Arp1alpha (centractin) differentially affects microtubule organization during mitosis and interphase

Dynactin is a large multisubunit complex that regulates cytoplasmic dynein-mediated functions. To gain insight into the role of dynactin's most abundant component, Arp1alpha was transiently overexpressed in mammalian cells. Arp1alpha overexpression resulted in a cell cycle delay at prometaphase. Intracellular dynactin, dynein and nuclear/mitotic apparatus (NuMA) protein were recruited to multiple foci associated with ectopic cytoplasmic aggregates of Arp1alpha in transfected cells. These ectopic aggregates nucleated supernumerary microtubule asters at prometaphase. Point mutations were generated in Arp1alpha that identified specific amino acids required for the prometaphase delay and for the formation of supernumerary microtubule asters. The mutant Arp1alpha proteins formed aggregates in cells that colocalized with dynactin and dynein peptides, but in contrast to wild-type Arp1alpha, NuMA localization remained unaffected. Although expression of mutant Arp1alpha proteins had no effect on mitotic cells, in interphase cells expression of the mutants resulted in disruption of the microtubule network. Immunoprecipitation studies demonstrated that overexpressed Arp1alpha interacts with dynactin and NuMA proteins in cell extracts, and that these interactions are destabilized in the Arp1alpha mutants. We conclude that the amino acids altered in the Arp1alpha mutant proteins participate in stabilizing interactions between overexpressed Arp1alpha and components of the endogenous dynactin complex as well as the NuMA protein.

Mitosis in filamentous fungi: how we got where we are

This review traces the principal advances in the study of mitosis in filamentous fungi from its beginnings near the end of the 19(th) century to the present day. Meiosis and mitosis had been accurately described and illustrated by the second decade of the present century and were known to closely resemble nuclear divisions in higher eukaryotes. This information was effectively lost in the mid-1950s, and the essential features of mitosis were then rediscovered from about the mid-1960s to the mid-1970s. Interest in the forces that separate chromatids and spindle poles during fungal mitosis followed closely on the heels of detailed descriptions of the mitotic apparatus in vivo and ultrastructurally during this and the following decade. About the same time, fundamental studies of the structure of fungal chromatin and biochemical characterization of fungal tubulin were being carried out. These cytological and biochemical studies set the stage for a surge of renewed interest in fungal mitosis that was issued in by the age of molecular biology. Filamentous fungi have provided model studies of the cytology and genetics of mitosis, including important advances in the study of mitotic forces, microtubule-associated motor proteins, and mitotic regulatory mechanisms.

Colocalization of cytoplasmic dynein with dynactin and CLIP-170 at microtubule distal ends

Cytoplasmic dynein is a minus end-directed microtubule motor responsible for centripetal organelle movement and several aspects of chromosome segregation. Our search for cytoplasmic dynein-interacting proteins has implicated the dynactin complex as the cytoplasmic dynein ‘receptor’ on organelles and kinetochores. Immunofluorescence microscopy using a total of six antibodies generated against the p150Glued, Arp1 and dynamitin subunits of dynactin revealed a novel fraction of dynactin-positive structures aligned in linear arrays along the distal segments of interphase microtubules. Dynactin staining revealed that these structures colocalized extensively with CLIP-170. Cytoplasmic dynein staining was undetectable, but extensive colocalization with dynactin became evident upon transfer to a lower temperature. Overexpression of the dynamitin subunit of dynactin removed Arp1 from microtubules but did not affect microtubule-associated p150Glued or CLIP-170 staining. Brief acetate treatment, which has been shown to affect lysosomal and endosomal traffic, also dispersed the Golgi apparatus and eliminated the microtubule-associated staining pattern. The effect on dynactin was rapidly reversible and, following acetate washout, punctate dynactin was detected at microtubule ends within 3 minutes. Together, these findings identify a region along the distal segments of microtubules where dynactin and CLIP-170 colocalize. Because CLIP-170 has been reported to mark growing microtubule ends, our results indicate a similar relationship for dynactin. The functional interaction between dynactin and cytoplasmic dynein further suggests that this these regions represent accumulations of cytoplasmic dynein cargo-loading sites involved in the early stages of minus end-directed organelle transport.

Specific isoforms of actin-binding proteins on distinct populations of Golgi-derived vesicles

Golgi membranes and Golgi-derived vesicles are associated with multiple cytoskeletal proteins and motors, the diversity and distribution of which have not yet been defined. Carrier vesicles were separated from Golgi membranes, using an in vitro budding assay, and different populations of vesicles were separated using sucrose density gradients. Three main populations of vesicles labeled with beta-COP, gamma-adaptin, or p200/myosin II were separated and analyzed for the presence of actin/actin-binding proteins. beta-Actin was bound to Golgi cisternae and to all populations of newly budded vesicles. Centractin was selectively associated with vesicles co-distributing with beta-COP-vesicles, while p200/myosin II (non-muscle myosin IIA) and non-muscle myosin IIB were found on different vesicle populations. Isoforms of the Tm5 tropomyosins were found on selected Golgi-derived vesicles, while other Tm isoforms did not colocalize with Tm5 indicating the association of specialized actin filaments with Golgi-derived vesicles. Golgi-derived vesicles were shown to bind to F-actin polymerized from cytosol with Jasplakinolide. Thus, newly budded, coated vesicles derived from Golgi membranes can bind to actin and are customized for differential interactions with microfilaments by the presence of selective arrays of actin-binding proteins.

Self-regulated polymerization of the actin-related protein Arp1

The actin-related protein Arp1 (or centractin, actin RPV) is the major subunit of dynactin, a key component of the cytoplasmic dynein motor machinery [1] [2] [3]. Of the ubiquitously expressed members of the Arp superfamily, Arp1 is most similar to conventional actin [4] [5] [6] and, on the basis of conserved sequence features, is predicted to bind ATP and possibly polymerize. In vivo, all cytosolic Arp1 sediments at 20S [7] suggesting that it assembles into oligomers, most likely dynactin - a multiprotein complex known to contain eight or nine Arp1 monomers in a 37 nm filament [8]. The uniform length of Arp1 polymers suggests a novel assembly mechanism that may be governed by a 'ruler' activity. In dynactin, the Arp1 filament is bounded by actin-capping protein at one end and a heterotetrameric protein complex containing the p62 subunit (D.M. Eckley, S.R. Gill, J.B.B., J.E. Heuser, T.A.S., unpublished observations) at the other [8]. In the present study, we analyzed the behavior of highly purified, native Arp1. Arp1 was found to polymerize rapidly into short filaments that were similar, but not identical, in length to those in dynactin. With time, these filaments appeared to anneal to form longer assemblies but never attained the length of conventional actin filaments.

The "8-kD" cytoplasmic dynein light chain is required for nuclear migration and for dynein heavy chain localization in Aspergillus nidulans

The heavy chain of cytoplasmic dynein is required for nuclear migration in Aspergillus nidulans and other fungi. Here we report on a new gene required for nuclear migration, nudG, which encodes a homologue of the "8-kD" cytoplasmic dynein light chain (CDLC). We demonstrate that the temperature sensitive nudG8 mutation inhibits nuclear migration and growth at restrictive temperature. This mutation also inhibits asexual and sexual sporulation, decreases the intracellular concentration of the nudG CDLC protein and causes the cytoplasmic dynein heavy chain to be absent from the mycelial tip, where it is normally located in wild-type mycelia. Coimmunoprecipitation experiments with antibodies against the cytoplasmic dynein heavy chain (CDHC) and the nudG CDLC demonstrated that some fraction of the cytoplasmic dynein light chain is in a protein complex with the CDHC. Sucrose gradient sedimentation analysis, however, showed that not all of the NUDG protein is complexed with the heavy chain. A double mutant carrying a cytoplasmic dynein heavy chain deletion plus a temperature-sensitive nudG mutation grew no more slowly at restrictive temperature than a strain with only the CDHC deletion. This result demonstrates that the effect of the nudG mutation on nuclear migration and growth is mediated through an interaction with the CDHC rather than with some other molecule (e.g., myosin-V) with which the 8-kD CDLC might theoretically interact.

The role of the dynactin complex in intracellular motility

Dynactin is a multisubunit complex that binds to the minus-end-directed microtubule motor cytoplasmic dynein and may provide a link between the motor and its cargo. Results from genetic studies in Saccharomyces cerevisiae, Neurospora crassa, Aspergillus nidulans, and Drosophila have suggested that cytoplasmic dynein and dynactin function in the same cellular pathways. p150Glued, a vertebrate homologue of the Drosophila gene Glued, is the largest polypeptide in the dynactin complex with multiple protein interactions. Centractin, the most abundant dynactin subunit polypeptide, forms an actin-like filament at the base of the complex. Studies on dynamitin, the 50-kDa dynactin subunit, predict a role for dynactin in mitotic spindle assembly. Other subunits of dynactin have also been cloned and characterized; these studies have provided insight into the role of the complex in essential cellular processes.

Analysis of EST-driven gene annotation in human genomic sequence

We have performed a systematic analysis of gene identification in genomic sequence by similarity search against expressed sequence tags (ESTs) to assess the suitability of this method for automated annotation of the human genome. A BLAST-based strategy was constructed to examine the potential of this approach, and was applied to test sets containing all human genomic sequences longer than 5 kb in public databases, plus 300 kb of exhaustively characterized benchmark sequence. At high stringency, 70%-90% of all annotated genes are detected by near-identity to EST sequence; >95% of ESTs aligning with well-annotated sequences overlap a gene. These ESTs provide immediate access to the corresponding cDNA clones for follow-up laboratory verification and subsequent biologic analysis. At lower stringency, up to 97% of annotated genes were identified by similarity to ESTs. The apparent false-positive rate rose to 55% of ESTs among all sequences and 20% among benchmark sequences at the lowest stringency, indicating that many genes in public database entries are unannotated. Approximately half of the alignments span multiple exons, and thus aid in the construction of gene predictions and elucidation of alternative splicing. In addition, ESTs from multiple cDNA libraries frequently cluster over genes, providing a starting point for crude expression profiles. Clone IDs may be used to form EST pairs, and particularly to extend models by associating alignments of lower stringency with high-quality alignments. These results demonstrate that EST similarity search is a practical general-purpose annotation technique that complements pattern recognition methods as a tool for gene characterization.

The presence of the 50-kDa subunit of dynactin complex in the nerve growth cone

Membrane proteins in growth cone-enriched and growth cone-non-enriched fractions prepared from neonatal mouse brain were separated by lectin-affinity and ion exchange chromatographies, 2D-PAGE, and SDS-PAGE. Partial amino acid sequences of the proteins concentrated in the growth cone-enriched fraction were determined. We found that one such protein, gmp23-48k, corresponds to the 50-kDa subunit (p50) of the dynactin complex. An antibody raised against gmp23-48k strongly reacted with growth cones of differentiated neuronal precursor cells. Immunoblot analyses revealed that gmp23-48k was present both in membrane and in soluble fractions of neonatal brain. However, the amount of gmp23-48k in the membrane fraction greatly decreased in adult brain. These results suggest a special role of membrane-associated gmp23-48k/p50 in synapse formation during brain development.

Primary structure of mouse actin-related protein 1 (Arp1) and its tissue expression

Different types of actin-related proteins which constitute an actin-superfamily together with conventional actin have recently been described (Mullins et al., 1996). Among them, Arp1 exhibits the highest homology with conventional actin. With the aim of clarifying the cellular function of Arp1 in mammalian cells, we cloned the cDNA encoding mouse alpha-Arp1, one of the variants of Arp1, from a mouse diaphragm cDNA library; two types of alpha-Arp1 cDNAs, which are probably generated by alternative RNA splicing from a single gene, were obtained and the entire sequences were determined. They differed only in the presence or absence of an insertion of 1.3 kb in the 3'-non-cooling region but shared a common open reading frame. The deduced amino acid sequence was identical with that of human alpha-Arp1. Northern blot analysis showed that the alpha-Arp1 mRNA corresponding to the longer cDNA is transcribed not only in various non-muscle tissues but also in muscle tissues, while the transcript corresponding to the shorter one becomes expressed only in skeletal muscle as development progresses. It is suggested that alpha-Arp1 may play some role in muscle, as judged by the significant level of its expression.

Centractin (ARP1) associates with spectrin revealing a potential mechanism to link dynactin to intracellular organelles

Centractin (Arp1), an actin-related protein, is a component of the dynactin complex. To investigate potential functions of the protein, we used transient transfections to overexpress centractin in mammalian cells. We observed that the overexpressed polypeptide formed filamentous structures that were significantly longer and more variable in length than those observed in the native dynactin complex. The centractin filaments were distinct from conventional actin in subunit composition and pharmacology as demonstrated by the absence of immunoreactivity of these filaments with an actin-specific antibody, by resistance to treatment with the drug cytochalasin D, and by the inability to bind phalloidin. We examined the transfected cells for evidence of specific associations of the novel centractin filaments with cellular organelles or cytoskeletal proteins. Using immunocytochemistry we observed the colocalization of Golgi marker proteins with the centractin polymers. Additional immunocytochemical analysis using antibodies to non-erythroid spectrin (fodrin) and Golgi-spectrin (beta I sigma *) revealed that spectrin colocalized with the centractin filaments in transfected cells. Biochemical assays demonstrated that spectrin was present in dynactin-enriched cellular fractions, was coimmunoprecipitated from rat brain cytosol using antibodies to dynactin subunits, and was coeluted with dynactin using affinity chromatography. Immunoprecipitations and affinity chromatography also revealed that actin is not a bona fide component of dynactin. Our results indicate that spectrin is associated with the dynactin complex. We suggest a model in which dynactin associates with the Golgi through an interaction between the centractin filament of the dynactin complex and a spectrin-linked cytoskeletal network.

Motor proteins: a dynamic duo

The interactions between the microtubule motor cytoplasmic dynein and its putative regulator dynactin have been shown to be dynamic and complex.

Molecular characterization of the 50-kD subunit of dynactin reveals function for the complex in chromosome alignment and spindle organization during mitosis

Dynactin is a multi-subunit complex which has been implicated in cytoplasmic dynein function, though its mechanism of action is unknown. In this study, we have characterized the 50-kD subunit of dynactin, and analyzed the effects of its overexpression on mitosis in living cells. Rat and human cDNA clones revealed p50 to be novel and highly conserved, containing three predicted coiled-coil domains. Immunofluorescence staining of dynactin and cytoplasmic dynein components in cultured vertebrate cells showed that both complexes are recruited to kinetochores during prometaphase, and concentrate near spindle poles thereafter. Overexpression of p50 in COS-7 cells disrupted mitosis, causing cells to accumulate in a prometaphase-like state. Chromosomes were condensed but unaligned, and spindles, while still bipolar, were dramatically distorted. Sedimentation analysis revealed the dynactin complex to be dissociated in the transfected cultures. Furthermore, both dynactin and cytoplasmic dynein staining at prometaphase kinetochores was markedly diminished in cells expressing high levels of p50. These findings represent clear evidence for dynactin and cytoplasmic dynein codistribution within cells, and for the presence of dynactin at kinetochores. The data also provide direct in vivo evidence for a role for vertebrate dynactin in modulating cytoplasmic dynein binding to an organelle, and implicate both dynactin and dynein in chromosome alignment and spindle organization.

The actin-related proteins

A family of proteins has been discovered over the past three years whose members have clear sequence homology to actin but are distinguished from actin by their structural and functional diversity. The ranks of this family, whose members are known as the actin-related proteins (arps), are expanding rapidly. Arps are but one branch of a larger superfamily which includes the actins, hsp/hsc70s, sugar kinases and several cell cycle proteins from bacteria. The existence of the superfamily has been inferred from tertiary structural data. In the case of the arps, their identification and classification has been based upon primary structural data. Placing the arps in a functional context is proving a slower process, although genetic and biochemical analyses are converging in several cases. In the past year, different arps have been linked to functions mediated by actin filaments (arp2 and arp3), microtubules (arp1) and the structural elements of chromatin (arp4 and arp6).

Cytoplasmic dynein binds dynactin through a direct interaction between the intermediate chains and p150Glued

Cytoplasmic dynein is a retrograde microtubule motor thought to participate in organelle transport and some aspects of minus end-directed chromosome movement. The mechanism of binding to organelles and kinetochores is unknown. Based on homology with the Chlamydomonas flagellar outer arm dynein intermediate chains (ICs), we proposed a role for the cytoplasmic dynein ICs in linking the motor protein to organelles and kinetochores. In this study two different IC isoforms were used in blot overlay and immunoprecipitation assays to identify IC-binding partners. In overlays of complex protein samples, the ICs bound specifically to polypeptides of 150 and 135 kD, identified as the p150Glued doublet of the dynactin complex. In reciprocal overlay assays, p150Glued specifically recognized the ICs. Immunoprecipitations from total Rat2 cell extracts, rat brain cytosol, and rat brain membranes further identified the dynactin complex as a specific target for IC binding. using truncation mutants, the sites of interaction were mapped to amino acids 1-123 of IC-1A and amino acids 200-811 of p150Glued. While cytoplasmic dynein and dynactin have been implicated in a common pathway by genetic analysis, our findings identify a direct interaction between two specific component polypeptides and support a role for dynactin as a dynein "receptor". Our data also suggest, however, that this interaction must be highly regulated.

Sequences, structural models, and cellular localization of the actin-related proteins Arp2 and Arp3 from Acanthamoeba

We cloned and sequenced the two actin-related proteins (Arps) present in the profilin-binding complex of Acanthamoeba (Machesky, L. M., S. J. Atkinson, C. Ampe, J. Vandekerckhove, and T. D. Pollard. 1994, J. Cell Biol. 127:107-115). The sequence of Arp2 is more similar to other Arp2s than to actin, while the sequence of Arp3 is more similar to other Arp3s than to actin. Phylogenetic analysis of all known Arps demonstrates that most group into three major families, which are likely to be shared across all eukaryotic phyla. Together with conventional actins, the Arps form a larger family distinct from structurally related ATPases such as Hsp70's and sugar kinases. Atomic models of the Arps based on their sequences and the structure of actin provide some clues about function. Both Arps have atoms appropriately placed to bind ATP and divalent cation. Arp2, but not Arp3, has a conserved profilin-binding site. Neither Arp has the residues required to copolymerize with actin, but an Arp heterodimer present in the profilin-binding complex might serve as a pointed end nucleus for actin polymerization. Both Acanthamoeba Arps are soluble in cell homogenates, and both are concentrated in the cortex of Acanthamoeba. The cellular concentrations are 1.9 microM Arp2 and 5.1 microM Arp3, substoichiometric to actin (200 microM) but comparable to many actin-binding proteins.