ADrosophila homologue of theSchizosaccharomyces pombe act2 gene

Reference genes to study the sex-biased expression of genes regulating Drosophila metabolism

Sex is an important variable in biology. Notable differences have been observed between male and female Drosophila in regulation of metabolism, in response to nutritional challenges, and in phenotypes relevant for obesity and metabolic disorders. The differences between males and females can be expected to result from differences in gene expression. We observed that expression levels of reference genes commonly used for normalization of qRT-PCR results such as GAPDH, β-actin, and 18SrRNA, show prominent sexual dimorphism. Since this will impact relative expression and conclusions related to that, we performed a systematic analysis of candidate reference genes with the objective of identifying reference genes with stable expression in male and female Drosophila. These reference genes (LamCa, βTub60D and βTub97EF) were then used to assess sex-specific differences in expression of metabolism associated genes. Additionally, we evaluated the utility of these reference genes following a nutritional challenge and showed that LamCa and βtub97EF are stably expressed between sexes and under different nutritional conditions and are thus suitable as reference genes. Our results highlight the importance of evaluating the stability of reference genes when sex-specific differences in gene expression are studied, and identify structural genes as a category worth exploring as reference genes in other species. Finally, we also uncovered hitherto unknown sexually dimorphic expression of a number of metabolism-associated genes, information of interest to others working in the field of metabolic disorders.

Molecular mechanisms of cell-cell spread of intracellular bacterial pathogens

Several bacterial pathogens, including Listeria monocytogenes, Shigella flexneri and Rickettsia spp., have evolved mechanisms to actively spread within human tissues. Spreading is initiated by the pathogen-induced recruitment of host filamentous (F)-actin. F-actin forms a tail behind the microbe, propelling it through the cytoplasm. The motile pathogen then encounters the host plasma membrane, forming a bacterium-containing protrusion that is engulfed by an adjacent cell. Over the past two decades, much progress has been made in elucidating mechanisms of F-actin tail formation. Listeria and Shigella produce tails of branched actin filaments by subverting the host Arp2/3 complex. By contrast, Rickettsia forms tails with linear actin filaments through a bacterial mimic of eukaryotic formins. Compared with F-actin tail formation, mechanisms controlling bacterial protrusions are less well understood. However, recent findings have highlighted the importance of pathogen manipulation of host cell–cell junctions in spread. Listeria produces a soluble protein that enhances bacterial protrusions by perturbing tight junctions. Shigella protrusions are engulfed through a clathrin-mediated pathway at ‘tricellular junctions’—specialized membrane regions at the intersection of three epithelial cells. This review summarizes key past findings in pathogen spread, and focuses on recent developments in actin-based motility and the formation and internalization of bacterial protrusions.

The ARP2/3 complex: an actin nucleator comes of age

The cellular functions of the actin cytoskeleton require precise regulation of both the initiation of actin polymerization and the organization of the resulting filaments. The actin-related protein-2/3 (ARP2/3) complex is a central player in this regulation. A decade of study has begun to shed light on the molecular mechanisms by which this powerful machine controls the polymerization, organization and recycling of actin-filament networks, both in vitro and in the living cell.

A subset of dynamic actin rearrangements in Drosophila requires the Arp2/3 complex

The Arp2/3 complex has been shown to dramatically increase the slow spontaneous rate of actin filament nucleation in vitro, and it is known to be important for remodeling the actin cytoskeleton in vivo. We isolated and characterized loss of function mutations in genes encoding two subunits of the Drosophila Arp2/3 complex: Arpc1, which encodes the homologue of the p40 subunit, and Arp3, encoding one of the two actin-related proteins. We used these mutations to study how the Arp2/3 complex contributes to well-characterized actin structures in the ovary and the pupal epithelium. We found that the Arp2/3 complex is required for ring canal expansion during oogenesis but not for the formation of parallel actin bundles in nurse cell cytoplasm and bristle shaft cells. The requirement for Arp2/3 in ring canals indicates that the polymerization of actin filaments at the ring canal plasma membrane is important for driving ring canal growth.

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.

Identification of human estrogen-inducible transcripts that potentially mediate the apoptotic response in breast cancer

Hormone manipulation has been used for several decades with the purpose of inducing breast cancer regression. On the one hand, hormone ablation and antiestrogen administration were used on the rationale that estrogens induce proliferation of their target cells. Before the advent of the antiestrogen tamoxifen, on the other hand, the estrogen agonist DES was used to obtain clinical remissions. The rationale for the use of diethylstilbestrol (DES) was totally empirical. In fact, the efficacy of both treatments was comparable. A mechanistic explanation for estrogen-induced regression is urgently needed in order to provide a rationale for its use in therapeutic fields, and to develop markers to identify this phenotype in order to recognize responsive tumors. In this report, we use E8CASS cells (a MCF7 variant) as a model to study estrogen-mediated regression. The proliferation rate of E8CASS cells is decreased by estrogens. In order to isolate mRNA sequences induced by estradiol, a subtracted library was prepared from E8CASS cells grown in the presence and absence of estrogens. Twenty nine differentially expressed unique sequences were found. Seven of them were homologous to known genes, 12 of them were homologous to expressed sequence tags (EST), and 10 sequences had no homologues in the databases. The two sequences showing the highest induction by estradiol (E9 and E43) were chosen for further analysis. The sequence of the E43 coding region has 96% homology to the bovine actin2 gene and 100% identity to bovine actin2 protein, and it is homologous to the human actin-related protein 3 (Arp3). It has been suggested that Arp3 is involved in actin nucleation. The phenotype of E8CASS cells is clearly affected by estrogen treatment. It is likely that E43 may be involved in these morphological changes. The E9 cDNA is a putative zinc-finger protein of the PHD family of transcriptional transactivators. A member of this family, Requiem, is involved in apoptosis. The E9 mRNA is highly expressed in E8CASS cells treated with estrogens, a treatment which results in decreased proliferation rate and increased DNA degradation. This correlation suggests that E9 may be a mediator of estrogen-induced regression of breast cancer.

Genomic structure and sequence of the pufferfish (Fugu rubripes) gene encoding an actin-related protein

Using a polymerase chain reaction (PCR)-generated gene fragment as a probe, we isolated and sequenced a gene encoding an actin-related protein belonging to the Arp3 family from the pufferfish, Fugu rubripes. The Fugu Arp3 gene spans 3.7kb from the transcription start site to the polyadenylation signal. This is the first report of the genomic sequence of a vertebrate Arp3 gene. The Fugu Arp3 gene consists of 12 exons and 11 introns compared to Drosophila homologue, which has six exons and five introns. The protein sequence encoded by the Fugu gene is 97% and 80% identical to bovine and Drosophila homologues, respectively. The Fugu Arp3 gene is expressed in a wide range of tissues, with higher levels detected in the gills, kidney, ovary, skin and testis. Three different size transcripts of the Arp3 gene (1.4kb, 1.8kb and 2.2kb) were identified in various tissues.

Identification of ACT4, a novel essential actin-related gene in the yeast Saccharomyces cerevisiae

Actin molecules are major cytoskeleton components of all eukaryotic cells. All conventional actins that have been identified so far are 374-376 amino acids in size and exhibit at least 70% amino acid sequence identity when compared with one another. In the yeast Saccharomyces cerevisiae, one conventional actin gene ACT1 and three so-called actin-related genes, ACT2, ACT3 and ACT5, have been identified. We report here the discovery of a new actin-related gene in this organism, which we have named ACT4. The deduced protein, Act4, of 449 amino acids, exhibits only 33.4%, 26.7%, 23.4% and 29.2% identity to Act1, Act2, Act3 and Act5, respectively. In contrast, it is 68.4% identical to the product of the Schizosaccharomyces pombe Act2 gene and has a similar level of identity to other Sch. pombe Act2 homologues. This places Act4 in the Arp3 family of actin-related proteins. ACT4 gene disruption and tetrad analysis demonstrate that this gene is essential for the vegetative growth of yeast cells. The act4 mutants exhibit heterogenous morphological phenotypes. We hypothesize that Act4 may have multiple roles in the cell cycle.

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).

Phylogenesis of fission yeasts. Contradictions surrounding the origin of a century old genus

The phylogenesis of fungi is controversial due to their simple morphology and poor fossilization. Traditional classification supported by morphological studies and physiological traits placed the fission yeasts in one group with ascomycetous yeasts. The rRNA sequence comparisons, however, revealed an enormous evolutionary gap between Saccharomyces and Schizosaccharomyces. As shown in this review, the protein sequences also show a large gap which is almost as large as that separating Schizosaccharomyces from higher animals. Since the two yeasts share features (both cytological and molecular) in common which are also characteristic of ascomycetous fungi, their separation must have taken place later than the sequence differences may suggest. Possible reasons for the paradox are discussed. The sequence data also suggest a slower evolutionary rate in the Schizosaccharomyces lineage than in the Saccharomyces branch. In the fission yeast lineage two ramifications can be supposed. First S. japonicus (Hasegawaea japonica) branched off, then S. octosporus (Octosporomyces octosporus) separated from S. pombe.

An actin-related protein from Dictyostelium discoideum is developmentally regulated and associated with mitochondria

An actin-related protein (ACLA) has been identified in the cellular slime mould Dictyostelium discoideum. The complete cDNA sequence indicates that within the actin superfamily it belongs to the ARP3 family of actin-related proteins together with Arp66B from Drosophila melanogaster, Actin2 from Bos taurus, act2 from Schizosaccharomyces pombe and possibly act2 from Caenorhabditis elegans. The ACLA mRNA is regulated during development, showing a maximum between 2 and 4 h after starvation. The protein has been expressed in E. coli and antibodies raised against it. Immunofluorescence microscopy shows that ACLA protein co-localises with mitochondria; the protein copurifies with Dictyostelium mitochondria.