What's new in edition 52 part April 2008 Gene sites new with this edition

The Interactive Fly was first released July/August 1996, with updates provided at approximately one month intervals, through September 1997 (edition 13). Updating quarterly started with edition 14. With edition 40, the Interactive Fly began to schedule updates three times a year: fall, winter and spring.

Gene sites new with this edition of the Interactive Fly:

Asterless

Centrosomes, the major organizers of the microtubule network in most animal cells (see The centrosome cycle in mammalian cells from Azimzadeh, 2007), are composed of centrioles embedded in a web of pericentriolar material (PCM). Recruitment and stabilization of PCM on the centrosome is a centriole-dependent function. Compared to the considerable number of PCM proteins known, the molecular characterization of centrioles is still very limited. Only a few centriolar proteins have been identified so far in Drosophila, most related to centriole duplication. asterless (asl) has been cloned and found to encode a 120 kD highly coiled-coil protein that is a constitutive pancentriolar and basal body component. Loss of asl function impedes the stabilization/maintenance of PCM at the centrosome. In embryos deficient for Asl, development is arrested right after fertilization. Asl shares significant homology with Cep 152, a protein described as a component of the human centrosome for which no functional data is yet available. The cloning of asl offers new insight into the molecular composition of Drosophila centrioles and a possible model for the role of its human homolog. In addition, the phenotype of asl-deficient flies reveals that a functional centrosome is required for Drosophila embryo development (Varmark, 2007).

Capsuléen

Although arginine modification has been implicated in a number of cellular processes, the in vivo requirement of protein arginine methyltransferases (PRMTs) in specific biological processes remain to be clarified. In this study the Drosophila PRMT Capsuléen, homologous to human PRMT5, has been characterized. During Drosophila oogenesis, catalytic activity of Capsuléen is necessary for both the assembly of the nuage surrounding nurse cell nuclei and the formation of the pole plasm at the posterior end of the oocyte. In particular, the nuage and pole plasm localization of Tudor, an essential component for germ cell formation, are abolished in csul mutant germ cells. The spliceosomal Sm proteins have been identified as in vivo substrates of Capsuléen and it is demonstrated that Capsuléen, together with its associated protein Valois, is essential for the synthesis of symmetric di-methylated arginyl residues in Sm proteins. Finally, Tudor can be targeted to the nuage in the absence of Sm methylation by Capsuléen, indicating that Tudor localization and Sm methylation are separate processes. These results thus reveal the role of a PRMT in protein localization in germ cells (Anne, 2007).

Ceramidase

Neutral ceramidase, a key enzyme of sphingolipid metabolism (see Long-chain base synthesis resulting in ceramide formation from Shayman, 2000), hydrolyzes ceramide to sphingosine. These sphingolipids are critical structural components of cell membranes and act as second messengers in diverse signal transduction cascades. This study isolated and characterized functional null mutants of Drosophila ceramidase. Secreted ceramidase functions in a cell-nonautonomous manner to maintain photoreceptor homeostasis. In the absence of ceramidase, photoreceptors degenerate in a light-dependent manner, are defective in normal endocytic turnover of rhodopsin, and do not respond to light stimulus. Consistent with a cell-nonautonomous function, overexpression of ceramidase in tissues distant from photoreceptors suppresses photoreceptor degeneration in an arrestin mutant and facilitates membrane turnover in a rhodopsin null mutant. Furthermore, the results show that secreted ceramidase is internalized and localizes to endosomes. These findings establish a role for a secreted sphingolipid enzyme in the regulation of photoreceptor structure and function (Acharya, 2008).

Cortex

Meiosis is a highly specialized cell division that requires significant reorganization of the canonical cell-cycle machinery and the use of meiosis-specific cell-cycle regulators. The anaphase-promoting complex (APC, a machine for degrading proteins; see APC subunits Cdc27 and morula; for review see Acquaviva, 2006) and a conserved APC adaptor/activator, Cdc20 (also known as Fizzy), are required for anaphase progression in mitotic cells. The APC has also been implicated in meiosis, although it is not yet understood how it mediates these non-canonical divisions. Cortex (Cort) is a diverged Fzy homologue that is expressed in the female germline of Drosophila, where it functions with the Cdk1-interacting protein Cks30A to drive anaphase in meiosis II. This study shows that Cort functions together with the canonical mitotic APC adaptor Fzy to target the three mitotic cyclins (A, B and B3) for destruction in the egg and drive anaphase progression in both meiotic divisions. In addition to controlling cyclin destruction globally in the egg, Cort and Fzy appear to both be required for the local destruction of cyclin B on spindles. Cyclin B associates with spindle microtubules throughout meiosis I and meiosis II, and dissociates from the meiotic spindle in anaphase II. Fzy and Cort are required for this loss of cyclin B from the meiotic spindle. These results lead to a model in which the germline-specific APC^Cort cooperates with the more general APC^Fzy, both locally on the meiotic spindle and globally in the egg cytoplasm, to target cyclins for destruction and drive progression through the two meiotic divisions (Swan, 2007).

Dilute class unconventional myosin

Sensory neuron terminal differentiation tasks apical secretory transport with delivery of abundant biosynthetic traffic to the growing sensory membrane. Drosophila Rab11 is essential for rhodopsin transport in developing photoreceptors and it was asked if myosin V (Didum) and the Drosophila Rab11 interacting protein, dRip11 (lethal (1) G0003), also participate in secretory transport. Reduction of either protein impairs rhodopsin transport, stunting rhabdomere growth and promoting accumulation of cytoplasmic rhodopsin. MyoV-reduced photoreceptors also develop ectopic rhabdomeres inappropriately located in basolateral membrane, indicating a role for MyoV in photoreceptor polarity. Binary yeast two hybrids and in vitro protein-protein interaction predict a ternary complex assembled by independent dRip11 and MyoV binding to Rab11. It is proposed that this complex delivers morphogenic secretory traffic along polarized actin filaments of the subcortical terminal web to the exocytic plasma membrane target, the rhabdomere base. A protein trio conserved across eukaryotes thus mediates normal, in vivo sensory neuron morphogenesis (Li, 2007).

Dynamin related protein 1

The role of mitochondria in Drosophila programmed cell death remains unclear, although certain gene products that regulate cell death seem to be evolutionarily conserved. This study found that developmental programmed cell death stimuli in vivo and multiple apoptotic stimuli ex vivo induce dramatic mitochondrial fragmentation upstream of effector caspase activation, phosphatidylserine exposure, and nuclear condensation in Drosophila cells. Unlike genotoxic stress, a lipid cell death mediator induces an increase in mitochondrial contiguity prior to fragmentation of the mitochondria. Dynamin related protein 1 (Drp1), is important for mitochondrial disruption. Using genetic mutants and RNAi-mediated knockdown of drp-1, it was found that Drp1 not only regulates mitochondrial fission in normal cells, but mediates mitochondrial fragmentation during programmed cell death. Mitochondria in drp-1 mutants fail to fragment, resulting in hyperplasia of tissues in vivo and protection of cells from multiple apoptotic stimuli ex vivo. Thus, mitochondrial remodeling is capable of modifying the propensity of cells to undergo death in Drosophila (Goyal, 2007).

Eater and Nimrod C1

Hemocytes, the blood cells of Drosophila, participate in the humoral and cellular immune defense reactions against microbes and parasites. The plasmatocytes, one class of hemocytes, are phagocytically active and play an important role in immunity and development by removing microorganisms as well as apoptotic cells. Located on the surface of circulating and sessile plasmatocytes, Nimrod C1 (NimC1) is involved in the phagocytosis of bacteria. Suppression of NimC1 expression in plasmatocytes inhibit the phagocytosis of Staphylococcus aureus. Conversely, overexpression of NimC1 in S2 cells stimulated the phagocytosis of both S. aureus and Escherichia coli. NimC1 is a 90-100 kDa single-pass transmembrane protein with ten characteristic EGF-like repeats (NIM repeats). The nimC1 gene is part of a cluster of ten related nimrod genes at 34E on chromosome 2, and similar clusters of nimrod-like genes are conserved in other insects such as Anopheles and Apis. The Nimrod proteins are related to other putative phagocytosis receptors such as Eater, which is also located on phagocytic cells and mediates resistence to bacteria (Kocks, 2005), Draper from Drosophila, and CED-1 from C. elegans. Together, they form a superfamily that also includes proteins that are encoded in the human genome (Kurucz, 2007).

Exo84

The polarized architecture of epithelial tissues involves a dynamic balance between apical and basolateral membrane domains. This study shows that epithelial polarity in the Drosophila embryo requires the exocyst complex subunit homolog Exo84. Exo84 activity is essential for the apical localization of the Crumbs transmembrane protein, a key determinant of epithelial apical identity. Adherens junction proteins become mislocalized at the cell surface in Exo84 mutants in a pattern characteristic of defects in apical, but not basolateral, components. Loss of Crumbs from the cell surface precedes the disruption of Bazooka and Armadillo localization in Exo84 mutants. Moreover, Exo84 mutants display defects in apical cuticle secretion that are similar to crumbs mutants and are suppressed by a reduction in the basolateral proteins Dlg and Lgl. In Exo84 mutants at advanced stages of epithelial degeneration, apical and adherens junction proteins accumulate in an expanded recycling endosome compartment. These results suggest that epithelial polarity in the Drosophila embryo is actively maintained by exocyst-dependent apical localization of the Crumbs transmembrane protein (Blankenship, 2007).

G protein αi subunit 65A

Drosophila neuroblasts divide asymmetrically by aligning their mitotic spindle with cortical cell polarity to generate distinct sibling cell types. Neuroblasts asymmetrically localize Gαi, Pins, and Mud proteins; Pins/Gαi direct cortical polarity, whereas Mud is required for spindle orientation. It is currently unknown how Gαi-Pins-Mud binding is regulated to link cortical polarity with spindle orientation. This study shows that Pins forms a "closed" state via intramolecular GoLoco-tetratricopeptide repeat (TPR) interactions, which regulate Mud binding. Biochemical, genetic, and live imaging experiments show that Gαi binds to the first of three Pins GoLoco motifs to recruit Pins to the apical cortex without "opening" Pins or recruiting Mud. However, Gαi and Mud bind cooperatively to the Pins GoLocos 2/3 and tetratricopeptide repeat domains, respectively, thereby restricting Pins-Mud interaction to the apical cortex and fixing spindle orientation. It is concluded that Pins has multiple activity states that generate cortical polarity and link it with mitotic spindle orientation (Nipper, 2007).

Hen1

Small silencing RNAs repress gene expression by a set of related mechanisms collectively called RNA-silencing pathways. In the RNA interference (RNAi) pathway, small interfering mRNA (siRNAs) defend cells from invasion by foreign nucleic acids, such as those produced by viruses. In contrast, microRNAs (miRNAs) sculpt endogenous mRNA expression. A third class of small RNAs, Piwi-interacting RNAs (piRNAs), defends the genome from transposons. This study reports that Drosophila piRNAs contain a 2'-O-methyl group on their 3' termini; this is a modification previously reported for plant miRNAs and siRNAs and mouse and rat piRNAs. Plant small-RNA methylation is catalyzed by the protein HEN1 (Yang, 2006; Li, 2005; Yu, 2005). DmHen1, the Drosophila homolog of HEN1, termed Pimet (piRNA methyltransferase) by Saito (2007) in a parallel study, methylates the termini of siRNAs and piRNAs. Without DmHen1, the length and abundance of piRNAs are decreased, and piRNA function is perturbed. Unlike plant HEN1, DmHen1 acts on single strands, not duplexes, explaining how it can use as substrates both siRNAs, which derive from double-stranded precursors, and piRNAs which do not. 2'-O-methylation of siRNAs may be the final step in assembly of the RNAi-enzyme complex, RISC, occurring after the Argonaute-bound siRNA duplex is converted to single-stranded RNA (Horwich, 2007; Saito, 2007).

Holes in muscle/Meso18E

Tissue development requires the controlled regulation of cell-differentiation programs. In muscle, the Mef2 transcription factor binds to and activates the expression of many genes and has a major positive role in the orchestration of differentiation. However, little is known about how Mef2 activity is regulated in vivo during development. This study characterized a gene, Holes in muscle (Him; Flybase name meso18E), which is part of this control in Drosophila. Him expression rapidly declines as embryonic muscle differentiates, and consistent with this, Him overexpression inhibits muscle differentiation. This inhibitory effect is suppressed by mef2, implicating Him in the mef2 pathway. Him downregulates the transcriptional activity of Mef2 in both cell culture and in vivo. Furthermore, Him protein binds Groucho, a conserved, transcriptional corepressor, through a WRPW motif and requires this motif and groucho function to inhibit both muscle differentiation and Mef2 activity during development. Together, these results identify a mechanism that can inhibit muscle differentiation in vivo. It is concluded that a balance of positive and negative inputs, including Mef2, Him, and Groucho, controls muscle differentiation during Drosophila development and suggest that one outcome is to hold developing muscle cells in a state with differentiation genes poised to be expressed (Liotta, 2007).

Mad2

The spindle assembly checkpoint is essential to maintain genomic stability during cell division. The role of the putative Drosophila Mad2 homologue was examined in the spindle assembly checkpoint and mitotic progression. Depletion of Mad2 by RNAi from S2 cells shows that it is essential to prevent mitotic exit after spindle damage, demonstrating its conserved role. Mad2 has been shown to block mitotic exit by sequestering Cdc20 (Fizzy in Drosophila). Mad2-depleted cells also show accelerated transit through prometaphase and premature sister chromatid separation, fail to form metaphases, and exit mitosis soon after nuclear envelope breakdown with extensive chromatin bridges that result in severe aneuploidy. Interestingly, preventing Mad2-depleted cells from exiting mitosis by a checkpoint-independent arrest allows congression of normally condensed chromosomes. More importantly, a transient mitotic arrest is sufficient for Mad2-depleted cells to exit mitosis with normal patterns of chromosome segregation, suggesting that all the associated phenotypes result from a highly accelerated exit from mitosis. Surprisingly, if Mad2-depleted cells are blocked transiently in mitosis and then released into a media containing a microtubule poison, they arrest with high levels of kinetochore-associated BubR1, properly localized cohesin complex and fail to exit mitosis revealing normal spindle assembly checkpoint activity. This behavior is specific for Mad2 because BubR1-depleted cells fail to arrest in mitosis under these experimental conditions. Taken together these results strongly suggest that Mad2 is exclusively required to delay progression through early stages of prometaphase so that cells have time to fully engage the spindle assembly checkpoint, allowing a controlled metaphase-anaphase transition and normal patterns of chromosome segregation (Orr, 2007).

Microcephalin

Microcephalin (MCPH1) is mutated in primary microcephaly, an autosomal recessive human disorder of reduced brain size. It encodes a protein with three BRCT domains that has established roles in DNA damage signalling and the cell cycle, regulating chromosome condensation. Significant adaptive evolutionary changes in primate MCPH1 sequence suggest that changes in this gene could have contributed to the evolution of the human brain. To understand the developmental role of microcephalin its function in Drosophila has been studied. Drosophila MCPH1 is cyclically localised during the cell cycle, co-localising with DNA during interphase, but not with mitotic chromosomes. mcph1 mutant flies have a maternal effect lethal phenotype, due to mitotic arrest occurring in early syncytial cell cycles. Mitotic entry is slowed from the very first mitosis in such embryos, with prolonged prophase and metaphase stages; and frequent premature separation as well as detachment of centrosomes. As a consequence, centrosome and nuclear cycles become uncoordinated, resulting in arrested embryonic development. Phenotypic similarities with abnormal spindle (asp) and centrosomin (cnn) mutants (whose human orthologues are also mutated in primary microcephaly), suggest that further studies in the Drosophila embryo may establish a common developmental and cellular pathway underlying the human primary microcephaly phenotype (Brunk, 2007).

Mushroom bodies tiny

The p21 activated kinase (Pak) family of protein kinases are involved in many cellular functions like re-organisation of the cytoskeleton, transcriptional control, cell division, and survival. These pleiotropic actions are reflected in a plethora of known interacting proteins and phosphorylation substrates. Yet, the integration of a single Pak protein into signalling pathways controlling a particular developmental process are less well studied. For two of the three known Pak proteins in Drosophila melanogaster, D-Pak and Mushroom bodies tiny (Mbt), distinct functions during eye development have been established. This study undertook a genetic approach to identify proteins acting in the Mbt signalling pathway during photoreceptor cell morphogenesis. The genetic screen identified the actin depolymerisation factor Twinstar/Cofilin as one target of Mbt signalling. Twinstar/Cofilin becomes phosphorylated upon activation of Mbt. However, biochemical and genetic experiments question the role of the LIM domain protein kinase (Limk) as a major link between Mbt and Twinstar/Cofilin as it has been suggested for other PAK proteins. Constitutive activation of Mbt not only disturbs the actin cytoskeleton but also affects adherins junctions organisation indicating a requirement of the protein in cell adhesion dependent processes during photoreceptor cell differentiation (Menzel, 2007).

Niemann-Pick Type C-2

Mutations in either of the two human Niemann-Pick type C (NPC) genes, NPC1 and NPC2, cause a fatal neurodegenerative disease associated with abnormal cholesterol accumulation in cells. npc1a, the Drosophila NPC1 ortholog, NPC1, regulates sterol homeostasis and is essential for molting hormone (20-hydroxyecdysone; 20E) biosynthesis. While only one npc2 gene is present in yeast, worm, mouse and human genomes, a family of eight npc2 genes (npc2a-h) exists in Drosophila. Among the encoded proteins, Npc2a (Flybase term: Niemann-Pick Type C-2, abbreviated NPC2) has the broadest expression pattern and is most similar in sequence to vertebrate Npc2. Mutation of npc2a results in abnormal sterol distribution in many cells, as in Drosophila npc1a or mammalian NPC mutant cells. In contrast to the ecdysteroid-deficient, larval-lethal phenotype of npc1a mutants, npc2a mutants are viable and fertile with relatively normal ecdysteroid level. Mutants in npc2b (CG3153), another npc2 gene, are also viable and fertile, with no significant sterol distribution abnormality. However, npc2a; npc2b double mutants are not viable but can be rescued by feeding the mutants with 20E or cholesterol, the basic precursor of 20E. It is concluded that npc2a functions redundantly with npc2b in regulating sterol homeostasis and ecdysteroid biosynthesis, probably by controlling the availability of sterol substrate. Moreover, npc2a; npc2b double mutants undergo apoptotic neurodegeneration, thus constituting a new fly model of human neurodegenerative disease (Huang, 2007).

Nipped-B

The cohesin complex is a chromosomal component required for sister chromatid cohesion that is conserved from yeast to man. The similarly conserved Nipped-B protein is needed for cohesin to bind to chromosomes. In higher organisms, Nipped-B and cohesin regulate gene expression and development by unknown mechanisms. Using chromatin immunoprecipitation, it was found that Nipped-B and cohesin bind to the same sites throughout the entire non-repetitive Drosophila genome. They preferentially bind transcribed regions and overlap with RNA polymerase II. This contrasts sharply with yeast, where cohesin binds almost exclusively between genes. Differences in cohesin and Nipped-B binding between Drosophila cell lines often correlate with differences in gene expression. For example, cohesin and Nipped-B bind the Abd-B homeobox gene in cells in which it is transcribed, but not in cells in which it is silenced. They bind to the Abd-B transcription unit and a downstream regulatory region and thus could regulate both transcriptional elongation and activation. It is proposed transcription facilitates cohesin binding, perhaps by unfolding chromatin, and that Nipped-B then regulates gene expression by controlling cohesin dynamics. These mechanisms are likely involved in the etiology of Cornelia de Lange syndrome, in which mutation of one copy of the NIPBL gene encoding the human Nipped-B ortholog causes diverse structural and mental birth defects (Misulovin, 2008).

Off-schedule

During spermatogenesis, cells coordinate differentiation with the meiotic cell cycle to generate functional gametes. The gene off-schedule (ofs) was identified as being essential for this coordinated control. During the meiotic G₂ phase, Drosophila ofs mutant germ cells do not reach their proper size and fail to execute meiosis or significant differentiation. The accumulation of four cell cycle regulators -- Cyclin A, Boule, Twine and Roughex -- is altered in these mutants, indicating that ofs reveals a novel branch of the pathway controlling meiosis and differentiation. Ofs is homologous to eukaryotic translation initiation factor eIF4G. The level of ofs expression in spermatocytes is much higher than for the known eIF4G ortholog (known as eIF-4G or eIF4G), suggesting that Ofs substitutes for this protein. Consistent with this, assays for association with mRNA cap complexes, as well as RNA-interference and phenotypic-rescue experiments, demonstrate that Ofs has eIF4G activity. Based on these studies, it is speculated that spermatocytes monitor G₂ growth as one means to coordinate the initiation of meiotic division and differentiation (Franklin-Dumont, 2007).

PDGF- and VEGF-related factor 2 and PDGF- and VEGF-related factor 3

Drosophila hemocytes are highly motile macrophage-like cells that undergo a stereotypic pattern of migration to populate the whole embryo by late embryogenesis. The migratory patterns of hemocytes at the embryonic ventral midline are orchestrated by chemotactic signals from the PDGF/VEGF ligands Pvf2 and Pvf3; these directed migrations occur independently of phosphoinositide 3-kinase (PI3K) signaling. In contrast, using both laser ablation and a novel wounding assay that allows localized treatment with inhibitory drugs, PI3K is shown to be essential for hemocyte chemotaxis toward wounds and Pvf signals and PDGF/VEGF receptor expression are not required for this rapid chemotactic response. These results demonstrate that at least two separate mechanisms operate in Drosophila embryos to direct hemocyte migration and show that although PI3K is crucial for hemocytes to sense a chemotactic gradient from a wound, it is not required to sense the growth factor signals that coordinate their developmental migrations along the ventral midline during embryogenesis (Wood, 2007).

Plexin B

The semaphorin gene family has been shown to play important roles in axonal guidance in both vertebrates and invertebrates. Both transmembrane (Sema1a, Sema1b, Sema5c) and secreted (Sema2a, Sema2b) forms of semaphorins exist in Drosophila. Two Sema receptors, plexins (PlexinA and PlexinB), have also been identified. Many questions remain concerning the axon guidance functions of the secreted semaphorins, including the identity of their receptors. The well-characterized sensory system of the Drosophila embryo was used to address these problems. Novel sensory axon defects were found in sema2a loss-of-function mutants in which particular axons misproject and follow inappropriate pathways to the CNS. plexB loss-of-function mutants show similar phenotypes to sema2a mutants and sema2a interacts genetically with plexB, supporting the hypothesis that Sema2a signals through PlexB receptors. Sema2a protein is expressed by larval oenocytes, a cluster of secretory cells in the lateral region of the embryo and the sema2a mutant phenotype can be rescued by driving Sema2a in these cells. Ablation of oenocytes results in sensory axon defects similar to the sema2a mutant phenotype. These data support a model in which Sema2a, while being secreted from oenocytes, acts in a highly localized fashion: It represses axon extension from the sensory neuron cell body, but only in regions in direct contact with oenocytes (Bates, 2007).

Polycomblike

PcG protein complex PRC2 is thought to be the histone methyltransferase (HMTase) responsible for H3-K27 trimethylation at Polycomb target genes. This study reports the biochemical purification and characterization of a distinct form of Drosophila PRC2 that contains the Polycomb group protein Polycomblike (Pcl). Like PRC2, Pcl-PRC2 is an H3-K27-specific HMTase that mono-, di- and trimethylates H3-K27 in nucleosomes in vitro. Analysis of Drosophila mutants that lack Pcl unexpectedly reveals that Pcl-PRC2 is required to generate high levels of H3-K27 trimethylation at Polycomb target genes but is dispensable for the genome-wide H3-K27 mono- and dimethylation that is generated by PRC2. In Pcl mutants, Polycomb target genes become derepressed even though H3-K27 trimethylation at these genes is only reduced and not abolished, and even though targeting of the Polycomb protein complexes PhoRC and PRC1 to Polycomb response elements is not affected. Pcl-PRC2 is thus the HMTase that generates the high levels of H3-K27 trimethylation in Polycomb target genes that are needed to maintain a Polycomb-repressed chromatin state (Nekrasov, 2007).

Pox meso

The Pax gene Pox meso (Poxm) was the first and so far only gene whose initial expression was shown to occur specifically in the anlage of the somatic mesoderm, yet its role in somatic myogenesis remained unknown. This study shows that it is one of the crucial genes regulating the development of the larval body wall muscles in Drosophila. It has two distinct functions expressed during different phases of myogenesis. The early function, partially redundant with the function of lethal of scute [l(1)sc], demarcates the 'Poxm competence domain', a domain of competence for ventral and lateral muscle development and for the determination of at least some adult muscle precursor cells. The late function is a muscle identity function, required for the specification of muscles DT1, VA1, VA2 and VA3. These results led to a reinterpretation of the roles of l(1)sc and twist in myogenesis and to the proposal of a solution of the 'l(1)sc conundrum' (Duan, 2007).

Rab-protein 6

The Drosophila body axes are defined by the precise localization and the restriction of molecular determinants in the oocyte. Polarization of the oocyte during oogenesis is vital for this process. The directed traffic of membranes and proteins is a crucial component of polarity establishment in various cell types and organisms. This study investigated the role of the small GTPase Rab6 in the organization of the egg chamber and in asymmetric determinant localization during oogenesis. Exocytosis is affected in rab6-null egg chambers, which display a loss of nurse cell plasma membranes. Rab6 is also required for the polarization of the oocyte microtubule cytoskeleton and for the posterior localization of oskar mRNA. In vivo, Rab6 is found in a complex with Bicaudal-D, and Rab6 and Bicaudal-D cooperate in oskar mRNA localization. Thus, during Drosophila oogenesis, Rab6-dependent membrane trafficking is doubly required; first, for the general organization and growth of the egg chamber, and second, more specifically, for the polarization of the microtubule cytoskeleton and localization of oskar mRNA. These findings highlight the central role of vesicular trafficking in the establishment of polarity and in determinant localization in Drosophila (Coutelis, 2007).

Rad21

Every time a cell divides, it is essential that both daughters receive the complete genetic information of their mother. In eukaryotes, the tight connection between the two copies of every chromosome generated by DNA replication (the sister chromatids) ensures the attachment of their kinetochores to spindle microtubules emanating from opposite poles so that the sister chromatids will be segregated into different daughter cells. Cohesin is a highly conserved multisubunit complex that holds sister chromatids together in mitotic cells (see Proposed functions for sister chromatid cohesion proteins in Dorsett, 2007; see A Ring for Holding Sister Chromatids Together?). At the metaphase to anaphase transition, proteolytic cleavage of the α kleisin subunit (Rad21) by separase causes cohesin's dissociation from chromosomes and triggers sister-chromatid disjunction. To investigate cohesin's function in postmitotic cells, where it is widely expressed, fruit flies were created whose Rad21 can be cleaved by TEV protease. Cleavage causes precocious separation of sister chromatids and massive chromosome missegregation in proliferating cells, but not disaggregation of polytene chromosomes in salivary glands. Crucially, cleavage in postmitotic neurons is lethal. In mushroom-body neurons, it causes defects in axon pruning, whereas in cholinergic neurons it causes highly abnormal larval locomotion. These data demonstrate essential roles for cohesin in nondividing cells and also introduce a powerful tool by which to investigate protein function in metazoa (Pauli, 2008).

Set2

In Drosophila, X chromosome dosage compensation requires the male-specific lethal (MSL) complex, which associates with actively transcribed genes on the single male X chromosome to upregulate transcription 2-fold. On the male X chromosome, or when MSL complex is ectopically localized to an autosome, histone H3K36 trimethylation (H3K36me3) is a strong predictor of MSL binding. Mutants lacking Set2, the H3K36me3 methyltransferase, were isolated, and it was found that Set2 is an essential gene in both sexes of Drosophila. In set2 mutant males, MSL complex maintains X specificity but exhibits reduced binding to target genes. Furthermore, recombinant MSL3 protein preferentially binds nucleosomes marked by H3K36me3 in vitro. These results support a model in which MSL complex uses high-affinity sites to initially recognize the X chromosome and then associates with many of its targets through sequence-independent features of transcribed genes (Larschan, 2007).

Sex peptide receptor

Mating in many species induces a dramatic switch in female reproductive behaviour. In most insects, this switch is triggered by factors present in the male's seminal fluid. How these factors exert such profound effects in females is unknown. Identified here is a receptor for the Drosophila melanogaster Sex peptide (SP, also known as Acp70A), the primary trigger of post-mating responses in this species. Females that lack the sex peptide receptor (SPR, also known as CG16752), either entirely or only in the nervous system, fail to respond to SP and continue to show virgin behaviours even after mating. SPR is expressed in the female's reproductive tract and central nervous system. The behavioural functions of SPR map to the subset of neurons that also express the fruitless gene, a key determinant of sex-specific reproductive behaviour. SPR is highly conserved across insects, opening up the prospect of new strategies to control the reproductive and host-seeking behaviours of agricultural pests and human disease vectors (Yapici, 2008).

Six4

Patterning of the Drosophila embryonic mesoderm requires the regulation of cell type-specific factors in response to dorsoventral and anteroposterior axis information. For the dorsoventral axis, the homeodomain gene, tinman, is a key patterning mediator for dorsal mesodermal fates like the heart. However, equivalent mediators for more ventral fates are unknown. This study shows that Six4, which encodes a Six family transcription factor, is required for the appropriate development of most cell types deriving from the non-dorsal mesoderm: the fat body, somatic cells of the gonad, and a specific subset of somatic muscles. Misexpression analysis suggests that Six4 and its likely cofactor, Eyes absent, are sufficient to impose these fates on other mesodermal cells. At stage 10, the mesodermal expression patterns of Six4 and tin are complementary, being restricted to the dorsal and non-dorsal regions respectively. These data suggest that Six4 is a key mesodermal patterning mediator at this stage that regulates a variety of cell-type-specific factors and hence plays an equivalent role to tin. At stage 9, however, Six4 and tin are both expressed pan-mesodermally. At this stage, tin function is required for full Six4 expression. This may explain the known requirement for tin in some non-dorsal cell types (Clark, 2006).

Spd-2

In C. elegans, genome-wide screens have identified just five essential centriole-duplication factors: SPD-2, ZYG-1, SAS-5, SAS-6, and SAS-4. These proteins are widely believed to comprise a conserved core duplication module. In worm embryos, SPD-2 is the most upstream component of this module, and it is also essential for pericentriolar material (PCM) recruitment to the centrioles (Pelletier, 2004; Kemp, 2004; Pelletier, 2006; Delattre, 2006). Drosophila Spd-2 is a component of both the centrioles and the PCM and has a role in recruiting PCM to the centrioles. Spd-2 appears not, however, to be essential for centriole duplication in somatic cells. Moreover, PCM recruitment in Spd-2 mutant somatic cells is only partially compromised, and mitosis appears unperturbed. In contrast, Spd-2 is essential for proper PCM recruitment to the fertilizing sperm centriole, and hence for microtubule nucleation and pronuclear fusion. Spd-2 therefore appears to have a particularly important role in recruiting PCM to the sperm centriole. It is speculated that the SPD-2 family of proteins might only be absolutely essential for the recruitment of centriole duplication factors and PCM to the centriole(s) that enter the egg with the fertilizing sperm (Dix, 2007).

Spindly

The eukaryotic spindle assembly checkpoint (SAC) monitors microtubule attachment to kinetochores and prevents anaphase onset until all kinetochores are aligned on the metaphase plate. In higher eukaryotes, cytoplasmic dynein is involved in silencing the SAC by removing the checkpoint proteins Mad2 and the Rod-Zw10-Zwilch complex (RZZ) from aligned kinetochores (Howell, 2001; Wojcik, 2001). Using a high throughput RNA interference screen in Drosophila melanogaster S2 cells, a new protein (Spindly) has been identified that accumulates on unattached kinetochores and is required for silencing the SAC. After the depletion of Spindly, dynein cannot target to kinetochores, and, as a result, cells arrest in metaphase with high levels of kinetochore-bound Mad2 and RZZ. A human homologue of Spindly serves a similar function. However, dynein's nonkinetochore functions are unaffected by Spindly depletion. These findings indicate that Spindly is a novel regulator of mitotic dynein, functioning specifically to target dynein to kinetochores (Griffis, 2007).

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