What's new in edition 49 part April 2007 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:

borealis

The protein kinase Aurora-A is required for centrosome maturation, spindle assembly, and asymmetric protein localization during mitosis. Borealis (Bora, so named for aurora borealis to indicate its similarity with aurora-A) is a conserved protein that is required for the activation of Aurora-A at the onset of mitosis. In the Drosophila peripheral nervous system, bora mutants show defects during asymmetric cell division identical to those observed in aurora-A. Furthermore, overexpression of bora can rescue defects caused by mutations in aurora-A. Bora is conserved in vertebrates, and both Drosophila and human Bora can bind to Aurora-A and activate the kinase in vitro. In interphase cells, Bora is a nuclear protein, but upon entry into mitosis, Bora is excluded from the nucleus and translocates into the cytoplasm in a Cdc2-dependent manner. A model is presented here in which activation of Cdc2 initiates the release of Bora into the cytoplasm where it can bind and activate Aurora-A (Hutterer, 2006).

Calcium/calmodulin-dependent protein kinase

The ability of CaMKII to act as a molecular switch, becoming Ca²⁺ independent after activation and autophosphorylation at T287, is critical for experience-dependent synaptic plasticity. This study shows that Caki, the Drosophila homolog of CASK, also known as Camguk, can act as a gain controller on the transition to calcium-independence in vivo. Genetic loss of dCASK significantly increases synapse-specific, activity-dependent autophosphorylation of CaMKII T287. In wild-type adult animals, simple and complex sensory stimuli cause region-specific increases in pT287. dCASK-deficient adults have a reduced dynamic range for activity-dependent T287 phosphorylation and have circuit-level defects that result in inappropriate activation of the kinase. dCASK control of the CaMKII switch occurs via its ability to induce autophosphorylation of T306 in the kinase's Calmodulin (CaM) binding domain. Phosphorylation of T306 blocks Ca²⁺/CaM binding, lowering the probability of intersubunit T287 phosphorylation, which requires CaM binding to both the substrate and catalytic subunits. dCASK is the first CaMKII-interacting protein other than CaM found to regulate this plasticity-controlling molecular switch (Hodge, 2006).

centrosomin's beautiful sister

The eukaryotic Golgi complex is an essential organelle involved in many cellular processes, including lipid biosynthesis, protein modification and intracellular membrane trafficking. A major question and area of debate in cell biology is how the Golgi complex is inherited by daughter cells during mitosis. This has led to two different mechanistic models. One model proposes that the Golgi is a derivative of the endoplasmic reticulum (ER) and arises de novo from the ER during late telophase, whereas the other model proposes the Golgi complex is a unique organelle that arises by a template-based mechanism, requiring existing Golgi subunits for reassembly after mitosis. This study examines the localization and function of a Golgi protein encoded by centrosomin's beautiful sister (cbs) during cleavage in Drosophila. Cbs contains a GRIP domain that is 57% identical to vertebrate Golgin-97. Cbs undergoes a dramatic relocalization during mitosis from the cytoplasm to an association with chromosomes from late prometaphase to early telophase, by a transport mechanism that requires the GRIP domain and Arl1, the product of the Arf72A locus. Additionally, Cbs remains independent of the endoplasmic reticulum throughout cleavage. The use of RNAi, Arf72A mutant analysis and ectopic expression of the GRIP domain, shows that cycling of Cbs during mitosis is required for the centrosome cycle. The normal cycling of Cbs is important for normal centrosome maturation, replication and spindle attachment during mitosis, and the linkage of this process to the normal nuclear cycle The effects on the centrosome cycle depend on Cbs concentration and Cbs transport from the cytoplasm to DNA. When Cbs levels are reduced centrosomes fail to mature, and when Cbs transport is impeded by ectopic expression of the GRIP domain, centrosomes undergo hypertrophy. It is proposed that, Cbs is a trans-Golgi protein that links Golgi inheritance to the cell cycle and the Drosophila Golgi is more vertebrate-like than previously recognized (Eisman, 2006).

Cytochrome c proximal and Cytochrome c distal

Cytochrome C has two apparently separable cellular functions: respiration and caspase activation during apoptosis. While a role of the mitochondria and cytochrome C in the assembly of the apoptosome and caspase activation has been established for mammalian cells, the existence of a comparable function for cytochrome C in invertebrates remains controversial. Drosophila possesses two cytochrome c genes, cyt-c-d and cyt-c-p. Only cyt-c-d is required for caspase activation in an apoptosis-like process during spermatid differentiation, whereas cyt-c-p is required for respiration in the soma. However, both cytochrome C proteins can function interchangeably in respiration and caspase activation, and the difference in their genetic requirements can be attributed to differential expression in the soma and testes. Furthermore, orthologues of the apoptosome components, Ark (Apaf-1) and Dronc (caspase-9), are also required for the proper removal of bulk cytoplasm during spermatogenesis. Finally, several mutants that block caspase activation during spermatogenesis were isolated in a genetic screen, including mutants with defects in spermatid mitochondrial organization. These observations establish a role for the mitochondria in caspase activation during spermatogenesis (Arama, 2006).

Ik2

Caspase activation has been extensively studied in the context of apoptosis. However, caspases also control other cellular functions, although the mechanisms regulating caspases in nonapoptotic contexts remain obscure. Drosophila IAP1 (DIAP1) is an endogenous caspase inhibitor that is crucial for regulating cell death during development. Drosophila IKK-related kinase (DmIKKε; FlyBase name, Ik2) as a regulator of caspase activation in a nonapoptotic context. DmIKKε promotes degradation of DIAP1 through direct phosphorylation. Knockdown of DmIKKε in the proneural clusters of the wing imaginal disc , in which nonapoptotic caspase activity is required for proper sensory organ precursor (SOP) development, stabilizes endogenous DIAP1 and affects Drosophila SOP development. These results demonstrate that DmIKKε is a determinant of DIAP1 protein levels and that it establishes the threshold of activity required for the execution of nonapoptotic caspase functions (Kuranaga, 2006).

Liprin-α

In the Drosophila visual system, the color-sensing photoreceptors R7 and R8 project their axons to two distinct layers in the medulla. Loss of the receptor tyrosine phosphatase LAR from R7 photoreceptors causes their axons to terminate prematurely in the R8 layer. This study has identified a null mutation in the Liprin-α (for LAR-interacting protein) gene based on a similar R7 projection defect. Liprin-α physically interacts with the inactive D2 phosphatase domain of LAR, and this domain is also essential for R7 targeting. However, another LAR-dependent function, egg elongation, requires neither Liprin-α nor the LAR D2 domain. Although human and Caenorhabditis elegans Liprin-α proteins have been reported to control the localization of LAR, LAR localizes to focal adhesions in Drosophila S2R+ cells and to photoreceptor growth cones in vivo independently of Liprin-α. In addition, Liprin-α overexpression or loss of function can affect R7 targeting in the complete absence of LAR. Despite its reported role in axonal transport of synaptic vesicle components (Miller, 2005), Liprin-α is not required for the transport of endogenous LAR protein to the growth cones of larval R1–R6 photoreceptors. It is concluded that Liprin-α does not simply act by regulating LAR localization but also has LAR-independent functions (Hofmeyer, 2006). A second publication (Choe, 2006) reports similar results.

Male-specific lethal 1

In Drosophila melanogaster, X chromosome dosage compensation is achieved by doubling the transcription of most X-linked genes. The male-specific lethal (MSL) complex is required for this process and binds to hundreds of sites on the male X chromosome. The MSL1 protein is essential for X chromosome binding and serves as a central scaffold for MSL complex assembly. The amino-terminal region of MSL1 binds to hundreds of sites on the X chromosome in normal males but only to approximately 30 high-affinity sites in the absence of endogenous MSL1. Binding to the high-affinity sites requires a basic motif at the amino terminus that is conserved among Drosophila species. X chromosome binding also requires a conserved leucine zipper-like motif that binds to MSL2. A glycine-rich motif between the basic and leucine-zipper-like motifs mediates MSL1 self-association in vitro and binding of the amino-terminal region of MSL1 to the MSL complex assembled on the male X chromosome. It is proposed that the basic region may mediate DNA binding and that the glycine-rich region may promote the association of MSL complexes to closely adjacent sites on the X chromosome (Li, 2005).

NMDA receptor 1 and NMDA receptor 2

N-methyl-D-aspartate (NMDA) receptors are one of three pharmacologically distinct subtypes of ionotropic receptors that mediate a majority of excitatory neurotransmission in the brain via the endogenous amino acid, L-glutamate. NMDARs form heteromeric complexes usually comprised of the essential NR1 subunit and various NR2 subunits. Molecular and electrophysiological properties of NMDARs suggest that they may be the Hebbian 'coincidence detectors' hypothesized to underlie associative learning. Because of the nonspecificity of drugs that modulate NMDAR function or the relatively chronic genetic manipulations of various NMDAR subunits from mammalian studies, conclusive evidence for such an acute role for NMDARs in adult behavioral plasticity, however, is lacking. Moreover, a role for NMDARs in memory consolidation remains controversial. The Drosophila genome encodes two NMDAR homologs, dNR1 and dNR2. When coexpressed in Xenopus oocytes or Drosophila S2 cells, dNR1 and dNR2 form functional NMDARs with several of the distinguishing molecular properties observed for vertebrate NMDARs, including voltage/Mg²⁺-dependent activation by glutamate. Both proteins are weakly expressed throughout the entire brain but show preferential expression in several neurons surrounding the dendritic region of the mushroom bodies. Hypomorphic mutations of the essential dNR1 gene disrupt olfactory learning, and this learning defect is rescued with wild-type transgenes. Importantly, Pavlovian learning is disrupted in adults within 15 hr after transient induction of a dNR1 antisense RNA transgene. Extended training is sufficient to overcome this initial learning defect, but long-term memory (LTM) specifically is abolished under these training conditions. In conclusion, this study uses a combination of molecular-genetic tools to (1) generate genomic mutations of the dNR1 gene, (2) rescue the accompanying learning deficit with a dNR1⁺ transgene, and (3) rapidly and transiently knockdown dNR1⁺ expression in adults, thereby demonstrating an evolutionarily conserved role for the acute involvement of NMDARs in associative learning and memory (Xia, 2005).

Rhomboid-7

In addition to being energy generators, mitochondria control many cellular processes including apoptosis. They are dynamic organelles, and the machinery of membrane fusion and fission is emerging as a key regulator of mitochondrial biology. A novel and conserved mitochondrial rhomboid intramembrane protease has been identified that controls membrane fusion in Saccharomyces cerevisiae by processing the dynamin-like GTPase, Mgm1, thereby releasing it from the membrane (McQuibban, 2003). The genetics of mitochondrial membrane dynamics has until now focused primarily on yeast. In Drosophila, the mitochondrial rhomboid (Rhomboid-7) is required for mitochondrial fusion during fly spermatogenesis and muscle maturation, both tissues with unusual mitochondrial dynamics. Mutations in Drosophila optic atrophy 1-like (Opa1-like or CG8479), the ortholog of yeast mgm1, display similar phenotypes, suggesting a shared role for Rhomboid-7 and Opa1-like, as with their yeast orthologs. Loss of human OPA1 leads to dominant optic atrophy, a mitochondrial disease leading to childhood onset blindness. rhomboid-7 mutant flies have severe neurological defects, evidenced by compromised signaling across the first visual synapse, as well as light-induced neurodegeneration of photoreceptors that resembles the human disease. rhomboid-7 mutant flies also have a greatly reduced lifespan (McQuibban, 2006).

RPS-protein kinase-II

Although p90 ribosomal S6 kinase (RSK) is known as an important downstream effector of the ribosomal protein S6 kinase/extracellular signal-regulated kinase (Ras/ERK) pathway, its endogenous role, and precise molecular function remain unclear. Using gain-of-function and null mutants of RSK, its physiological role was successfully characterized in Drosophila. Surprisingly, RSK-null mutants are viable, but exhibit developmental abnormalities related to an enhanced ERK-dependent cellular differentiation such as ectopic photoreceptor- and vein-cell formation. Conversely, overexpression of RSK dramatically suppresses the ERK-dependent differentiation, which is further augmented by mutations in the Ras/ERK pathway. Consistent with these physiological phenotypes, RSK negatively regulates ERK-mediated developmental processes and gene expressions by blocking the nuclear localization of ERK in a kinase activity-independent manner. In addition, RSK-dependent inhibition of ERK nuclear migration is mediated by the physical association between ERK and RSK. Collectively, these studies reveal a novel regulatory mechanism of the Ras/ERK pathway by RSK, which negatively regulates ERK activity by acting as a cytoplasmic anchor in Drosophila (Kim, 2006).

Rtf1

The Rtf1 subunit of the Paf1 complex is required for proper monoubiquitination of histone H2B and methylation of histone H3 on lysines 4 (H3K4) and 79 in yeast Saccharomyces cerevisiae. Using RNAi, the role of Rtf1 in histone methylation and gene expression was examined in Drosophila. Drosophila Rtf1 (dRtf1) is required for proper gene expression and development. Furthermore, RNAi-mediated reduction of dRtf1 results in a reduction in histone H3K4 trimethylation levels on bulk histones and chromosomes in vivo, indicating that the histone modification pathway via Rtf1 is conserved among yeast, Drosophila, and human. Recently, it was demonstrated that histone H3K4 methylation mediated via the E3 ligase Bre1 is critical for transcription of Notch target genes in Drosophila. This study demonstrates that the dRtf1 component of the Paf1 complex functions in Notch signaling (Tenney, 2006).

Sarah

The Drosophila modulatory calcineurin-interacting protein (MCIP) sarah (sra) is essential for meiotic progression in oocytes. Activation of mature oocytes initiates development by releasing the prior arrest of female meiosis, degrading certain maternal mRNAs while initiating the translation of others, and modifying egg coverings. In vertebrates and marine invertebrates, the fertilizing sperm triggers activation events through a rise in free calcium within the egg. In insects, egg activation occurs independently of sperm and is instead triggered by passage of the egg through the female reproductive tract; it is unknown whether calcium signaling is involved. MCIPs [also termed regulators of calcineurin (RCNs), calcipressins, or DSCR1 (Down syndrome critical region 1)] are highly conserved regulators of calcineurin, a Ca2+/calmodulin-dependent protein phosphatase 1 and 2. Although overexpression experiments in several organisms have revealed that MCIPs inhibit calcineurin activity, their in vivo functions remain unclear. Eggs from sra null mothers are arrested at anaphase of meiosis I. This phenotype was due to loss of function of sra specifically in the female germline. Sra is physically associated with the catalytic subunit of calcineurin, and its overexpression suppresses the phenotypes caused by constitutively activated calcineurin, such as rough eye or loss of wing veins. Hyperactivation of calcineurin signaling in the germline cells resulted in a meiotic-arrestphenotype, which can also be suppressed by overexpression of Sra. All these results support the hypothesis that Sra regulates female meiosis by controlling calcineurin activity in the germline. This is the first unambiguous demonstration that the regulation of calcineurin signaling by MCIPs plays a critical role in a defined biological process (Takeo, 2006; Horner, 2006).

Serotonin receptor 1A and Serotonin receptor 1B

Although sleep is an important process essential for life, its regulation is poorly understood. The recently developed Drosophila model for sleep provides a powerful system to genetically and pharmacologically identify molecules that regulate sleep. Serotonin is an important neurotransmitter known to affect many behaviors, but its role in sleep remains controversial. Flies were generated with genetically altered expression of each of three Drosophila serotonin receptor subtypes (5-HT1A, 5-HT1B, and d5-HT2) and they were assayed for baseline sleep phenotypes. The data indicated a sleep-regulating role for the 5-HT1A receptor. 5-HT1A mutant flies had short and fragmented sleep, which was rescued by expressing the receptor in adult mushroom bodies, a structure associated with learning and memory in Drosophila. Neither the d5-HT2 receptor nor the 5-HT1B receptor, which was previously implicated in circadian regulation, had any effect on baseline sleep, indicating that serotonin affects sleep and circadian rhythms through distinct receptors. Elevating serotonin levels, either pharmacologically or genetically, enhanced sleep in wild-type flies. In addition, serotonin promoted sleep in some short-sleep mutants, suggesting that it can compensate for some sleep deficits. These data show that serotonin promotes baseline sleep in Drosophila. They also link the regulation of sleep behavior by serotonin to a specific receptor in a distinct region of the fly brain (Yuan, 2006).

SnoN

A screen for modifiers of Dpp adult phenotypes led to the identification of the Drosophila homolog of the Sno oncogene (dSno; termed snoN in FlyBase). The SnoN locus is large, transcriptionally complex and contains a recent retrotransposon insertion that may be essential for SnoN function. This is an intriguing possibility from the perspective of developmental evolution. SnoN is highly transcribed in the embryonic central nervous system and transcripts are most abundant in third instar larvae. SnoN mutant larvae have proliferation defects in the optic lobe of the brain very similar to those seen in baboon (Activin type I receptor) and Smad2 mutants. This suggests that SnoN is a mediator of Baboon signaling. SnoN binds to Medea and Medea/SnoN complexes have enhanced affinity for Smad2. Alternatively, Medea/SnoN complexes have reduced affinity for Mad such that, in the presence of SnoN, Dpp signaling is antagonized. It is proposed that SnoN functions as a switch in optic lobe development, shunting Medea from the Dpp pathway to the Activin pathway to ensure proper proliferation. Pathway switching in target cells is a previously unreported mechanism for regulating TGFß signaling and a novel function for Sno/Ski family proteins (Takaesu, 2006).

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