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Bicaudal-C (Bic-C) encodes a KH-type RNA binding protein required maternally for anterior patterning of the Drosophila oocyte and correct migration of the centripetal follicle cells. In Drosophila, premature translation of the germ-plasm determinant Oskar in Bic-C mutant oocytes suggests a function for Bic-C in post-transcriptional gene regulation. / Purification and microarray analysis of Bic-C containing ribonucleoprotein complexes revealed that Bic-C associates with multiple transcripts encoding functionally-related components of the Wnt/Frizzled/Dishevelled signaling pathway that regulate actin dynamics, in addition to its own mRNA. Using transgenic reporter constructs, Bic-C was demonstrated to destabilize its own mRNA via cis-acting 5' UTR elements. When auto-regulation was bypassed and Bic-C was over-expressed in the female germline, premature cytoplasmic streaming was induced, disrupting axial patterning through displacement of both Gurken (Grk) and oskar. These phenotypes can also be induced by disruption of the actin cytoskeleton with pharmacological agents and are similar to those described for hypomorphic mutant alleles of orb, which encodes a CPEB-like protein that promotes polyadenylation of target mRNAs. The Bic-C overexpression phenotypes require its RNA binding activity, are substantially enhanced by mutations affecting orb and poly(A) polymerase, and are suppressed by mutations affecting the deadenylase CCR4 and its accessory protein NOT3. Co-immunoprecipitation experiments demonstrate that Bic-C associates with components of the deadenylase complex and with components of an ER-associated RNP complex that includes Me31B, PABP and Trailer-hitch. The latter complex is involved in Grk exocytosis. Accordingly, Grk secretion is defective in Bic-C mutants. / Taken together, these results support a model whereby Bic-C antagonizes Orb function by negatively regulating the expression of Orb target mRNAs, through recruitment of the deadenylase machinery, that are involved in coordinating cytoplasmic movements. Furthermore, this work identifies a novel function of Bic-C in dorsal/ventral patterning by promoting Grk secretion.

I have investigated the mechanisms by which developmental asymmetry arises, using oocyte determination in Drosophila melanogaster as a model system. The Bicaudal-D (Bic-D) gene is required early in oogenesis for the asymmetric localization of specific mRNAs and proteins and for the differentiation of an oocyte from one of a cluster of 16 interconnected germarial cells. To better understand how Bic-D functions in creating this asymmetry, I took two approaches. First, I examined the role of Bic-D in the asymmetric localization of mRNA and other cellular components during later oogenesis. Second, I molecularly and genetically characterized a gene that interacts with Bic-D in oocyte determination. To determine the role of Bic-D in later oogenesis, I used an inducible source of Bic-D activity to selectively rescue the block at oocyte determination in Bic-D null mutants. Using this system, I find that Bic-D is indeed required in the later stages of oogenesis for the localization of specific mRNAs at both the anterior and posterior of the oocyte. Bic-D is also required for oocyte growth and nuclear positioning, processes which also depend on microtubules. / In the second part of this thesis, I describe the characterization of a Bic-D interacting gene which I have identified as the Drosophila homologue of the human Lissencephaly-1 (Lis-1) gene, DLis-1. Human Lis-1 is the causative gene for Miller-Dieker Syndrome and is required for neuronal migration in the developing brain, while fungal homologues have been implicated in dynein dependent nuclear migration. Like Bic-D , DLis-1 is essential for oocyte determination and for intracellular localization throughout oogenesis. DLis-1 is required for correct positioning of the oocyte nucleus, and appears to function upstream of dynein in this process. Immunolocalization studies suggest that DLis-1 functions as part of a cortical anchor that links microtubules and the oocyte nucleus, via dynein and microtubules, to the cell cortex. DLis-1 and Bic-D are also required for nuclear positioning during neural development in Drosophila, supporting a model in which the neuronal migration defects in Miller-Dieker Syndrome are due to a disruption of dynein/Lis-1 dependent nuclear migration.

loki was identified in our lab in a screen for novel serine-threonine protein kinases that are specifically expressed in the ovary. loki transcripts are expressed in early embryos and adult ovaries. In the ovaries, loki transcripts accumulate in the oocyte during early oogenesis, and by stage 8, are found to localize anteriorly to the oocyte-nurse cell interface. In order to understand the role of loki in Drosophila development, I created a loki mutant through excision of a P-element that had inserted 700bp upstream of loki. loki mutants display no apparent phenotype. / I was also able to identify the transcription unit responsible for valois (vls) function since it is located downstream from loki in the 38B region. vls is a member of the posterior group gene family and vls mutant mother produce embryos that lack pole cells and show abdomen patterning defects. Further characterization of vls indicates that it is a fairly direct activator of Vasa protein: vls function is required between Oskar protein and Vasa in the posterior patterning pathway since Vasa fails to localize to the posterior pole of vls mutant eggchambers. I also find that Vasa is differentially modified in vls ovaries and embryos, even though it is not yet known whether this modification is required for localization, proper function or both processes.

Thesis (Ph.D.)--University of Adelaide, Dept. of Molecular Biosciences, 2002. / "May 2002" Bibliography: leaves 157-176.

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