Oskar encodes a product involved in pole cell
determination which is expressed in the ovary (oocyte , egg chamber , germarium region 2b , nurse cell and 2 other
listed tissues) and pole granule ).
oskar mRNA is localized to the posterior pole of the Drosophila
oocyte.
General function
RNA binding
Comment
The site of oskar RNA and protein localization within the oocyte determines
where in the embryo primordial germ cells form and where the abdomen
develops. Initiation of oskar RNA localization requires the activity of several
genes. Rongo C, et al showed that ovaries mutant for any of these genes lack Oskar protein.
Using various transgenic constructs they have determined that sequences required
for oskar RNA localization and translational repression map to the oskar 3'UTR,
while sequences involved in the correct temporal activation of translation reside
outside the oskar 3'UTR. Upon localization of oskar RNA and protein at the
posterior pole, Oskar protein is required to maintain localization of oskar RNA
throughout oogenesis. Stable anchoring of a transgenic reporter RNA at the
posterior pole is disrupted by oskar nonsense mutations. They propose that initially
localization of oskar RNA permits translation into Oskar protein and that
subsequently Oskar protein regulates its own RNA localization through a positive
feedback mechanism.
Cellular localization
Cytoplasmic
Comment
Ovarian function
Germ cell development, Oogenesis
Comment
Translational Activation of Oskar mRNA: Reevaluation of the Role and Importance of a 5' Regulatory Element. Kanke M et al. (2015) Local translation of oskar (osk) mRNA at the posterior pole of the Drosophila oocyte is essential for axial patterning of the embryo, and is achieved by a program of translational repression, mRNA localization, and translational activation. Multiple forms of repression are used to prevent Oskar protein from accumulating at sites other than the oocyte posterior. Activation is mediated by several types of cis-acting elements, which presumably control different forms of activation. We characterize a 5' element, positioned in the coding region for the Long Osk isoform and in the extended 5' UTR for translation of the Short Osk isoform. This element was previously thought to be essential for osk mRNA translation, with a role in posterior-specific release from repression. From our work, which includes assays which separate the effects of mutations on RNA regulatory elements and protein coding capacity, we find that the element is not essential, and conclude that there is no evidence supporting a role for the element only at the posterior of the oocyte. The 5' element has a redundant role, and is only required when Long Osk is not translated from the same mRNA. Mutations in the element do disrupt the anchoring function of Long Osk protein through their effects on the amino acid sequence, a confounding influence on interpretation of previous experiments.//////////////////
Kim-Ha J, et al reported that
transcripts of one of oskar become localized to the posterior pole of
oocytes shortly after the oocyte begins to differentiate visibly. Analysis of various
posterior-group mutants reveals that localization of oskar mRNA is an early step
in the posterior localization pathway. In addition, we find that nonsense oskar
mutations disrupt osk mRNA localization, while missense oskar mutations do not.
Kim-Ha J, et al reported translational regulation of oskar mRNA by bruno, an ovarian
RNA-binding protein, is essential.
Oskar (osk) protein directs the deployment of nanos (nos), the posterior
body-patterning morphogen in Drosophila. To avoid inappropriate activation of
nos, osk activity must appear only at the posterior pole of the oocyte, where the
osk mRNA becomes localized during oogenesis. The translation
of osk mRNA is, and must be, repressed prior to its localization; absence of
repression allows osk protein to accumulate throughout the oocyte, specifying
posterior body patterning throughout the embryo. Translational repression is
mediated by an ovarian protein, bruno, that binds specifically to bruno response
elements (BREs), present in multiple copies in the osk mRNA 3'UTR. Addition of
BREs to a heterologous mRNA renders it sensitive to translational repression in
the ovary.
Expression regulated by
Comment
Ovarian localization
Primordial Germ Cell, Oocyte
Comment
Splicing of oskar RNA in the nucleus is coupled to its cytoplasmic localization.
Hachet O, .
oskar messenger RNA localization at the posterior pole of the Drosophila oocyte is essential for germline and abdomen formation in the future embryo. The nuclear shuttling proteins Y14/Tsunagi and Mago nashi are required for oskar mRNA localization, and they co-localize with oskar mRNA at the posterior pole of the oocyte. Their human homologues, Y14/RBM8 and Magoh, are core components of the exon-exon junction complex (EJC). The EJC is deposited on mRNAs in a splicing-dependent manner, 20-24 nucleotides upstream of exon-exon junctions, independently of the RNA sequence. This indicates a possible role of splicing in oskar mRNA localization, challenging the established notion that the oskar 3' untranslated region (3'UTR) is sufficient for this process. Here we show that splicing at the first exon-exon junction of oskar RNA is essential for oskar mRNA localization at the posterior pole. We revisit the issue of sufficiency of the oskar 3'UTR for posterior localization and show that the localization of unrelated transcripts bearing the oskar 3'UTR is mediated by endogenous oskar mRNA. Our results reveal an important new function for splicing: regulation of messenger ribonucleoprotein complex assembly and organization for mRNA cytoplasmic localization.