oskar

oskar is a gene required for the development of the Drosophila embryo. It defines the posterior pole during early embryogenesis. Its two isoforms, short and long, play different roles in Drosophila embryonic development.

This article is about the Drosophila gene. For the human receptor gene, see OSCAR (gene). For the name, see Oscar (name). For other uses, see Oscar (disambiguation).

Maternal effect protein oskar
Identifiers
Organism	Drosophila melanogaster
Symbol	osk
UniProt	P25158
Search for Structures Swiss-model Domains InterPro

Translational-level regulation

oskar is translationally repressed prior to reaching the posterior pole of the oocyte by Bruno, which binds to three bruno response elements (BREs) on the 3' end of the transcribed oskar mRNA.[1] The Bruno inhibitor has two distinct modes of action: recruiting the Cup eIF4E binding protein, which is also required for oskar mRNA localization due to interactions with the Barentsz microtubule-linked transporter,[2] and promoting oligomerization of oskar mRNA.[3] Oskar mRNA harbours a stem-loop structure in the 3’UTR, called the oocyte entry signal (OES), that promotes dynein-based mRNA accumulation in the oocyte.[4]

P granule formation

oskar plays role in recruiting other germ line genes to the germ plasm for PGC (primordial germ cell) specification. oskar mRNA locates to the posterior end of an oocyte and, once translated, the short isoform of oskar (Short oskar) recruits germ plasm components such as the protein Vasa and the RNA-binding proteins of the Piwi family, among many others.[5] The long isoform of oskar (Long oskar) has been implicated in creating an actin network on the posterior pole end.

A second role has been discovered that relates to the formation of P granules, or germ granules. These ribonucleoprotein granules are found in every species' germ line cells. Although they are mobile, they typically localize to the nuclei and sit on nuclear pores. This positioning makes them ideal mRNA regulators, as the mRNA must pass through to exit the nucleus[6]. Translational regulation also makes sense due to the granules' close association with ribosomes. These P granules are phase-transition entities, which means that they can display both liquid-like and hydrogel-like properties.[5] This allows them to be very versatile structures, able to dissolve, condense, and exchange their protein content with their environment at will. Recent studies have shown that the short isoform of oskar has another function as the nucleator of nuclear germ granules. oskar recruits vasa to these round granules, then promotes the localization to the nucleus. oskar was ablated to explore the function of these nuclear germ granules. The results showed that the division of PGCs was compromised without oskar, meaning that the P granules play a role in the cell cycle of germ cells.[5] It is still unclear exactly how the nuclear granules interact with certain factors and what factors (proteins, regulators, inhibitors) they interact with in order to regulate cell division.

Domain families

OSK
Identifiers
Symbol	OSK
Pfam	PF17182
InterPro	IPR033447
CATH	5a4a
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

OST-HTH/LOTUS
Identifiers
Symbol	OST-HTH
Pfam	PF12872
InterPro	IPR025605
PROSITE	PS51644
CATH	5a48
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

oskar contains two RNA-binding protein domains: the OSK RNA-binding domain and the OST-HTH/LOTUS domain. The former is structurally related to SGNH hydrolases but lack the active site residues. The latter is a winged helix-turn-helix domain also found in human TDRD5/TDRD7. The OST-HTH domain in oskar is mainly responsible for recruiting the Vasa helicase by binding to it.[7]

References

1. Kim-Ha J, Kerr K, Macdonald PM (May 1995). "Translational regulation of oskar mRNA by bruno, an ovarian RNA-binding protein, is essential". Cell. 81 (3): 403–12. doi:10.1016/0092-8674(95)90393-3. PMID 7736592.
2. Wilhelm JE, Hilton M, Amos Q, Henzel WJ (December 2003). "Cup is an eIF4E binding protein required for both the translational repression of oskar and the recruitment of Barentsz". The Journal of Cell Biology. 163 (6): 1197–204. doi:10.1083/jcb.200309088. PMC 2173729. PMID 14691132.
3. Chekulaeva M, Hentze MW, Ephrussi A (February 2006). "Bruno acts as a dual repressor of oskar translation, promoting mRNA oligomerization and formation of silencing particles". Cell. 124 (3): 521–33. doi:10.1016/j.cell.2006.01.031. PMID 16469699.
4. Jambor H, Mueller S, Bullock SL, Ephrussi A (April 2014). "A stem-loop structure directs oskar mRNA to microtubule minus ends". RNA. 20 (4): 429–39. doi:10.1261/rna.041566.113. PMC 3964905. PMID 24572808.
5. Kistler KE, Trcek T, Hurd TR, Chen R, Liang FX, Sall J, et al. (September 2018). "Drosophila primordial germ cells". eLife. 7. doi:10.7554/eLife.37949. PMC 6191285. PMID 30260314.
6. Wang JT, Seydoux G (July 2014). "P granules". Current Biology. 24 (14): R637–R638. doi:10.1016/j.cub.2014.06.018. PMC 4966529. PMID 25050955.
7. Jeske M, Bordi M, Glatt S, Müller S, Rybin V, Müller CW, Ephrussi A (July 2015). "The Crystal Structure of the Drosophila Germline Inducer Oskar Identifies Two Domains with Distinct Vasa Helicase- and RNA-Binding Activities". Cell Reports. 12 (4): 587–98. doi:10.1016/j.celrep.2015.06.055. PMID 26190108.