''oskar'' is a

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

required for the development of the ''

Drosophila ''Drosophila'' (), from Ancient Greek δρόσος (''drósos''), meaning "dew", and φίλος (''phílos''), meaning "loving", is a genus of fly, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or p ...

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

. It defines the posterior pole during early

embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...

. Its two isoforms, short and long, play different roles in Drosophila embryonic development. ''oskar'' was named after the main character from the

Günter Grass Günter Wilhelm Grass (; 16 October 1927 – 13 April 2015) was a German novelist, poet, playwright, illustrator, graphic artist, sculptor, and recipient of the 1999 Nobel Prize in Literature. He was born in the Free City of Danzig (now Gda ...

novel

The Tin Drum ''The Tin Drum'' (, ) is a 1959 novel by Günter Grass, the first book of his Danzig Trilogy. It was adapted into a 1979 film, which won both the 1979 Palme d'Or and the Academy Award for Best Foreign Language Film in 1980. To "beat a ti ...

, who refuses to grow up.

Evolutionary history

''oskar'' displays a unique evolutionary origin resulting from a Horizontal Domain Transfer from a probably bacterial endosymbiont onto an ancestral insect genome. The OSK domain is of bacterial origin and fused with the LOTUS domain through a linker domain. This event must have happened just prior to the divergence with the

Crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

, the insect's sister group, as ''oskar'' can be found as early as the

Zygentoma Zygentoma are an order in the class Insecta, and consist of about 550 known species. The Zygentoma include the so-called silverfish or fishmoths, and the firebrats. A conspicuous feature of the order are the three long caudal filaments. The t ...

but does not seem to exist in

Translational-level regulation

''oskar'' is translationally repressed prior to reaching the posterior pole of the oocyte by

Bruno Bruno may refer to: People and fictional characters * Bruno (name), including lists of people and fictional characters with either the given name or surname * Bruno, Duke of Saxony (died 880) * Bruno the Great (925–965), Archbishop of Cologn ...

, which binds to three bruno response elements (BREs) on the 3' end of the transcribed ''oskar'' mRNA. The Bruno inhibitor has two distinct modes of action: recruiting the

Cup A cup is an open-top vessel (container) used to hold liquids for drinking, typically with a flattened hemispherical shape, and often with a capacity of about . Cups may be made of pottery (including porcelain), glass, metal, wood, stone, pol ...

eIF4E Eukaryotic translation initiation factor 4E, also known as eIF4E, is a protein in humans encoded by the ''EIF4E'' gene. eIF4E plays a central role in translation initiation and is involved in regulating protein synthesis. Its activity influences ...

binding protein, which is also required for ''oskar'' mRNA localization due to interactions with the Barentsz microtubule-linked transporter, and promoting oligomerization of ''oskar'' mRNA. 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.

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. 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. Translational regulation also makes sense due to the granules' close association with

ribosomes Ribosomes () are macromolecular machines, found within all cells, that perform biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA molecules to fo ...

. These P granules are phase-transition entities, which means that they can display both liquid-like and hydrogel-like properties. 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. 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

''oskar'' contains two

RNA-binding protein RNA-binding proteins (often abbreviated as RBPs) are proteins that bind to the double or single stranded RNA in cell (biology), cells and participate in forming ribonucleoprotein complexes. RBPs contain various structural motifs, such as RNA reco ...

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 Helix-turn-helix is a DNA-binding domain (DBD). The helix-turn-helix (HTH) is a major structural motif capable of binding DNA. Each monomer incorporates two alpha helix, α helices, joined by a short strand of amino acids, that bind to the majo ...

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.

References

{{Reflist Developmental genes and proteins Drosophila melanogaster genes