Identification of OORP-T, a novel oocyte-specific gene encoding a protein with a conserved oxysterol binding protein domain in rainbow trout.

Genes specifically expressed in oocytes are important for the development of oocytes and early embryos. By analyzing expressed sequence tags (ESTs) from a rainbow trout oocyte cDNA library, we identified a novel EST sequence that does not show homology to any sequences in the GenBank. Analysis of tissue distribution by RT-PCR revealed that this gene was only expressed in unfertilized oocytes. Sequencing of the EST clone identified a cDNA of 3,163 bp. Northern blot analysis showed the novel gene has a single transcript of 3.4 kb. Additional 5' sequence was obtained by 5' RACE, extending the novel cDNA to 3,333 bp. Analysis of the full-length cDNA identified an open reading frame (ORF) encoding a protein of 564 amino acids. The novel protein contains a conserved oxysterol binding protein (OSBP) domain at the C terminus that is characteristic of OSBP-related proteins (ORPs) implicated in lipid metabolism. Therefore, we named the novel gene as Oocyte-specific Oxysterol binding protein Related-Protein of Trout (OORP-T). In situ hybridization showed that the OORP-T mRNA appears to be confined to the cytoplasm of vitellogenic oocytes. Transcription of OORP-T appears to start during pre-vitellogenesis and increases steadily, reaching its peak in the late vitellogenic stage. OORP-T transcript is abundantly present in unfertilized eggs but the level drops significantly in day 2 embryos and continues to decline in day 7 embryos after which it remains low. We propose that OORP-T may play an important role in the utilization of yolk-derived lipid products during oocyte development and early stages of embryonic development in rainbow trout.