An3 mRNA Encodes an RNA Helicase that Colocalizes with Nucleoli in Xenopus Oocytes in a Stage-Specific Manner

An3 is a maternal mRNA localized to the animal hemisphere of oocytes and early embryos. We have analyzed the enzymatic activity and the subcellular localization of the protein encoded by An3 mRNA during Xenopus oogenesis. Antibodies raised using recombinant full-length and truncated An3 protein recognized a single protein in Xenopus and single proteins from HeLa cells, Drosophila, mouse testes, and Saccharomyces cerevisiae. An3 protein immunoprecipitated from stage IV and stage VI oocytes had ATP-dependent RNA helicase activity. The subcellular location of An3 protein changed during oocyte development. In previtellogenic oocytes, An3 was present throughout the nucleus; cytoplasmic localization was relatively sparse. Nuclear localization in midvitellogenic oocytes was primarily nucleolar; cytoplasmic staining increased relative to earlier stages. In stage VI oocytes, An3 protein was detected only in the cytoplasm. The temporal change in An3 protein localization is consistent with a role in the production of large maternal pools of rRNA during oogenesis.