Identification of an amphibian oocyte nuclear protein as a candidate for a role in embryonic DNA replication

Monoclonal antibodyB(24/3) recognizes a nuclear protein of 104 kD in germinal vesicles of newt oocytes. Immunohistostaining of oocytes at different stages of growth shows an accumulation of B₂₄ protein throughout oogenesis. During developmentd B₂₄ protein is located inside embryonic cell nuclei from the onset of cleavage onwards. It gradually decreases from gastrulation and disappears at the tailbud stage. The NvB₂₄ 17.1clone was isolated from an ovary expression library of the newt Notophthalmus viridescens and then sequenced: the open reading frame is capable of encoding a polypeptide of 744 amino acids. Northern blot experiments have shown that the 17.1 clone recognizes a single transcript of about 3 Kb in the ovary. In situ hybridization experiments showed that B₂₄ mRNA transcription starts from previtellogenic oocytes, and is followed by the appearance and gradual accumulation of B₂₄ protien in germinal vesicles of medium and large size oocytes. Keeping in mind the sequence similarity shown by the B₂₄ protien to the mouse P1 protein as well as to the budding yeast Mcm3 and fission yeast cdc21 proteins, B₂₄ protein can be speculated to play a role in the events of DNA replication during early amphibian embryogenesis. As B₂₄ antigen is located in the sphere organelles both inserted on the lampbrush chromosomes and free in the oocyte nucleoplasm, an additional possible role of B₂₄ protein could be related to assembling and/or storing snRNPs during oogenesis.