Molecular characterization of a novel nucleolar protein in starfish oocytes which is phosphorylated before and during oocyte maturation

In response to 1-methyladenine, a maturation-inducing substance, starfish o ocytes undergo reinitiation of meiosis with germinal vesicle breakdown thro ugh activation of p34(cdc2)-cyclin B, which results in the dispersal of the nucleolus. Little information has been elucidated thus far on nucleolar pr oteins that are phosphorylated by p34cdc2-cyclin B during meiotic maturatio n. Here, we describe a novel nucleolar protein of the starfish Asterina pec tinifera oocyte, which is designated ANO39 and which is phosphorylated duri ng meiotic maturation. A full-length ANO39 cDNA of 2106 base pairs encodes a polypeptide of 346 amino acids having a calculated M-r of 39 005. The amo unt of ANO39 is kept nearly constant during oocyte maturation and embryogen esis up to the midgastrula stage. The transcript encoding ANO39 was present in growing oocytes but not in full-grown ones, as evidenced by Northern bl ot hybridization. Ser145 is specifically phosphorylated when ANO39 is incub ated in vitro with purified starfish p34cdc2-cyclin B. This phosphorylation site corresponds to that is phosphorylated during meiotic maturation in vi vo. Immunoblot analysis using phosphoserine145-specific antibody as a probe revealed that some populations of ANO39 of the immature oocytes at the G(2 ) stage have been already phosphorylated on Ser145 and Ser145 is maximally phosphorylated during meiotic maturation.