Localization of c-mos mRNA around the animal pole in the zebrafish oocyte with Zor-1/Zorba.

In oocytes, many maternally supplied products are stored, and these products play important roles in cell cycle regulation and early development. Mos protein, which is coded on the c-mos gene, promotes oocyte maturation and is involved in MAP-kinase signaling pathway. In Xenopus, maternally supplied c-mos mRNA undergoes poly(A) addition, and translational activation via CPE (cytoplasmic polyadenylation element) and CPEB (CPE binding protein). The elongated poly(A) is shortened and the c-mos mRNA is degraded during early embryogenesis via EDEN (embryo deadenylation element) and EDEN-BP (EDEN-binding protein). We cloned the full-length zebrafish c-mos gene, which is conserved at the protein coding region in vertebrates. c-mos mRNA has two putative CPE sequences in its 3'UTR, which binds to zebrafish CPEB homologous protein, Zor-1. We could not observe EDEN sequence, and could not detect interaction between c-mos mRNA and zebrafish EDEN-BP homologous protein, Brul, even though immuno precipitation and RT-PCR experiments suggested that c-mos mRNA interacts with Zor-1 in vivo. Interestingly, we found c-mos mRNA is located in the animal cortex of zebrafish oocyte, where Zor-1 protein exists. Taken together, these results suggest that the animal cortex is the central core of oocyte maturation in zebrafish.