Requirements for nuclear translocation and nucleolar accumulation of nucleolin of Xenopus laevis

The intracellular localization of germinal vesicle proteins is regulated during early Xenopus development. Here we have analyzed the determinants that control the localization of nucleolin, a nucleolar protein of vertebrates. Our immunological analyses and isolation of cDNAs have revealed the presence of a second nucleolin gene in addition to that previously identified. The two nucleolin polypeptides of 95 kDa and 90 kDa molecular mass are both expressed in oocytes and are found predominantly in the nucleoli. During oocyte maturation, both polypeptides are hyperphosphorylated and found distributed throughout the cytoplasm. Hyperphosphorylated forms and cytoplasmic localization of nucleolin both prevail in early embryos up to the midblastula transition. Subsequently, the proteins are found in the nuclei. Accumulation in nucleoli is only detected at later stages, beginning with gastrulation. Thus nucleolar localization is temporally uncoupled from nuclear translocation. Consistent with these observations, our molecular analyses have revealed that the nuclear location signal which is present in nucleolin and which is sufficient for nuclear location, is not sufficient for nucleolar accumulation. Nucleolar accumulation requires, in addition to the nuclear location signal, the presence of the RNA binding domains and of an RG-rich domain, which is also thought to interact with RNA.