DNA cloning and amino acid sequence determination of a major constituent protein of mammalian nucleoli. Correspondence of the nucleoplasmin-related protein NO38 to mammalian protein B23.

Using a cDNA probe encoding the nucleolar protein NO38 of Xenopus laevis, we have isolated clones that code for the corresponding mammalian protein from cDNA libraries of mouse embryonal carcinoma and fetal liver cells. The murine cDNA-derived amino acid sequence defines a polypeptide of 292 amino acids (including the initial methionine) of a total molecular weight of 32,560 and identifies a single approximately 1.5 kb mRNA on Northern blot hybridization. This polypeptide, which is highly homologous to the Xenopus protein NO38, displays an organization in three major domains: (1) an aminoterminal portion of 119 amino acids, which shows a striking homology to nucleoplasmin of Xenopus; (2) a central portion of 68 amino acids that contains two extended acidic domains, a shorter of 13 residues and a longer of 29 residues, separated by an interval enriched in positively charged amino acids; (3) a carboxyterminal portion of 105 amino acids, which is almost identical to the reported partial amino acid sequence of human and rat nucleolar protein termed B23. The sequence comparisons show that the murine protein is the mammalian counterpart to the nucleolar protein NO38 of Xenopus and is compatible with the idea that both proteins NO38 represent the amphibian and murine equivalents to the human and rat nucleolar phosphoprotein B23. Special sequence features and predicted conformations of this protein are discussed in relation to the specific localization and the possible functions of this major nucleolar protein.