A constitutive nucleolar protein identified as a member of the nucleoplasmin family.

Abstract

Using monoclonal antibodies we have localized a polypeptide, appearing on gel electrophoresis with a Mr of approximately 38,000 and a pI of approximately 5.6, to the granular component of the nucleoli of Xenopus laevis oocytes and a broad range of cells from various species. The protein (NO38) also occurs in certain distinct nucleoplasmic particles but is not detected in ribosomes and other cytoplasmic components. During mitosis NO38‐containing material dissociates from the nucleolar organizer region and distributes over the chromosomal surfaces and the perichromosomal cytoplasm; in telophase it re‐populates the forming nucleoli. With these antibodies we have isolated from a X. laevis ovary lambda gt11 expression library a cDNA clone encoding a polypeptide which, on one‐ and two‐dimensional gel electrophoresis, co‐migrates with authentic NO38. The amino acid sequence deduced from this clone defines a polypeptide of 299 amino acids of mol. wt 33,531 which is characterized by the presence of two domains exceptionally rich in aspartic and glutamic acid, one of them flanked by two putative karyophilic signal heptapeptides. Comparison with other protein sequences shows that NO38 is closely related to the histone‐binding, karyophilic protein nucleoplasmin: the first 124 amino acids have 58 amino acid positions in common. Protein NO38 also shows striking homologies to the phosphopeptide region of rat nucleolar protein B23 and the carboxyterminal region of human B23. We propose that protein NO38, which forms distinct homo‐oligomers of approximately 7S and Mr of approximately 230,000, is a member of a family of karyophilic proteins, the ‘nucleoplasmin family’. It is characterized by its specific association with the nucleolus and might be involved in nuclear accumulation, nucleolar storage and pre‐rRNA assembly of ribosomal proteins in a manner similar to that discussed for the role of nucleoplasmin in histone storage and chromatin assembly.

A Novel Helicase-Type Protein in the Nucleolus: Protein NOH61

“…Even after longer exposure, protein NOH61 could not be detected in fractions of precursors for the 40S small ribosomal subunit. In contrast, nucleolar proteins NO38 ( Figure 5C) and NO29 could be identified as components of both preribosomal particles as previously described (Schmidt-Zachmann et al, 1987;Zirwes et al, 1997). It should be noted that protein NOH61 was not detectable in mature cytoplasmic ribosomes, as determined by immunoblotting experiments on 40S and 60S ribosomal subunits and by immunocytochemistry (our unpublished results).…”

“…In metaphase and anaphase, i.e., upon desintegration of nucleolar structures, immunofluorescence was noted on the surfaces of the chromosomes, in addition to a diffuse staining of the perichromosomal cytoplasm (our unpublished results). This pattern of distribution during mitosis is similar to the mitotic behavior described for proteins NO38 and NO29 (Schmidt-Zachmann et al, 1987;Zirwes et al, 1997) and is characteristic for a subset of nucleolar proteins, primarily those located in the granular component of the nucleolus (Olson, 1990). Because protein NOH61 belongs to the family of putative ATP-dependent RNA helicases, we have analyzed its intracellular distribution after treatment with nucleases and the transcription inhibitor AMD (Figure 8).…”

“…The protein was only present in trace amounts in the HSS fraction containing soluble nuclear proteins, not detectable in the egg extract, but was present again in somatic cells ( Figure 4B). The identity and integrity of the fractions were ascertained by reprobing the nitrocellulose filter with mAb No-185 directed against nucleolar protein NO38 (Schmidt- Zachmann et al, 1987), known to be enriched in the LSP and HSP but absent from the HSS fraction ( Figure 4C; cf. Schmidt-Zachmann et al, 1998).…”

Apoptosis induced by the myelodysplastic syndrome-associated NPM-MLF1 chimeric protein

“…NPM is a highly conserved nonribosomal RNAbinding nucleolar phosphoprotein, one of which function is a shuttle protein, transporting ribosomal ribonucleoproteins (rRNPs) between the nucleolus and the cytoplasm during the assembly of ribosomes (Schmidt-Zachmann et al, 1987;Borer et al, 1989). NPM forms a hexameric homo-oligomer (assembled in a head-to-head/tail-to-tail fashion) as it shuttles between subcellular compartments (Yung and Chan, 1987;Schmidt-Zachmann and Franke, 1988;Liu and Chan, 1991).…”

Evidence for the ability of nucleophosmin/B23 to bind ATP

“…Nucleophosmin\B23, also called protein B23, NO38 or numatrin [1][2][3], is a major nucleolar phosphoprotein that is significantly more abundant in tumour and proliferating cells than in normal resting cells [3,4]. Nucleophosmin\B23 is localized in granular regions of the nucleolus [5], associated with preribosomal particles [1,6], and forms hexamers [7] that might be essential for the assembly of ribosomes.…”