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.
Small nucleolus-related bodies which occur in the nUcleoplasm of " micronuclei " lacking nucleolar organizers have been studied by immunofluorescence microscopy . These bodies stained specifically with three different antibodies directed against proteins that are normally associated with the dense fibrillar component of functional nucleoli, but not with antibodies specific for certain proteins of the granular component or the fibrillar centers . Our data show that, in the absence of rRNA genes , the various constituent proteins characteristic of the dense fibrillar component spontaneously assemble into spherical entities but that the subsequent fusion of these bodies into larger structures is prevented in these micronuclei . The similarity between these nucleolus-related bodies of micronuclei and the prenucleolar bodies characteristic of early stages of nucleologenesis during mitotic telophase is discussed.
Nucleolin, a major nucleolar phosphoprotein, is presumed to function in rDNA transcription, rRNA packaging and ribosome assembly. Its primary sequence was highly conserved during evolution and suggests a multi-domain structure. To identify structural elements required for nuclear uptake and nucleolar accumulation of nucleolin, we used site-directed mutagenesis to introduce point- and deletion-mutations into a chicken nucleolin cDNA. Following transient expression in mammalian cells, the intracellular distribution of the corresponding wild-type and mutant proteins was determined by indirect immunofluorescence microscopy. We found that nucleolin contains a functional nuclear localization signal (KRKKEMANKSAPEAKKKK) that conforms exactly to the consensus proposed recently for a bipartite signal (Robbins, J., Dilworth, S.M., Laskey, R.A. and Dingwall, C. (1991) Cell 64, 615–623). Concerning nucleolar localization, we found that the N-terminal 250 amino acids of nucleolin are dispensible, but deletion of either the centrally located RNA-binding motifs (the RNP domain) or the glycine/arginine-rich C terminus (the GR domain) resulted in an exclusively nucleoplasmic distribution. Although both of these latter domains were required for correct subcellular localization of nucleolin, they were not sufficient to target non-nucleolar proteins to the nucleolus. From these results we conclude that nucleolin does not contain a single, linear nucleolar targeting signal. Instead, we propose that the protein uses a bipartite NLS to enter the nucleus and then accumulates within the nucleolus by virtue of binding to other nucleolar components (probably rRNA) via its RNP and GR domains.
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