Nucleolus: Structure and Function

Abstract

The nucleolus is the nuclear subdomain that assembles ribosomal subunits in eukaryotic cells. The nucleolar organiser regions of chromosomes, which contain the genes for pre‐ribosomal ribonucleic acid (rRNA), serve as the foundation for nucleolar structure. The nucleolus disassembles at the beginning of mitosis, its components disperse in various parts of the cell and reassembly occurs during telophase and early G1 phase. Ribosome assembly begins with transcription of pre‐rRNA. During transcription ribosomal and nonribosomal proteins attach to the RNA. Subsequently, there is modification and cleavage of pre‐rRNA and incorporation of more ribosomal proteins and 5S rRNA into maturing pre‐ribosomal complexes. The nucleolus also contains proteins and RNAs that are not related to ribosome assembly and a number of new functions for the nucleolus have been identified. These include assembly of signal recognition particles, sensing cellular stress and transport of human immunodeficiency virus 1 (HIV‐1) messenger RNA.

Key Concepts:

  • The nucleolus, whose primary function is to assemble ribosomes, is the largest structure in the cell nucleus.

  • The nucleolus organiser regions of chromosomes, which contain the genes for pre‐rRNA, are the foundation for the nucleolus.

  • All active nucleoli contain at least two ultrastructural components, the nucleolar dense fibrillar component representing early pre‐ribosomal complexes and the granular component containing more mature pre‐ribosomal particles.

  • Most nucleoli in higher eukaryotes also contain fibrillar centres, which are the interphase equivalents of the nucleolus organiser regions.

  • The nucleolus disassembles at the beginning of mitosis and begins to reassemble in telophase.

  • Ribosome assembly begins with the transcription of pre‐rRNA by RNA polymerase I.

  • Ribosomal and nonribosomal proteins and 5S RNA associate with the pre‐rRNA during and after transcription.

  • The pre‐rRNA is modified and processed into rRNA with the aid of nonribosomal proteins and small nucleolar RNAs.

  • The nucleolus has numerous other functions including assembly of signal recognition particles, modification of transfer RNAs and sensing cellular stress.

Keywords: nucleolus; ribosome biogenesis; pre‐rRNA; RNA processing; SnoRNAs

Figure 1.

Electron micrographs of nucleoli from human HeLa cells showing examples of two different types of subnucleolar organisation of the components in cell nucleoli. (a) Reticulated type of nucleolus, which is often found in transcriptionally active cells. These nucleoli have relatively small fibrillar centres (F) surrounded by strands of dense fibrillar component (D). The nucleolar interstices (I) appear as regions of lower electron density and contain a few strands of intranucleolar chromatin. The small punctate granular components (G) are scattered throughout the nucleolus. (b) More common form of structural organisation, in which the nucleoli have large and prominent fibrillar centres (F) surrounded by dense fibrillar components (D). Relatively large areas are covered by the granular components (G), which are more clearly separated from the dense fibrillar components than in reticulated nucleoli. In (b) a Cajal body (arrowheads) seems to be attached to the nucleolus through clumps of perinucleolar chromatin, which were detected by immunogold staining using an antibody against single‐stranded DNA. Reproduced from Olson MOJ, Dundr M and Szebeni A (2000) The nucleolus: an old factory with unexpected capabilities. Trends in Cell Biology10: 189–196, with permission from Elsevier. Scale bars, 200 nm.

Figure 2.

Principal steps in vertebrate ribosome assembly. The process begins in the nucleolus with transcription of 47S pre‐rRNA. During and after transcription, nonribosomal proteins and snoRNAs associate with the pre‐rRNA transcript. Methylation and pseudouridylation of the nascent pre‐rRNA is guided by the snoRNAs. 5S rRNA, which is synthesised in the nucleoplasm and ultimately becomes a component of the 60S subunit, is added to the maturing complex at an undefined stage. The pre‐rRNA undergoes a series of cleavages that result ultimately in 18S, 5.8S and 28S rRNAs (see Figure ). The complex is split into two precursor particles corresponding to the small (40S) and large (60S) ribosomal subunits. Ribosomal proteins are added to the precursor complexes at various stages of assembly. The nearly mature subunits are transported to the cytoplasm, where final maturation takes place and the small and large subunits are incorporated into ribosomes.

Figure 3.

Features of pre‐rRNA processing in vertebrates. The pathway shown is compiled from reviews by Eichler and Craig and Sollner‐Webb et al.. The sizes of the intermediates are those found in mouse; human cells follow a very similar but slightly different pathway. The 47S pre‐rRNA transcript contains a 5′ external transcribed spacer (5′ ETS) region, two internal transcribed spacer (ITS1 and ITS2) segments and a 3′ external transcribed spacer (3′ ETS) region, all of which are removed to generate 18S, 5.8S and 28S rRNAs. The first processing event is indicated by the arrow near the 5′ end of the 47S transcript. The mature rRNAs are produced by sequential endonuclease cleavage, with some of the mature rRNA termini generated by exonuclease digestion. The distribution of the mature rRNAs in the ribosomal subunits is shown below. The 5.8S rRNA is associated with the 5′ end of 28S rRNA through hydrogen bonding. It associates with ribosomal protein L5 before being incorporated into the large ribosomal subunit.

Figure 4.

Nontraditional roles of the nucleolus. Several examples are illustrated in the figure. A variety of cellular processes occur in the nucleolus including tRNA maturation and partial assembly of signal recognition particles (SRPs) and telomerase complexes. The yeast cell cycle is regulated through nucleolar sequestration of the cdc14p protein phosphatase in the RENT complex, which also contains the proteins Sir2p and Net1p. The activity of tumour‐suppressor protein p53 is also regulated by sequestration in the nucleolus of components that control its degradation (MDM2 and E2F1), which bind to the ARF protein. The HIV‐1 Rev protein interacts with protein B23 in the nucleolus and also recruits the nuclear export factor hCRM1 and nucleoporins Nup98 and Nup214 to the nucleolus. Proteins Sir3p and Sir4p relocate from telomeres to nucleoli during aging in yeast, presumably in response to accumulation of extrachromosomal circles of rDNA. Sir2p also participates in chromatin silencing and suppression of extrachromosomal circle formation. Reproduced from Olson et al., with permission from Elsevier.

Figure 5.

The HIV‐1 Rev protein accumulates in the nucleoli of HIV‐1 infected cells. HeLa cells were transfected with the HIV‐1 molecular clone PNL4‐3, which expresses the Rev protein in a cascade of viral progression. Twenty‐four hours after transfection the HIV‐1 Rev protein detected by a specific antibody is localised predominantly in the nucleoli (green). Viral replication was monitored by an antibody against the HIV‐1 p24‐core protein (red). The overlay of all three labellings is shown in the lower right panel. Bar, 2 μm. Reproduced from Olson , with permission from Springer.

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Busch H and Smetana K (1970) The Nucleolus. New York: Academic Press.

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Henras AK, Soudet J and Gérus M (2008) The post‐transcriptional steps of eukaryotic ribosome biogenesis. Cellular and Molecular Life Sciences 65: 2334–2359.

Paule M (1998) Transcription of Eukaryotic ribosomal RNA Genes by RNA polymerase I. New York: Springer.

Politz JC, Yarovoi S, Kilroy SM et al. (2000) Signal recognition particle components in the nucleolus. Proceedings of the National Academy of Sciences of the USA 97: 55–60.

Scheer U and Hock R (1999) Structure and function of the nucleolus. Current Opinion in Cell Biology 11: 385–390.

Wolin SL and Matera AG (1999) The trials and travels of tRNA. Genes & Development 13: 1–10.

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Olson, Mark OJ(Dec 2010) Nucleolus: Structure and Function. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005975.pub2]