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 non‐ribosomal proteins attach to the rRNA. 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 harbour 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 non‐ribosomal 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 non‐ribosomal 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 that 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 with permission from Olson MOJ, Dundr M and Szebeni A (2000) © Elsevier
Figure 2. Principal steps in vertebrate ribosome assembly. The process begins in the nucleolus with transcription of 47S pre‐rRNA. During and after transcription, non‐ribosomal 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. An overview of ribosome biogenesis, both under normal and stress conditions. (a) Under basal conditions, rRNA is transcribed and processed in the nucleolus. RPs are imported into the nucleolus, where they are assembled together with processed rRNA into the large and small ribosomal subunits (60S and 40S, respectively). Mdm2 binds p53 and polyubiquitinates it, sending it to proteasomal degradation, possibly assisted by nucleolar‐mediated export. (b) Inhibition of different steps in ribosome biogenesis can cause unassembled RPs an 5S rRNA to bind Mdm2 and prevent p53 degradation. Reproduced with permission from Golomb et al., © 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 with permission from Olson © Springer.
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Olson, Mark OJ, and Dundr, Miroslav(Feb 2015) Nucleolus: Structure and Function. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005975.pub3]