The Nucleolus is probably the most striking and well studies sub-compartment of the eukaryotic nucleus that, in addition to its role in ribosomal RNA biogenesis, has been implicated in control of cellular survival and proliferation (reviewed in Carmo-Fonseca et al., 2000) . Mammalian nuclei usually contain between 1-4 nucleoli, which together can take up as much as a third of the nuclear volume. As a structure the nucleolus consists of three morphologically distinct components: the fibrillar centres (FC), which contain hundreds of rRNA genes in tandem arrays found at several chromosomal loci (termed nucleolar organising regions (NORs) ; the dense fibrillar component (DFC), which contains actively transcribing rRNA genes and nascent rRNA transcripts; and the granular component (GC), which is the site of late processing events in the

biogenesis of rRNAs (Fig. 1; Carmo-Fonseca et al., 2000) .

The Nucleolus is a Dynamic Structure

The maintenance of nucleolar structure is intimately coupled to on-going RNA polymerase I transcription, which when inhibited leads to a rapid and dramatic re-organisation of this nuclear organelle (reviewed in Olson et al., 2000). In addition, the RNA polymerase II inhibitor DRB produces a dramatic reorganisation of the nucleolus, whereby the rRNA genes form bead-like structures, possibly implicating pol II transcription in nucleolar organisation. As a caveat, it must be noted that DRB is a kinase inhibitor (i.e. casein kinase II), which suggests that the morphological changes caused by DRB may not be directly linked to the inhibition of pol II transcription.

Although yeast nucleoli remain intact during mitosis, the nucleoli of metazoans are highly dynamic and undergo rounds of dissasembly and reassembly during the cell cycle (Carmo-Fonseca et al., 2000; Olson et al., 2000). Upon entry into mitosis, an ordered disassembly of first the GC and then the DFC of the nucleolus occurs. At this time certain nucleolar components (e.g. UBF, a key regulator of pol I transcription) remain attached to chromosomal NORs. A number of nucleolar proteins, including

Nucleolar Ultranstructure

A) Mammalian nucleoli typically contain several fibrillar centres (FC), each encircled by a layer of dense fibrillar component (DFC), which in turn is surrounded by the granular component (GC). B) By combining cryofixation and cryosubstitution, it is possible to identify morphological subcompartments in the nucleolus of S. cerevisiae that are similar to those of nucleoli of higher eukaryotes. (Fig. 1. Carmo-Fonseca et al., 2000 ).
Click to enlarge
(Original image courtesy of I. Léger-Silvestre and N. Gas).
pre-rRNA processing components, associate with mitotic chromosomes during prophase but eventually localise to extrachromosomal nucleolus-derived foci (NDF) (e.g. nucleolin ) during the progression from anaphase to telophase. During late telophase NDF disappear and some processing components become associated with pre-nucleolar bodies (PNBs) (e.g. fibrillarin), which appear prior to the initiation of pol I transcription. Once RNA pol I transcription begins, PNBs (or their components) are recruited to NORs resulting in the formation of the nucleolus.

Nucleolar associated structures: the Perinucleolar Compartment, Sam68 and Cajal bodies

Several sub-nuclear compartments are often associated with nucleoli, including the Cajal and Sam68 bodies, as well as the perinucleolar compartment (PNC). Originally called the nucleolar accessory body, Cajal bodies are often found in close proximity or juxtaposed to nucleoli. Many Cajal body proteins are also found in nucleoli including p80 coilin, fibrillarin and Nopp140. It is believed that Cajal bodies play a role in snRNP biogenesis and in the trafficking of snoRNPs and snRNPs, which appear to move through the Cajal body en route to nucleoli or splicing speckles (respectively) (Sleeman and Lamond 1999) .

The PNC, first described by the localisation of the polypyrimidine tract binding protein (PTB), is a dynamic perinucleolar structure predominately observed in transformed cells (reviewed in Huang et al., 2000). The presence of hnRNP proteins, splicing factors and snRNAs in the PNC suggests that this structure is involved in multiple aspects of RNA metabolism. The PNC is also enriched in RNase MRP and RNase P RNAs, which have been shown to be involved in rRNA processing. Therefore, the PNC may act as a resevoir for these molecules during rRNA biogenesis in transformed cells. Equally enigmatic are the Sam68 bodies, which are also found predominately in transformed cell lines and are implicated in RNA metabolism (Huang et al., 2000). The significance of the association of these bodies with nucleoli is unknown.

The Nucleolus: Storage Site or Cell Cycle Regulator?

Beyond its role in ribosomal RNA biogenesis, the nucleolus appears to act as a storage site or reservoir for a number of proteins that do not have roles in rRNA metabolism (Carmo-Fonseca et al., 2000; Olson et al., 2000) . The nucleolar sequestration of proteins appears to be a common means of cell cycle regulation among eukaryotes. For example, the tumour suppressor ARF, whose expression is upregulated by oncogenic proteins such as RAS , Myc and E1A, is a nucleolar protein (Tao and Levine, 1999). ARF can bind and recruit MDM2 to the nucleolus thereby preventing the MDM2-dependent degredation of p53, which is normally exported to the cytoplasm for degradation in association with MDM2. Increased levels of p53 can lead to cell cycle arrest. Therefore, nucleolar anchoring of MDM2 by ARF provides one means of regulating the cell cycle.

Similarly, in S. cerevisiae the exit from mitosis is regulated by the nucleolar sequestration of the protein phosphatase Cdc14p, which is released from the nucleolus to promote both the degradation of the cyclin subunit Clb and the accumulation of protein-kinase inhibitor during late anaphase (Visintin et al., 1999). However, this mechanism for controlling mitotic exit is specific for lower eukaryotes like yeast, because as discussed above, nucleoli are dispersed during mitosis in metazoan nuclei.

The Nucleolus and Disease

Nucleolar porteins are know to be mutated in a number of disease including TCOF in Treacher Collins (Marsh et al., 1998) and ataxin 7 in a form of spinocerevellar ataxia (Kaytor et al., 1999). In addition, several proteins that localise to the nucleolus are involved in cancer including both the Werner's syndrome (WRN) and the Bloom's syndrome (BLM) gene products, which are DNA helicases that affect genome stability (reviewed in Mohaghegh and Hickson, 2001). Furthermore, nucleolar proteins are common auto-antigens in patients with hepatocellular carcinoma (HCC), gastrointestinal, lung, and ovarian cancers (Imai et al., 1992) .

  • The Nucleolus--from Gwen Childs

  • EM Images of Nucleoli--University of Mainz EM Atlas

  • Nucleoli Images from the Lamond lab (Dundee, Scotland)

  • Nucleolar Protein Database--Lamond Lab (browse by name, 1D gel slice or domain/motif). This site uses JAVA and FLASH and may load slowly over a dialup connection. Full data table of nucleolar proteinsin PDF format is available, which can be searched by name and Unigene cluster using Adobe Acrobat.

  • Nucleolus Images--From the

  • Published Movies of Nucleolar Movement
    JCB 150:433-- The Dynamics of Postmitotic Reassembly of the Nucleolus. Dundr et al. (2000)

    JCB 153:1097 Nucleolar Assembly of the rRNA Processing Machinery in Living Cells" Savino et al. (2001)


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    Huang, S. (2000) Review: Perinucleolar Structures. J. Struc. Biol. 129(2-3):233-240

    Imai H, Ochs RL, Kiyosawa K, Furuta S, Nakamura RM, Tan EM. (1992) Nucleolar antigens and autoantibodies in hepatocellular carcinoma and other malignancies. Am. J. Pathol. 140(4):859-870

    Kaytor MD, Duvick LA, Skinner PJ, Koob MD, Ranum LP, Orr HT. (1999) Nuclear localization of the spinocerebellar ataxia type 7 protein, ataxin-7. Hum. Mol. Genet. 8(9):1657-1664.

    Marsh KL, Dixon J, Dixon MJ(1998) Mutations in the Treacher Collins syndrome gene lead to mislocalization of the nucleolar protein treacle. Hum. Mol. Genet. 7(11):1795-1800.

    Mohaghegh P, Hickson ID. (2001)DNA helicase deficiencies associated with cancer predisposition and premature ageing disorders. Hum. Mol. Genet. 10(7):741-6

    Olson MO, Dundr M, Szebeni A.(2000) The nucleolus: an old factory with unexpected capabilities. Trends Cell Biol. 10(5):189-196

    Sleeman JE, and Lamond AI. (1999) Newly assembled snRNPs associate with coiled bodies before speckles, suggesting a nuclear snRNP maturation pathway. Curr. Biol. 9(19):1065-1074

    Tao W, Levine AJ.(1999) P19(ARF) stabilizes p53 by blocking nucleo-cytoplasmic shuttling of Mdm2. Proc Natl Acad Sci U S A. 96(12):6937-6941.

    Visintin R, Hwang ES, Amon A.(1999) Cfi1 prevents premature exit from mitosis by anchoring Cdc14 phosphatase in the nucleolus. Nature. 398(6730):818-823.