Conventional and nonconventional roles of the nucleolus

Abstract

As the most prominent of subnuclear structures, the nucleolus has a well-established role in ribosomal subunit assembly. Additional nucleolar functions, not related to ribosome biogenesis, have been discovered within the last decade. Built around multiple copies of the genes for preribosomal RNA (rDNA), nucleolar structure is largely dependent on the process of ribosome assembly. The nucleolus is disassembled during mitosis at which time preribosomal RNA transcription and processing are suppressed; it is reassembled at the end of mitosis in part from components preserved from the previous cell cycle. Expression of preribosomal RNA (pre-rRNA) is regulated by the silencing of individual rDNA genes via alterations in chromatin structure or by controlling RNA polymerase I initiation complex formation. Preribosomal RNA processing and posttranscriptional modifications are guided by a multitude of small nucleolar RNAs. Nearly completed ribosomal subunits are exported to the cytoplasm by an established nuclear export system with the aid of specialized adapter molecules. Some preribosomal and nucleolar components are transiently localized in Cajal bodies, presumably for modification or assembly. The nonconventional functions of nucleolus include roles in viral infections, nuclear export, sequestration of regulatory molecules, modification of small RNAs, RNP assembly, and control of aging, although some of these functions are not well established. Additional progress in defining the mechanisms of each step in ribosome biogenesis as well as clarification of the precise role of the nucleolus in nonconventional activities is expected in the next decade.

NuMA Influences Higher Order Chromatin Organization in Human Mammary Epithelium

“…1996; Dillon and Festenstein, 2002;Olson et al, 2002;Garagna et al, 2004). Examination of the immunostaining for heterochromatin marker H4K20m (Schotta et al, 2004) in acinar HMECs, both in 3D culture and on sections of normal breast tissue, indicated that the perinucleolar concentration of heterochromatin is also a trait of the differentiation of HMECs ( Figure 4B).…”

An architectural perspective of vitamin D responsiveness

“…The components of higher order nuclear architecture, including nuclear pores [22], the nuclear matrix, and subnuclear domains, contribute to the subnuclear distribution and activities of genes and regulatory factors [21,23]. Compartmentalization of regulatory complexes is illustrated by focal organization of PML bodies [24], Runx bodies [25,26], the nucleolus [27], and chromosomes [28], as well as by the punctate intranuclear distribution of sites for replication [29], DNA repair [30], transcription [31], and the processing of gene transcripts [32][33][34]. There is emerging recognition that nuclear structure and function are casually related.…”

The nucleolus: reviewing oldies to have new understandings

“…Thus, many non-traditional functions of the nucleolus have been proposed [5][6][7]9]. These include signal recognition particle assembly, small RNA modification, RNA editing, telomerase maturation, nuclear export, cell cycle control, and stress sensor [17][18][19][20][21][22]. The nucleolar proteins unrelated to ribosome assembly mostly contain an RNA-binding motif or have a chaperone function.…”