The Dynamic Proteome of the Nucleolus

The primary function of the nucleolus is as the site of ribosome subunit biogenesis in eukaryotic cells. Nucleoli reassemble at the end of mitosis around the tandemly repeated clusters of rDNA genes forming a subnuclear compartment that locally concentrates the dedicated transcription and processing machineries that are responsible for generating ribosome subunits. The process of assembling a ribosome subunit requires the initial transcription of the ribosomal DNA (rDNA) genes by a specialized RNA polymerase – RNA pol I. These rDNA genes are arranged in arrays of head-to-tail tandem repeats, termed nucleolar organizer regions (NORs). In humans, approximately 400 copies of 43-kb repeat units are distributed along all acrocentric chromosomes (chromosomes 13, 14, 15, 21 and 22) to form NORs. In many cell types, only a subset of rDNA genes are transcriptionally active, even though inactive rDNAs are still assembled into nucleoli. The initial 47S ribosomal RNA (rRNA) precursor transcript transcribed by RNA pol I is subsequently cleaved to form the mature 28S, 18S and 5.8S rRNAs, post-transcriptionally modified through interaction with small nucleolar ribonucleoproteins (snoRNPs) and additional protein processing factors. Finally, the processed and modified rRNAs are assembled with the many ribosomal proteins, prior to interaction with the export machinery and transport to the cytoplasm. The isolation and characterization of organelles by subcellular fractionation is a well-established technique in cell biology. Many organelles have been isolated and analysed in the past century (see, e.g. Spector et al. (1997) for reviews and protocols). These studies have provided invaluable information on the functions and properties of individual organelles. With recent advances in mass spectrometry based proteomic