The Nucleolus and the Four Ribonucleoproteins of Translation

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Abstract
The nucleolus is a large and ultrastructurally complex intranuclear structure that typically has the most concentrated mass per unit volume of any region in the cell (Vincent 1955; Goessens 1984; Hadjiolov 1985). Nucleoli arise from the transcriptional activity of the ribosomal RNA genes and contain a multitude of proteins and small RNAs that mediate processing and modification of rRNA and ribonucleoprotein assembly of nascent ribosomal subunits (Warner 1990; Bachellerie et al. 1995; Shaw and Jordan 1995; Maden and Hughes 1997; Scheer and Hock 1999; Weinstein and Steitz 1999). Although the central role of the nucleolus in ribosome biosynthesis remains a resoundingly confirmed principle of eukaryotic molecular cell biology, within two years the nucleolus has been implicated in aspects of eukaryotic cell biology beyond rRNA biosynthesis, i.e., gene silencing, cell cycle progression, and senescence (Pederson 1998a; Cockell and Gasser 1999; Garcia and Pillus 1999). Moreover, it has been found that some RNAs unrelated to rRNA biosynthesis, primarily RNA polymerase III transcripts, also traffic through the nucleolus (Pederson 1998b). In this article, we summarize recent findings that point to an additional important role for the nucleolus in the assembly, and perhaps quality control, of the multiple ribonucleoproteins involved in protein synthesis, rather than simply serving as a site for ribosome synthesis, per se.