Ribosome assembly is a complex process, the most important features of which are outlined in Figure 6–45. In addition to its central role in ribosome biogenesis, the nucleolus is the site where other noncoding RNAs are produced and other
RNA–protein complexes are assembled. For example, the U6 snRNP, which func tions in pre-mRNA splicing (see Figure 6–28), is composed of one RNA molecule and at least seven proteins. The U6 snRNA is chemically modified by snoRNAs in the nucleolus before its final assembly there into the U6 snRNP. Other important RNA–protein complexes, including telomerase (encountered in Chapter 5) and the signal-recognition particle (which we discuss in Chapter 12), are assembled at the nucleolus. Finally, the tRNAs (transfer RNAs) that carry the amino acids for protein synthesis are processed there as well; like the rRNA genes, the genes encoding tRNAs are clustered in the nucleolus. Thus, the nucleolus can be thought of as a large factory at which different noncoding RNAs are transcribed, processed, and assembled with proteins to form a large variety of ribonucleoprotein complexes.
Figure 6–44 Nucleolar fusion. These light micrographs of human fibroblasts grown in culture show various stages of nucleolar fusion. After mitosis, each of the 10 human chromosomes that carry a cluster of rRNA genes begins to form a tiny nucleolus, but these rapidly coalesce as they grow to form the single large nucleolus typical of many interphase cells. (Courtesy of E.G. Jordan and
J. McGovern.)
The Nucleus Contains a Variety of Subnuclear Aggregates
Although the nucleolus is the most prominent structure in the nucleus, several other nuclear bodies have been observed and studied (Figure 6–46). These include Cajal bodies (named for the scientist who first described them in 1906) and interchromatin granule clusters (also called “speckles”). Like the nucleolus, these other nuclear structures lack membranes and are highly dynamic depending on the needs of the cell. Their assembly is likely mediated by the association of low complexity protein domains, as described in Chapter 3 (see Figure 3–36). Their appearance is the result of the tight association of protein and RNA components involved in the synthesis, assembly, and storage of macromolecules involved in gene expression. Cajal bodies are sites where the snRNPs and snoRNPs undergo their final maturation steps, and where the snRNPs are recycled and their RNAs are “reset” after the rearrangements that occur during splicing (see p. 321). In contrast, the interchromatin granule clusters have been proposed to be stockpiles of fully mature snRNPs and other RNA processing components that are ready to be used in the production of mRNA.
Scientists have had difficulties in working out the function of these small sub-nuclear structures, in part because their appearances can change dramatically as cells traverse the cell cycle or respond to changes in their environment. Moreover,
Figure 6–45 The function of the nucleolus in ribosome and other ribonucleoprotein synthesis. The 45S precursor rRNA is packaged in a large ribonucleoprotein particle containing many ribosomal proteins imported from the cytoplasm. While this particle remains at the nucleolus, selected components are added and others discarded as it is processed into immature large and small ribosomal subunits. The two ribosomal subunits attain their final functional form only after each is individually transported through the nuclear pores into the cytoplasm. Other ribonucleoprotein complexes, including telomerase shown here, are also assembled in the nucleolus.