12 The Ever-Growing World of Small Nuclear Ribonucleoproteins

Abstract

Small ribonucleoproteins (RNPS)—defined as tight complexes of one or more proteins with a short RNA molecule (usually 60–300 nucleotides)—inhabit every compartment of eukaryotic cells. Those that reside in the nucleus, the small nuclear RNPs (snRNPs), can themselves be divided into several families. There are snRNPs of the nucleoplasm, whose major business is the generation of messenger RNAs for export to the cytoplasm. A different set of snRNPs, called snoRNPs, reside in the cell nucleolus, the subnuclear locale responsible for the synthesis, maturation, and assembly of rRNAs into ribosomal subunits, which are then exported to function in cytoplasmic protein synthesis. A third class (scaRNPs), related both functionally and structurally to snoRNPs, is localized to Cajal bodies, foci in the nucleoplasm that are way stations in the biogenesis of snRNPs and snoRNPs. Thus, vertebrate cells contain hundreds of distinct species of snRNPs with abundances ranging between 10³ (for scaRNPs directing snRNA modification) to over 10⁶ (for snRNPs of the major spliceosome), and more are continually emerging. All of those whose functions have been assigned play roles in gene expression, underscoring the pivotal participation of RNA molecules in the evolution of the gene expression apparatus. The one exception is the telomerase snRNP, essential for genome maintenance (see Chapter 15). Curiously, snRNPs are often the target of autoantibodies present in the sera of patients suffering from rheumatic disease (Table 1).

The most challenging question for snRNP investigators continues to be the contribution of the RNA component of each snRNP to function.