The growing world of small nuclear ribonucleoproteins

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 two families. There are snRNPs of the nucleoplasm, whose major business is the generation of messenger RNAs for export to the cyto-plasm. A different set of snRNPs, called snoRNPs, reside in the cell nu-cleolus, 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. The last two years have pro-duced the extraordinary realization that vertebrate cells contain about 200 distinct kinds of snRNPs with abundances between 104 (for snoRNPs di-recting rRNA modification) to over 106 (for snRNPs of the major spliceo-some). 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 23). Curiously, snRNPs are often the target of autoantibodies present in the sera of patients suffering from rheumatic disease (as well as in other mammals afflicted with autoimmunity). These autoantibodies almost al-ways target epitopes on the protein component(s) of the particles, and since an antigenic protein often associates with multiple snRNAs (small nuclear RNAs), they define families of snRNPs related in structure (and usually also in function) (see Table 1). For instance, U1, U2, U4, U5, U11, U12, U4atac, and U7 are all Sm snRNPs and participate in mRNA bio-genesis within the nucleoplasm. Similarly, all known fibrillarin-associated small nucleolar RNPs (snoRNPs) are involved in pre-rRNA cleavage or The RNA World, Second Edition 1999 Cold Spring Harbor Laboratory Press 0-87969-561-7 For conditions see www.cshlpress.com/copyright. 488 Y.-T. Yu et al. Table 1 Small nuclear ribonucleoproteins (a)Metazoan snRNP