14 The Diverse World of Small Ribonucleoproteins

Abstract

Among the major players in the functioning of present-day eukaryotic cells are small complexes consisting of RNA and protein. Small ribonucleoprotein (RNP) particles, defined as tight complexes between one or more proteins and a small RNA molecule (chain length ≤ ~300 nucleotides), come in a variety of shapes and sizes. Since they can be very abundant (up to 10 7 copies per mammalian cell, as many as ribosomes) and are often highly conserved from yeast to man, deciphering their cellular roles has been an important challenge for molecular biologists. Small RNPs inhabit all cellular compartments that have been examined: the nucleoplasm, the nucleoli, the cytoplasm, and the mitochondria. Further classification of small RNPs (Table 1) has been made possible by the discovery that they are often targeted by autoantibodies found in the sera of patients with rheumatic disease, such as systemic lupus erythematosus. Usually the autoepitopes reside on the protein rather than the RNA moieties of small RNPs, meaning that particles of the same class possess common polypeptide constituents. The autoantibodies also provide potent tools for probing the structures and functions of small RNPs. The vast majority of small RNPs whose functions have been deciphered play roles somewhere along the pathway of gene expression (see Table 1). For example, small n uclear RNPs (snRNPs) of the nucleoplasm are involved in pre-mRNA processing or tRNA biogenesis. Small nucleolar RNPs contribute to the maturation of rRNA. S mall RNPs of the c ytoplasm (scRNPs) function in the control of translation or disposition of newly synthesized proteins.