Ribonucleoprotein particles in cellular processes

THE general way of thinking about the pathway of expression of genetic information in eukaryotes, from DNA to protein via RNA, instills a protein-centric point of view, as if the protein is the real goal and the RNA is just the intermediate. In fact, most of the RNA in the cell is not mRNA but rather part of modular structures, RNA-protein machines, that are termed ribonucleoprotein (RNP) particles or ribonucleoprorein complexes. The ribosome is one well-known example of such a complex. What is new and surprising is the discovery of the tremendous variety, complexity, and versatility of RNP particles which is reflected by the wide range of cellular processes in which they are involved. RNPs are involved in each step along the pathway of gene expression in eukaryotes, mRNAs are formed from precursor transcripts (hnRNAs or pre-mRNAs) by splicing of introns, and transported to the cytoplasm where each needs to accumulate to the precise level that will produce the correct amount of the particular protein. What has emerged over the past few years is that key components involved in processing pre-mRNA are complexes of unique small RNAs (snRNAs) with specific proteins, small nuclear ribonucleoprotein particles (snRNPs). The nuclear precursor transcripts themselves are in fact also RNPs, being complexed with specific proteins, the hnRNP proteins, as are cytoplasmic mRNAs which are associated with the mRNP proteins. Parts of the protein synthetic machinery, particularly ribosomes, are also complexes of proteins with RNA. The continual and dynamic association of messenger RNAs and their precursor hnRNA molecules with specific sets of proteins in the cell underscores the importance of understanding the molecular nature of these hnRNP and mRNP complexes. Recent work from numerous laboratories has led to the discovery of many additional RNP particles that are involved in an unexpected variety of cellular processes including protein targeting, priming of mitochondrial DNA synthesis, and transcription termination (see Table I). Here we review some of the most recent findings on the structure, composition, and function of RNP particles in the eukaryotic cell. Ribosomes, mRNPs, and viral RNPs are not discussed here. Also, since several reviews (1-6) have been published over the last two years that discuss pre-mRNA processing, we confine the discussion to only the most recent data in this rapidly advancing field.