It is now well established that most RNAs are tightly associated with one or more proteins to generate nuclear or cytoplasmic ribonucleoprotein (RNP) complexes. Over 30 years ago, Lerner, Steitz, and coworkers found that the proteic components of these complexes made up some of the antigens recognized by sera of patients affected by systemic lupus erythematosus and other rheumatic diseases. These studies, based on sera derived from several patients, allowed the classification of mammalian small RNPs into several distinct groups.
As a result, several RNP complexes composed of a small RNA chain and of one or more proteins have been identified so far. Most of them are localized within nuclei (small nuclear RNPs or snRNPs) and their function, related to nuclear RNA processing, is well characterized. More recently, a minor RNP subset localized in the cytoplasm (small cytoplasmic RNP or scRNP) has also been identified. Structural characterizations have highlighted that different classes of scRNPs exist and are conserved through evolution (they can also be synthesized by viruses).
Although the functions of these particles are not completely characterized, different studies suggest their implication in the control of small RNA transcription by RNA polymerase III, protection from nuclease-degradation, and regulation of protein translation.
