hnRNP Protein Distribution in Various Differentiated Vertebrate Cells

Abstract

While there have been very extensive studies on the pathway of nuclear RNA molecules from synthesis to their appearance as messenger RNA on cytoplasmic polyribosomes, there have been far fewer attempts made to examine the native forms of nuclear RNA molecules undergoing processing (for review, see Martin et al. 1980). Electron microscope studies (Miller and Bakken 1972; Angelier and Lacroix 1975; Puvion-Dutilleul et al. 1977) suggest that as nuclear hnRNA is synthesized, and is still in its nascent state attached to RNA polymerase and the chromatin fibril, protein molecules are bound which fold the RNA chains presumably facilitating their removal from the template. These proteins confer on the nascent RNA chain a particulate substructure. In separate biochemical experiments it has been suggested that the bulk of hnRNA present in the nucleus of higher eukaryotic cells can be extracted as ribonucleoprotein complexes (hnRNP) which also have a substructure that is revealed by limited nuclease digestion (Samarina et al. 1968; Martin et al. 1974, 1978). The greater part of the hnRNA which can be extracted is contained in a relatively homogeneous substructure, the 30 S RNP complex. The protein component of purified 30 S RNP complexes is relatively simple, consisting of a set of similar polypeptides of 4–6 distinguishable species, all having molecular weights in the range 34,000–40,000 (Martin et al. 1974; Beyer et al. 1977; Billings and Martin 1978). These polypeptides appear to have been conserved in size and amino acid composition during the evolution of higher eukaryotes (Martin et al. 1974). Within the limit of resolution so far achieved there is a great deal of similarity observed between the morphology of the isolated 30 S RNP complexes and the substructures observed on nascent RNA protein fibrils. While it is very likely that other proteins are also involved in the binding to specific sites along nascent RNA molecules the ubiquity and relative ease of isolation of the 30 S RNP core proteins make them an obvious choice for initial studies of changes in RNA binding proteins within cells in different physiological states.