The structure of ribonucleoprotein particles from rat liver nuclei

During or immediately after transcription of chromatin the high molecular weight pre-mRNA is complexes with proteins and low molecular weight RNA (lmwRNA). In the presence of a cytosolic RNase inhibitor pre-mRNA-protein complexes, designated as polyparticles, can be isolated from rat liver nuclei. The polyparticles are characterized by a maximum of their sedimentation coefficient of around 90 S, a protein to RNA ratio of 4.1, and a density in CsCl of 1.4 g/cm3. A set of 6--10 basic proteins of molecular weights between 30 and 45 kd as well as a multitude of polypeptides of higher molecular weights is associated with the rapidly labeled, polydisperse, high molecular weight RNA and several lmwRNA species. In order to study the structure of these very complex nuclear RNP complexes, the polyparticles were incubated at various concentrations of sodium chloride, urea or proteinases of different specificities (trypsin, chymotrypsin, proteinase K), recentrifuged through a sucrose layer and analyzed with respect to their sedimentation behavior, their protein to RNA ratios and their protein- and RNA components. Rhe results of these experiments led us to the proposal of a structural model which is presented here.