Characterization of 20‐S and 40‐S Non‐Polysomal Cytoplasmic Messenger Ribonucleoprotein Particles from Rat Liver

Abstract

Two populations of free messenger ribonucleoprotein (mRNP) particles, sedimenting at 20 S and 40 S, respectively, were isolated from a rat liver postpolysomal supernatant. After treatment with 0.5 M KCl and recentrifugation through a sucrose layer, the mRNP particles were characterized for their low MW RNA and protein components. Very different RNA patterns were found for the 40 S and 20 S particles. Four distinct low-MW RNA species of .apprx. 105, 139, 187 and 256 nucleotides were found as components of the 40 S mRNP. The 20 S mRNP particles contain 1 major low-MW RNA species of .apprx. 243 nucleotides and a characteristic pattern of low-MW RNA similar to the one found in nuclear ribonucleoprotein particles. In contrast to the low-MW RNA found in nuclear RNP particles, most of the low-MW RNA species present in 20 S and 40 S mRNA particles are rapidly labeled after [3H]orotate administration. Whereas the low-MW RNA composition of 20 S and 40 S mRNP particles is very different, the protein patterns of both mRNP complexes are similar. Six major polypeptides, with the following MW of 117,000, 79,800, 76,700, 53,800, 43,900, 36,300, and several minor ones were found in 20 S and 40 S mRNP. In a cell-free system from wheat germs neither 20 S nor 40 S mRNP particles stimulated the incorporation of [3H]leucine into proteins. Phenol-extracted RNA from 20 S and 40 S mRNP stimulated total protein synthesis 16- and 3-fold, respectively. The RNA from both mRNP pools directed the synthesis of albumin in vitro.