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

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 with respect to their low-molecular-weight RNA and protein components. 40-S and 20-S particles show very different RNA patterns. Four distinct low-molecular-weight RNA species of approximately 105, 139, 187 and 256 nucleotides were found as components of the 40-S mRNPs. The 20-S mRNP particles contain one major low-Mr RNA species of approximately 243 nucleotides and a characteristic pattern of low-Mr RNAs similar to the one found in nuclear ribonucleoprotein particles. In contrast to the low-Mr RNAs found in nuclear RNP particles most of the low-Mr RNA species present in 20-S and 40-S mRNP particles are rapidly labeled after [3H]orotate administration. Whereas the low-Mr RNA composition of 20-S and 40-S mRNP particles is very different, the protein patterns of both mRNP complexes are very similar. Six major polypeptides with the following molecular weights of 117000, 79800, 76700, 53800, 43900, 36300 and several minor ones were found in both 20-S and 40-S mRNPs. In a cell-free system from wheat germs neither 20-S nor 40-S mRNP particles stimulated the incorporation of [3H]leucine into proteins. However, phenol-extracted RNA from 20-S and 40-S mRNPs stimulated total protein synthesis 16-fold and 3-fold, respectively. Furthermore, the RNA from both mRNP pools directed the synthesis of albumin in vitro.