Free cytoplasmic messenger ribonucleoprotein complexes from rabbit reticulocytes

Interaction of cytoplasmic messenger-RNA with proteins: their possible function in the regulation of translation

In the present report we summarize recent results concerning our studies on messenger ribonucleoprotein particles and RNA-binding proteins from the cytoplasm of mouse erythroblasts and mouse Krebs II ascites cells.

Non-polysomal poly(A)-containing messenger ribonucleoproteins of cryptobiotic gastrulae of Artemia salina

Non-polysomal poly(A)-containing messenger ribonucleoprotein (mRNP) of Artemia salina has been isolated by thermal chromatography on oligo(dT)-cellulose in moderate (250 mM) and low (50 mM NaCl and 5 mM MgCl2) ionic strength. The purified particles sedimented between 5 S and 30 S and banded at a density of 1.38-1.40 g/cm3 and 1.26-1.27 g/cm3 in CsCl and sucrose isopycnic centrifugation, respectively. The translatability of the mRNP in a cell-free system depended on the conditions of isolation. The protein composition of the free mRNP is independent of the conditions used in oligo(dT)-cellulose chromatography. The proteins have Mr of 87,000, 76,000, 65,000, 50,000, 45,000, 38,000 and 23,500. A specific set of proteins is associated wtih different ribonucleoproteins, although some proteins are present on multiple particles. The main 17 +/- 2-S particle is composed of proteins with Mr of 87,000, 76,000, 45,000 and 38,000. Approximately the same proteins were present on free mRNP and mRNP isolated from non-polysomal mRNP-ribosome complexes. Poly(A)-binding proteins have Mr of 38,000 and 23,500. The 38,000-Mr protein comprised at least 60% of the total mRNP protein. Poly(A)-binding proteins with Mr of 38,000 and 76,000 are also present in a free state in the cytoplasm. A relation between the main poly(A)-binding mRNP protein and the helix-destabilizing protein HD40 [Marvil, D. K., Nowak, L., and Szer, W. (1980) J. Biol. Chem. 255, 6466-6472] is discussed.

Cytoplasmic processing events in the polyadenylate region of Physarum messenger RNA

Cytoplasmic processing events in the poly(A) region of mRNA from Physarum polycephalum are reviewed. Two classes of poly-containing RNA [poly(A)+ RNA] exist in the cytoplasm. One contains very short poly(A) sequences, averaging about 15 adenylate residues, while the other contains relatively long poly(A) sequences, averaging about 60 residues. Molecules with short poly(A) sequences are found exclusively in the polysomes while those with long poly(A) sequences are restricted to the free cytoplasmic mRNP. Since proteins are associated with only the long poly(A) sequences the poly(A) . protein complex is also restricted to the free mRNP. The long poly(A) sequences are relatively short-lived. They are degraded by two distinct processes, a shortening process in which 15-20 residues are gradually removed and a turnover process in which long poly(A) tracts are rapidly converted to the short sequences. This process, along with the dissociation of the poly(A) . protein complex, occurs when poly(A)+ RNA molecules located in free mRNP are transferred to the polysomes. Poly(A) . protein complex dissociation appears to precede poly(A) turnover during translational selection. The significance of these processing events in relation to mRNA maturation is discussed.

Purification of messenger ribonucleoprotein particles from rabbit reticulocytes by zonal centrifugation in metrizamide

A large-scale purification procedure for messenger ribonucleoprotein (mRNP) particles from rabbit reticulocyte polysomes is described. The mRNP particles were dissociated from polysomes by treatment with urea and separated by differential centrifugation under conditions of high ionic strength. Zonal centrifugation in a metrizamide buoyant density gradient was the final purification step. One major class of mRNA particle was observed. The RNA was defined as mRNA by polyacrylamide gel electrophoresis and by globin production in a cell-free protein-synthesizing system. The twenty-three different proteins associated with the particle were a discrete set of proteins, which ranged in molecular weight from 175,000 to 23,500. The relative amount of each peptide in the particle was determined from a gel scan of the stained protein by computer simulation. None of the polypeptides comigrated with proteins from the 40-S and 60-S ribosomal subunits when analyzed by two-dimensional polyacrylamide gel electrophoresis.

Cross-linking of mRNA to proteins by irradiation of intact cells with ultraviolet light

Ehrlich ascites tumor cells were irradiated with ultraviolet light to cross-link intracellular RNAs to their tightly bound proteins. The efficiency of such cross-linking in vivo was measured by two independent methods, namely by phenol/chloroform extraction and by isopycnic centrifugation. After short irradiation times more than 70% of the ribosomal RNAs and more than 80% of the mRNA were found to be cross-linked to protein. The size of the cross-linked RNAs was essentially unchanged, indicating that the irradiation does not lead to extensive fragmentation of the RNA. Covalent poly(A)-containing mRNA-protein complexes were isolated by oligo(dT)-cellulose chromatography in the presence of sodium dodecylsulphate. The complexed proteins were freed from RNA by extensive nuclease degradation and analysed by polyacrylamide gel electrophoresis. The results show that some specific proteins are associated with poly(A)-containing RNA in vivo.