Characterization of the association of specific proteins with messenger ribonucleic acid

Isolation of messenger ribonucleic acid (mRNA)-protein particles from cytosol or dissociated polyribosomes yielded complexes in which several proteins were consistently associated with mRNA. Some of the mRNA-associated proteins appeared to have a high affinity for mRNA since they remained complexed to mRNA during centrifugation in CsCl gradients. Quantitation of RNA and protein in polyribosomal mRNPs suggested that each molecule of mRNA bound a molecule of each of the two major proteins of 78 000 and 52 000 apparent molecular weights and/or one or more of several minor proteins found in mRNPs. Of the several mRNP proteins, only the protein of 78 000 apparent molecular weight appeared to form a stable complex with the polyriboadenylic acid [poly(A)]-tract of mRNA, suggesting that the remaining mRNA-associated proteins bind to other regions which may be common to many or all mRNAs. Binding of [3H]poly(A)-rich RNA to mRNP proteins was effectively inhibited by unlabeled poly(A)-rich RNA or the homopolymer polyriboguanylic acid [poly(G)], but not by poly(A) or other natural or synthetic mRNAs. The properties of non-poly(A)-dependent binding of mRNA by mRNP protein were similar to those of mRNA binding by the guanosine triphosphate dependent Met-tRNAfMet-binding protein.