In vitro translation of globin: effect of proteins purified by affinity chromatography on polyadenylate-Sepharose

By means of affinity chromatography on poly(adenylic acid) (poly(A))-fixed Sepharose, protein fractions having strong affinity to poly(A) were prepared from postribosomal supernatants of rabbit reticulocyte and rat liver. These fractions contained several proteins similar by electrophoretic analysis to rabbit globin messenger ribonucleoprotein. Protein fractions from both sources were shown to form ribonucleoprotein complexes with rabbit globin mRNA, and these complexes sedimented at the same rate as native globin messenger ribonucleoprotein. Binding of the proteins to RNA was not highly specific, since not only poly(A) but also other polynucleotides as poly(C) or poly(U) were bound to these proteins. Ribosomal RNAs, tRNA, or DNAs did not bind the proteins. In order to ascertain the function of the poly(A)-Sepharose purified proteins, their effects on translation of globin mRNA was studied in vitro. Addition of rabbit reticulocyte protein to globin mRNA resulted in no more than a slight stimulation of both alpha- and beta-chain synthesis. Poly(A)-Sepharose purified protein from rat liver, however, caused a marked preferential reduction of alpha-chain synthesis. These results showed that at least some proteins in the poly(A)-Sepharose purified proteins affect the translation of globin. This inference suggested a possibility that protein moiety in globin mRNP might be involved in control of globin synthesis.