Studies on the biosynthesis of structural proteins of liver ribosomes: II. The incorporation of labelled amino acid into basic structural proteins of liver ribosomes by a cell free system

Abstract

1. A cell-free system, consisting of rat-liver polysomes and cell sap, incorporated [ 14C]amino acids into a basic protein fraction (Fraction I) of liver ribosomes. The Fraction I was prepared by CM-cellulose chromatography of the acetic acidsoluble fraction of ribosomes freed from nascent proteins. 2. The distribution pattern of the radioactivity in starch-gel electrophoresis of the labelled Fraction I was examined. A higher activity was generally associated with the slower-moving proteins. The activity was rather low in the fast-moving proteins. 3. The labelled Fraction I was fractionated into two components by Sephadex G-200 column chromatography. Component I, eluted first, had higher specific activity than Component II which was eluted later. On the subsequent starch-gel electrophoresis, Component I gave a slow-moving and rather diffuse pattern and a high radioactivity was found near the origin. Component II gave distinct fast-moving protein bands and some of the radioactivity was associated with these protein bands. 4. Two-dimensional starch-gel electrophoresis of the labelled Component II revealed that the radioactivity was incorporated into most of the protein spots and no significant activity was found in other areas. The distribution of the radioactivity in each spot was not uniform. 5. It is proposed that a considerable part of “basic structural proteins” of liver ribosomes are synthesized on polysomes through the general protein-synthetic pathway involving amino acyl-soluble RNA as the intermediate.