STUDIES ON BIOSYNTHESIS OF STRUCTURAL PROTEINS OF LIVER RIBOSOMES .I. INCORPORATION OF LABELLED AMINO ACIDS INTO BASIC STRUCTURAL PROTEINS OF LIVER RIBOSOMES IN VIVO

Rat-llver ribosomes were labelled with 14C-labelled amino acids-in vivo. The ribosomes were freed from the nascent protein fraction by EDTA treatment and basic structural proteins were extracted by acetic acid. The labelled basic proteins were subjected to CM-cellulose chromatography to give Fraction I protein which was further fractionated by starch-gel electrophoresis. Starch-gel electrophoresis of Fraction 1 labelled for 10 min., 2 hr. and 12 hr. showed that this fraction consisted of proteins with different metabolic rates and that slow-moving (less basic) proteins, were in general more active than fast-moving (more basic) proteins. Two-dimensional starch-gel electrophoresis of Fraction I resulted in the separation of this fraction Into about 30 protein components. The finding that the incorporation of the label occurred in the majority of the protein spots and was negligible in other areas, provides strong evidence that the bulk of the basic structural proteins undergo metabolic renewal. The heterogeneity in metabolic rate among these proteins was further demonstrated. The time course of the specific activity of Fraction I, labelled in vivo for various time intervals, showed that the rate of incorporation into Fraction 1 is rather slow, reaching the maximum about 6 hr. after the injection. In contrast the incorporation into nascent proteins was very rapid, reaching a maximum at 10 min. The semi-logarithmic plots of the decreasing specific activities exhibited by Fraction I and other cullular components showed that the average renewal rate of the basic structural protein of ribosomes is comparable for the total proteins of mitochondria, nucleus and cell sap.