Protein synthesis in erythroid cells: II. Polyribosome function in intact reticulocytes

Abstract

In the present study, the function of polyribosomes in intact reticulocytes has been investigated. The pattern of increase in the specific activity of ribosomes of cells incubated with [14C]leucine indicates that the initiation and growth of polypeptide chains occurs predominantly on polyribosomes. The relative rates of incorporation of leucine and tryptophan on polyribosomes of various sizes were determined. The results suggest that reticulocyte polyribosomes corresponding to sedimentation coefficients of 110 s to 240 s are involved in globin synthesis. Inhibition of protein synthesis with puromycin is associated with a release of nascent peptide chains from ribosomes and a rapid breakdown of polyribosomes to 78 s ribosomes and smaller units. On the other hand, inhibition of protein synthesis by amino acid starvation, which is not associated with a release of nascent protein from ribosomes, does not alter the rate of polyribosome breakdown. These results support the concept that S-RNA with the attached peptide chain is important to the stability of polyribosomes in the intact cell. In reticulocytes there appear to be at least two processes involved in polyribosome breakdown. One of these mechanisms is associated with completion of peptide chains; the other accompanies cell maturation and does not require protein synthesis.