Ribosome and Polyribosome Disappearance during in vivo Erythroid Maturation

Abstract

THE maturation of the mammalian erythroid cell is marked by loss of the ability to synthesize protein¹. The predominant site of protein synthesis within the cell is the polyribosome, an aggregate of two or more single ribosomes². Although the decline in protein synthesis which accompanies erythroid maturation is apparently related both to a reduction in the cell content of polyribosomes and to a decrease in their functional capacity^3–6, there has been disagreement over the rates of polyribosome and single ribosome disappearance. It has been claimed, on the basis of in vitro³ and in vivo^5,6 investigations, that erythroid maturation is accompanied by a selective loss of heavier polyribosomes, which results in an increasing proportion of smaller aggregates and single ribosomes as the reticulocyte ages. Other investigations have shown that there is a relatively constant proportion of polyribosomes in reticulocytes of different ages^4,7. We have developed a system which allows longitudinal investigation of a population of reticulocytes as they mature in vivo, unaffected by continued production of new erythroid cells. Investigations of ribosome behaviour in this system indicate that with maturation all classes of ribosomes disappear at proportional rates, and that in particular-there is no orderly shift of polyribosomes from larger to smaller aggregates.