Membrane-bound ribosomes in HeLa cells: I. Their proportion to total cell ribosomes and their association with messenger RNA

Abstract

The ribosomes associated with the endoplasmic reticulum in HeLa cells have been the object of an electron microscopic and biochemical investigation. An appreciable amount of elements of rough endoplasmic reticulum, in the form of tubules, vesicles, and short cisternae scattered throughout the cytoplasm, has been observed by electron microscopic examination of thin sections of these cells. From the distribution of rRNA among the subcellular fractions, it has been estimated that from 10 to 15% (and possibly as many as 20%) of the total ribosomes in HeLa cells are associated with elements of rough endoplasmic reticulum. 65 to 70% of these ribosomes can be recovered as polysomes after sodium deoxycholate treatment and are presumably engaged in protein synthesis in vivo. Treatment with EDTA releases essentially all the small subunits and about 70% of the larger subunits of the endoplasmic reticulum-bound ribosomes: these respond, therefore, to EDTA treatment similarly to ribosomes of the rough endoplasmic reticulum from rat liver.