Heterogeneity of native ribosomal 60-S subunits in Ehrlich ascites tumor cells cultured in vitro.

Native large ribosomal subunits in cultured Ehrlich ascites tumor cells analyzed by high-resolution CsCl isopycnic centrifugation consist of at least two classes of particles with densities of 1.57 g/cm3 (LI) and 1.59 g/cm3 (LII), respectively. A wash with 0.5 M KCl converts LI into particles with the density of LII particles. Incubation of derived large subunits (density 1.59 g/cm3) with 0.5 M KCl wash of reticulocyte ribosomes leads to the formation of particles with the density of LI particles. A protein with a molecular weight of 57000 present in the high-KCl wash of 60-S native subunits was virtually absent in the KCl wash of 40-S subunits or polyribosomes suggesting that specific protein factors may be present on some native 60-S subunits. Possible functions of these protein factors are discussed.