Evidence for heterogeneity of ribosomes within the HeLa cell

The proteins of several subsets of ribosomal subunits isolated from HeLa cells were separated by high resolution electrophoresis in polyacrylamide slab gels containing sodium dodecyl sulfate. Comparisons were made among native subunits, and subunits derived from single ribosomes, free polysomes, and membrane-bound polysomes. At least 10 heterogeneously distributed proteins were identified among the 40 S subunit proteins, compared wtih 17 to 20 proteins that occurred in all 40 S subunit classes. All 60 S subunits shared 34 separable proteins. One protein was missing only from membrane-bound 60 S subunits and another protein was present only in membrane-bound 60 S subunits. It is suggested that these two proteins regulate the binding of 60 S subunits to the endoplasmic reticulum.