Specific Dissociation of Ribosomes from Saccharomyces cerevisiae by a Protein Factor

A ribosome fraction enriched in “native” subunits by differential centrifugation yields, upon washing in a buffer containing ammonium chloride and magnesium ions, a protein mixture which can dissociate cytoplasmic ribosomes from Saccharomyces cerevisiae into 40 S and 60 S subunits. This heat dependent reaction cannot be related to any marked ribonuclease activity in the protein mixture. Furthermore, the mixture is unable to dissociate Escherichia coli or rabbit reticulocyte ribosomes; this property is unique, since proteolytic enzymes readily dissociate reticulocyte ribosomes as well as yeast ribosomes. Within 30 min of incubation, the factor‐dependent dissociation reaches a plateau, due either to the stoichiometric mode of action of the factor, or to the inability of ribosomes to support complete dissociation under the ionic conditions of the experiments. These two points are discussed.