Properties of Membrane-bound Ribosomes in Reticulocytes

An average of 17% of rabbit reticulocyte ribosomes are closely bound to the cell membrane. Evidence is presented to indicate that membrane binding of reticulocyte ribosomes is not an artifact. Membrane-bound reticulocyte ribosomes differ in physical and chemical properties from free reticulocyte ribosomes in having a high coefficient of sedimentation (greater than 400 S) and a high protein to RNA ratio (greater than 5:1) and in being relatively resistant to breakdown by deoxycholate. The protein to RNA ratio of the rapidly sedimenting ribonucleoprotein aggregates varies with the degree of maturity of the cell, being greatest in older cells. Membrane-bound ribosomes are active in protein synthesis both in the intact cell and the cell-free system, but the degree of activity is less than that of free ribosomes. Their susceptibility to inhibitors of protein synthesis, such as puromycin, 10-3 m, or sodium fluoride, 10-2 m, is less than that of free ribosomes. Membrane-bound ribosomes do not break down to subunits after exposure to EDTA, and are relatively resistant to degradation by pancreatic ribonuclease. Binding to the cell membrane is not dependent upon magnesium ion concentration, an RNA component susceptible to ribonuclease action or the presence of peptidyl-transfer RNA on the ribosomes. The data suggest that membrane binding of reticulocyte ribosomes may be involved in the mechanism of ribosomal degradation in the erythroid cell.