Physical Properties and Protein Constituents of Cytoplasmic and Mitochondrial Ribosomes of Xenopus laevis

Abstract

Abstract Cytoplasmic and mitochondrial ribosomes were isolated from ovaries of Xenopus laevis. Subunits prepared from these ribosomes have relatively low poly(U)-directed poly(Phe) synthesizing activity when tested separately, but regain at least 80% of the activity of the original ribosomes when mixed. Buoyant densities in analytical CsCl gradients and chemical measurements of RNA and protein content indicate that cytoplasmic large (1.626 g per cm3; 58% RNA) and small (1.553 g per cm3; 50% RNA) subunits are less protein rich and contain a smaller mass of protein than mitochondrial large (1.447 g per cm3; 32% RNA) and small (1.448 g per cm3; 20% RNA) subunits. We suggest that in comparison to 80 S cytoplasmic ribosomes, the lower sedimentation coefficient of 55 to 60 S mitochondrial ribosomes is due equally to their lower particle mass (3.5 versus 4.0 x 106 daltons) and to their lower density. Proteins extracted from separated subunits were fractionated and their number and molecular weights were estimated by two-dimensional polyacrylamide gel electrophoresis. Proteins present in different subunits were compared by co-electrophoresis of trace amounts of 125I-labeled and stainable amounts of unabeled proteins. Cytoplasmic large and small subunits contain 37 and 34 proteins, respectively; at least 65, but more likely all 71, of these proteins are distinct. Mitochondrial large and small subunits contain 40 and 44 proteins, respectively, at least 77, but more likely all 84, of these proteins are distinct. Cytoplasmic and mitochondrial ribosomal subunits share few, if any, of these 155 proteins. Xenopus mitochondrial DNA can encode at most onesixth of the mitochondrial ribosomal proteins. Therefore, most, if not all, mitochondrial ribosomal proteins are encoded by nuclear DNA.