Proteins of rat liver free and membrane-bound ribosomes Modification of two large sub-unit proteins by a factor detached from ribosomes at high ionic strength

Abstract

We have compared the protein composition of sub-units obtained from rat liver ribosomes prepared under different ionic strength conditions. Analysis of ribosomal proteins by one-dimensional polyacrylamide gel electrophoresis showed that differences in the protein content of large (60 S) sub-units occurred according to whether the ribosomes were exposed to low ionic strength media or were prepared in the absence of added salts. These differences may be accounted for by the presence of a protease in both free and membrane-bound ribosomes, the activity of which is normally only observed under low ionic strength conditions and which cleaves a small portion of two large sub-unit proteins in a specific manner. The factor responsible for the alteration in electrophoretic mobility can be released from the ribosomes by exposure to high ionic strength conditions such as those used to dissociate polysomes into sub-units in the presence of puromycin. Free and membrane-bound ribosomes, prepared under conditions in which this protease activity was inhibited and in the absence of detergents, were compared by both one-dimensional and two-dimensional polyacrylamide gel electrophoresis. Analysis by one-dimensional polyacrylamide-sodium dodcyl sulfate gel electrophoresis resolved 17 bands for 40-S sub-units and 26 bands for 60-S subunits. The profiles of free and membrane-bound ribosomal particles were identical. Two-dimensional analysis resolved 26 small sub-unit proteins and 38 large sub-unit proteins for both free and membrane-bound ribosomes. The protein complement of free and bound ribosomal sub-units was identical. These results suggest that there is no structural difference between free and bound ribosomes and that the two populations are interchangeable.