A Comparison of the Proteins of Rat Skeletal Muscle and Liver Ribosomes by Two-Dimensional Polyacrylamide Gel Electrophoresis OBSERVATIONS ON THE PARTITION OF PROTEINS BETWEEN RIBOSOJIAL SUBUNITS AND A DE- SCRIPTION OF TWO ACIDIC PROTEINS IN THE LARGE SUBUNIT*

We have determined the number of proteins in rat skeletal muscle 80 S ribosomes and ribosomal subparticles by twodimensional polyacrylamide gel electrophoresis. The proteins were compared with those from liver ribosomes. The small subunit of muscle ribosomes contained 31 proteins: it had two proteins (SJ and S,,,2) which were not found in liver 40 S particles; one protein (S31*) was always found in the small subunit of muscle ribosomes but was present in the 40 S subparticle of liver ribosomes only if they were prepared in buffer containing 500 InM potassium and 3 mM magnesium (it was absent if the cation concentrations during preparation were 830 and 12.5 mM, respectively); muscle small subparticles lacked SlO and S22; finally, S8 was absent if muscle subunits were prepared in buffer containing lower concentrations of potassium and magnesium (500 and 3 mM) whereas it was present if the cation concentration was higher (830 mM potassium and 12.5 mM magnesium). The large subunit of muscle ribosomes had 38 proteins: it lacked two (Ll and L41*) that always were present in liver 60 S subunits; and lacked one (L20) found only in the large subunit of liver ribosomes prepared in buffer containing 830 mM potassium and 12.5 mbr magnesium; L20 is also absent from the large subunit of liver ribosomes if they are prepared in 500 mM magnesium and 3 mM potassium. We have detected two acidic proteins (L40* and L41*) in the large subunit of ribosomes that may be homologous with Escherichio coli L7 and L12. Preparations of muscle 80 S monomers contained five proteins (1, 2, 3, Ll, M4) not present in either subunit; one of the five proteins (M4) was absent from liver ribosomes. We estimate muscle ribosomes have 67 to 74 different proteins; liver ribosomes have 69 to 74. Most of the apparent variance between the proteins of muscle and liver ribosomes can be accounted for either by differential partition of selected proteins to one or the other