Study on mammalian ribosomal protein reactivity in situ. III. Effect of trypsin on 40S and 60S subunits.

Rat liver 40S and 60S ribosomal subunits were treated with increasing concentrations of trypsin. The activity of both trypsin-treated subunits, when assayed for polyphenylalanine synthesis, progressively decreased, but the 60S subunits were inactivated at much lower trypsin concentrations than were the 40S ones. The sedimentation coefficients of trypsin-treated subunits were identical to those of control subunits when sucrose gradients containing 0.5 M KCl were used. When the sucrose gradients were prepared with a low salt buffer (80 mM KCl), dimer formation was observed with control subunits, but not with trypsin-treated ones. Two-dimensional gel electrophoresis analysis of the proteins extracted from trypsin-treated subunits revealed that all ribosomal proteins in the subunits were accessible to the enzyme. However, several proteins were more resistant to trypsin in compact subunits than when they were free or in unfolded subunits. Proteins of the 60S subunits were generally digested by lower trypsin concentrations than those of the 40S subunits. From the quantitative measurements of the undigested proteins, a classification of the proteins from both subunits according to their trypsin sensitivity was established. These results were compared with those previously obtained concerning ribosomal protein reactivity to chemical reagents.