Structure and Function of Rat‐Liver Ribosomes

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Abstract
Rat liver ribosomes and 60-S ribosome subunits were treated with the primary-amino-group-specific reagent 2-methoxy-5-nitrotropone. Important differences in the sensitivity of several ribosomal activities to inactivation by the reagent were observed. While elongation-factor-dependent activites are totally abolished in the treated particles, peptidyl transferase activity is either unaffected in 60-S ribosomal subunits or even strongly stimulated in 80-S ribosomes. Analysis of the ribosomal proteins modified by nitrotropone made it possible to draw some conclusions on their accesibility in the ribosomal structure and to relate some proteins with their involvment in the ribosome active centers. Proteins L3, L13, L15 and L23 seem to be in a rather well protected position while proteins L10, L35, L37, X1 and X2 are totally exposed to the reagent. The protein accessibility also depends on the ribosome conformation, proteins L14 and L17, for instance, being sensitive in 80-S ribosomes and protected in 60-S subunits. In relation to the implication of proteins in functional centers, these data together with other data obtained from protein-deficient core particles seem to indicate a possible role of proteins L21 and/or L26 in the peptidyl transferase center.