Change of protein reactivity in mammalian ribosomal subunits as a function of temperature

Reductive methylation of rat liver ribosomal subunits was carried out at different temperatures and the reaction was followed as a function of time. The percentage of protein lysine residues which were methylated reached a definite plateau for each temperature, and was considerably increased by heating (four times at 40 degrees C). This increase was not observed when free ribosomal proteins were heated under the same conditions. Ribosomal subunits kept their biological activity (elongation steps of protein synthesis) even when methylated to a high extent. Half of the subunit activity was still found when 67% of the lysine residues were methylated. Proteins were divided into different classes on the basis of their alkylation in response to temperature. Those which reacted poorly within subunits even at high temperature should interact directly with rRNA. The results suggest that the protein reactivity modifications induced by heating (up to 37 degrees C), which were often totally or partially reversible, reflect a conformational change of the proteins themselves within the subunits. Besides protein unfolding, higher temperatures produced structural modification and inactivation of the subunits, which were restrained by the presence of aminoacyl tRNA.