Invariable stoichiometry of ribosomal proteins in mouse brain tissues with aging.

Across phyla, the ribosomes-the central molecular machines for translation of genetic information-exhibit an overall preserved architecture and a conserved functional core. The natural heterogeneity of the ribosome periodically phases a debate on their functional specialization and the tissue-specific variations of the ribosomal protein (RP) pool. Using sensitive differential proteomics, we performed a thorough quantitative inventory of the protein composition of ribosomes from 3 different mouse brain tissues, i.e., hippocampus, cortex, and cerebellum, across various ages, i.e., juvenile, adult, and middle-aged mouse groups. In all 3 brain tissues, in both monosomal and polysomal ribosome fractions, we detected an invariant set of 72 of 79 core RPs, RACK1 and 2 of the 8 RP paralogs, the stoichiometry of which remained constant across different ages. The amount of a few RPs punctually varied in either one tissue or one age group, but these fluctuations were within the tight bounds of the measurement noise. Further comparison with the ribosomes from a high-metabolic-rate organ, e.g., the liver, revealed protein composition identical to that of the ribosomes from the 3 brain tissues. Together, our data show an invariant protein composition of ribosomes from 4 tissues across different ages of mice and support the idea that functional heterogeneity may arise from factors other than simply ribosomal protein stoichiometry.