Proteomics-based characterization of ribosome heterogeneity in adult mouse organs

While long thought to be invariable in all cellular organisms, evidence has emerged that the translation process, i.e. protein assembly from mRNA sequence decoding, is regulated by variable features of the translation machinery. Notably, ribosomes, the functional units of protein synthesis, display variations in their composition, depending on the developmental stage, cell type or physiopathological context, thus providing a new level of actionable regulation of gene expression. Yet, a comprehensive map of the heterogeneity of ribosome composition in ribosomal proteins (RPs) in different organs and tissues is not available. In this work, we explored tissue-specific ribosome heterogeneity using mass spectrometry-based quantitative proteomic characterization of ribosomal fractions purified from 14 adult mouse organs and tissues. We performed crossed clustering and statistical analyses of RP composition to highlight stable, variable and tissue-specific RPs across organs and tissues. Focusing on specific RPs, we validated their relative abundance with a targeted proteomic approach, which gave further insight into the tissue-specific ribosome RP signature. Finally, we investigated the origin of RP variations in ribosome fraction of the different tissues, by comparing RP relative abundances in our proteomic dataset and in three independent transcriptomic datasets. Interestingly, we found that, in some tissues, the RP abundance in purified ribosomes does not always correlate with the corresponding RP transcript level, arguing for a translational regulation of RP expression, and/or a regulated incorporation of RPs into ribosomes. Altogether, our data support the notion of a tissue-specific RP signature of ribosomes, which opens avenues to study how specific ribosomal composition provides an additional level of regulation to control gene expression in different tissues and organs.