  Understanding the regulation and structure of ribosomes is essential to
understanding protein synthesis and its deregulation in disease. While
ribosomes are believed to have a fixed stoichiometry among their core ribosomal
proteins (RPs), some experiments suggest a more variable composition. Testing
such variability requires direct and precise quantification of RPs. We used
mass-spectrometry to directly quantify RPs across monosomes and polysomes of
mouse embryonic stem cells (ESC) and budding yeast. Our data show that the
stoichiometry among core RPs in wild-type yeast cells and ESC depends both on
the growth conditions and on the number of ribosomes bound per mRNA.
Furthermore, we find that the fitness of cells with a deleted RP-gene is
inversely proportional to the enrichment of the corresponding RP in polysomes.
Together, our findings support the existence of ribosomes with distinct protein
composition and physiological function.
