PT - JOURNAL ARTICLE AU - Nikolai Slavov AU - Stefan Semrau AU - Edoardo Airoldi AU - Bogdan A. Budnik AU - Alexander van Oudenaarden TI - Differential stoichiometry among core ribosomal proteins AID - 10.1101/005553 DP - 2015 Jan 01 TA - bioRxiv PG - 005553 4099 - http://biorxiv.org/content/early/2015/07/20/005553.short 4100 - http://biorxiv.org/content/early/2015/07/20/005553.full AB - Understanding the regulation and structure of ribosomes is essential to under-standing 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.