Median based absolute quantification of proteins using Fully Unlabelled Generic Internal Standard

By reporting molar abundances of proteins, absolute quantification determines their stoichiometry in complexes, pathways or networks and also relates them to abundances of non-protein biomolecules. Typically, absolute quantification relies either on protein- specific isotopically labelled peptide standards or on a semiempirical calibration against the average abundance of peptides chosen from arbitrary selected standard proteins. Here we developed a generic protein standard FUGIS (Fully unlabelled Generic Internal Standard) that requires no isotopic labelling, synthesis of standards or external calibration and is applicable to proteins of any organismal origin. FUGIS is co-digested with analysed proteins and enables their absolute quantification in the same LC-MS/MS run. By using FUGIS, median based absolute quantification (MBAQ) workflow provides similar quantification accuracy compared to isotopically-labelled peptide standards and outperforms methods based on external calibration or selection of best ionized reporter peptides (e.g. Top3 quantification) with a median quantification error below 20%