How Many Proteins are Missed in Quantitative Proteomics Based on MS/MS Sequencing Methods?

Current bottom-up quantitative proteomics methods based on MS/MS sequencing of peptides are shown to be strongly dependent on sample preparation. Using cytosolic proteins from MCF-7 breast cancer cells, it is shown that protein pre- fractionation based on pI and MW is more effective than pre-fractionation using only MW in increasing the number of observed proteins (947 vs. 704 proteins) and the number of spectral counts per protein. Combination of MS data from the different pre-fractionation methods results in further improvements (1238 proteins). We discuss that at present the main limitation on quantitation by MS/MS sequencing is not MS sensitivity and protein abundance, but rather extensive peptide overlap and limited MS/MS sequencing through- put, and that this favors internally calibrated methods such as SILAC, ICAT or ITRAQ over spectral counting methods in attempts to drastically improve proteome coverage of biological samples.