Defeating major contaminants in Fe-IMAC phosphopeptide enrichment

Here we demonstrate that biomolecular contaminants, such as nucleic acid molecules, can seriously interfere with IMAC based phosphopeptide enrichments. We address and largely solve this issue, developing a robust protocol implementing methanol/chloroform protein precipitation and enzymatic digestion using benzonase, which degrades all forms of DNA and RNA, prior to IMAC-column loading. This simple procedure resulted in a drastic increase of enrichment sensitivity, enabling the identification of around 17,000 unique phosphopeptides and 12,500 unambiguously localized phosphosites in human cell-lines from a single LC-MS/MS run, constituting a 50% increase when compared to the standard protocol. The improved protocol was also applied to bacterial samples, increasing the number of identified bacterial phosphopeptides even more strikingly, by a factor 10, when compared to the standard protocol. For E. coli we detected around 1300 unambiguously localized phosphosites per LC-MS/MS run. The preparation of these ultra-pure phosphopeptide samples only requires marginal extra costs and sample preparation time and should thus be adoptable by every laboratory active in the field of phosphoproteomics.