Direct MALDI-MS/MS of phosphopeptides affinity-bound to immobilized metal ion affinity chromatography beads

Immobilized metal ion affinity chromatography (IMAC) is a useful method to selectively isolate and enrich phosphopeptides from a peptide mixture. Mass spectrometry is a very suitable method for exact molecular weight determination of IMAC-isolated phosphopeptides, due to its inherent high sensitivity. Even exact molecular weight determination, however, is not sufficient for identification of the phosphorylation site if more than one potential phosphorylation site is present on a peptide. The previous method of choice for sequencing the affinity-bound peptides was electrospray tandem mass spectrometry (ESI-MS/MS). This method required elution and salt removal prior to MS analysis of the peptides, which can lead to sample loss. Using a matrix-assisted laser desorption/ionization (MALDI) source coupled to an orthogonal injection quadrupole time-of-flight (QqTOF) mass spectrometer with true MS/MS capabilities, direct sequencing of IMAC-enriched peptides has been performed on IMAC beads applied directly to the MALDI target. The utility of this new method has been demonstrated on a protein with unknown phosphorylation sites, where direct MALDI-MS/MS of the tryptic peptides bound to the IMAC beads resulted in the identification of two novel phosphopeptides. Using this technique, the phosphorylation site determination is unambiguous, even with a peptide containing four potentially phosphorylated residues. Direct analysis of phosphorylated peptides on IMAC beads does not adversely affect the high-mass accuracy of an orthogonal injection QqTOF mass spectrometer, making it a suitable technique for phosphoproteomics.