Isolation and characterization of glycosylphosphatidylinositol-anchored peptides by hydrophilic interaction chromatography and MALDI tandem mass spectrometry

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are posttranslationally processed proteins that become tethered to the extracellular leaflet of the plasma membrane via a C-terminal glycan-like moiety. Since the first GPI-AP was described in the 1970s, more than 500 GPI-APs have been reported in a range of species, including plants, microbes, and mammals. GPI-APs are probably involved in cell signaling, cell recognition, and cell remodeling processes, and they may potentially serve as cell surface antigens or vaccine targets in pathogenic microorganisms or transformed mammalian cells. Due to the structural complexity and physicochemical properties of GPI-APs, their identification and structural characterization is a demanding analytical task. Here, we report a simple, fast and sensitive method for isolation and structural analysis of GPI-anchors using a combination of hydrophilic interaction liquid chromatography and matrix-assisted laser desorption/ionization (MALDI) quadrupole time-of-flight tandem mass spectrometry. This method allowed analysis of GPI peptides derived from low picomole levels of the porcine kidney membrane dipeptidase. Furthermore, it allowed unambiguous assignment of the omega site via amino acid sequencing of the modified peptides. GPI-anchor-specific diagnostic ions were observed by MALDI-MS/MS at m/z 162, 286, 422, and 447, corresponding to glucosamine, mannose ethanolamine phosphate, glucosamine inositol phosphate, and mannose ethanolamine phosphate glucosamine, respectively. Thus, the methodology described herein may enable sensitive and specific detection of GPI-anchored peptides in large-scale proteomic studies of plasma membrane proteins.