Global and site-specific detection of human integrin alpha 5 beta 1 glycosylation using tandem mass spectrometry and the StrOligo algorithm

Glycans are oligosaccharides associated with proteins, and are known to confer specific functions and conformations on glycoproteins. As protein tridimensional structures are related to function, the study of glycans and their impact on protein folding can provide important information to the field of proteomics. The subdiscipline of glycomics (or glycoproteomics) is rapidly growing in importance as glycans in proteins have shown to be involved in protein-protein or protein-(drug, virus, antibody) interactions. Glycomics studies most often aim at identifying glycosylation sites, and thus are performed on deglycosylated proteins resulting in loss of site-specific details concerning the glycosylation. In order to obtain such details by mass spectrometry (MS), either whole glycoproteins must be digested and analyzed as mixtures of peptides and glycopeptides, or glycans must be isolated from glycopeptide fractions and analyzed as pools. This article describes parallel experiments involving both approaches, designed to take advantage of the StrOligo algorithm functionalities with the aim of characterizing glycosylation microheterogeneity on a specific site. A hybrid quadrupole-quadrupole-time-of-flight (QqTOF) instrument equipped with a matrix-assisted laser desorption/ionization (MALDI) source was used. Glycosylation of alpha 5 beta 1 subunits of human integrin was studied to test the methodology. The sample was divided in two aliquots, and glycans from the first aliquot were released enzymatically, labelled with 2-aminobenzamide, and identified using tandem mass spectrometry (MS/MS) and the StrOligo program. The other aliquot was digested with trypsin and the resulting peptides separated by reversed-phase high-performance liquid chromatography (HPLC). A specific collected fraction was then analyzed by MS before and after glycan release. These spectra allowed, by comparison, detection of a glycopeptide (several glycoforms) and elucidation of peptide sequence. Compositions of glycans present were proposed, and identification of possible glycan structures was conducted using MS/MS and StrOligo.