Site-specific glycoproteomics confirms that protein structure dictates formation of N-glycan type, core fucosylation and branching

Growing evidence indicates that the individualized and highly reproducible N-glycan repertoires on each protein glycosylation site modulate function. Relationships between protein structures and the resulting N-glycoforms have previously been observed, but remain to be quantitatively confirmed and examined in detail to define the responsible mechanisms in the conserved mammalian glycosylation machinery. Here, we investigate this relationship by manually extracting and analyzing quantitative and qualitative site-specific glycoprofiling data from 117 research papers. Specifically, N-glycan structural motifs were correlated with the structure of the protein carriers, focusing on the solvent accessibility of the individual glycosylation sites and the physicochemical properties of the surrounding polypeptide chains. In total, 474 glycosylation sites from 169 mammalian N-glycoproteins originating from different tissues/body fluids were investigated. It was confirmed statistically that the N-glycan type, degree of core fucosylation and branching are strongly influenced by the glycosylation site accessibility. For these three N-glycan features, glycosylation sites carrying highly processed glycans were significantly more solvent-accessible than those carrying less processed counterparts. The glycosylation site accessibilities could be linked to molecular signatures at the primary and secondary protein levels, most notably to the glycoprotein size and the proportion of glycosylation sites located in accessible beta-turns. In addition, the subcellular location of the glycoproteins influenced the formation of the N-glycan structures. These data confirm that protein structures dictate site-specific formation of several features of N-glycan structures by affecting the biosynthetic pathway. Mammals have, as such, evolved mechanisms enabling proteins to influence the N-glycans they present to the extracellular environment.