Analysis of protein landscapes around N-glycosylation sites from the PDB repository for understanding the structural basis of N-glycoprotein processing and maturation

Asparagine-linked glycans (N-glycans) are attached onto nascent glycoproteins in the endoplasmic reticulum (ER) and subsequently processed by a set of processing enzymes in the ER and Golgi apparatus. Accumulating evidence has shown that not all N-glycans on glycoproteins are uniformly processed into mature forms (hybrid and complex types in mammals) through the ER and Golgi apparatus, and a certain set of glycans remains unprocessed as an "immature" form (high-mannose type in mammals). Much attention has been paid to environmental factors regulating N-glycoprotein maturation, such as the expression levels of glycosyltransferases/glycosidases. On the other hand, the influence of the 3D structure of the carrier glycoprotein on N-glycan maturation has been investigated mostly using individual model glycoproteins. To obtain more insights into N-glycoprotein maturation, we herein analyze glycoprotein structures extracted from the Protein Data Bank. We confirm that site-specific N-glycan processing is largely explained by the solvent accessibility of the glycosylated Asn residue and of the conjugated N-glycan. Potential bias of protein structural features toward immature or mature forms was explored within a range of concentric circles of fully folded glycoproteins. There does appear to be bias in amino acid composition and secondary structure. Most notably, gamma-branched amino acid residues (Asn+Asp+Leu) occur more frequently on unstructured loop regions of immature glycoproteins. Structural features of the protein surface around the N-glycosylated site do seem to affect N-glycan processing and maturation.