Chaperone-Like Functions of High-Mannose Type and Complex-Type N-Glycans and Their Molecular Basis

Abstract

It has recently become apparent that high-mannose type N-glycans directly promote protein folding, whereas complex-type ones play a crucial role in the stabilization of protein functional conformations through hydrophobic interactions with the hydrophobic protein surfaces. Here an attempt was made to understand more deeply the molecular basis of these chaperone-like functions with the aid of information obtained from spacefill models of N-glycans. The promotion of protein folding by high-mannose N-glycans seemed to be based on their unique structure, which includes a hydrophobic region similar to the cyclodextrin cavity. The promotive features of high-mannose N-glycans newly observed under various conditions furnished strong support for the view that both intra- and extramolecular high-mannose N-glycans are directly involved in the promotion of protein folding in the endoplasmic reticulum. Further, it was revealed that the N-acetyllactosamine units in complex-type N-glycans have an amphiphilic structure and greatly contribute to the formation of extensive hydrophobic surfaces and, consequently, to the N-glycan-protein hydrophobic interactions. The processing of high-mannose type N-glycans to complex-type ones seems to be an ingenious device to enable the N-glycans to perform these two chaperone-like functions.

Expression of terminal α2-6–linked sialic acid on von Willebrand factor specifically enhances proteolysis by ADAMTS13

“…4,[29][30][31][32]39,40 Subtle variations in glycan structures directly influence glycoprotein conformation, by altering conformational freedom of the local peptide backbone. 29,41,42 We observed that the ability of sialic acid to enhance ADAMTS13 cleavage became less marked at progressively higher urea concentrations, suggesting that sialic acid may influence the conformation of VWF and that removal of N-linked sialic acid results in the A2 domain adopting a conformation that is less permissive for ADAMTS13 cleavage. A putative role for ␣2-6-linked sialic acid in determining VWF conformation is supported by previous studies showing that neuraminidase-treated VWF induced spontaneous platelet aggregation in the absence of ristocetin.…”

Bombay phenotype is associated with reduced plasma-VWF levels and an increased susceptibility to ADAMTS13 proteolysis

“…16 Previous studies have demonstrated that N-linked glycan structures directly influence the folding of glycoproteins by reducing conformational freedom of the local peptide backbone. 17,18 We hypothesized that glycan changes may alter the conformation of VWF and thus alter accessibility to the ADAMTS13 cleavage site. To investigate this hypothesis, we repeated ADAMTS13 cleavage assays over a range of urea concentrations (0.5-4 M) to mimic changes in shear forces responsible for unraveling VWF multimer.…”

N-linked glycosylation of VWF modulates its interaction with ADAMTS13

“…The glycan moieties of other proteins, such as erythropoietin, have been shown to confer stability on the molecule by forming hydrophobic interactions between glycan and protein. 31 It has been suggested that N-linked glycans form hydrophobic planes, where the degree of hydrophobicity is increased with increased branching of the glycan chain. 31 This hypothesis would also provide an explanation for the observation made by ourselves 15 and Bowen 16 that ABO/ Bombay blood group affects susceptibility to ADAMTS13 cleavage.…”