Structural basis for the presence of a monoglucosylated oligosaccharide in mature glycoproteins

Arylphorin is an insect hexameric storage protein. The structures of the oligosaccharides attached to this protein have recently been determined. However, their precise functions remain to be established. Proteolysis and MALDI MS studies disclose that the amino acid residues Asn196 and Asn344 are N-glycosylated with Glc1Man9GlcNAc2 and Man5-6GlcNAc2 oligosaccharides, respectively. Interestingly, significant variations in the amounts of glycans involving Glc1Man9GlcNAc2 are evident in arylphorins purified from larvae reared at different seasons. The data suggest that the metabolism of larvae and local protein structure contribute to glycan development. Three-dimensional model of the protein speculated that N-glycosidic linkage to Asn196 in the Glc1Man9GlcNAc2 structure was buried inside the twofold axis of the hexamer, whereas oligosaccharide linkages to Asn344 were completely exposed to solvent. This finding is in agreement with previous biochemical data showing that limited Glc1Man9GlcNAc2 was released by protein-N-glycosidase F under non-denaturing conditions, in contrast to Man5-6GlcNAc2 oligosaccharides.