Malectin: A Novel Carbohydrate-binding Protein of the Endoplasmic Reticulum and a Candidate Player in the Early Steps of ProteinN-Glycosylation

Abstract

N-Glycosylation starts in the endoplasmic reticulum (ER) where a 14-sugar glycan composed of three glucoses, nine mannoses, and two N-acetylglucosamines (Glc₃Man₉GlcNAc₂) is transferred to nascent proteins. The glucoses are sequentially trimmed by ER-resident glucosidases. The Glc₃Man₉GlcNAc₂moiety is the substrate for oligosaccharyltransferase; the Glc₁Man₉GlcNAc₂and Man₉GlcNAc₂intermediates are signals for glycoprotein folding and quality control in the calnexin/calreticulin cycle. Here, we report a novel membrane-anchored ER protein that is highly conserved in animals and that recognizes the Glc₂-N-glycan. Structure determination by nuclear magnetic resonance showed that its luminal part is a carbohydrate binding domain that recognizes glucose oligomers. Carbohydrate microarray analyses revealed a uniquely selective binding to a Glc₂-N-glycan probe. The localization, structure, and binding specificity of this protein, which we have named malectin, open the way to studies of its role in the genesis, processing and secretion of N-glycosylated proteins.