Chapter Ten - Mannose 6-Phosphate Receptor Homology Domain-Containing Lectins in Mammalian Endoplasmic Reticulum-Associated Degradation

Abstract

Quality control of glycoproteins synthesized in the endoplasmic reticulum (ER) is mediated by lectins and molecular chaperones. N-linked Glc(3)Man(9)GlcNAc(2) oligosaccharides attached to the nascent polypeptides are processed and recognized by lectins in the ER. OS-9 and XTP3-B/Erlectin, mannose 6-phosphate receptor homology (MRH) domain-containing lectins in mammals, were recently identified as ER luminal glycoproteins that participate in ER-associated degradation (ERAD) of misfolded proteins. Frontal affinity chromatography (FAC) and cell-surface expressed lectin assay revealed that both OS-9 and XTP3-B recognize high-mannose type N-glycans that lack the terminal mannose on the C branch. Furthermore, these lectins associate with the HRD1-SEL1L ubiquitin ligase complex on the ER membrane. In this chapter, we describe the FAC methods used to analyze the carbohydrate-recognition specificity of OS-9 and methods to examine the interaction and the effect on ERAD of these proteins in vivo. We also discuss the structure and function of OS-9 and XTP3-B, and the effect of these lectins on ERAD.