The Glycosylation of Human Serum IgD and IgE and the Accessibility of Identified Oligomannose Structures for Interaction with Mannan-Binding Lectin

Abstract

Analysis of the glycosylation of human serum IgD and IgE indicated that oligomannose structures are present on both Igs. The relative proportion of the oligomannose glycans is consistent with the occupation of one N-linked site on each heavy chain. We evaluated the accessibility of the oligomannose glycans on serum IgD and IgE to mannan-binding lectin (MBL). MBL is a member of the collectin family of proteins, which binds to oligomannose sugars. It has already been established that MBL binds to other members of the Ig family, such as agalactosylated glycoforms of IgG and polymeric IgA. Despite the presence of potential ligands, MBL does not bind to immobilized IgD and IgE. Molecular modeling of glycosylated human IgD Fc suggests that the oligomannose glycans located at Asn354 are inaccessible because the complex glycans at Asn445 block access to the site. On IgE, the additional CH2 hinge domain blocks access to the oligomannose glycans at Asn394 on one H chain by adopting an asymmetrically bent conformation. IgE contains 8.3% Man5GlcNAc2 glycans, which are the trimmed products of the Glc3Man9GlcNAc2 oligomannose precursor. The presence of these structures suggests that the CH2 domain flips between two bent quaternary conformations so that the oligomannose glycans on each chain become accessible for limited trimming to Man5GlcNAc2 during glycan biosynthesis. This is the first study of the glycosylation of human serum IgD and IgE from nonmyeloma proteins.