Structure of the N-linked oligosaccharides of MHC class I molecules from cells deficient in the antigenic peptide transporter. Implications for the site of peptide association
Hayes, B.K.; Esquivel, F.; Bennink, J.R.; Yewdell, J.W.; Varki, A.
Journal of Immunology 155(8): 3780-3787
1995
ISSN/ISBN: 0022-1767 PMID: 7561082 Document Number: 440349
Class I molecules are N-linked glycoproteins encoded by the MHC. They carry cytosolic protein-derived peptides to the cell surface, displaying them to enable immune surveillance of cellular processes. Peptides are delivered to class I molecules by the transporter associated with Ag processing (TAP). Peptide association is known to occur before exposure of class I molecules to the medial Golgi-processing enzyme a-mannosidase II, but there is limited information regarding the location or timing of peptide binding within the earlier regions of the endoplasmic reticulum (ER)-Golgi pathway. A reported association of newly synthesized class I molecules with the ER chaperonin calnexin raises the possibility of persistence of the monoglycosylated N-linked oligosaccharide (NLO) Glc-1Man-8GIcNAc-2, known to be recognized by this lectin. To explore these matters, we determined the structure of the NLOs on the subset of newly synthesized class I molecules awaiting the loading of peptide. We pulse-labeled murine MHC H-2D-b class I molecules in RMA/S cells, which lack one of the TAP subunits, causing the great majority of the molecules to be retained for prolonged periods in an early secretory compartment, awaiting peptide binding. MHC molecules pulse-labeled with (3H)glucosamine were isolated, the NLOs specifically released and structurally analyzed by a variety of techniques. Within the chosen window of biosynthetic time, most D-b molecules from parental RMA cells carried mature NLOs of the biantennary complex-type, with one to two sialic acid residues. In RMA/S cells, such chains were in the minority, the majority consisting of the precursor forms Man-8GIcNAc, and Man-9GlcNAc-2. No glucosylated forms were detected, nor were the later processing intermediates Man-5-7GlcNAc, or GlcNAc-1Man-4-5GlcNAc-2. Thus, most D-b molecules in TAP-deficient cells are retained in an early compartment of the secretory pathway, before the point of first access to the Golgi alpha-mannosidase 1, which trims alpha-1-2 linked mannose residues, but beyond the point where the alpha-1-3-linked glucose residue is finally removed by the ER glucosidase 11. Thus, structural analysis of NLOs on class I molecules within a defined biosynthetic window has established a biochemical measure of the timing of peptide association.