title: Glycosylation of MUC1 influences the binding of a therapeutic antibody by altering the conformational equilibrium of the antigen.
abstract: In cancer cells, the glycoprotein Mucin 1 (MUC1) undergoes abnormal, truncated glycosylation. The truncated glycosylation exposes cryptic peptide epitopes that can be recognized by antibodies. Since these immunogenic regions are cancer specific, they represent ideal targets for therapeutic antibodies. We investigated the role of tumor-specific glycosylation on antigen recognition by the therapeutic antibody AR20.5. We explored the affinity of AR20.5 to a synthetic cancer-specific MUC1 glycopeptide and peptide. The antibody bound to the glycopeptide with an order of magnitude stronger affinity than the naked peptide. Given these results, we postulated that AR20.5 must specifically bind the carbohydrate as well as the peptide. Using X-ray crystallography, we examined this hypothesis by determining the structure of AR20.5 in complex with both peptide and glycopeptide. Surprisingly, the structure revealed that the carbohydrate did not form any specific polar contacts with the antibody. The high affinity of AR20.5 for the glycopeptide and the lack of specific binding contacts support a hypothesis that glycosylation of MUC1 stabilizes an extended bioactive conformation of the peptide recognized by the antibody. Since high affinity binding of AR20.5 to the MUC1 glycopeptide may not driven by specific antibody-antigen contacts, but rather evidence suggests that glycosylation alters the conformational equilibrium of the antigen, which allows the antibody to select the correct conformation. This study suggests a novel mechanism of antibody-antigen interaction and also suggests that glycosylation of MUC1 is important for the generation of high affinity therapeutic antibodies.
authors.affiliation: Department of Chemistry, California State University Fresno, 2555 E San Ramon Ave, Fresno, CA 93740, USA.
authors.forename: Mohammadreza
authors.initials: M
authors.lastname: Movahedin
authors.affiliation: Department of Chemistry, California State University Fresno, 2555 E San Ramon Ave, Fresno, CA 93740, USA.
authors.forename: Teresa M
authors.initials: TM
authors.lastname: Brooks
authors.affiliation: Complex Carbohydrate Research Center, 315 Riverbend Road, Athens, GA 30602, USA.|Department of Chemistry, University of Georgia, 140 Cedar street, Athens, GA 30602, USA.
authors.forename: Nitin T
authors.initials: NT
authors.lastname: Supekar
authors.affiliation: Department of Chemistry, California State University Fresno, 2555 E San Ramon Ave, Fresno, CA 93740, USA.
authors.forename: Naveen
authors.initials: N
authors.lastname: Gokanapudi
authors.affiliation: Complex Carbohydrate Research Center, 315 Riverbend Road, Athens, GA 30602, USA.|Department of Chemistry, University of Georgia, 140 Cedar street, Athens, GA 30602, USA.|Department of Chemical Biology and Drug Discovery, Utrecht Institute for Pharmaceutical Sciences, and Bijvoet Center for Biomolecular Research, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.
authors.forename: Geert-Jan
authors.initials: GJ
authors.lastname: Boons
authors.affiliation: Department of Chemistry, California State University Fresno, 2555 E San Ramon Ave, Fresno, CA 93740, USA.
authors.forename: Cory L
authors.initials: CL
authors.lastname: Brooks
mesh_terms: D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000276:immunology*; D000915:Antibody Affinity; D001666:Binding Sites, Antibody; D006031:Glycosylation; D006801:Humans; D006825:Hybridomas; D062105:Molecular Docking Simulation; D018396:Mucin-1 / Q000737:chemistry / Q000276:immunology* / Q000378:metabolism; D011485:Protein Binding; D011499:Protein Processing, Post-Translational
chemical_list: D000911:Antibodies, Monoclonal; D018396:Mucin-1; C499880:mAb-AR20.5
keywords: cancer; crystal structure; immunotherapy; molecular recognition; mucin