title: A role for collagen IV cross-links in conferring immune privilege to the Goodpasture autoantigen: structural basis for the crypticity of B cell epitopes.
abstract: The detailed structural basis for the cryptic nature (crypticity) of a B cell epitope harbored by an autoantigen is unknown. Because the immune system may be ignorant of the existence of such "cryptic" epitopes, their exposure could be an important feature in autoimmunity. Here we investigated the structural basis for the crypticity of the epitopes of the Goodpasture autoantigen, the alpha3alpha4alpha5 noncollagenous-1 (NC1) hexamer, a globular domain that connects two triple-helical molecules of the alpha3alpha4alpha5 collagen IV network. The NC1 hexamer occurs in two isoforms as follows: the M-isoform composed of monomer subunits in which the epitopes are accessible to autoantibodies, and the D-isoform composed of both monomer and dimer subunits in which the epitopes are cryptic. The D-isoform was characterized with respect to quaternary structure, as revealed by mass spectrometry of dimer subunits, homology modeling, and molecular dynamics simulation. The results revealed that the D-isoform contains two kinds of cross-links as follows: S-hydroxylysyl-methionine and S-lysyl-methionine cross-links, which stabilize the alpha3alpha5-heterodimers and alpha4alpha4-homodimers, respectively. Construction and analysis of a three-dimensional model of the D-isoform of the alpha3alpha4alpha5 NC1 hexamer revealed that crypticity is a consequence of the following: (a) sequestration of key residues between neighboring subunits that are stabilized by domain-swapping interactions, and (b) by cross-linking of subunits at the trimer-trimer interface, which stabilizes the structural integrity of the NC1 hexamer and protects against binding of autoantibodies. The sequestrated epitopes and cross-linked subunits represent a novel structural mechanism for conferring immune privilege at the level of quaternary structure. Perturbation of the quaternary structure may be a key factor in the etiology of Goodpasture disease.
authors.affiliation: Center for Matrix Biology, Vanderbilt University Medical Center, Nashville, Tennessee 37232, USA.
authors.forename: Roberto M
authors.initials: RM
authors.lastname: Vanacore
authors.forename: Amy-Joan L
authors.initials: AJ
authors.lastname: Ham
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Cartailler
authors.forename: Munirathinam
authors.initials: M
authors.lastname: Sundaramoorthy
authors.forename: Parvin
authors.initials: P
authors.lastname: Todd
authors.forename: Vadim
authors.initials: V
authors.lastname: Pedchenko
authors.forename: Yoshikazu
authors.initials: Y
authors.lastname: Sado
authors.forename: Dorin-Bogdan
authors.initials: DB
authors.lastname: Borza
authors.forename: Billy G
authors.initials: BG
authors.lastname: Hudson
mesh_terms: D000595:Amino Acid Sequence; D019867:Anti-Glomerular Basement Membrane Disease / Q000276:immunology*; D001324:Autoantigens / Q000737:chemistry / Q000276:immunology*; D024141:Collagen Type IV / Q000737:chemistry / Q000276:immunology*; D017124:Conserved Sequence; D003432:Cross-Linking Reagents; D019281:Dimerization; D018985:Epitopes, B-Lymphocyte / Q000276:immunology; D006801:Humans; D008969:Molecular Sequence Data; D020836:Protein Structure, Quaternary
chemical_list: D001324:Autoantigens; D024141:Collagen Type IV; D003432:Cross-Linking Reagents; D018985:Epitopes, B-Lymphocyte; C057112:type IV collagen alpha3 chain