Accumulating evidence has showed that
anti-CASPR2 autoantibodies occur in a long list of neurological
immune disorders including
limbic encephalitis (LE). Belonging to the well-known neurexin superfamily, CASPR2 has been suggested to be a central node in the molecular networks controlling neurodevelopment. Distinct from other subfamilies in the neurexin superfamily, the CASPR subfamily features a unique
discoidin (Disc) domain. As revealed by our and others' recent studies, CASPR2 Disc domain bears a major
epitope for
autoantibodies. However, structural information on CASPR2 recognition by
autoantibodies has been lacking. Here, we report the crystal structure of human CASPR2 Disc domain at a high resolution of 1.31 Å, which is the first atomic-resolution structure of the CASPR subfamily members. The Disc domain adopts a total β structure and folds into a distorted jellyroll-like barrel with a conserved
disulfide-bond interlocking its N- and C-termini. Defined by four loops and located in one end of the barrel, the "loop-tip surface" is totally polar and easily available for
protein docking. Based on structure-guided
epitope prediction, we generated nine mutants and evaluated their binding to
autoantibodies of cerebrospinal fluid from twelve patients with
limbic encephalitis. The quadruple mutant G69N/A71S/S77N/D78R impaired CASPR2 binding to
autoantibodies from eleven LE patients, which indicates that the loop L1 in the Disc domain bears hot spots for
autoantibody interaction. Structural mapping of autoepitopes within human CASPR2 Disc domain sheds light on how
autoantibodies could sequester CASPR2 ectodomain and antagonize its functionalities in the pathogenic processes.