STRUCTURAL DIFFERENCES BETWEEN THE HUMAN AND MOUSE 52-KD RO AUTOANTIGENS ASSOCIATED WITH POORLY CONSERVED AUTOANTIBODY ACTIVITY ACROSS SPECIES

Anti-nuclear autoantibodies found in human autoimmune diseases frequen tly cross-react with homologous autoantigens in distant species, suppo rting the notion that autoantibodies target conserved functional domai ns. However, the 52-kD Ro(SS-A) protein is an exception, in that human autoantibodies are not known to recognize any equivalent antigen in t he cells of rodents and other non-primate species. To understand this lack of cross-reactivity we have isolated cDNAs encoding the mouse 52- kD Ro molecule. The cDNA encoding mouse 52-kD Ro revealed an open read ing frame of 470 amino acids, with 70% sequence identity to the human 52-kD Ro antigen. The putative leucine-zipper and zinc-finger motifs p resent in human Ro52 were conserved in the mouse protein. Recombinant mouse 52-kD Ro protein reacted with human autoantibodies by ELISA and immunoblot, but with approximately 10-fold lower reactivity than recom binant human 52-kD Ro protein under the same conditions. Detection of both human and mouse 52-kD Ro by immunoblot was dependent on antigen c oncentration which was limiting in the cell equivalents generally used in immunoblot assays. Differential chaotropic disruption of antibody binding suggested a lower avidity of human autoantibody binding to the mouse 52-kD Ro protein compared with the human antigen. Thus the poor reactivity of native mouse 52-kD Ro with human autoantibodies is asso ciated with species divergence diffusely distributed throughout the pr imary structure of the 52-kD Ro molecule.