HUMAN AUTOANTIBODIES DIRECTED AGAINST THE RNA RECOGNITION MOTIF OF LA(SS-B) BIND TO A CONFORMATIONAL EPITOPE PRESENT ON THE INTACT LA (SS-B) RO (SS-A) RIBONUCLEOPROTEIN PARTICLE/

In systemic autoimmunity, the human B cell response to the La (SS-B) a utoantigen is polyclonal and directed to both conserved and human-spec ific epitopes. This study has further characterized the B cell epitope (s) present within the conserved central region of the La protein, LaC (amino acids 111-242) containing the RNA recognition motif (RRM, aa 1 11-187). Ten overlapping and non-overlapping protein fragments spannin g LaC were expressed in bacteria as NH2-terminal fusions with glutathi one-S-transferase. The fusion proteins were tested by ELISA for reacti vity with a panel of human anti-La sera in order to define the nature of the epitopes. Ninety-two percent of patient sera containing anti-La antibodies reacted with the region of La containing the RRM. Fine map ping of this reactivity using deletion mutants indicated that the dele tion of 19 amino acids from either the NH2-terminal or COOH-terminal r egion of the RRM was associated with loss of antibody reactivity, sugg esting that the immunodominant epitope expressed in this region is dis continuous. Autoantibodies affinity-purified from the La RRM fragment to remove other specificities immunoprecipitated newly synthesized nat ive La (SS-B)/Ro (SS-A) complexes, providing additional evidence that autoantibodies were recognizing a conformational epitope. The findings indicate that the human autoantibody response to La involves recognit ion of a conformational determinant involving the conserved RRM region without necessarily interfering with the RNA-dependent association of the La/Ro ribonucleoprotein.