Autoantibody to the leucine zipper region of 52 kDa Ro/SSA binds native 60kDa Ro/SSA: Identification of a tertiary epitope with components from 60 kDa Ro/SSA and 52 kDa Ro/SSA

Anti-Ro (or SSA) is found in the sera of patients with autoimmune rheumatic illnesses. All patients with anti-Ro defined by precipitation bind a 60 00 0 Da antigen (60 kDa Ro), whereas some patients also bind a 52 000 Da molec ule (52 kDa Ro). In general, antibody binding is directed against native 60 kDa Ro and denatured 52 kDa Ro. The mechanism by which anti-52 kDa Ro aris es in the setting of anti-60 kDa Ro is unknown. Conflicting data exist as t o the existence of a physical interaction between the two proteins in cells and as to cross-reacting antibodies. Antibodies were affinity purified fro m a peptide within the leucine zipper region of 52 kDa Ro. These purified a ntibodies binding the 197-207 peptide from 52 kDa Ro (anti-52LZ) bound nati ve 60 kDa Ro as well as denatured 52 kDa Ro. In addition, anti-52LZ also bo und up to four regions from the sequence of 60 kDa Ro and a single conforma tional epitope of 60 kDa Ro. Thus, these primary sites represent components of the tertiary epitope. We hypothesized that if this was the case, these peptides making up a tertiary epitope would show molecular interaction. In fact, peptides from 60 kDa Ro have a molecular interaction with the 52 kDa Ro peptide as well as full-length 52 kDa Ro when assessed by surface plasmo n resonance. The leucine-zipper region peptide from 52 kDa Ro bound three o f the four peptides from 60 kDa Ro. These data suggest that these two molec ular species, 60 and 52 kDa Ro, form a conformational epitope. This relatio nship may explain why anti-52 kDa Ro is found in association with anti-60 k Da Ro.