Autoreactive epitope profiles of the proliferating cell nuclear antigen define two classes of autoantibodies

The proliferating cell nuclear antigen (PCNA) is a conserved protein required for cellular DNA replication. PCNA was first recognized using serum from the autoimmune disease SLE. To analyze the regions on PCNA that confer autoantibody binding, we modified the cDNA encoding full-length PCNA to generate a series of amino terminal, carboxyl terminal and internally deleted constructs, which were transcribed and then translated in vitro using the wheat germ cell-free translation system. An immunoprecipitation assay was used to study the ability of these mutated forms of PCNA to bind anti-PCNA Abs from patients with SLE. Eight of the ten sera studied required a protein of nearly full length for binding: antigenicity was abrogated by removal of 39 amino acids from the amino terminus, by various internal deletions, or by the removal of 15 (but not 11) amino acids from the carboxyl terminus. The remaining two sera exhibited an Ab-binding specificity to the carboxyl-terminally truncated proteins similar to that of the majority of anti-PCNA sera, but their specificity was different in the amino terminus: these sera were able to recognize PCNA lacking over 40% of sequence in the amino terminus, but they did not bind proteins with short internal deletions in that region. Thus, these epitopes appear to be conformational and distinguish two classes of autoimmune sera.