Protein–Protein Interactions Between Native Ro52 and Immunoglobulin G Heavy Chain

Using a yeast two‐hybrid system to search for proteins interacting with Ro52 autoantigen, we identified a novel protein–protein interaction. Two different cDNA clones, which interacted with Ro52 in the yeast twohybrid system, were identified and isolated from a human B‐cell library. Surprisingly, both clones encoded the heavy chain of human IgG1. The expression of both HIS3 and b‐galactosidase reporter genes in yeast suggested that the interaction between Ro52 and IgG occurred in vivo. In vitro studies utilizing recombinant Ro52 and purified immunoglobulins indicated that the interaction was immunoglobulin class and subclass specific. Ro52 interacted with IgG1 and IgG4, but not with IgG2, IgG3, IgA or IgM. Ro52 could also precipitate IgG directly from serum. The identified cDNA clones did not include the variable region of IgG, which suggested a non‐classical interaction independent of antibody specificity. We further mapped the domain of Ro52 responsible for this interaction to the C‐terminus rfp‐like region. In conclusion, our data support an unusual interaction between native Ro52 and IgG. The potential biological significance of this unusual protein–protein interaction is discussed.