Analysis of the mi-2 autoantigen of dermatomyositis

OBJECTIVE: To determine the biochemical structure and antigenic components of Mi-2 autoantigen, the target of autoantibodies in 15-20% of dermatomyositis patients. METHODS: Immunoprecipitation from 35S-labeled HeLa cell extract, immunoblotting, and purification from bovine thymus by immunoaffinity chromatography. RESULTS: All 46 sera that had anti-Mi-2 autoantibodies demonstrated by immunodiffusion immunoprecipitated a major protein of approximately 240 kd. Additional proteins of 200, 150, 72, 65, 63, 50, and 34 kd appeared to be part of the antigen. Fractions of purified bovine Mi-2 with antigenic activity showed high molecular weight bands comparable with immunoprecipitated HeLa Mi-2. Twenty-four of 47 anti-Mi-2 positive sera reacted with the 240-kd protein by immunoblot against anti-Mi-2 immunoprecipitates. CONCLUSION: Mi-2 antigen consists of multiple proteins, of which the 240-kd protein appears to be the major reactive component.