MOLECULAR ANALYSIS OF A MAJOR ANTIGENIC REGION OF THE 240-KD PROTEIN OF MI-2 AUTOANTIGEN

Anti-Mi-2 autoantibody is strongly associated with dermatomyositis and found in sera of 20% of patients. Mi-2 antigen contains at least eigh t components and previous evidence suggested that the 240-kD protein w as the antigenic component for at least some sera. In this study, anti -Mi-2 patient sera were used to screen human thymocyte and HeLa cell l ambda gt11 expression libraries, and two clones from each had plaques specifically reactive with anti-Mi-2 sera. Studies with affinity-purif ied antibody supported the identification of the clones, All of 44 ant i-Mi-2 sera reacted with the plaques, but none of 44 control sera reac ted significantly, The cDNAs were identical, and full sequencing of on e revealed an open reading frame spanning a 1,054-bp insert, Rescreeni ng the library with the cDNA yielded a 1,589-bp cDNA that continued th e open reading frame, The Mi-2 cDNA hybridized to a single 7.5-8.0 kb mRNA of HeLa cells, by Northern blot, Rabbit antiserum directed at a p ortion of the cDNA product reacted with HeLa 240-kD Mi-2 protein, The sequence was notable for four potential zinc-fingers and several charg ed regions. The protein encoded by the cDNA produced in vitro reacted with only one of five of the Mi-2 sera. These findings indicate that t he Mi-2 240 kD is a novel protein that is antigenic for all Mi-2 sera, and strongly suggests that a major common epitope is conformational i n nature.