Mitochondrial antigen M2 is a cluster of four major mitochondrial innermembrane proteins of approximately 74, 52, 45 and 39 kDa. Contains the E2 subunit of branched chain 2-oxo acid dehydrogenase, the E2 subunit of the 2-oxo dehydrogenase complex, protein X and the E1a and E1b subunits of the pyruvate dehydrogenase complex. Other antigens (Sm, RNP, SSA, Jo-1 and SSB) are not detected with ELISA assays.