Component of the branched-chain alpha-ketoacid dehydrogenase (BCKADH) complex, that catalyzes the overall conversion of branched-chain alpha-ketoacids to acyl-CoA and CO(2). (R)-N(6)-lipoyl-L-lysyl-[dihydrolipoyllysine-residue (2-methylpropanoyl)transferase] + 3-methyl-2-oxobutanoate + H(+) = (R)-N(6)-(S(8)-2-methylpropanoyldihydrolipoyl)-L-lysyl-[dihydrolipoyllysine-residue (2-methylpropanoyl)transferase] + CO2 Heteromer of E1 alpha (BkdA) and beta (BkdB) subunits. Part of the BCKADH complex, consisting of multiple copies of BkdA/BkdB (E1), BkdC (E2) and Lpd (E3). Up-regulated upon nutrient starvation. Is also highly up-regulated in a DlaT-deficient strain. Part of the bkdABC operon.