(A) Digitonin-solubilized mitochondria from doxycycline-treated 143B cells, COX1∆ and COX2∆ cybrids were analyzed by BN-PAGE followed by CI-IGA assays (upper panels) or immunoblotting (medium and lower panels) with antibodies against CI subunits NDUFA9 and NDUFS1. The identities of MRC complexes and SCs are as in Figure 1. 