文摘
Deletion of the yeast mitochondrial gene COX2 encoding subunit 2 (Cox2) of cytochrome c oxidase (CcO) results in loss of respiration (¦¤cox2 strain). Supekova et al. (2010) transformed a ¦¤cox2 strain with a vector expressing Cox2 with a mitochondrial targeting sequence (MTS) and the point mutation W56R (Cox2W56R), restoring respiratory growth. Here, the CcO carrying the allotopically-expressed Cox2W56R was characterized. Yeast mitochondria from the wild-type (WT) and the ¦¤cox2 + Cox2W56R strains were subjected to Blue Native electrophoresis. In-gel activity of CcO and spectroscopic quantitation of cytochromes revealed that only 60 % of CcO is present in the complemented strain, and that less CcO is found associated in supercomplexes as compared to WT. CcOs from the WT and the mutant exhibited similar subunit composition, although activity was 20-25 % lower in the enzyme containing Cox2W56R than in the one with Cox2WT. Tandem mass spectrometry confirmed that W56 was substituted by R56 in Cox2W56R. In addition, Cox2W56R exhibited the same N-terminus than Cox2WT, indicating that the MTS of Oxa1 and the leader sequence of 15 residues were removed from Cox2W56R during maturation. Thus, Cox2W56R is identical to Cox2WT except for the point mutation W56R. Mitochondrial Cox1 synthesis is strongly reduced in ¦¤cox2 mutants, but the Cox2W56R complemented strain led to full restoration of Cox1 synthesis. We conclude that the cytosol-synthesized Cox2W56R follows a rate-limiting process of import, maturation or assembly that yields lower steady-state levels of CcO. Still, the allotopically-expressed Cox2W56R restores CcO activity and allows mitochondrial Cox1 synthesis to advance at WT levels.