Characterization of a highly diverged mitochondrial ATP synthase F(o) subunit in Trypanosoma brucei

The mitochondrial F(1)F(o) ATP synthase of the parasite Trypanosoma brucei has been previously studied in detail. This unusual enzyme switches direction in functionality during the life cycle of the parasite, acting as an ATP synthase in the insect stages, and as an ATPase to generate mitochondrial membrane potential in the mammalian bloodstream stages. Whereas the trypanosome F(1) moiety is relatively highly conserved in structure and composition, the F(o) subcomplex and the peripheral stalk have been shown to be more variable. Interestingly, a core subunit of the latter, the normally conserved subunit b, has been resistant to identification by sequence alignment or biochemical methods. Here, we identified a 17 kDa mitochondrial protein of the inner membrane, Tb927.8.3070, that is essential for normal growth, efficient oxidative phosphorylation, and membrane potential maintenance. Pull-down experiments and native PAGE analysis indicated that the protein is both associated with the F(1)F(o) ATP synthase and integral to its assembly. In addition, its knockdown reduced the levels of F(o) subunits, but not those of F(1), and disturbed the cell cycle. Finally, analysis of structural homology using the HHpred algorithm showed that this protein has structural similarities to F(o) subunit b of other species, indicating that this subunit may be a highly diverged form of the elusive subunit b.