The OXA2a Insertase of Arabidopsis Is Required for Cytochrome Maturation.

In yeast () and human () mitochondria, Oxidase assembly protein1 (Oxa1) is the general insertase for protein insertion from the matrix side into the inner membrane while Cytochrome oxidase assembly protein18 (Cox18/Oxa2) is specifically involved in the topogenesis of the complex IV subunit, Cox2. Arabidopsis () mitochondria contain four OXA homologs: OXA1a, OXA1b, OXA2a, and OXA2b. OXA2a and OXA2b are unique members of the Oxa1 superfamily, in that they possess a tetratricopeptide repeat (TPR) domain at their C termini. Here, we determined the role of OXA2a by studying viable mutant plants generated by partial complementation of homozygous lethal transfer-DNA insertional mutants using the developmentally regulated () promoter. The plants displayed growth retardation due to a reduction in the steady-state abundances of both -type cytochromes, cytochrome and cytochrome The observed reduction in the steady-state abundance of complex III could be attributed to cytochrome being one of its subunits. Expression of a soluble heme lyase from an organism with cytochrome maturation system III could functionally complement the lack of OXA2a. This implies that OXA2a is required for the system I cytochrome maturation of Arabidopsis. Due to the interaction of OXA2a with Cytochrome maturation protein CcmF C-terminal-like protein (CCMF) in a yeast split-ubiquitin based interaction assay, we propose that OXA2a aids in the membrane insertion of CCMF, which is presumed to form the heme lyase component of the cytochrome maturation pathway. In contrast with the crucial role played by the TPR domain of OXA2b, the TPR domain of OXA2a is not essential for its functionality.