The products of the mitochondrial orf25 and orfB genes are FO components in the plant F1FO ATP synthase

The FO portion of the mitochondrial ATP synthase contains a range of different subunits in bacteria, yeast and mammals. A search of the Arabidopsis genome identified sequence orthologs for only some of these subunits. Blue native polyacrylamide gel electrophoresis separation of Arabidopsis mitochondrial respiratory chain complexes revealed intact F1FO, and separated F1 and FO components. The subunits of each complex were analysed by mass spectrometry and matched to Arabidopsis genes. In the F1FO complex a series of nine known subunits were identified along with two additional proteins matching the predicted products of the mitochondrial encoded orfB and orf25 genes. The F1 complex contained the five well‐characterised F1 subunits, while four subunits in the FO complex were identified: subunit 9, d subunit, and the orfB and orf25 products. Previously, orfB has been suggested as the plant equivalent of subunit 8 based on structural and sequence similarity. We propose that orf25 is the plant b subunit based on structural similarity and its presence in the FO complex. Chimerics of orf25, orfB, subunit 9 and subunit 6 have been associated with cytoplasmic male sterility in a variety of plant species, our additional findings now place all these proteins in the same protein complex.

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