The products of the mitochondrial orf25 and orfB genes are F O components in the plant F 1F O ATP synthase

Abstract

The F O 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 F 1F O, and separated F 1 and F O components. The subunits of each complex were analysed by mass spectrometry and matched to Arabidopsis genes. In the F 1F O 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 F 1 complex contained the five well-characterised F 1 subunits, while four subunits in the F O 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 F O 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.