Su e of the yeast F1Fo-ATP synthase forms homodimers

The yeast F1Fo-ATP synthase forms a dimeric complex in the mitochondrial inner membrane. Dimerization of two F1Fo monomeric complexes involves the physical association of two membrane-embedded F-o sectors and in a manner, which is dependent on the F-o subunit, Su e. Sequence analysis of Su e protein family members indicated the presence of a conserved coiled-coil motif. As this motif is often the basis for protein homodimerization events, it was hypothesized that Su e forms homodimers in the inner membrane and that formation of Su e dimers between two neighboring F-o complexes would facilitate dimerization of the F1Fo-ATP synthase complex (Arnold, I., Pfeiffer, K., Neupert, W., Stuart, R. A., and Schagger, H. (1998) EMBO J. 17, 7170-7178). Using a histidine-tagged derivative of yeast Su e, Su e-His(12), combined with cross-linking and affinity purification approaches, we have directly demonstrated the ability of the yeast Su e protein to form homodimers. Functionality of the Su e-His(12) derivative was confirmed by its ability to assemble into the F1Fo-ATP synthase complex and to support its dimerization in the Deltasu e null mutant yeast cells. The close association of two neighboring Su e proteins was also demonstrated using cross-linking with Cu2+, which binds and cross-links a unique Cys residue in neighboring Su e proteins. Finally, we propose a model for the molecular basis of the homodimerization of the Su e proteins.