TRANSMEMBRANE HELIX-HELIX INTERACTIONS IN F(0) SUGGESTED BY SUPPRESSOR MUTATIONS TO ALA(24)-]ASP ASP(61)-]GLY MUTANT OF ATP SYNTHASE SUBUNIT-C/

A mutant of ATP synthase subunit c was isolated in which the essential aspartate was exchanged from position 61 on transmembrane helix-2 to position 24 on transmembrane helix-1 (Miller, M. J., Oldenburg, M., an d Fillingame, R. H. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 4900-49 041. The H+ transporting ATP synthase function of the Ala24 --> Asp/As p61 --> Gly mutant is not optimal, and cells grow more slowly than wil d type. Twenty-three third-site suppressor mutants with optimized func tion were isolated in this study. Ten of the optimizing mutations were located to helix-2 of subunit c, and seven of these fell in residues Phe53, Met57, and Met65. The side chains of these three residues are p roposed to form a hydrophobic surface on transmembrane helix-2, which participates in the presentation or occlusion of the essential asparta te carboxyl group during proton translocation. The other 13 optimizing mutations were located to subunit a, and 10 of these fell in residues Ala217, Ile221, and Leu224. These three residues are proposed to lie on one face of a transmembrane alpha-helix that includes the essential Arg210 residue. This helix is proposed to interact with the transmemb rane bihelical unit of subunit c during protonation and deprotonation of the essential Asp24 in the mutant or Asp61 in wild type.