MOLECULAR ARCHITECTURE OF THE C-SUBUNIT OLIGOMER IN THE MEMBRANE DOMAIN OF F-ATPASES PROBED BY TRYPTOPHAN SUBSTITUTION MUTAGENESIS

Subunit c of the proton-transporting ATP synthase of Escherichia coli forms an oligomeric complex in the membrane domain that functions in t ransmembrane proton conduction. Ln order to gain some insight into the architecture of this oligomeric complex, the transmembrane region in the C-terminal membrane-spanning segment was analysed by a site-direct ed mutagenesis approach. Tryptophan substitution of consecutive residu es in positions 61 to 72 of subunit c was used to identify residues or iented towards a helix-helix surface or an accessible phase in the oli gomeric complex. Mutants were analysed in functional assays of ATP hyd rolysis, ATP synthesis and ATP-dependent proton transport. Function wa s disrupted according to a pattern that identified inter- and intramol ecular contacts in the c-subunit oligomer. Screening experiments on mi nimal medium support the helix-helix contacts found in the functional assays. The results add strong constraints to the potential orientatio n of the monomers in the oligomeric complex and are discussed against the background of different structural models that have been proposed for the c-subunit oligomer. (C) 1998 Academic Press