The T9176G mutation of human mtDNA gives a fully assembled but inactive ATP synthase when modeled in Escherichia coli. 
A new mutation in human F(1)F(0) ATPase6, T9176G, which changes Leu 217 to an Arg, has been described in two siblings with Leigh syndrome [Carrozzo et al. ( 2000) Neurology, in press].  This mutation was modeled in Escherichia coli by changing Leu 259 (the equivalent residue) to Arg and the properties of the altered ECF(1)F(0) were compared to those of previously characterized ATPase6 mutants also modeled in the E. coli enzyme.  The L259R change produced a fully assembled ECF(1)F(0) which had no significant ATP hydrolysis, ATP synthesis or proton pumping functions.  This is very different from previously described human ATPase6 mutations.  The presence of Arg at position 259 in subunit a did not make membranes permeable to protons.  We conclude that the mutation inhibits functioning by blocking the rotary motor action of the enzyme.