enzyme complexes, indicating that these regions were not important for function. A

fourth deletion, from residues al20-124 WAS concluded to be important for function, but not

assemble because high levels of a subunit were found in the membrane, but the enzyme

was not functional.

 The importance of the carboxyl-terminus was also analyzed by constructing a series

of early termination codons (164). Sequence alignment of the a subunit demonstrates

that many bacterial homologues contain glutamate and histidine residues at the extreme

carboxyl-terminus (glu-glu-his in E. coli). However, truncation of the Einal four residues

had no effect, and truncation of the Einal nine residues were tolerated at 250C, suggesting

that the extreme carboxyl terminus of the a subunit did not significantly contribute to

proton conduction or functional interactions with other subunits.

 Proton translocation. The first indication that the a subunit was directly involved

in proton translocation appeared nearly two decades ago when mutations constructed in

the a subunit (aser206-leu and ahis245-tyr) WeTO found to affect Fo-mediated proton pumping

without influencing F1Fo ATP synthase assembly (165). Since then, not including the

cysteine mutations described above, more than 75 missense mutations have been

constructed and analyzed in or near the conserved regions of the a subunit to chart the

amino acids involved in proton translocation. In general, mutation of a conserved amino

acid residue impaired Fo-mediated proton translocation, but the severity of the defects

varied (166).

 The only F1Fo ATP synthase a subunit residue that is strictly conserved amongst all

species, from bacteria to humans, and cannot endure any amino acid substitution, whether

basic, acidic or nonpolar, was aarg210 (145-147). Mutations at this site abolished both