Characterization of the First Cytoplasmic Loop of Subunit a of the Escherichia coli ATP Synthase by Surface Labeling, Cross-linking, and Mutagenesis *

The first cytoplasmic loop of subunit a of the Escherichia coli ATP synthase has been analyzed by cysteine substitution mutagenesis. 13 of the 26 residues tested were found to be accessible to the reaction with 3-( N -maleimidylpropionyl)-biocytin. The other 13 residues predominantly found in the central region of the polypeptide chain between the two transmembrane spans were more resistant to labeling by 3-( N -maleimidylpropionyl)-biocytin while in membrane vesicle preparations. This region of subunit a contains a conserved residue Glu-80, which when mutated to lysine resulted in a significant loss of ATP-driven proton translocation. Other substitutions including glutamine, alanine, and leucine were much less detrimental to function. Cross-linking studies with a photoactive cross-linking reagent were carried out. One mutant, K74C, was found to generate distinct cross-links to subunit b, and the cross-linking had little effect on proton translocation. The results indicate that the first transmembrane span (residues 40–64) of subunit a is probably near one or both of the b subunits and that a less accessible region of the first cytoplasmic loop (residues 75–90) is probably near the cytoplasmic surface, perhaps in contact with b subunits.