Definition of Membrane Topology and Identification of Residues Important for Transport in Subunit a of the Vacuolar ATPase

Subunit a of the vacuolar H+-ATPases plays an important role in proton transport. This membrane-integral 100-kDa subunit is thought to form or contribute to proton-conducting hemi-channels that allow protons to gain access to and leave buried carboxyl groups on the proteolipid subunits (c, c', and c) during proton translocation. We previously demonstrated that subunit a contains a large N-terminal cytoplasmic domain followed by a C-terminal domain containing eight transmembrane (TM) helices. TM7 contains a buried arginine residue (Arg-735) that is essential for proton transport and is located on a helical face that interacts with the proteolipid ring. To further define the topology of the C-terminal domain, the accessibility of 30 unique cysteine residues to the membrane-permeant reagent N-ethylmaleimide and the membrane-impermeant reagent polyethyleneglycol maleimide was determined. The results further define the borders of transmembrane segments in subunit a. To identify additional buried polar and charged residues important in proton transport, 25 sites were individually mutated to hydrophobic amino acids, and the effect on proton transport was determined. These and previous results identify a set of residues important for proton transport located on the cytoplasmic half of TM7 and TM8 and the lumenal half of TM3, TM4, and TM7. Based upon these data, we propose a tentative model in which the cytoplasmic hemi-channel is located at the interface of TM7 and TM8 of subunit a and the proteolipid ring, whereas the lumenal hemi-channel is located within subunit a at the interface of TM3, TM4, and TM7.