Direct evidence for the cytoplasmic location of the NH2- and COOH-terminal ends of the Neurospora crassa plasma membrane H+-ATPase.

Reconstituted proteoliposomes containing Neurospora plasma membrane H+-ATPase molecules oriented predominantly with their cytoplasmic portion facing outward have been used to determine the location of the NH2 and COOH termini of the H+-ATPase relative to the lipid bilayer. Treatment of the proteoliposomes with trypsin in the presence of the H+-ATPase ligands Mg2+, ATP, and vanadate produces approximately 97-, 95-, and 88-kDa truncated forms of the H+-ATPase similar to those already known to result from cleavage at Lys24, Lys36, and Arg73 at the NH2-terminal end of the molecule. These results establish that the NH2-terminal end of the H+-ATPase polypeptide chain is located on the cytoplasmic side of the membrane. Treatment of the same proteoliposome preparation with trypsin in the absence of ligands releases approximately 50 water-soluble peptides from the proteoliposomes. Separation of the released peptides by high performance liquid chromatography and spectral analysis of the purified peptides identified only a few peptides with the properties expected of a COOH-terminal, tryptic undecapeptide with the sequence SLEDFVVSLQR, and NH2-terminal amino acid sequence analysis identified this peptide among the possible candidates. Quantitative considerations indicate that this peptide must have come from H+-ATPase molecules oriented with their cytoplasmic portion facing outward, and could not have originated from a minor population of H+-ATPase molecules of reverse orientation. These results directly establish that the COOH-terminal end of the H+-ATPase is also located on the cytoplasmic side of the membrane. These findings are important for elucidating the topography of the membrane-bound H+-ATPase and are possibly relevant to the topography of other aspartyl-phosphoryl-enzyme intermediate ATPases as well.