Inhibition and Labeling of the Plant Plasma Membrane H-ATPase with N-Ethylmaleimide.

H(+)-ATPase activity in plasma membranes isolated from Avena sativa root cells is inhibited by N-ethylmaleimide, a covalent modifier of protein sulfhydryl groups. The rate of inhibition is reduced by ADP, MgADP, and MgATP, but even at 40 millimolar ADP the enzyme is only partially protected against inactivation. When plasma membranes are treated wth N-[2-(3)H]ethylmaleimide and analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis, prominent radioactive bands appear at M(r)=100,000 and several other positions. However, only radioactivity in the M(r)=100,000 protein is reduced by the presence of MgADP. These results provide independent evidence that the M(r)=100,000 polypeptide which is observed in purified preparations of the enzyme is the catalytic subunit of the H(+)-ATPase. When tryptic peptides are produced from N-[2-(3)H]ethylmaleimide labeled M(r)=100,000 protein and separated by reverse phase high performance liquid chromatography, two radioactive peaks are observed for which N-[2-(3)H]ethylmaleimide incorporation is reduced in the presence of MgADP.