Purification and properties of the ATphase Solubilized from Membranes of an Acidothermophilic Archaeobacterium, sulfolobus acidocaldarius1

Abstract

A novel ATPase was solubilized from membranes of an acidothermophilic archaebacterium, Sulfolobus acidocaldarius, with low ionic strength buffer containing EDTA. The enzyme was purified to homogeneity by hydrophobic chromatography and gel filtration. The molecular weight of the purified enzyme was estimated to be 360,000. Polyacrylamide gel electrophoresis of the purified enzyme in the presence of sodium dodecyl sulfate revealed that it consisted of three kinds of subunits, α, β, and γ, whose molecular weights were approximately 69,000, 54,000, and 28,000, respectively, and the most probable subunit stoichiometry was α₃β₃γ₃. The purified ATPase hydrolyzed ATP, GTP, ITP, and CTP but not UTP, ADP, AMP, or p-nitrophenylphosphate. The enzyme was highly heat stable and showed an optimal temperature of 85°C. It showed an optimal pH of around 5, very little activity at neutral pH, and another small activity peak at pH 8.5. The ATPase activity was significantly stimulated by bisulfite and bicarbonate ions, the optimal pH remaining unchanged. The Lineweaver-Burk plot was linear, and the K_m for ATP and the V_max were estimated to be 1.6 mM and 13 μmol Pi-mg.⁻¹.min⁻¹, respectively, at pH 5.2 at 60°C in the presence of bisulfite. The chemical modification reagent, 7-chloro-4-nitrobenzo-2-oxa-l,3-diazole, caused inactivation of the ATPase activity although the enzyme was not inhibited by N,N′-dicyclohexylcarbodiimide, N-ethyl-maleimide, azide or vanadate. These results suggest that the ATPase purified from membranes ofS. acidocaldarius resembles other archaebacterial ATPases, although a counterpart of theγ subunit has not been found in the latter. The relationship of the S. acidocaldarius ATPase to other ion-transporting ATPases, such as F₀F₁ type or E₁E₂ type ATPases, was discussed.