A plasma‐membrane associated ATPase from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius

Thermoacidophilic archaebacteria have gained much interest because of their phylogenetic distance to eubacteria and eukaryotes and also because of their unique living conditions. Investigation of the energy‐converting system therefore offers a key for understanding the evolutionary position and environmental adaptation of these unusual bacteria. A plasma‐membrane‐associated adenosine triphosphatase with specific activities of 0.3–0.6 μmol min −1 (mg protein) −1 has been detected in the thermoacidophilic archaebacterium Sulfolobus acidocaldarius (DSM 639). The enzyme exhibits two optima at pH 5.5 and 8.0, sulfite activation leads to only one optimum at pH 6.25. In the presence of the divalent cations Mg 2+ or Mn 2+ it hydrolyzes ATP with highest reactivity and also other purine and pyrimidine nucleotides, but not ADP and pyrophosphate. A specific stimulation by monovalent cations is not observed. The ATPase activity is not inhibited by N, N ′‐dicyclohexylcarbodiimide, azide or vanadate, but it is by the vacuolar ATPase inhibitor nitrate with an [I] 50 of 8 mM. Linear Arrhenius plots up to 75°C reflect pronounced adaptation to the hot environment of the archaebacterium. The solubilized ATPase as localized by activity staining in non‐denaturating gels and further analyzed by sodium dodecyl sulfate electrophoresis is composed of two major polypeptides of 65 and 51 kDa reminiscent of the α and β subunits of eubacterial and eukaryotic F 0 F 1 ‐ATPases. The ATPase is suggested as a probable candidates for a reversibly acting ATP synthase responsible for oxidative phosphorylation found in Sulfolobus acidocaldarius.