Purification and characterization of an ATPase from Natronococcus occultus, a haloalkaliphilic Archaeon
Natronococcus occultus is a member of the haloalkaliphilic Archaea which exist in desert soda lakes, an environment characterized by both high sodium chloride concentrations and pH. As a result, it has developed a unique biochemistry and novel enzymes in order to cope with these extreme conditions. An ATPase from N. occultus was purified and its properties were determined. The native molecular mass of the ATPase was around 130 kDa and was composed of 74 kDa and 61 kDa subunits. Enzyme activity was specific to the hydrolysis of ATP, with slight activity towards GTP, CTP, and ITP hydrolysis. The enzyme had a salt requirement; enzyme activity varied over a broad range of NaCl and KC1 concentrations (maximal activity at 2.5 M for both salts). The enzyme was also active in the presence of 1.5 M Na2SC>4 and 1 M (NH4 )2SC>4 and not active in the presence of Na2SC>3 or sodium citrate. The enzyme had a requirement for Mg2+, but other divalent cations could substitute to a lesser extent. The ATPase was alkaliphilic, with maximal activity at pH 9.0. Enzyme activity decreased substantially within 24 h at room temperature. In addition, the enzyme was cold-labile, making storage difficult. The ATPase from N. occultus was inhibited by NBD-C1, NaN3 , and ouabain, and was sensitive to nitrate, vanadate, DCCD, and bafilomycin Ai. It was not inhibited by thiocyanide or NEM. Addition of dithiothreitol resulted in an artificial four-fold increase in ATPase activity. The ATPase had a Kk of 0.5 mM and appeared to be non-competitively inhibited by NaN3 with a Ki of 3.1 mM. The ATPase from N. occultus possessed some unique characteristics, but overall it can be classified as an A-ATPase.