Purification of a crenarchaeal ATP synthase in the light of the unique bioenergetics of Ignicoccus species.

In this study, the ATP synthase of Ignicoccus hospitalis was purified, characterized and structurally compared to the respective enzymes of the other Ignicoccus species to shed light on energy conservation in this unique group of Archaea. The crenarchaeal genus Ignicoccus comprises three described species: I. hospitalis and I. islandicus from hot marine sediments near Iceland and I. pacificus from a hydrothermal vent system in the Pacific Ocean. This genus is unique among all Archaea due to the unusual cell envelope consisting of two membranes that enclose a large intermembrane compartment (IMC). I. hospitalis is the best studied member of this genus, mainly for being the only known host for the potentially parasitic archaeon Nanoarchaeum equitans I. hospitalis grows chemolithoautotrophically, and its sole energy yielding reaction is the reduction of elemental sulfur with molecular hydrogen, forming large amounts of hydrogen sulfide. This reaction generates an electro-chemical gradient, which is used by the ATP synthase, located in the outer cellular membrane, to generate ATP inside the IMC. The genome of I. hospitalis encodes for nine subunits of an A-type ATP synthase, which we could identify in the purified complex. Although the maximal in vitro activity of the I. hospitalis enzyme was measured around pH 6, the optimal stability of the A1 AO complex seems to be at pH 9. Interestingly, the soluble A1 subcomplexes of the different Ignicoccus species exhibit significant differences in their apparent molecular masses in native electrophoresis, although their behavior in gel filtration and mass spectrometry chromatography is very similar. IMPORTANCE The Crenarchaeota represent one of the major phyla within the domain of the Archaea. This study describes the successful purification of a crenarchaeal ATP synthase. So far, all information about A-type ATP synthases stem from euryarchaeal enzymes. The fact that it has not been possible to purify this enzyme complex from a member of the Crenarchaeota until now, points towards significant differences in their stability, possibly caused by structural alterations. Furthermore, the study subject I. hospitalis has a particular importance among the Crenarchaeota, since it is the only known host of N. equitans The energy metabolism in this system is still poorly comprehended and our results can help understand the unique relationship between these two microbes.