Energized outer membrane and spatial separation of metabolic processes in the hyperthermophilic Archaeon Ignicoccus hospitalis

Abstract

ATP synthase catalyzes ATP synthesis at the expense of an electrochemical ion gradient across a membrane that can be generated by different exergonic reactions. Sulfur reduction is the main energyyielding reaction in the hyperthermophilic strictly anaerobic Crenarchaeon Ignicoccus hospitalis. This organism is unusual in having an inner and an outer membrane that are separated by a huge intermembrane compartment. Here we show, on the basis of immuno-EM analyses of ultrathin sections and immunofluorescence experiments with whole I. hospitalis cells, that the ATP synthase and H 2 :sulfur oxidoreductase complexes of this organism are located in the outer membrane. These two enzyme complexes are mandatory for the generation of an electrochemical gradient and for ATP synthesis. Thus, among all prokaryotes possessing two membranes in their cell envelope (including Planctomycetes, Gram-negative bacteria), I. hospitalis is a unique organism, with an energized outer membrane and ATP synthesis within the periplasmic space. In addition, DAPI staining and EM analyses showed that DNA and ribosomes are localized in the cytoplasm, leading to the conclusion that in I. hospitalis energy conservation is separated from information processing and protein biosynthesis. This raises questions regarding the function of the two membranes, the interaction between these compartments, and the general definition of a cytoplasmic membrane.Archaea | ATP synthase | ATPase | immunolabeling | sulfur reductase

AMP-Forming Acetyl Coenzyme A Synthetase in the Outermost Membrane of the Hyperthermophilic Crenarchaeon Ignicoccus hospitalis

“…As a consequence, ATP is generated from ADP and P i in the IMC. Since biosynthesis processes such as replication, transcription, and translation take place in the cytoplasm ATP has to be imported from the ICM (38). However, it is unknown at present, how the exchange of ATP and ADP across the inner (cytoplasmic) membrane is achieved.…”

“…Therefore, it was surprising that immunolabeling experiments on ultrathin sections of I. hospitalis cells not only confirmed the membrane association but even more showed an unexpected subcellular location of the ACS at the outermost membrane. As a consequence, the outermost membrane of I. hospitalis is now known not only to harbor the energy conserving machinery (ATP synthase and H 2 :sulfur oxidoreductase) (38) but also the acetate activating ACS complex which consumes ATP. Figure 7 summarizes the identified reactions and the localization of the corresponding enzyme complexes in the IMC and in the outermost membrane of I. hospitalis.…”

“…These two organisms form an "intimate association," the only known cultivable coculture of two Archaea (23,30). Recently, an important and highly unexpected feature of I. hospitalis was described: its outermost membrane inhabits the primary (H 2 :sulfur oxidoreductase) and secondary (A 1 A O ATP synthase) proton pumps necessary for energy conservation and therefore must be energized (38). Simultaneously, it was shown that in I. hospitalis DNA and ribosomes are exclusively located inside its inner membrane, leading to a spatial separation of energy conservation and information processing.…”

“…Simultaneously, it was shown that in I. hospitalis DNA and ribosomes are exclusively located inside its inner membrane, leading to a spatial separation of energy conservation and information processing. As a consequence, the compartment between the two membranes was renamed the "intermembrane compartment" (IMC) instead of the "periplasmic space" (38). It is the site of energy conservation and may contribute significantly to further metabolic activities of the cell.…”

Origin of life: LUCA and extracellular membrane vesicles (EMVs)

“…Only a few archaeal groups are diderm (Klingl 2014). The only diderm archaea that has been studied in detail is Ignicoccus hospitalis (Huber et al 2002;Küper et al 2010). The volume of Ignicoccus inter-membrane space can be large (20-1000 nm) and contains numerous vesicles that bud from the inner membrane and fuse with the OM (Näther & Rachel 2004).…”