Ignicoccus hospitalis and Nanoarchaeum equitans represent the first obligate symbiotic / parasitic community within the Archaea. While I. hospitalis is closely related to the other Ignicoccus species, N. equitans represents the first and so far only cultivated representative of the Nanoarchaeota phylum. In its genome (only 490 kbp) many biosynthetic pathways are rudimentary or missing. Lipid analyses demonstrated that N. equitans receives its membrane lipids from I. hospitalis; ongoing studies indicate that this may also be true for amino acids and carbohydrates. In this proposal, we will focus on key questions of the cell-cell interaction: enzymes involved in energy conversion, ATP generation/ hydrolysis, and structural aspects of the contact site. We will characterize the ATPase of I. hospitalis and then focus on the N. equitans enzyme, a probably rudimentary ATPase of five subunits only, according to genome data. Both projects will include: determining structure and composition of the ATPase, immun-localizing the enzyme, and studying their hydrolysing and synthesizing activity in inverted membrane vesicles. The questions to be answered are: is the N. equitans enzyme a minimal and therefore ancient ATPase? Does it contain further subunits originating from the Ignicoccus enzyme? Is N. equitans an energy predator using ATP from its host? In addition, we are in the process of identifying molecules 3 which are involved in the interaction between these two archaeal cells. It is our aim to directly visualize the cell-cell contact site, using serial sections, tomographic reconstructions, and immunolabeling of sections. Molecular tools are antibodies directed against a variety of proteins which are likely to be part of the cell surface or integrated in the contact site, in particular S-layer, outer-membrane-proteins , flagella , ATP synthases, and AAA proteins.
