Comparative genomics reveals electron transfer and syntrophic mechanisms differentiating methanotrophic and methanogenic archaea
Fig 9
Overview of proposed EET pathways.
Comparison between EET systems known from gram-negative bacteria and proposed analogous systems in ANME archaea. (A) EET systems in gram-negative bacteria involve membrane-bound quinol:cytochrome c oxidoreductases (CbcL, ImcH, CymA, NetD), small soluble cytochromes apparently involved in electron transport through the periplasmic space (PpcA, Stc, PdsA), and a beta-barrel/decaheme cytochrome c protein complex (MtrCAB) that acts as an electron conduit by which electrons can transit through the outer membrane to the extracellular space filled with additional cytochrome c such as OmcZ and filaments of OmcS. (B) Analogous protein complexes found in ANME genomes that appear optimized for the challenges associated with EET in the archaeal cell architecture. MpH2:cytochrome c oxidoreductases are likely present in the form of gene clusters containing VhtC cytochrome b subunits together with large 7 or 11 heme-binding MHC proteins (Mco). Other potential options for this step could include the NapH homologs sporadically distributed through ANME genomes or through the action of the unique cytochrome b gene found in ANME Rnf clusters. Electron transfer through the outer proteinaceous S-layer requires a different mechanism than the beta-barrel/decaheme cytochrome strategy evolved in the EET-capable bacteria containing an outer membrane. This step is expected to be overcome by the giant ANME-specific MHC proteins containing S-layer domains allowing them to integrate into the S-layer structure. Very close homologs of OmcZ are found in ANME (see Fig 10). For details of S-layer MHC fusions, see Fig 11. ANME, anaerobic methanotrophic; EET, extracellular electron transfer; Mco, methanophenazine-cytochrome c oxidoreductase; MHC, multiheme c-type cytochrome; Rnf, Rhodobacter nitrogen fixation.