title: Methanogenesis and the K+ transport system are activated by divalent cations in ammonia-treated cells of Methanospirillum hungatei.
abstract: We describe a K+ transport system in Methanospirillum hungatei cells depleted of cytoplasmic K+ via an ammonia/K+ exchange reaction (Sprott, G. D., Shaw, K. M., and Jarrell, K. F. (1984) J. Biol. Chem. 259, 12602-12608). Ammonia-treated cells contained low concentrations of ATP and were unable to make CH4 or to transport 86Rb+. All of these properties were restored by CaCl2, MgCl2, or MnCl2, and not by CoCl2 or NiCl2. The Rb+ transport system had a Km of 0.42 and Vmax of 29 nmol/min X mg; K+ inhibited competitively. Both H2 and CO2 were required for appreciable transport, whereas air, valinomycin, or nigericin were potent inhibitors. The influx of Rb+ was electrogenic and associated with proton efflux, producing a delta pH (alkaline inside) in acidic media. In the absence of K+ (or Rb+), the activation of CH4 synthesis by Mg2+ produced little change in the cytoplasmic pH, showing that methanogenesis did not elicit a net efflux of protons. The pH optimum for transport was in the range 6.0-7.3 where the transmembrane pH gradient would contribute minimally to the proton motive force. Protonophores at pH 6.3 caused a partial decline in CH4 synthesis and the ATP content and dramatically collapsed Rb+ transport. These and other inhibitor experiments, coupled with the fact that the Rb+ gradient was too large to be in equilibrium with the proton motive force alone, suggest a role for both ATP and the proton motive force in Rb+ transport. Also, a role for K+ in osmoregulation is indicated.
mesh_terms: D000641:Ammonia / Q000494:pharmacology*; D001693:Biological Transport, Active / Q000187:drug effects; D002413:Cations, Divalent; D019605:Euryarchaeota / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D008697:Methane / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D012413:Rubidium / Q000378:metabolism
chemical_list: D002413:Cations, Divalent; D015636:Magnesium Chloride; D000641:Ammonia; D008274:Magnesium; D012413:Rubidium; D008697:Methane; D011188:Potassium