Nitrogen is, of course, an essential element for life, and the primary mode by which microbes acquire nitrogen is by ammonia uptake through the trimeric ammonia channel AmtB. Two crystal structures of bacterial AmtB (and one of archaeal Amt1) revealed that each monomer features a passageway for ammonia (NH3) and a pseudosymmetric pair of five transmembrane helix domains (for more on how this dual topology arrangement might have evolved, see Rapp et al., Reports, Science Express, 25 January 2007). When intracellular nitrogen is limiting, the primary AmtB regulator—the trimeric protein GlnK—dissociates to allow ammonia entry; an unregulated channel (and persistent ammonia influx) would be fatal given its effect on pH homeostasis.
Gruswitz et al. and Conroy et al. have solved the structure (at 2.0 and 2.5 Å, respectively) of the bacterial AmtB-GlnK complex, and Yildiz et al. have contributed an electron microscopic study of the archaeal Amt1-GlnK1 complex. They find that an extended region of GlnK referred to as the T loop, which contains an arginine residue at its tip, inserts into the cytoplasmic vestibule of AmtB and physically impedes the passage of ammonia. All five hydrogens of the guanidinium cation find partners on the interior surface of the AmtB pore, which seems fitting because the channel has, after all, adapted to conduct ammonia. How is such an intimate association disrupted? As intracellular nitrogen drops, the small molecule 2-oxoglutarate accumulates and binds to GlnK (precisely where is not quite clear yet) along with Mg-ATP; these interactions and perhaps ATP hydrolysis may be involved in the extension and retraction of the T loop. — GJC
Proc. Natl. Acad. Sci. U.S.A. 104, 42; 1213 (2007); EMBO J. 10.1038/sj.emboj.7601492 (2007).