title: A cytosolic trans-activation domain essential for ammonium uptake.
abstract: Polytopic membrane proteins are essential for cellular uptake and release of nutrients. To prevent toxic accumulation, rapid shut-off mechanisms are required. Here we show that the soluble cytosolic carboxy terminus of an oligomeric ammonium transporter from Arabidopsis thaliana serves as an allosteric regulator essential for function; mutations in the C-terminal domain, conserved between bacteria, fungi and plants, led to loss of transport activity. When co-expressed with intact transporters, mutants inactivated functional subunits, but left their stability unaffected. Co-expression of two inactive transporters, one with a defective pore, the other with an ablated C terminus, reconstituted activity. The crystal structure of an Archaeoglobus fulgidus ammonium transporter (AMT) suggests that the C terminus interacts physically with cytosolic loops of the neighbouring subunit. Phosphorylation of conserved sites in the C terminus are proposed as the cognate control mechanism. Conformational coupling between monomers provides a mechanism for tight regulation, for increasing the dynamic range of sensing and memorizing prior events, and may be a general mechanism for transporter regulation.
authors.affiliation: Carnegie Institution, 260 Panama St, Stanford, California 94305, USA.
authors.forename: D
authors.initials: D
authors.lastname: Loqué
authors.forename: S
authors.initials: S
authors.lastname: Lalonde
authors.forename: L L
authors.initials: LL
authors.lastname: Looger
authors.forename: N
authors.initials: N
authors.lastname: von Wirén
authors.forename: W B
authors.initials: WB
authors.lastname: Frommer
mesh_terms: D000494:Allosteric Regulation; D017360:Arabidopsis / Q000166:cytology / Q000235:genetics / Q000378:metabolism*; D019611:Archaeoglobus fulgidus / Q000737:chemistry; D001692:Biological Transport; D027682:Cation Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017124:Conserved Sequence / Q000235:genetics; D003600:Cytosol / Q000737:chemistry*; D008958:Models, Molecular; D046912:Multiprotein Complexes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D010940:Plant Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D015533:Transcriptional Activation
chemical_list: D027682:Cation Transport Proteins; D046912:Multiprotein Complexes; D010940:Plant Proteins; D021122:Protein Subunits; D000644:Quaternary Ammonium Compounds; C088952:ammonium transporters, plant