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1: Nature. 2007 Mar 8;446(7132):195-8. Epub 2007 Feb 11.
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A cytosolic trans-activation domain essential for ammonium uptake.

Loqué D, Lalonde S, Looger LL, von Wirén N, Frommer WB.

Carnegie Institution, 260 Panama St, Stanford, California 94305, USA.

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.

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PMID: 17293878 [PubMed - indexed for MEDLINE]