1. Carnegie Institution, 260 Panama St, Stanford, California 94305, USA
2. Institute for Plant Nutrition, University of Hohenheim, Stuttgart 70593, Germany
3. These authors contributed equally to this work.

Correspondence to: W. B. Frommer¹ Correspondence and requests for materials should be addressed to W.B.F. (Email: wfrommer@stanford.edu).

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)¹ suggests that the C terminus interacts physically with cytosolic loops of the neighbouring subunit. Phosphorylation of conserved sites in the C terminus² 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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