De Michele, R. et al. eLife 2, e00800 (2013).

Ammonium is an important source of nitrogen for bacteria, fungi and plants, and excreted as a waste product by animals. Ammonium transport is regulated by ammonium transporters, but the functions of such proteins have proved difficult to study. De Michele et al. now report ammonium transport activity sensors for Arabidopsis ('AmTrac') and for yeast ('MepTrac') by engineering a circularly permuted GFP into the middle of an ammonium transporter sequence. When the protein undergoes a conformational change during transport, the fluorescent properties of the GFP are altered and can be read out by fluorescence detection. The concept could be applied to generate activity sensors for a variety of other transporters, receptors and enzymes.