(A) Representation of the ammonium transport protein studied by Frommer and co-workers (De Michele et al., 2013). This protein, which contains 11 transmembrane helices, transports ammonium into the cell (red arrow). De Michele et al. inserted a variant of green fluorescent protein (cpGFP; green) into the cytosolic loop between helix V (shown in pink) and helix VI (blue) to create a fluorescent sensor that reports the binding of ammonium to the protein and/or the transport of ammonium through the internal pore. (B) Representative transport and channel proteins that share an internal pseudo-twofold symmetry. LacY (left; Abramson et al., 2003) and AmtB (centre; Khademi et al., 2004) are transport proteins; human aquaporin 5 (right; Horsefield et al., 2008) is a channel protein. All three are shown as ribbon diagrams viewed from the cytoplasmic side: light pink indicates the N-terminal half and light blue indicates the C-terminal half; the yellow ribbon in AmtB is not part of either half. In each protein, the substrate (lactose, ammonium or water) is thought to be translocated through the interface between the two halves (marked by red stars). AmtB and AQP5 can assemble in the plasma membrane into compact trimers and tetramers respectively (grey). Figure prepared by Yi Wang based on images from the Protein Data Bank: www.rcsb.org/pdb/home/home.do.