Figure 1: Structure of UapA.

(a) Topology diagram of UapA. α-helices are represented by cylinders and β-strands by arrows. The substrate-binding site between the amino termini of TMs 3 and 10 is indicated by the schematic of xanthine (cyan). The protein is arranged into two domains, a gate domain consisting of TMs 5, 6, 7, 12, 13 and 14 (blue), and a core domain consisting of TMs 1, 2, 3, 4, 8, 9, 10 and 11 (red). The half helices and short β-strand regions of TMs 3 and 10 (both part of the core domain) are coloured yellow and green, respectively. (b) Ribbon representation of the UapA monomer. The gate domain is shown in blue while most of the core domain is shown in red. The amphipathic helices that link the core and gate domains are shown in grey. Xanthine is shown in cyan as a space-filling model and the disulphide bond is shown in orange sticks. Only the N-terminal (residues 12–65) and C-terminal (residues 546–574) ends are missing from the structure. (c) UapA dimer from the cytoplasmic side. One monomer is shown as in b, with the helices numbered, and the other is shown in surface representation with the same colouring as in a. The xanthine in the surface representation is labelled. (d) Surface representation of the dimer looking through the membrane. The pale blue lines indicate the likely location of the membrane.