Structural properties of Pnu transporters. (a) Sequence alignment of TM1–3 and TM5–7 of PnuC (N. mucosa) and PnuT (S. woodyi). Conserved residues in the substrate binding site and the potential gate regions are colored yellow and cyan, respectively. (b) Substrate binding site of PnuCNm (Protein Data Bank ID 4QTN) viewed from the membrane plane. A single protomer of PnuCNm is shown, with the two structurally related three-helix domains TM1–3 and TM5–7 colored blue and cyan, respectively, and TM4 in light gray. The nonconserved N-terminal TM is not shown for clarity. The residues of the binding site and potential gates are colored as in panel (a), and shown in stick representation. Residue numbering according to the sequence of PnuTSw, the numbers in brackets are from PnuCNm. The substrate nicotinamide riboside of PnuC is shown in black stick representation. (c) Domain structure of PnuCNm. The protein is viewed from the periplasmic side along an axis perpendicular to the plane of the membrane. The two structurally related three-helix domains TM1–3 and TM5–7 are colored blue and cyan, respectively.
