Homology model of RhAG. (A) Structural model of the RhAG trimeric complex looking from the outside of the cell. The helices 2 through 12 are displayed as Cα ribbon and are numbered for one RhAG monomer. The twin-histidine motif (His175 and His334), the phe-gate (Phe120 and Phe225), and residues mutated in OHSt patients (Ile61 and Phe65) are labeled. (B) Side view of the channel in one monomer of wild-type RhAG. Helices 3, 4, and 5 of the gray monomer have been removed to show the putative conductance pathway. In addition to the residues labeled in panel A, side-chains of residues that contribute to the hydrophobic environment of the Ile61 side-chain are marked. The dashed box indicates the area displayed in panels C through E. (C) Close-up view of the central pore in one monomer of wild-type RhAG. A cross-section of the central pore of one monomer of wild-type RhAG is shown. The protein surface is colored gray, with darker areas representing either solvent accessible surface or cavities within the protein. Side-chains of key residues are shown and labeled as in panel B. (D) Close-up view of the central pore in one monomer of Ser65 RhAG. A cross-section of the central pore of one monomer of Ser65 RhAG is shown and labeled as in panel B. The replacement of Phe65 with Ser65 is predicted to open the pore structure to a minimum width of about 3.6 Å between the cytoplasm and the phe-gate. This is sufficiently wide to allow hydrated Na+ (3.58 Å), K+ (3.31 Å), and NH4 (3.31 Å) ions to pass.27 (E) Close-up view of the central pore in one monomer of Arg61 RhAG. A cross-section of the central pore of one monomer of Arg61 RhAG is shown and labeled as in panel B. The replacement of Ile61 with Arg61 is predicted to open the pore structure to a minimum width of about 1.5 Å between the cytoplasm and the phe-gate. However, the incorporation of the positively charged guanidinium group from the arginine into a hydrophobic region may lead to more pronounced structural changes than those observed in our model. (F) The central pore of one monomer of wild-type RhAG looking from the outside of the cell. The wild-type model is viewed looking down through the central pore. The model is colored and labeled as in panel B. (G) The central pore of one monomer of Ser65 RhAG looking from the outside of the cell. The Ser65 model is viewed looking down through the central pore. Replacement of Phe65 with Ser65 opens up the pore structure beyond the phe-gate. The model is colored and labeled as in panel B. (H) The central pore of one monomer of Arg61 RhAG looking from the outside of the cell. The Arg61 model is displayed showing the hydrophobic environment of the Arg61 side-chain and its relationship to the central pore. The model is colored and labeled as in panel B.
