Visualization of cross-links on structural models. (A) TM1 of the hP2X1R in the closed state and residues 25–29 mutated to cysteines with BMB (10.9 Å) as cross-linker. (B) Cartoon representation of TM1 and N-terminal region (one subunit) for an ab initio model with residues 25 in cross-linking distance of the hP2X1R in the closed state (gray) and the hP2X1R in the ATP-bound state homology model (black). Main chain atoms for the residues 25–29 are shown as spheres, colored in rainbow order; zoom-in box refers to the superimposition of the N-terminal region. (C) Cartoon representation of TM1 and N-terminal region (trimer) for the ab initio model of the hP2X1R in the closed state (gray) and the hP2X1R in the ATP-bound state homology model (black). (D) Same representation as C but with TM2 and C-terminal region added. (E) Transmembrane region of the hP2X1R G30C homology model in the closed state viewed along the membrane. The N-terminal TM-helices are shown in blue, C-terminal TM helices in white. The C30 side-chain is shown as spheres with the cysteine sulfur atom in yellow. The red double arrows indicate the actual C30-C30 sulfur-sulfur distance as derived from the homology model (28 Å), and the C30-C30 sulfur-sulfur distance that is required to accommodate a G30C-G30C cross-link (12 Å). (F) As in E, but rotated by 90°, view perpendicular to the membrane from the intracellular side.