Figure 9.

Model of three TM2 helices to illustrate positions of key residues. Viewed from the outside, the circles pass through the Cα atoms of Asn³³³ (outer circle) and Thr³³⁶ (inner circle) from each chain. Viewed from the inside, the circle passes through the Cα atoms of Thr³³⁹. The structure shown is a threaded model of rat P2X2 generated using a default script in Modeler9v6 from the zebrafish P2X4 receptor in the closed state (pdb 3H9V) (Kawate et al., 2009) and indicating side-chains from Asn³³³ to Ser³⁴⁰ only. The side-chain of Ser³⁴⁰ is not seen, but the position of Ser³⁴⁰ Cα is indicated (white dots). The results indicate that Ser³⁴⁰ is exposed in the open channel permeation pathway, which implies that the three TM2 helices twist counter-clockwise during channel opening.