The Dynamic Behavior of the P2X4 Ion Channel in the Closed Conformation.

We present the results of a detailed molecular dynamics study of the closed form of the P2X4 receptor. The fluctuations observed in the simulations were compared with the changes that occur in the transition from the closed to the open structure. To get further insight on the opening mechanism, the actual displacements were decomposed into interchain motions and intrachain deformations. This analysis revealed that the iris-like expansion of the transmembrane helices mainly results from interchain motions that already take place in the closed conformation. However, these movements cannot reach the amplitude required for the opening of the channel because they are impeded by interactions occurring around the ATP binding pocket. This suggests that the union of ATP produces distortions in the chains that eliminate the restrictions on the interchain displacements, leading to the opening of the pore.