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Positively Charged Residues in the Head Domain of P2X4 Receptors Assist the Binding of ATP

P2X receptors are a family of trimeric cationic channels located in the membrane of mammalian cells. They open in response to the binding of ATP. The modifications that occur during the close-to-open transition have been described in detail, for several members of the family. However, the order in which these variations take place, and the residues involved in the intermediate stages, are still unknown. Here we present the results of Umbrella Sampling simulations aimed to elucidate the sequence of conformational changes that occur during the reversible binding of ATP to the P2X4 receptor. The simulations also provided information about the interactions that develop in the course of the process. In particular, they revealed the existence of a metastable state which assists the binding process. This state is stabilized by positively charged residues located in the head domain of the receptor. Based on these findings we propose a novel mechanism for the capture of ATP by P2X4 receptors.