 in the closed structure of the p2x cation channel , three -helical transmembrane domains cross the membrane obliquely : in rat p2x2 receptors , these intersect at thr339 . replacing thr339 by lysine in one , two or three subunits progressively increased chloride permeability and reduced unitary conductance . 
 this implies that the closed - open transition involves a symmetrical separation of the three subunits , and that thr339 from each contributes symmetrically to the open channel permeation pathway . 
Supplementary Material