Figure 3. State-dependent accessibility of extracellular Ag⁺ in P2X₂ receptor channels.

a, P2X₂-3T is insensitive to Ag⁺ in both the absence and presence of ATP. Same experimental paradigm as in Fig. 1a. b, c, Superimposed scaled current traces for I332C and T336C showing that these channels are rapidly modified by 7 nM Ag⁺ in the presence of ATP. d, Time course for modification of I332C and T336C in the absence of ATP. Red and blue circles indicate the normalized currents in response to 2 s application of ATP. Ag⁺ was applied for 2 s (I332C) or 20 s (T336C) in the absence of ATP and washed out before each ATP application to assay channel activity. Note the different time scales for the two mutants. e, Summary of Ag⁺ modification rates for seven residues in TM2 in the absence (black filled squares) or presence (open circles) of an EC₅₀ concentration of ATP. Each point is the mean value from four to seven experiments and error bars are S.E.M. (in most cases these are smaller than the symbols). Fractional inhibition values for Ag⁺ in the presence of ATP are as follows: 78±7% (I332C), 91±4% (T336C), 93±3% (T339C), 68±5% (I341C), 94±3% (V343C), 82±3% (S345C) and 83±7% (F346C).