Polar residues of the second transmembrane domain influence cation permeability of the ATP-gated P2X2 receptor

P2X receptors are simple polypeptide channels that mediate fast purinergic depolarizations in both nerve and muscle. Although the depolarization results mainly from the influx of Na+, these channels also conduct a significant Ca2+ current that is large enough to evoke transmitter release from presynaptic neurons. We sought to determine the molecular basis of this Ca2+ conductance by a mutational analysis of recombinant P2X(2) receptors. Wild type and 31 mutant P2X(2) receptors were expressed in HEK-293 cells and studied under voltage-clamp. We found that the relative Ca2+ permeability measured from the reversal potentials of ATP-gated currents was unaffected by neutralizing fixed charge (Asp(315), Asp(349)) near the mouths of the channel pore. By contrast, mutations that changed the character or side chain volume of three polar residues (Thr(336), Thr(339), Ser(340)) within the pore led to significant changes in P-Ca/P-Cs. The largest changes occurred when Thr339 and Ser340 were replaced with tyrosine; these mutations almost completely abolished Ca2+ permeability, reduced P-Li/P-Cs by about one-half, and shifted the relative permeability sequence of Cs+, Rb+, K+, and Na+ to their relative mobility in water. Our results suggest that the permeability sequence of the P2X(2) receptor arises in part from interactions of permeating cations with the polar side chains of three amino acids located in a short stretch of the second transmembrane domain.