Ion selectivity filter regulates local anesthetic inhibition of G-protein-gated inwardly rectifying K+ channels

The weaver mutation (G156S) in G-protein-gated inwardly rectifying K+ (GIRK) channels alters ion selectivity and reveals sensitivity to inhibition by a charged local anesthetic, QX-314, applied extracellularly. In this paper, disrupting the ion selectivity in another GIRK channel, chimera I1G1(M), generates a GIRK channel that is also inhibited by extracellular local anesthetics. I1G1(M) is a chimera of IRK1 (G-protein-insensitive) and GIRK1 and contains the hydrophobic domains (M1-pore-loop-M2) of GIRK1 (G1(M)) with the N- and C-terminal domains of IRK1 (I1). The local anesthetic binding site in I1G1(M) is indistinguishable from that in GIRK2(wv) channels. Whereas chimera I1G1(M) loses K+ selectivity, although there are no mutations in the pore-loop complex, chimera I1G2(M), which contains the hydrophobic domain from GIRK2, exhibits normal K+ selectivity. Mutation of two amino acids that are unique in the pore-loop complex of GIRK1 (F137S and A143T) restores K+ selectivity and eliminates the inhibition by extracellular local anesthetics, suggesting that the pore-loop complex prevents QX-314 from reaching the intrapore site. Alanine mutations in the extracellular half of the M2 transmembrane domain alter QX-314 inhibition, indicating the M2 forms part of the intrapore binding site. Finally, the inhibition of G-protein-activated currents by intracellular QX-314 appears to be different from that observed in nonselective GIRK channels. The results suggest that inward rectifiers contain an intrapore-binding site for local anesthetic that is normally inaccessible from extracellular charged local anesthetics.