MULTIPLE RESIDUES SPECIFY EXTERNAL TETRAETHYLAMMONIUM BLOCKADE IN VOLTAGE-GATED POTASSIUM CHANNELS

We have mapped residues in the carboxyl half of the P region of a volt age-gated K+ channel that influence external tetraethylammonium (TEA) block. Fifteen amino acids were substituted with cysteine and expresse d in oocytes from monomeric or heterodimeric cRNAs. From a total of si x mutant channels with altered TEA sensitivity, three were susceptible to modification by extracellularly applied charged methanethiosulfona tes (MTSX). Another residue did not affect TEA block but was protected from MTSX by TEA. MTSX modification of position Y380C, thought to for m the TEA binding site, affected TEA affinity only moderately, and thi s effect could be reversed by additional charge transfer from an oppos itely charged MTSX analog. The results show that TEA block is modulate d from multiple sites, including residues located deep in the pore and that several side chains besides that of Y380 are exposed at the TEA receptor.