CHANGES IN ACTIVATION GATING OF I-SK POTASSIUM CURRENTS BROUGHT ABOUTBY MUTATIONS IN THE TRANSMEMBRANE SEQUENCE

Expression of the rat kidney I-sK protein in Xenopus oocytes produces slowly-activating potassium channel currents. We have investigated the relationship between structure and function of the single putative me mbrane-spanning domain using site-directed mutagenesis. Six mutants we re constructed in which consecutive individual amino acids (53 to 58) of the transmembrane region were substituted by cysteine. Expression o f four of these mutants in Xenopus oocytes resulted in currents which were similar to wild-type. However, for one mutant (position 55) activ ation curves were shifted in a hyperpolarising direction and for anoth er mutant (position 58) activation curve were shifted in a depolarisin g direction. This suggests that the hydrophobic phenylalanine residues at positions 55 and 58 may play a critical role in I-sK activation ga ting. This spacing of functional amino acids at every third residue ma y indicate an a-helical conformation for the membrane-spanning domain ofI(sK). Furthermore, these results also indicate that one face of the helix may represent a region of subunit association.