Individual subunits contribute independently to slow gating of bovine EAG potassium channels

The bovine ether a go-go gene encodes a delayed rectifier potassium channel . In contrast to other delayed rectifiers, its activation kinetics is large ly determined by the holding potential and the concentration of extracellul ar Mg2+, giving rise to slowly activating currents with a characteristic si gmoidal rising phase. Replacement of a single amino acid in the extracellul ar linker between transmembrane segments S3 and S4 (L322H) strongly reduced the prepulse dependence and accelerated activation by 1 order of magnitude . In addition, compared with the wild type, the half-activation voltage of this mutant was shifted by more than 30 mV to more negative potentials. We used dimeric and tetrameric constructs of the bovine eag1 gene to analyze c hannels with defined stoichiometry of mutated and wild-type subunits within the tetrameric channel complexes. With increasing numbers of mutated subun its, the channel activation was progressively accelerated, and the sig moid icity of the current traces was reduced, Based on a quantitative analysis, we show that the slow gating, typical for EAG channels, is mediated by inde pendent conformational transitions of individual subunits, which gain their voltage dependence from the S4 segment. At a given voltage, external Mg2increases the probability of a channel subunit to be in the slowly activati ng conformation, whereas mutation L322H strongly reduces this probability.