Single-channel characteristics of wild-type I-Ks channels and channels formed with two minK mutants that cause long QT syndrome

I-Ks channels are voltage dependent and K+ selective. They influence cardia c action potential duration through their contribution to myocyte repolariz ation. Assembled from minK and KvLQT1 subunits, I-ks channels are notable f or a heteromeric ion conduction pathway in which both subunit types contrib ute to pore formation. This study was undertaken to assess the effects of m inK on pore function. We first characterized the properties of wild-type hu man I-Ks channels and channels formed only of KvLQR1 subunits. Channels wer e expressed in Xenopus laevis oocytes or Chinese hamster ovary cells and cu rrents recorded in excised membrane patches or whole-cell mode. Unitary con ductance estimates were dependent on bandwidth due to rapid channel "flicke r." At 25 kHz in symmetrical 100-mM KCl, the single-channel conductance of I-Ks channels ws similar to 16 pS (corresponding to similar to 0.8 pA at 50 mV) as judged by noise-variance analysis; this was fourfold greater than t he estimated conductance of homomeric KvLQT1 channels. Mutant I-Ks channels formed with D76N and S74L minK subunits are associated with long QT syndro me. When compared with wild type, mutant channels showed lower unitary curr ents and diminished open probabilities with only minor changes in ion perme abilities. Apparently, the mutations altered single-channel currents at a s ite in the pore distinct from the ion selectivity apparatus. Patients carry ing these mutant minK genes are expected to manifest decreased K+ flux thro ugh I-Ks channels due to lowered single-channel conductance and altered gat ing.