Allosteric effects of mutations in the extracellular S5-P loop on the grating and ion permeation properties of the hERG potassium channel

The hERG channel has an unusually long (39 amino acids) extracellular loop between the transmembrane S5 segment and the pore region that may play a role in channel function. We explored this possibility by mutating two histidine residues in this region (H578 and H587, referred to as H-1 and H-2) to various residues and examined the resulting changes in channel function. Both positions. could tolerate drastic changes in sidechain properties (proline, cysteine, glutamate and lysine), indicating that they are solvent exposed. None of the H-1 mutations affected hERG channel function. On the other hand, although replacing H-2 with glutamate had little or no effect on hERG properties, putting a proline or lysine at this position disrupted the C-type inactivation process and the pore's K selectivity. There was also a hyperpolarizing shift in the voltage dependence of activation. The phenotype of the H2C mutant was similar to that of H2P or H2K. However, dithiothreitol (DTT, a thiol-reducing agent) treatment converted the H2C phenotype to that of the wild-type channel. These observations suggest:that the peptide backbone conformation around position 587 in the extracellular S5-P loop of hERG channel can affect the channel's gating and ion selectivity functions.