The Influence of Surface Charges on Quaternary Ammonium Block of Shaker K⁺ Channels

Abstract.

Block of K⁺ channels can be influenced by the ability of charged residues on the protein surface to accumulate cationic blocking ions to concentrations greater than those in bulk solution. We examined the ionic strength dependence of extracellular block of Shaker K⁺ channels by tetraethylammonium ions (TEA⁺) and by a trivalent quaternary ammonium ion, gallamine³⁺. Wild-type and mutant channels were expressed in Xenopus oocytes and currents recorded with the cut-open oocyte technique. Channel block by both compounds was substantially increased when the bathing electrolyte ionic strength was lowered, but with a much larger effect for trivalent gallamine. These data were quantitatively well described by a simple electrostatic model, accounting for accumulation of blocking ions near the pore of the channel by surface charges. The surface charge density of the wild-type channel consistent with the results was −0.1 e nm⁻². Shaker channels with T449Y mutations have an increased affinity for both TEA and gallamine but the ionic strength dependence of block was described with the same surface charge density as wild-type channels. Much of the increased sensitivity of Shaker K⁺ channels to gallamine may be due to a larger local accumulation of the trivalent ion. The negative charge at position 431 contributes to the sensitivity of channels to TEA (MacKinnon & Yellen, 1990). A charge reversal mutation at this location had little effect on the ionic strength dependence of quaternary ammonium ion block, suggesting that the charge on this amino acid may directly affect binding affinity but not local ion accumulation.