Rescue and Gating of a Disease Mutation at an M2 glycine in Kir6.2 of ATP-Sensitive Potassium (KATP) Channels

Abstract

A glycine in the M2 helix of inwardly-rectifying potassium (Kir) channels was hypothesized to bend M2 and gate the intracellular helix-bundle crossing. Bacterial crystal structures position the glycine near the selectivity filter at the extracellular end of the pore. Our previous work characterized a mis-sense mutation at this glycine position identified in a patient with congenital hyperinsulinism (Pinney SE et al., 2008) that would generate Kir6.2 G156R mutant KATP channels. Mutant channels showed near WT surface expression in mammalian cells but no channel activity from inside-out excised patches when heterologously expressed in vitro. Further, additional mutations at G156 produce functional channels only if residues are small and uncharged. To test the hypothesis that the arginine mutation affects permeation by inhibiting conduction through the pore due to its size and charge, we generated double mutant G156R/N160D channels. Double mutant channels were functional; in addition, G156R/N160D did not show strong rectification in contrast to N160D, suggesting electrostatic interaction between the two residues. Single channel activity of double mutant channels exhibit altered intraburst gating kinetics compared to WT, suggesting the mutations or their interaction affects the selectivity filter. However, double mutant channels were sensitive to inhibitor ATP and activators MgADP and long-chain acyl Coenzyme A similar to WT channels. Collectively, our results demonstrate functional rescue of the putative glycine hinge position caused by a disease mutation in KATP channels.