ATP binding without hydrolysis switches sulfonylurea receptor 1 (SUR1) to outward-facing conformations that activate K _ATP channels

Abstract

Neuroendocrine-type ATP-sensitive K ⁺ (K _ATP) channels are metabolite sensors coupling membrane potential with metabolism, thereby linking insulin secretion to plasma glucose levels. They are octameric complexes, (SUR1/Kir6.2) ₄, comprising sulfonylurea receptor 1 (SUR1 or ABCC8) and a K ⁺-selective inward rectifier (Kir6.2 or KCNJ11). Interactions between nucleotide-, agonist-, and antagonist-binding sites affect channel activity allosterically. Although it is hypothesized that opening these channels requires SUR1-mediated MgATP hydrolysis, we show here that ATP binding to SUR1, without hydrolysis, opens channels when nucleotide antagonism on Kir6.2 is minimized and SUR1 mutants with increased ATP affinities are used. We found that ATP binding is sufficient to switch SUR1 alone between inward- or outward-facing conformations with low or high dissociation constant, K _D , values for the conformation-sensitive channel antagonist [ ³H]glibenclamide ([ ³H]GBM), indicating that ATP can act as a pure agonist. Assembly with Kir6.2 reduced SUR1's K _D for [ ³H]GBM. This reduction required the Kir N terminus (KNtp), consistent with KNtp occupying a “transport cavity,” thus positioning it to link ATP-induced SUR1 conformational changes to channel gating. Moreover, ATP/GBM site coupling was constrained in WT SUR1/WT Kir6.2 channels; ATP-bound channels had a lower K _D for [ ³H]GBM than ATP-bound SUR1. This constraint was largely eliminated by the Q1179R neonatal diabetes-associated mutation in helix 15, suggesting that a “swapped” helix pair, 15 and 16, is part of a structural pathway connecting the ATP/GBM sites. Our results suggest that ATP binding to SUR1 biases K _ATP channels toward open states, consistent with SUR1 variants with lower K _D values causing neonatal diabetes, whereas increased K _D values cause congenital hyperinsulinism.