The open structure of a neonatal diabetes-causing variant of the pancreatic KATP channel determined using cryo-electron microscopy.

Abstract

Pancreatic ATP-sensitive potassium (KATP) channels, comprising four Kir6.2 and four SUR1 subunits, control insulin secretion in β-cells by coupling glucose metabolism with membrane excitability. Regulation of KATP channels involves orchestrated interactions of their subunits, Kir6.2 and SUR1, and ligands. ATP and PIP2 are known to inhibit and stimulate channel activity, respectively. Although Kir6.2 harbors the primary binding sites for ATP and PIP2, SUR1 is required to confer high channel sensitivity to both ligands. Mutations in channel proteins that disrupt ATP and PIP2 gating cause insulin secretion disease. Kir6.2-Q52R is a neonatal diabetes mutation that results in channels with extremely high open probability and low ATP sensitivity. Previous studies suggest the Kir6.2-Q52R mutation stabilizes channel opening in a SUR1-dependent manner, but the structural mechanism is unknown. Here, we determined the cryoEM structure of the Kir6.2-Q52R4SUR14 channel in the absence of nucleotides or pharmacological ligands. This 3.3 Å resolution structure reveals that Kir6.2 is open, with its cytoplasmic domain (CTD) corkscrewed up to the membrane. The side-chain of Kir6.2-Q52R is flipped away from the ATP-binding site and embedded into SUR1-TMD0 in a cation-pi interaction with SUR1-W51 at the membrane. The cryo-EM map has clear density showing the first intracellular loop of SUR1-TMD0 (ICL1, 52-60) is ordered and forms main chain interactions between Kir6.2-βA (the CTD β-strand just N-terminal to the interfacial helix) and SUR1-ICL1, tethering the CTD to TMD0 to stabilize the open state. Our study provides structural insights into the mechanism by which the Kir6.2-Q52R mutation causes severe neonatal diabetes and how Kir6.2 and SUR1 interact to stabilize KATP channels in the open state.