Ligand binding and conformational changes of SUR1 subunit in pancreatic ATP-sensitive potassium channels

Abstract

ATP-sensitive potassium channels (KATP) are energy sensors on the plasma membrane. By sensing the intracellular ADP/ATP ratio of β-cells, pancreatic KATP channels control insulin release and regulate metabolism at the whole body level. They are implicated in many metabolic disorders and diseases and are therefore important drug targets. Here, we present three structures of pancreatic KATP channels solved by cryo-electron microscopy (cryo-EM), at resolutions ranging from 4.1 to 4.5 Å. These structures depict the binding site of the antidiabetic drug glibenclamide, indicate how Kir6.2 (inward-rectifying potassium channel 6.2) N-terminus participates in the coupling between the peripheral SUR1 (sulfonylurea receptor 1) subunit and the central Kir6.2 channel, reveal the binding mode of activating nucleotides, and suggest the mechanism of how Mg-ADP binding on nucleotide binding domains (NBDs) drives a conformational change of the SUR1 subunit.Electronic supplementary materialThe online version of this article (10.1007/s13238-018-0530-y) contains supplementary material, which is available to authorized users.

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

“…Consistent with our earlier results (54,55) and cryo-EM structures (41,42,77,78), the high affinity of GBM binding strongly selects and stabilizes inward-facing conformations of SUR1 with a ⌬G 0 value of approximately Ϫ12.4 or Ϫ13.6 kcal/ mol for receptors or channels, respectively. The difference in binding energies of SUR1 versus SUR1/Kir6.2 reflects an increased stability of GBM-bound versus unliganded channels.…”

“…40) is clinically important. These physiologic and pharmacologic modulators all affect channel gating via allosteric interactions in the sense that their binding sites on SUR1 are coupled to and known to be physically distinct from one another based on structural studies (41,42). The available data are consistent with the idea that SUR1 exists in multiple conformations differing in affinity for ligands and ability to activate channel openings.How nucleotides regulate K ATP channels remains an open question.…”

“…The products of the dissociation constants along the two branches forming GBM-channel complexes must be equal. K chan , the dissociation constant for channels into subunits, is not known but must be low because channels are stable and complexes can be isolated (41,42,57,(77)(78)(79). The K G values for WT receptors versus channels gave an estimated value for ␣ of ϳ0.14, indicating channels complexed with GBM are ϳ7 times more stable than unliganded K ATP channels.…”

“…MgADP increases the open probability of WT K ATP channels and stabilizes more outward-facing receptor conformations (42); thus, the effect of MgADP on GBM binding was determined for WT SUR1 versus WT SUR1/WT Kir6.2 channels. Ortiz et al (54) used an excess of MgAMP to minimize conversion of added ADP to ATP by adenylate kinase and reported that MgADP stabilized SUR1 conformations with lower affinity for GBM somewhat better than MgATP.…”

“…Despite its structural relation to other ABC transporters, SUR1 is usually assumed to be a "regulatory protein," not a transporter. However, the localization of GBM bound in the central cavity of SUR1 by cryo-EM (41,42) together with earlier studies aimed at defining the GBM-binding site and defining the role(s) of the N terminus, KNtp, of Kir6.2 in GBM binding and channel gating (for a review, see Ref. 40) suggests that the transport idea deserves revisiting.…”