Role of the amino-terminal transmembrane domain of sulfonylurea receptor SUR2B for coupling to KIR6.2, ligand binding, and oligomerization

The Structural Basis for the Binding of Repaglinide to the Pancreatic KATP Channel

Repaglinide (RPG) is a short-acting insulin secretagogue widely prescribed for the treatment of type 2 diabetes. It boosts insulin secretion by inhibiting the pancreatic ATP-sensitive potassium channel (K_ATP). However, the mechanisms by which RPG binds to the K_ATP channel are poorly understood. Here, we describe two cryo-EM structures: the pancreatic K_ATP channel in complex with inhibitory RPG and adenosine-5'-(γ-thio)-triphosphate (ATPγS) at 3.3 Å and a medium-resolution structure of a RPG-bound mini SUR1 protein in which the N terminus of the inward-rectifying potassium channel 6.1 (Kir6.1) is fused to the ABC transporter module of the sulfonylurea receptor 1 (SUR1). These structures reveal the binding site of RPG in the SUR1 subunit. Furthermore, the high-resolution structure reveals the complex architecture of the ATP binding site, which is formed by both Kir6.2 and SUR1 subunits, and the domain-domain interaction interfaces.

Structural characterization of the L0 cytoplasmic loop of human multidrug resistance protein 6 (MRP6)

ABCC6 is a member of the C subfamily of ATP-binding cassette transporters whose mutations are correlated to Pseudoxanthoma elasticum, an autosomal recessive, progressive disorder characterized by ectopic mineralization and fragmentation of elastic fibers. Structural studies of the entire protein have been hindered by its large size, membrane association, and domain complexity. Studies previously performed have contributed to shed light on the structure and function of the nucleotide binding domains and of the N-terminal region. Here we report the expression in E. coli of the polypeptide E₂₀₅-G₂₇₉ contained in the cytoplasmic L0 loop. For the first time structural studies in solution were performed. Far-UV CD spectra showed that L0 is structured, assuming predominantly α-helix in TFE solution and turns in phosphate buffer. Fluorescence spectra indicated some flexibility of the regions containing aromatic residues. ¹H NMR spectroscopy identified three helical regions separated by more flexible regions.

Cryo-EM structure of the ATP-sensitive potassium channel illuminates mechanisms of assembly and gating

K_ATP channels are metabolic sensors that couple cell energetics to membrane excitability. In pancreatic β-cells, channels formed by SUR1 and Kir6.2 regulate insulin secretion and are the targets of antidiabetic sulfonylureas. Here, we used cryo-EM to elucidate structural basis of channel assembly and gating. The structure, determined in the presence of ATP and the sulfonylurea glibenclamide, at ~6 Å resolution reveals a closed Kir6.2 tetrameric core with four peripheral SUR1s each anchored to a Kir6.2 by its N-terminal transmembrane domain (TMD0). Intricate interactions between TMD0, the loop following TMD0, and Kir6.2 near the proposed PIP₂ binding site, and where ATP density is observed, suggest SUR1 may contribute to ATP and PIP₂ binding to enhance Kir6.2 sensitivity to both. The SUR1-ABC core is found in an unusual inward-facing conformation whereby the two nucleotide binding domains are misaligned along a two-fold symmetry axis, revealing a possible mechanism by which glibenclamide inhibits channel activity.

DOI:http://dx.doi.org/10.7554/eLife.24149.001

The hormone insulin reduces blood sugar levels by encouraging fat, muscle and other body cells to take up sugar. When blood sugar levels rise following a meal, cells within the pancreas known as beta cells should release insulin. In people with diabetes, the beta cells fail to release insulin, meaning that the high blood sugar levels are not corrected.

When blood sugar levels are high, beta cells generate more energy in the form of ATP molecules. The increased level of ATP causes channels called ATP-sensitive potassium (K_ATP) channels in the membrane of the cell to close. This triggers a cascade of events that leads to the release of insulin.

Some treatments for diabetes alter how the K_ATP channels work. For example, a widely prescribed medication called glibenclamide (also known as glyburide in the United States) stimulates the release of insulin by preventing the flow of potassium through K_ATP channels. It remains unknown exactly how ATP and glibenclamide interact with the channel’s molecular structure to stop the flow of potassium ions.

K_ATP channels are made up of two proteins called SUR1 and Kir6.2. To investigate the structure of the K_ATP channel, Martin et al. purified channels made of the hamster form of the SUR1 protein and the mouse form of Kir6.2, which each closely resemble their human counterparts. The channels were purified in the presence of ATP and glibenclamide and were then rapidly frozen to preserve their structure, which allowed them to be visualized individually using electron microscopy. By analyzing the images taken from many channels, Martin et al. constructed a highly detailed, three-dimensional map of the K_ATP channel. The structure revealed by this map shows how SUR1 and Kir6.2 work together and provides insight into how ATP and glibenclamide interact with the channel to block the flow of potassium, and hence stimulate the release of insulin.

An important next step will be to improve the structure to more clearly identify where ATP and glibenclamide bind to the K_ATP channel. It will also be important to study the structures of channels that are bound to other regulatory molecules. This will help researchers to fully understand how K_ATP channels located throughout the body operate under healthy and diseased conditions. This knowledge will aid in the design of more effective drugs to treat several devastating diseases caused by defective K_ATP channels.

DOI:http://dx.doi.org/10.7554/eLife.24149.002

New insights into the roles of the N-terminal region of the ABCC6 transporter

ABCC6 is a human ATP binding cassette (ABC) transporter of the plasma membrane associated with Pseudoxanthoma elasticum (PXE), an autosomal recessive disease characterized by ectopic calcification of elastic fibers in dermal, ocular and vascular tissues. Similar to other ABC transporters, ABCC6 encloses the core structure of four domains: two transmembrane domains (TMDs) and two nucleotide binding domains (NBDs) but also an additional N-terminal extension, including a transmembrane domain (TMD0) and a cytosolic loop (L0), which is only found in some members of ABCC subfamily, and for which the function remains to be established. To investigate the functional roles of this N-terminal region, we generated several domain deletion constructs of ABCC6, expressed in HEK293 and polarized LLC-PK1 cells. ABCC6 lacking TMD0 displayed full transport activity as the wild type protein. Unlike the wild type protein, ABCC6 without L0 was not targeted to the basolateral membrane. Moreover, homology modeling of L0 suggests that it forms an ATPase regulatory domain. Furthermore, we show that the expression of ABCC6 is linked to a cellular influx of Ca(2+). The results suggest that TMD0 is not required for transport function and that L0 maintains ABCC6 in a targeting-competent state for the basolateral membrane and might be involved in regulating the NBDs. These findings shed new light on a possible physiological function of ABCC6 and may explain some of the hallmarks of the clinical features associated with PXE that could contribute to the identification of novel pharmacological targets.

Structurally distinct ligands rescue biogenesis defects of the KATP channel complex via a converging mechanism

Small molecules that correct protein misfolding and misprocessing defects offer a potential therapy for numerous human diseases. However, mechanisms underlying pharmacological correction of such defects, especially in heteromeric complexes with structurally diverse constituent proteins, are not well understood. Here we investigate how two chemically distinct compounds, glibenclamide and carbamazepine, correct biogenesis defects in ATP-sensitive potassium (KATP) channels composed of sulfonylurea receptor 1 (SUR1) and Kir6.2. We present evidence that despite structural differences, carbamazepine and glibenclamide compete for binding to KATP channels, and both drugs share a binding pocket in SUR1 to exert their effects. Moreover, both compounds engage Kir6.2, in particular the distal N terminus of Kir6.2, which is involved in normal channel biogenesis, for their chaperoning effects on SUR1 mutants. Conversely, both drugs can correct channel biogenesis defects caused by Kir6.2 mutations in a SUR1-dependent manner. Using an unnatural, photocross-linkable amino acid, azidophenylalanine, genetically encoded in Kir6.2, we demonstrate in living cells that both drugs promote interactions between the distal N terminus of Kir6.2 and SUR1. These findings reveal a converging pharmacological chaperoning mechanism wherein glibenclamide and carbamazepine stabilize the heteromeric subunit interface critical for channel biogenesis to overcome defective biogenesis caused by mutations in individual subunits.