MgATP activates the β cell K _ATP channel by interaction with its SUR1 subunit

Abstract

ATP-sensitive potassium (K ATP ) channels in the pancreatic ␤ cell membrane mediate insulin release in response to elevation of plasma glucose levels. They are open at rest but close in response to glucose metabolism, producing a depolarization that stimulates Ca 2؉ inf lux and exocytosis. Metabolic regulation of K ATP channel activity currently is believed to be mediated by changes in the intracellular concentrations of ATP and MgADP, which inhibit and activate the channel, respectively. The ␤ cell K ATP channel is a complex of four Kir6.2 pore-forming subunits and four SUR1 regulatory subunits: Kir6.2 mediates channel inhibition by ATP, whereas the potentiatory action of MgADP involves the nucleotidebinding domains (NBDs) of SUR1. We show here that MgATP (like MgADP) is able to stimulate K ATP channel activity, but that this effect normally is masked by the potent inhibitory effect of the nucleotide. Mg 2؉ caused an apparent reduction in the inhibitory action of ATP on wild-type K ATP channels, and MgATP actually activated K ATP channels containing a mutation in the Kir6.2 subunit that impairs nucleotide inhibition (R50G). Both of these effects were abolished when mutations were made in the NBDs of SUR1 that are predicted to abolish MgATP binding and/or hydrolysis (D853N, D1505N, K719A, or K1384M). These results suggest that, like MgADP, MgATP stimulates K ATP channel activity by interaction with the NBDs of SUR1. Further support for this idea is that the ATP sensitivity of a truncated form of Kir6.2, which shows functional expression in the absence of SUR1, is unaffected by Mg 2؉ .ATP-sensitive K ϩ (K ATP ) channels couple cell metabolism to electrical activity. They thereby regulate insulin secretion from pancreatic ␤ cells, participate in the response to cardiac and cerebral ischemia, control vascular smooth muscle tone, modulate transmitter release at brain synapses, and mediate K ϩ fluxes across epithelial cells (1-3). It is thought that metabolic regulation is achieved, at least in part, by variation in the intracellular concentrations of the adenine nucleotides ATP and ADP. These nucleotides influence K ATP channel activity by interacting with two physically distinct sites on the K ATP channel: interaction with one site leads to channel inhibition whereas interaction with the other site causes an increase in channel activity (4, 5). ADP interacts with both sites, being stimulatory at low concentrations and inhibitory at high concentrations. Although it is well established that ATP interacts with the inhibitory site, it is not known whether it is also effective at the stimulatory site. The properties of the two sites differ in their requirement for Mg 2ϩ , the divalent cation being required for the potentiatory but not the inhibitory action of ADP (4, 6, 7).The ␤ cell K ATP channel consists of two types of subunit (8, 9): an inwardly rectifying K ϩ channel subunit (Kir6.2) and a sulfonylurea receptor subunit (SUR1), which assemble in a 4:4 stoichiometry to form an octameric channel (10-12). Both ...

ATP-sensitive K channel channel/enzyme multimer: Metabolic gating in the heart

“…Magnesium-dependent NBD2 ATPase activity was within the range reported for other ABC proteins [9,47]. Site-directed mutagenesis of the lysine residue in the Walker A motif of NBD2 or mutation in the Walker B aspartate suppressed this ATPase activity, and produced K ATP channels with a higher sensitivity to ATP compared to wild-type Kir6.2/SUR2A [47,48] indicating that the hydrolytically-driven transition from the MgATP-to the MgADP-liganded conformations of SUR influences the gating properties of K ATP channels.…”

“…Although K ATP channel inhibition is usually attributed exclusively to interaction of ATP with the channel pore, negative channel gating apparently could also be induced following binding of sulfonylurea drugs to SUR [48]. Whereas traditionally the effect of sulfonylurea is believed to be nucleotideindependent, binding of glyburide to SUR has been shown to modulate the nucleotide interaction with SUR [59], suggesting the involvement of ATPase-related intermediates in sulfonylurea-induced K ATP channel inhibition [60,61].…”

The ligand-sensitive gate of a potassium channel lies close to the selectivity filter

“…This effect is the opposite of that expected if the access of Ba 2+ to its binding site is impaired when the channel is shut by ATP, and implies that the gate lies above the Ba 2+ -binding site. Ba 2+ did not alter the half-maximal inhibitory concentration (IC 50 ) for ATP inhibition (6 ± 2 µM (n = 6) compared with 6 µM in the absence of Ba 2+ (Gribble et al, 1998)), which argues against an electrostatic (gating-independent) effect of ATP on Ba 2+ binding.…”

Mechanisms of action of glucagon-like peptide 1 in the pancreas

“…2 the K ATP channel consists of four pore forming units, K ir6.2 and four SUR1 (140 kDa) regulatory subunits. ATP binds to the cytoplasmic side of K ir6.2 subunit in an Mg 2+ -dependent manner initiating a conformational change that results in closure of the channel (Gribble et al, 1998). Depolarization and deactivation of the K ATP channels are entirely responsible for the first phase of insulin secretion as illustrated by the K ATP knockout mice (Miki et al, 2005) and separately by the SUR1 knockout mice (Miki et al, 2005;Shiota et al, 2002).…”