Activation of the KATP channel by Mg-nucleotide interaction with SUR1

The mechanism of adenosine triphosphate (ATP)-sensitive potassium (KATP) channel activation by Mg-nucleotides was studied using a mutation (G334D) in the Kir6.2 subunit of the channel that renders KATP channels insensitive to nucleotide inhibition and has no apparent effect on their gating. KATP channels carrying this mutation (Kir6.2-G334D/SUR1 channels) were activated by MgATP and MgADP with an EC50 of 112 and 8 µM, respectively. This activation was largely suppressed by mutation of the Walker A lysines in the nucleotide-binding domains of SUR1: the remaining small (∼10%), slowly developing component of MgATP activation was fully inhibited by the lipid kinase inhibitor LY294002. The EC50 for activation of Kir6.2-G334D/SUR1 currents by MgADP was lower than that for MgATP, and the time course of activation was faster. The poorly hydrolyzable analogue MgATPγS also activated Kir6.2-G334D/SUR1. AMPPCP both failed to activate Kir6.2-G334D/SUR1 and to prevent its activation by MgATP. Maximal stimulatory concentrations of MgATP (10 mM) and MgADP (1 mM) exerted identical effects on the single-channel kinetics: they dramatically elevated the open probability (PO > 0.8), increased the mean open time and the mean burst duration, reduced the frequency and number of interburst closed states, and eliminated the short burst states. By comparing our results with those obtained for wild-type KATP channels, we conclude that the MgADP sensitivity of the wild-type KATP channel can be described quantitatively by a combination of inhibition at Kir6.2 (measured for wild-type channels in the absence of Mg2+) and activation via SUR1 (determined for Kir6.2-G334D/SUR1 channels). However, this is not the case for the effects of MgATP

The KCNJ11 E23K Polymorphism and Progression of Glycaemia in Southern Chinese: A Long-Term Prospective Study

Context: The KCNJ11 E23K variant is associated with type 2 diabetes mellitus (T2DM) in cross-sectional studies, but conflicting findings have been reported from prospective studies. Objective: This study aimed to evaluate whether the E23K variant could predict glycaemic progression in a Southern Chinese population. Methods/Principal Findings: We performed a long-term prospective study on 1912 subjects from the Hong Kong Cardiovascular Risk Factors Prevalence Study (CRISPS). The KCNJ11 E23K variant was associated with the progression to prediabetes after a median interval of 12 years on multinomial logistic regression analysis, even after adjustment for traditional risk factors (OR 1.29, P age, sex, BMI and fasting plasma glucose [FPG] adjusted = 0.02). Based on Cox proportional hazard regression analysis, the E23K variant also predicted incident prediabetes (HR 1.18, P age, sex, BMI and FPG adjusted = 0.021). However, E23K was not associated with the progression to T2DM in either multinomial or Cox regression analysis, and the association of E23K with glycaemic progression to either prediabetes or T2DM was significant only in unadjusted Cox regression analysis (P = 0.039). In a meta-analysis of eight prospective studies including our own, involving 15680 subjects, the E23K variant was associated with incident T2DM (fixed effect: OR 1.10, P = 4×10 -3; random effect: OR 1.11, P = 0.035). Conclusions: Our study has provided supporting evidence for the role of the E23K variant in glycaemic progression in Chinese, with its effect being more evident in the early stage of T2DM, as the subjects progressed from normal glucose tolerance to prediabetes. © 2011 Cheung et al.published_or_final_versio

Potassium channel openers require ATP to bind to and act through sulfonylurea receptors.

KATP channels are composed of a small inwardly rectifying K+ channel subunit, either KIR6.1 or KIR6.2, plus a sulfonylurea receptor, SUR1 or SUR2 (A or B), which belong to the ATP-binding cassette superfamily. SUR1/KIR6.2 reconstitute the neuronal/pancreatic beta-cell channel, whereas SUR2A/KIR6.2 and SUR2B/KIR6.1 (or KIR6.2) are proposed to reconstitute the cardiac and the vascular-smooth-muscle-type KATP channels, respectively. We report that potassium channel openers (KCOs) bind to and act through SURs and that binding to SUR1, SUR2A and SUR2B requires ATP. Non-hydrolysable ATP-analogues do not support binding, and Mg2+ or Mn2+ are required. Point mutations in the Walker A motifs or linker regions of both nucleotide-binding folds (NBFs) abolish or weaken [3H]P1075 binding to SUR2B, rendering reconstituted SUR2B/KIR6.2 channels insensitive towards KCOs. The C-terminus of SUR affects KCO affinity with SUR2B approximately SUR1 > SUR2A. KCOs belonging to different structural classes inhibited specific [3H]P1075 binding to SUR2B in a monophasic manner, with the exception of minoxidil sulfate, which induced a biphasic displacement. The affinities of KCO binding to SUR2B were 3.5-8-fold higher than their potencies for activation of SUR2B/KIR6.2 channels. The results establish that SURs are the KCO receptors of KATP channels and suggest that KCO binding requires a conformational change induced by ATP hydrolysis in both NBFs