The carboxyl termini of K(ATP) channels bind nucleotides

Kir6.2-D323 and SUR2A-Q1336: an intersubunit interaction pairing for allosteric information transfer in the KATP channel complex

ATP-sensitive potassium (K_ATP) channels are widely expressed and play key roles in many tissues by coupling metabolic state to membrane excitability. The SUR subunits confer drug and enhanced nucleotide sensitivity to the pore-forming Kir6 subunit, and so information transfer between the subunits must occur. In our previous study, we identified an electrostatic interaction between Kir6 and SUR2 subunits that was key for allosteric information transfer between the regulatory and pore-forming subunit. In this study, we demonstrate a second putative interaction between Kir6.2-D323 and SUR2A-Q1336 using patch clamp electrophysiological recording, where charge swap mutation of the residues on either side of the potential interaction compromise normal channel function. The Kir6.2-D323K mutation gave rise to a constitutively active, glibenclamide and ATP-insensitive K_ATP complex, further confirming the importance of information transfer between the Kir6 and SUR2 subunits. Sensitivity to modulators was restored when Kir6.2-D323K was co-expressed with a reciprocal charge swap mutant, SUR-Q1336E. Importantly, equivalent interactions have been identified in both Kir6.1 and Kir6.2 suggesting this is a second important interaction between Kir6 and the proximal C terminus of SUR.

Computational Identification of Novel Kir6 Channel Inhibitors

KATP channels consist of four Kir6.x pore-forming subunits and four regulatory sulfonylurea receptor (SUR) subunits. These channels couple the metabolic state of the cell to membrane excitability and play a key role in physiological processes such as insulin secretion in the pancreas, protection of cardiac muscle during ischemia and hypoxic vasodilation of arterial smooth muscle cells. Abnormal channel function resulting from inherited gain or loss-of-function mutations in either the Kir6.x and/or SUR subunits are associated with severe diseases such as neonatal diabetes, congenital hyperinsulinism, or Cantú syndrome (CS). CS is an ultra-rare genetic autosomal dominant disorder, caused by dominant gain-of-function mutations in SUR2A or Kir6.1 subunits. No specific pharmacotherapeutic treatment options are currently available for CS. Kir6 specific inhibitors could be beneficial for the development of novel drug therapies for CS, particular for mutations, which lack high affinity for sulfonylurea inhibitor glibenclamide. By applying a combination of computational methods including atomistic MD simulations, free energy calculations and pharmacophore modeling, we identified several novel Kir6.1 inhibitors, which might be possible candidates for drug repurposing. The in silico predictions were confirmed using inside/out patch-clamp analysis. Importantly, Cantú mutation C166S in Kir6.2 (equivalent to C176S in Kir6.1) and S1020P in SUR2A, retained high affinity toward the novel inhibitors. Summarizing, the inhibitors identified in this study might provide a starting point toward developing novel therapies for Cantú disease.

Activation mechanism of ATP-sensitive K+ channels explored with real-time nucleotide binding

The response of ATP-sensitive K⁺ channels (K_ATP) to cellular metabolism is coordinated by three classes of nucleotide binding site (NBS). We used a novel approach involving labeling of intact channels in a native, membrane environment with a non-canonical fluorescent amino acid and measurement (using FRET with fluorescent nucleotides) of steady-state and time-resolved nucleotide binding to dissect the role of NBS2 of the accessory SUR1 subunit of K_ATP in channel gating. Binding to NBS2 was Mg²⁺-independent, but Mg²⁺ was required to trigger a conformational change in SUR1. Mutation of a lysine (K1384A) in NBS2 that coordinates bound nucleotides increased the EC₅₀ for trinitrophenyl-ADP binding to NBS2, but only in the presence of Mg²⁺, indicating that this mutation disrupts the ligand-induced conformational change. Comparison of nucleotide-binding with ionic currents suggests a model in which each nucleotide binding event to NBS2 of SUR1 is independent and promotes K_ATP activation by the same amount.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

A high-throughput TNP-ATP displacement assay for screening inhibitors of ATP-binding in bacterial histidine kinases

Bacterial histidine kinases (HK) are members of the GHKL superfamily, which share a unique adenosine triphosphate (ATP)-binding Bergerat fold. Our previous studies have shown that Gyrase, Hsp90, MutL (GHL) inhibitors bind to the ATP-binding pocket of HK and may provide lead compounds for the design of novel antibiotics targeting these kinases. In this article, we developed a competition assay using the fluorescent ATP analog, 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate. The method can be used for high-throughput screening of compound libraries targeting HKs or other ATP-binding proteins. We utilized the assay to screen a library of GHL inhibitors targeting the bacterial HK PhoQ, and discuss the applications of the 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate competition assay beyond GHKL inhibitor screening.

Electrophysiology of islet cells

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells. The effect of glucose is directly linked to mitochondrial metabolism as the ATP/ADP ratio determines the open probability of ATP-sensitive K(+) channels (K(ATP) channels). Nucleotide sensitivity and concentration in the direct vicinity of the channels are controlled by several factors including phospholipids, fatty acids, and kinases, e.g., creatine and adenylate kinase. Closure of K(ATP) channels leads to depolarization of beta-cells via a yet unknown depolarizing current. Ca(2+) influx during action potentials (APs) results in an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) that triggers exocytosis. APs are elicited by the opening of voltage-dependent Na(+) and/or Ca(2+) channels and repolarized by voltage- and/or Ca(2+)-dependent K(+) channels. At a constant stimulatory glucose concentration APs are clustered in bursts that are interrupted by hyperpolarized interburst phases. Bursting electrical activity induces parallel fluctuations in [Ca(2+)](c) and insulin secretion. Bursts are terminated by I(Kslow) consisting of currents through Ca(2+)-dependent K(+) channels and K(ATP) channels. This review focuses on structure, characteristics, physiological function, and regulation of ion channels in beta-cells. Information about pharmacological drugs acting on K(ATP) channels, K(ATP) channelopathies, and influence of oxidative stress on K(ATP) channel function is provided. One focus is the outstanding significance of L-type Ca(2+) channels for insulin secretion. The role of less well characterized beta-cell channels including voltage-dependent Na(+) channels, volume sensitive anion channels (VSACs), transient receptor potential (TRP)-related channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is discussed. A model of beta-cell oscillations provides insight in the interplay of the different channels to induce and maintain electrical activity. Regulation of beta-cell electrical activity by hormones and the autonomous nervous system is discussed. alpha- and delta-cells are also equipped with K(ATP) channels, voltage-dependent Na(+), K(+), and Ca(2+) channels. Yet the SSC of these cells is less clear and is not necessarily dependent on K(ATP) channel closure. Different ion channels of alpha- and delta-cells are introduced and SSC in alpha-cells is described in special respect of paracrine effects of insulin and GABA secreted from beta-cells.

Noble gas xenon is a novel adenosine triphosphate-sensitive potassium channel opener

BACKGROUND

Adenosine triphosphate-sensitive potassium (KATP) channels in brain are involved in neuroprotective mechanisms. Pharmacologic activation of these channels is seen as beneficial, but clinical exploitation by using classic K channel openers is hampered by their inability to cross the blood-brain barrier. This is different with the inhalational anesthetic xenon, which recently has been suggested to activate KATP channels; it partitions freely into the brain.

METHODS

To evaluate the type and mechanism of interaction of xenon with neuronal-type KATP channels, these channels, consisting of Kir6.2 pore-forming subunits and sulfonylurea receptor-1 regulatory subunits, were expressed in HEK293 cells and whole cell, and excised patch-clamp recordings were performed.

RESULTS

Xenon, in contrast to classic KATP channel openers, acted directly on the Kir6.2 subunit of the channel. It had no effect on the closely related, adenosine triphosphate (ATP)-regulated Kir1.1 channel and failed to activate an ATP-insensitive mutant version of Kir6.2. Furthermore, concentration-inhibition curves for ATP obtained from inside-out patches in the absence or presence of 80% xenon revealed that xenon reduced the sensitivity of the KATP channel to ATP. This was reflected in an approximately fourfold shift of the concentration causing half-maximal inhibition (IC50) from 26 +/- 4 to 96 +/- 6 microm.

CONCLUSIONS

Xenon represents a novel KATP channel opener that increases KATP currents independently of the sulfonylurea receptor-1 subunit by reducing ATP inhibition of the channel. Through this action and by its ability to readily partition across the blood-brain barrier, xenon has considerable potential in clinical settings of neuronal injury, including stroke.

Inward rectifier channel, ROMK, is localized to the apical tips of glial-like cells in mouse taste buds

Cells in taste buds are closely packed, with little extracellular space. Tight junctions and other barriers further limit permeability and may result in buildup of extracellular K(+) following action potentials. In many tissues, inwardly rectifying K channels such as the renal outer medullary K (ROMK) channel (also called Kir1.1 and derived from the Kcnj1 gene) help to redistribute K(+). Using reverse-transcription polymerase chain reaction (RT-PCR), we defined ROMK splice variants in mouse kidney and report here the expression of a single one of these, ROMK2, in a subset of mouse taste cells. With quantitative (q)RT-PCR, we show the abundance of ROMK mRNA in taste buds is vallate > foliate > > palate > > fungiform. ROMK protein follows the same pattern of prevalence as mRNA, and is essentially undetectable by immunohistochemistry in fungiform taste buds. ROMK protein is localized to the apical tips of a subset of taste cells. Using tissues from PLCbeta2-GFP and GAD1-GFP transgenic mice, we show that ROMK is not found in PLCbeta2-expressing type II/receptor cells or in GAD1-expressing type III/presynaptic cells. Instead, ROMK is found, by single-cell RT-PCR and immunofluorescence, in most cells that are positive for the taste glial cell marker, Ectonucleotidase2. ROMK is precisely localized to the apical tips of these cells, at and above apical tight junctions. We propose that in taste buds, ROMK in type I/glial-like cells may serve a homeostatic function, excreting excess K(+) through the apical pore, and allowing excitable taste cells to maintain a hyperpolarized resting membrane potential.

Reduced expression and abnormal localization of the K(ATP) channel subunit SUR2A in patients with familial hypokalemic periodic paralysis

Familial hypokalemic periodic paralysis is an autosomal-dominant channelopathy that features episodic attacks of flaccid paralysis with concomitant hypokalemia. Reduced activity of ATP-sensitive K(+) (K(ATP)) channels is suggested to be responsible for this disorder; however, the molecular mechanisms have not yet been elucidated. In this study, we investigated the molecular mechanism of reduced K(ATP) channel activity in skeletal muscle cells of patients with familial hypokalemic periodic paralysis. We examined the mRNA and protein levels of SUR2A, a K(ATP) channel subunit, in cells from patients (patient cells) and normal individuals (normal cells). Our results demonstrated that normal cells exposed to 50mM potassium buffer, which was used to induce depolarization, did not show significant change in the SUR2A mRNA levels; however, the protein level significantly increased in the cytosolic fraction. When the patient cells were exposed to 50mM potassium buffer, the SUR2A mRNA level significantly decreased. Further, the protein level of SUR2A significantly increased in the membrane fraction but decreased in the cytosolic fraction in patient cells. These findings suggest that abnormal localization of the SUR2A K(+) channel protein leads to reduced K(ATP) channel activity in familial hypokalemic periodic paralysis.

N type rapid inactivation in human Kv1.4 channels: functional role of a putative C-terminal helix

Voltage gated potassium channels are tetrameric membrane proteins, which have a central role in cellular excitability. Human Kv1.4 channels open on membrane depolarization and inactivate rapidly by a 'ball and chain' mechanism whose molecular determinants have been mapped to the cytoplasmic N terminus of the channel. Here we show that the other terminal end of the channel also plays a role in channel inactivation. Swapping the C-terminal residues of hKv1.4 with those from two non-inactivating channels (hKv1.1 and hKv1.2) affects the rates of inactivation, as well as the recovery of the channel from the inactivated state. Secondary structure predictions of the hKv1.4 sequence reveal a helical structure at its distal C-terminal. Complete removal or partial disruption of this helical region results in channels with remarkably slowed inactivation kinetics. The ionic selectivity and voltage-dependence of channel opening were similar to hKv1.4, indicative of an unperturbed channel pore. These results demonstrate that fast inactivation is modulated by structural elements in the C-terminus, suggesting that the process involves the concerted action of the N- and C-termini.

A comprehensive guide to the ROMK potassium channel: form and function in health and disease

The discovery of the renal outer medullary K+ channel (ROMK, K(ir)1.1), the founding member of the inward-rectifying K+ channel (K(ir)) family, by Ho and Hebert in 1993 revolutionized our understanding of potassium channel biology and renal potassium handling. Because of the central role that ROMK plays in the regulation of salt and potassium homeostasis, considerable efforts have been invested in understanding the underlying molecular mechanisms. Here we provide a comprehensive guide to ROMK, spanning from the physiology in the kidney to the organization and regulation by intracellular factors to the structural basis of its function at the atomic level.

ATP modulation of Ca2+ release by type-2 and type-3 inositol (1, 4, 5)-triphosphate receptors. Differing ATP sensitivities and molecular determinants of action

ATP enhances Ca(2+) release from inositol (1,4,5)-trisphosphate receptors (InsP(3)R). However, the three isoforms of InsP(3)R are reported to respond to ATP with differing sensitivities. Ca(2+) release through InsP(3)R1 is positively regulated at lower ATP concentrations than InsP(3)R3, and InsP(3)R2 has been reported to be insensitive to ATP modulation. We have reexamined these differences by studying the effects of ATP on InsP(3)R2 and InsP(3)R3 expressed in isolation on a null background in DT40 InsP(3)R knockout cells. We report that the Ca(2+)-releasing activity as well as the single channel open probability of InsP(3)R2 was enhanced by ATP, but only at submaximal InsP(3) levels. Further, InsP(3)R2 was more sensitive to ATP modulation than InsP(3)R3 under similar experimental conditions. Mutations in the ATPB sites of InsP(3)R2 and InsP(3)R3 were generated, and the functional consequences of these mutations were tested. Surprisingly, mutation of the ATPB site in InsP(3)R3 had no effect on ATP modulation, suggesting an additional locus for the effects of ATP on this isoform. In contrast, ablation of the ATPB site of InsP(3)R2 eliminated the enhancing effects of ATP. Furthermore, this mutation had profound effects on the patterns of intracellular calcium signals, providing evidence for the physiological significance of ATP binding to InsP(3)R2.

Genetic study of Saudi diabetes (GSSD): significant association of the KCNJ11 E23K polymorphism with type 2 diabetes

BACKGROUND

The E23K variant of KCNJ11 has been associated with type 2 diabetes (T2D) in several but not all populations studied. Thus far, despite a high incidence of T2D, the role of this variant in Arabs has not been established.

METHODS

We performed a case-control association study using 550 T2D Saudi patients (WHO criteria), and 335 controls (age>or=60; fasting plasma glucose<7 mmol/L). E23K genotyping was performed by using molecular beacon-based real time PCR assays.

RESULTS

The difference in K or risk allele frequency of cases and controls was significant with an OR of 1.7 (p=0.0001). The K allele is more common among T2D patients (21%) than in the age and sex matched controls (13.6%). This was consistent with a likely eventual conversion to T2D of younger normoglycemic individuals as they grow older.

CONCLUSIONS

Our results report for the first time a positive association of the E23K variant with T2D in an Arab population. Confirmation by a larger study is indicated.

Prokaryotic type II and type III pantothenate kinases: The same monomer fold creates dimers with distinct catalytic properties

Three distinct isoforms of pantothenate kinase (CoaA) in bacteria catalyze the first step in coenzyme A biosynthesis. The structures of the type II (Staphylococcus aureus, SaCoaA) and type III (Pseudomonas aeruginosa, PaCoaA) enzymes reveal that they assemble nearly identical subunits with actin-like folds into dimers that exhibit distinct biochemical properties. PaCoaA has a fully enclosed pantothenate binding pocket and requires a monovalent cation to weakly bind ATP in an open cavity that does not interact with the adenine nucleotide. Pantothenate binds to an open pocket in SaCoaA that strongly binds ATP by using a classical P loop architecture coupled with specific interactions with the adenine moiety. The PaCoaA*Pan binary complex explains the resistance of bacteria possessing this isoform to the pantothenamide antibiotics, and the similarity between SaCoaA and human pantothenate kinase 2 explains the molecular basis for the development of the neurodegenerative phenotype in three mutations in the human protein.

ABCC8 and ABCC9: ABC transporters that regulate K+ channels

The sulfonylurea receptors (SURs) ABCC8/SUR1 and ABCC9/SUR2 are members of the C-branch of the transport adenosine triphosphatase superfamily. Unlike their brethren, the SURs have no identified transport function; instead, evolution has matched these molecules with K(+) selective pores, either K(IR)6.1/KCNJ8 or K(IR)6.2/KCNJ11, to assemble adenosine triphosphate (ATP)-sensitive K(+) channels found in endocrine cells, neurons, and both smooth and striated muscle. Adenine nucleotides, the major regulators of ATP-sensitive K(+) (K(ATP)) channel activity, exert a dual action. Nucleotide binding to the pore reduces the activity or channel open probability, whereas Mg-nucleotide binding and/or hydrolysis in the nucleotide-binding domains of SUR antagonize this inhibitory action to stimulate channel openings. Mutations in either subunit can alter this balance and, in the case of the SUR1/KIR6.2 channels found in neurons and insulin-secreting pancreatic beta cells, are the cause of monogenic forms of hyperinsulinemic hypoglycemia and neonatal diabetes. Additionally, the subtle dysregulation of K(ATP) channel activity by a K(IR)6.2 polymorphism has been suggested as a predisposing factor in type 2 diabetes mellitus. Studies on K(ATP) channel null mice are clarifying the roles of these metabolically sensitive channels in a variety of tissues.

Inhibition of ATP binding to the carboxyl terminus of Kir6.2 by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs), the cytochrome P450 metabolites of arachidonic acid (AA), are potent and stereospecific activators of cardiac ATP-sensitive K(+)(K(ATP)) channels. EETs activate K(ATP) channels by reducing channel sensitivity to ATP. In this study, we determined the direct effects of EETs on the binding of ATP to K(ATP) channel protein. A fluorescent ATP analog, 2,4,6-trinitrophenyl (TNP)-ATP, which increases its fluorescence emission significantly upon binding with proteins, was used for binding studies with glutathione-S-transferase (GST) Kir6.2 fusion proteins. TNP-ATP bound to GST fusion protein containing the C-terminus of Kir6.2 (GST-Kir6.2C), but not to the N-terminus of Kir6.2, or to GST alone. 11,12-EET (5 muM) did not change TNP-ATP binding K(D) to GST-Kir6.2C, but B(max) was reduced by half. The effect of 11,12-EET was dose-dependent, and 8,9- and 14,15-EETs were as effective as 11,12-EET in inhibiting TNP-ATP binding to GST-Kir6.2C. AA and 11,12-dihydroxyeicosatrienoic acid (11,12-DHET), the parent compound and metabolite of 11,12-EET, respectively, were not effective inhibitors of TNP-ATP binding to GST-Kir6.2C, whereas the methyl ester of 11,12-EET was. These findings suggest that the epoxide group in EETs is important for modulation of ATP binding to Kir6.2. We conclude that EETs bind to the C-terminus of K(ATP) channels, inhibiting binding of ATP to the channel.

CFTR is required for PKA-regulated ATP sensitivity of Kir1.1 potassium channels in mouse kidney

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel plays vital roles in fluid transport in many epithelia. While CFTR is expressed along the entire nephron, its function in renal tubule epithelial cells remains unclear, as no specific renal phenotype has been identified in cystic fibrosis. CFTR has been proposed as a regulator of the 30 pS, ATP-sensitive renal K channel (Kir1.1, also known as renal outer medullar K [ROMK]) that is critical for K secretion by cells of the thick ascending limb (TAL) and distal nephron segments responsive to aldosterone. We report here that both ATP and glibenclamide sensitivities of the 30 pS K channel in TAL cells were absent in mice lacking CFTR and in mice homozygous for the deltaF508 mutation. Curcumin treatment in deltaF508-CFTR mice partially reversed the defect in ATP sensitivity. We demonstrate that the effect of CFTR on ATP sensitivity was abrogated by increasing PKA activity. We propose that CFTR regulates the renal K secretory channel by providing a PKA-regulated functional switch that determines the distribution of open and ATP-inhibited K channels in apical membranes. We discuss the potential physiological role of this functional switch in renal K handling during water diuresis and the relevance to renal K homeostasis in cystic fibrosis.

KATP channel: relation with cell metabolism and role in the cardiovascular system

ATP-sensitive potassium channel (K(ATP)) is one kind of inwardly rectifying channel composed of two kinds of subunits: the pore forming subunits and the regulatory subunits. K(ATP) channels exist in the sarcolemmal, mitochondrial and nuclear membranes of various tissues. Cell metabolism regulates K(ATP) gene expression and metabolism products regulate the channel by direct interactions, while K(ATP) controls membrane potentials and regulate cell activities including energy metabolism, apoptosis and gene expression. K(ATP) channels from different cell organelles are linked by some signal molecules and they can respond to common stimulation in a coordinate way. In the cardiovascular system K(ATP) has important functions. The most prominent is that opening of this channel can protect cardiac myocytes against ischemic injuries. The sarcolemmal K(ATP) may provide a basic protection against ischemia by energy sparing, while both the sarcolemmal K(ATP) and mitochondrial K(ATP) channels are necessary for the ischemia preconditioning. K(ATP) channels also have important functions including homeostasis maintenance and vascular tone regulation under physiological conditions. Further elucidation of the role of K(ATP) in the cardiovascular system will help us to regulate cell metabolism or prevent damage caused by abnormal channel functions.

Novel nucleotide-binding sites in ATP-sensitive potassium channels formed at gating interfaces

The coupling of cell metabolism to membrane electrical activity is a vital process that regulates insulin secretion, cardiac and neuronal excitability and the responses of cells to ischemia. ATP-sensitive potassium channels (K(ATP); Kir6.x) are a major part of this metabolic-electrical coupling system and translate metabolic signals such as the ATP:ADP ratio to changes in the open or closed state (gate) of the channel. The localization of the nucleotide-binding site (NBS) on Kir6.x channels and how nucleotide binding gates these K(ATP) channels remain unclear. Here, we use fluorescent nucleotide binding to purified Kir6.x proteins to define the peptide segments forming the NBS on Kir6.x channels and show that unique N- and C-terminal interactions from adjacent subunits are required for high-affinity nucleotide binding. The short N- and C-terminal segments comprising the novel intermolecular NBS are next to helices that likely move with channel opening/closing, suggesting a lock-and-key model for ligand gating.

Molecular determinants of cardiac K(ATP) channel activation by epoxyeicosatrienoic acids

We have previously reported that epoxyeicosatrienoic acids (EETs), the cytochrome P450 epoxygenase metabolites of arachidonic acid, are potent stereospecific activators of the cardiac K(ATP) channel. The epoxide group in EET is critical for reducing channel sensitivity to ATP, thereby activating the channel. This study is to identify the molecular sites on the K(ATP) channels for EET-mediated activation. We investigated the effects of EETs on Kir6.2delta C26 with or without the coexpression of SUR2A and on Kir6.2 mutants of positively charged residues known to affect channel activity coexpressed with SUR2A in HEK293 cells. The ATP IC50 values were significantly increased in Kir6.2 R27A, R50A, K185A, and R201A but not in R16A, K47A, R54A, K67A, R192A, R195A, K207A, K222A, and R314A mutants. Similar to native cardiac K(ATP) channel, 5 microM 11,12-EET increased the ATP IC50 by 9.6-fold in Kir6.2/SUR2A wild type and 8.4-fold in Kir6.2delta C26. 8,9- and 14,15-EET regioisomers activated the Kir6.2 channel as potently as 11,12-EET. 8,9- and 11,12-EET failed to change the ATP sensitivity of Kir6.2 K185A, R195A, and R201A, whereas their effects were intact in the other mutants. 14,15-EET had a similar effect with K185A and R201A mutants, but instead of R195A, it failed to activate Kir6.2R192A. These results indicate that activation of Kir6.2 by EETs does not require the SUR2A subunit, and the region in the Kir6.2 C terminus from Lys-185 to Arg-201 plays a critical role in EET-mediated Kir6.2 channel activation. Based on computer modeling of the Kir6.2 structure, we infer that the EET-Kir6.2 interaction may allosterically change the ATP binding site on Kir6.2, reducing the channel sensitivity to ATP.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Concerted gating mechanism underlying KATP channel inhibition by ATP

KATP channels assemble from four regulatory SUR1 and four pore-forming Kir6.2 subunits. At the single-channel current level, ATP-dependent gating transitions between the active burst and the inactive interburst conformations underlie inhibition of the KATP channel by intracellular ATP. Previously, we identified a slow gating mutation, T171A in the Kir6.2 subunit, which dramatically reduces rates of burst to interburst transitions in Kir6.2DeltaC26 channels without SUR1 in the absence of ATP. Here, we constructed all possible mutations at position 171 in Kir6.2DeltaC26 channels without SUR1. Only four substitutions, 171A, 171F, 171H, and 171S, gave rise to functional channels, each increasing Ki,ATP for ATP inhibition by >55-fold and slowing gating to the interburst by >35-fold. Moreover, we investigated the role of individual Kir6.2 subunits in the gating by comparing burst to interburst transition rates of channels constructed from different combinations of slow 171A and fast T171 "wild-type" subunits. The relationship between gating transition rate and number of slow subunits is exponential, which excludes independent gating models where any one subunit is sufficient for inhibition gating. Rather, our results support mechanisms where four ATP sites independently can control a single gate formed by the concerted action of all four Kir6.2 subunit inner helices of the KATP channel.

Expression, purification and spectroscopic studies of full-length Kir3.1 channel C-terminus

A polypeptide corresponding to the full-length C-terminal cytoplasmic domain of a G-protein-regulated inwardly rectifying potassium channel (Kir3.1) bearing a hexahistidine (His6) tag was produced by DNA recombinant overexpression techniques in Escherichia coli. This permitted the isolation of approximately 5 mg of pure protein per liter of bacterial culture. Further purification by size exclusion chromatography (SEC) of the C-terminal domain revealed that it exists predominantly as a dimer. The secondary structure was estimated using circular dichroism measurements that indicated the presence of approximately 35% beta-sheet and approximately 15% alpha-helix. G-protein betagamma subunits incubated with His-tagged Kir3.1 C-terminal domain, bound to immobilized metal affinity chromatography (IMAC) resin, copurified with the peak of specifically eluted recombinant protein. These observations demonstrate that full-length Kir3.1 C-terminus can be purified in a stable conformation capable of binding proteins known to activate Kir3 channels and may contain elements involved in channel assembly.

Syntaxin-1A binds the nucleotide-binding folds of sulphonylurea receptor 1 to regulate the KATP channel

ATP-sensitive potassium (KATP) channels in neuron and neuroendocrine cells consist of a pore-forming Kir6.2 and regulatory sulfonylurea receptor (SUR1) subunits, which are regulated by ATP and ADP. SNARE protein syntaxin 1A (Syn-1A) is known to mediate exocytic fusion, and more recently, to also bind and modulate membrane-repolarizing voltage-gated K+ channels. Here we show that Syn-1A acts as an endogenous regulator of KATP channels capable of closing these channels when cytosolic ATP concentrations were lowered. Botulinum neurotoxin C1 cleavage of endogenous Syn-1A in insulinoma HIT-T15 cells resulted in the increase in KATP currents, which could be subsequently inhibited by recombinant Syn-1A. Whereas Syn-1A binds both nucleotide-binding folds (NBF-1 and NBF-2) of SUR1, the functional inhibition of KATP channels in rat islet beta-cells by Syn-1A seems to be mediated primarily by its interactions with NBF-1. These inhibitory actions of Syn-1A can be reversed by physiologic concentrations of ADP and by diazoxide. Syn-1A therefore acts to fine-tune the regulation of KATP channels during dynamic changes in cytosolic ATP and ADP concentrations. These actions of Syn-1A on KATP channels contribute to the role of Syn-1A in coordinating the sequence of ionic and exocytic events leading to secretion.

A tale of two tails: cytosolic termini and K(+) channel function

The enormous variety of neuronal action potential waveforms can be ascribed, in large part, to the sculpting of their falling phases by currents through voltage-gated potassium channels. These proteins play several additional roles in other tissues such as the regulation of heartbeat and of insulin release from pancreatic cells as well as auditory signal processing in the cochlea. The functional channel is a tetramer with either six or two transmembrane segments per monomer. Selectivity filters, voltage sensors and gating elements have been mapped to residues within the transmembrane region. Cytoplasmic residues, which are accessible targets for signal transduction cascades and provide attractive means of regulation of channel activity, are now seen to be capable of modulating various aspects of channel function. Here we review structural studies on segments of the cytoplasmic tails of K(+) channels, as well as the range of modulatory activities of these tails.

Fluorescent and colored trinitrophenylated analogs of ATP and GTP

Fluorescent and colored trinitrophenylated (TNP) analogs of ATP and GTP can interact with nucleotide-requiring enzymes and proteins as a substitute for the parent nucleotide. These analogs have strong binding affinities for most nucleotide-requiring systems. Their bindings are easily detected by absorption and fluorescence changes in the visible region. Recent years have seen dramatic developments in the application of the TNP nucleotide analogs as spectroscopic probes for the study on the nucleotide-interacting properties of various enzymes and proteins including their mutants. This review is intended as a broad overview of currently extensively used applications of the nucleotide analogs in various biological systems.

Cardioprotection through a PKC-dependent decrease in myofilament ATPase

Activation of myocardial kappa-opioid receptor-protein kinase C (PKC) pathways may improve postischemic contractile function through a myofilament reduction in ATP utilization. To test this, we first examined the effects of PKC inhibitors on kappa-opioid receptor-dependent cardioprotection. The kappa-opioid receptor agonist U50,488H (U50) increased postischemic left ventricular developed pressure and reduced postischemic end-diastolic pressure compared with controls. PKC inhibitors abolished the cardioprotective effects of U50. To determine whether kappa-opioid-PKC-dependent decreases in Ca2+-dependent actomyosin Mg2+-ATPase could account for cardioprotection, we subjected hearts to three separate actomyosin ATPase-lowering protocols. We observed that moderate decreases in myofibrillar ATPase were equally cardioprotective as kappa-opioid receptor stimulation. Immunoblot analysis and confocal microscopy revealed a kappa-opioid-induced increase in myofilament-associated PKC-epsilon, and myofibrillar Ca2+-independent PKC activity was increased after kappa-opioid stimulation. This PKC-myofilament association led to an increase in troponin I and C-protein phosphorylation. Thus we propose PKC-epsilon activation and translocation to the myofilaments causes a decrease in actomyosin ATPase, which contributes to the kappa-opioid receptor-dependent cardioprotective mechanism.

TraG-like proteins of type IV secretion systems: functional dissection of the multiple activities of TraG (RP4) and TrwB (R388)

TraG-like proteins are essential components of type IV secretion systems. During secretion, TraG is thought to translocate defined substrates through the inner cell membrane. The energy for this transport is presumably delivered by its potential nucleotide hydrolase (NTPase) activity. TraG of conjugative plasmid RP4 is a membrane-anchored oligomer that binds RP4 relaxase and DNA. TrwB (R388) is a hexameric TraG-like protein that binds ATP. Both proteins, however, lack NTPase activity under in vitro conditions. We characterized derivatives of TraG and TrwB truncated by the N-terminal membrane anchor (TraGdelta2 and TrwBdelta1) and/or containing a point mutation at the putative nucleotide-binding site (TraGdelta2K187T and TraGK187T). Unlike TraG and TrwB, truncated derivatives behaved as monomers without the tendency to form oligomers or aggregates. Surface plasmon resonance analysis with immobilized relaxase showed that mutant TraGK187T was as good a binding partner as the wild-type protein, whereas truncated TraG monomers were unable to bind relaxase. TraGdelta2 and TrwBdelta1 bound ATP and, with similar affinity, ADP. Binding of ATP and ADP was strongly inhibited by the presence of Mg(2+) or single-stranded DNA and was competed for by other nucleotides. Compared to the activity of TraGdelta2, the ATP- and ADP-binding activity of the point mutation derivative TraGdelta2K187T was significantly reduced. Each TraG derivative bound DNA with an affinity similar to that of the native protein. DNA binding was inhibited or competed for by ATP, ADP, and, most prominently, Mg(2+). Thus, both nucleotide binding and DNA binding were sensitive to Mg(2+) and were competitive with respect to each other.

Identification of residues contributing to the ATP binding site of Kir6.2

The ATP-sensitive potassium (K(ATP)) channel links cell metabolism to membrane excitability. Intracellular ATP inhibits channel activity by binding to the Kir6.2 subunit of the channel, but the ATP binding site is unknown. Using cysteine-scanning mutagenesis and charged thiol-modifying reagents, we identified two amino acids in Kir6.2 that appear to interact directly with ATP: R50 in the N-terminus, and K185 in the C-terminus. The ATP sensitivity of the R50C and K185C mutant channels was increased by a positively charged thiol reagent (MTSEA), and was reduced by the negatively charged reagent MTSES. Comparison of the inhibitory effects of ATP, ADP and AMP after thiol modification suggests that K185 interacts primarily with the beta-phosphate, and R50 with the gamma-phosphate, of ATP. A molecular model of the C-terminus of Kir6.2 (based on the crystal structure of Kir3.1) was constructed and automated docking was used to identify residues interacting with ATP. These results support the idea that K185 interacts with the beta-phosphate of ATP. Thus both N- and C-termini may contribute to the ATP binding site.

Gating dependence of inner pore access in inward rectifier K(+) channels

Cation channel gating may occur either at or below the inner vestibule entrance or at the selectivity filter. To differentiate these possibilities in inward rectifier (Kir) channels, we examined cysteine accessibility in the ATP-gated Kir6.2 channel. MTSEA and MTSET both block channels and modify M2 cysteines with identical voltage dependence. If entry is restricted to open channels, modification rates will slow in ATP-closed channels, but because the reagent can be trapped in the pore following brief openings, this may not be apparent until open probability is extremely low (<0.01). When these conditions are met, modification does slow significantly, indicating gated access and highlighting an important caveat for interpretation of MTS-accessibility measurements: reagent "trapping" in nominally "closed" channels may obscure gated access.

Localization of the ATP/phosphatidylinositol 4,5 diphosphate-binding site to a 39-amino acid region of the carboxyl terminus of the ATP-regulated K+ channel Kir1.1

Intracellular ATP and membrane-associated phosphatidylinositol phospholipids, like PIP(2) (PI(4,5)P(2)), regulate the activity of ATP-sensitive K(+) (K(ATP)) and Kir1.1 channels by direct interaction with the pore-forming subunits of these channels. We previously demonstrated direct binding of TNP-ATP (2',3'-O-(2,4,6-trinitrophenylcyclo-hexadienylidene)-ATP) to the COOH-terminal cytosolic domains of the pore-forming subunits of Kir1.1 and Kir6.x channels. In addition, PIP(2) competed for TNP-ATP binding on the COOH termini of Kir1.1 and Kir6.x channels, providing a mechanism that can account for PIP(2) antagonism of ATP inhibition of these channels. To localize the ATP-binding site within the COOH terminus of Kir1.1, we produced and purified maltose-binding protein (MBP) fusion proteins containing truncated and/or mutated Kir1.1 COOH termini and examined the binding of TNP-ATP and competition by PIP(2). A truncated COOH-terminal fusion protein construct, MBP_1.1CDeltaC170, containing the first 39 amino acid residues distal to the second transmembrane domain was sufficient to bind TNP-ATP with high affinity. A construct containing the remaining COOH-terminal segment distal to the first 39 amino acid residues did not bind TNP-ATP. Deletion of 5 or more amino acid residues from the NH(2)-terminal side of the COOH terminus abolished nucleotide binding to the entire COOH terminus or to the first 49 amino acid residues of the COOH terminus. PIP(2) competed TNP-ATP binding to MBP_1.1CDeltaC170 with an EC(50) of 10.9 microm. Mutation of any one of three arginine residues (R188A/E, R203A, and R217A), which are conserved in Kir1.1 and K(ATP) channels and are involved in ATP and/or PIP(2) effects on channel activity, dramatically reduced TNP-ATP binding to MBP_1.1DeltaC170. In contrast, mutation of a fourth conserved residue (R212A) exhibited slightly enhanced TNP-ATP binding and increased affinity for PIP(2) competition of TNP-ATP (EC(50) = 5.7 microm). These studies suggest that the first 39 COOH-terminal amino acid residues form an ATP-PIP(2) binding domain in Kir1.1 and possibly the Kir6.x ATP-sensitive K(+) channels.

Compromised ATP binding as a mechanism of phosphoinositide modulation of ATP-sensitive K+ channels

Inhibition of ATP-sensitive K(+) (K(ATP)) channels by ATP, a process presumably initiated by binding of ATP to the pore-forming subunit, Kir6.2, is reduced in the presence of phosphoinositides (PPIs). Previous studies led to the hypothesis that PPIs compromise ATP binding. Here, this hypothesis was tested using purified Kir6.2. We show that PPIs bind purified Kir6.2 in an isomer-specific manner, that biotinylated ATP analogs photoaffinity label purified Kir6.2, and that this labeling is weakened in the presence of PPIs. Patch-clamp measurements confirmed that these ATP analogs inhibited Kir6.2 channels, and that PPIs decreased the level of inhibition. These results indicate that interaction of PPIs with Kir6.2 impedes ATP-binding activity. The PPI regulation of ATP binding revealed in this study provides a putative molecular mechanism that is potentially pivotal to the nucleotide sensitivity of K(ATP) channels.

A trail of research on potassium

A complex pump-leak system involving both active and passive transport mechanisms is responsible for the appropriate distribution of potassium (K) between the intra- and extracellular fluid compartments. In addition, the kidneys, and to a lesser extent the colon, safeguard maintenance of the narrow range of low K concentrations in the extracellular fluid. Early renal clearance studies showed that K is normally both reabsorbed and secreted by renal tubules, and that regulated secretion is the major source of K excretion. Studies at the tubule and cell level have localized secretion and reabsorption of K to principal and intercalated cells in the collecting ducts. Measurements of the electrochemical driving forces across individual cell membranes have permitted the characterization of specific ATPases, K channels and K cotransporters and also provided insights into the molecular structure of individual transporters that regulate K excretion.