Two Different Conformational States of the KirBac3.1 Potassium Channel Revealed by Electron Crystallography

Structure of the Ion Channel Kir7.1 and Implications for its Function in Normal and Pathophysiologic States

Hereditary defects in the function of the Kir7.1 in the retinal pigment epithelium are associated with the ocular diseases retinitis pigmentosa, Leber congenital amaurosis, and snowflake vitreal degeneration. Studies also suggest that Kir7.1 may be regulated by a GPCR, the melanocortin-4 receptor, in certain hypothalamic neurons. We present the first structures of human Kir7.1 and describe the conformational bias displayed by two pathogenic mutations, R162Q and E276A, to provide an explanation for the basis of disease and illuminate the gating pathway. We also demonstrate the structural basis for the blockade of the channel by a small molecule ML418 and demonstrate that channel blockade in vivo activates MC4R neurons in the paraventricular nucleus of the hypothalamus (PVH), inhibiting food intake and inducing weight loss. Preliminary purification, and structural and pharmacological characterization of an in tandem construct of MC4R and Kir7.1 suggests that the fusion protein forms a homotetrameric channel that retains regulation by liganded MC4R molecules.

The structure of a potassium-selective ion channel reveals a hydrophobic gate regulating ion permeation

Protein dynamics are essential to function. One example of this is the various gating mechanisms within ion channels, which are transmembrane proteins that act as gateways into the cell. Typical ion channels switch between an open and closed state via a conformational transition which is often triggered by an external stimulus, such as ligand binding or pH and voltage differences. The atomic resolution structure of a potassium-selective ion channel named NaK2K has allowed us to observe that a hydro-phobic residue at the bottom of the selectivity filter, Phe92, appears in dual conformations. One of the two conformations of Phe92 restricts the diameter of the exit pore around the selectivity filter, limiting ion flow through the channel, while the other conformation of Phe92 provides a larger-diameter exit pore from the selectivity filter. Thus, it can be concluded that Phe92 acts as a hydro-phobic gate, regulating the flow of ions through the selectivity filter.

New Structural insights into Kir channel gating from molecular simulations, HDX-MS and functional studies

Inward rectifier potassium (Kir) channels play diverse and important roles in shaping action potentials in biological membranes. An increasing number of diseases are now known to be directly associated with abnormal Kir function. However, the gating of Kir still remains unknown. To increase our understanding of its gating mechanism, a dynamical view of the entire channel is essential. Here the gating activation was studied using a recent developped in silico method, MDeNM, which combines normal mode analysis and molecular dynamics simulations that showed for the very first time the importance of interrelated collective and localized conformational movements. In particular, we highlighted the role played by concerted movements of the different regions throughout the entire protein, such as the cytoplasmic and transmembrane domains and the slide helices. In addition, the HDX-MS analysis achieved in these studies provided a comprehensive and detailed view of the dynamics associated with open/closed transition of the Kir channel in coherence with the theoretical results. MDeNM gives access to the probability of the different opening states that are in agreement with our electrophysiological experiments. The investigations presented in this article are important to remedy dysfunctional channels and are of interest for designing new pharmacological compounds.

Contributions of natural products to ion channel pharmacology

Covering: Up to 2020Ion channels are a vast super-family of membrane proteins that play critical physiological roles in excitable and non-excitable cells. Their biomedical importance makes them valuable and attractive drug targets for neurological, cardiovascular, gastrointestinal and metabolic diseases, and for cancer therapy and immune modulation. Current therapeutics target only a minor subset of ion channels, leaving a large unexploited space within the ion channel field. Natural products harnessed from the almost unlimited and diverse universe of compounds within the bioenvironment have been used to modulate channels for decades. In this review we highlight the impact made by natural products on ion channel pharmacology, specifically on K⁺, Na_V and Ca_V channels, and use case studies to describe the development of ion channel-modulating drugs from natural sources for the treatment of pain, heart disease and autoimmune diseases.

Conformational changes upon gating of KirBac1.1 into an open-activated state revealed by solid-state NMR and functional assays

Inward rectifier K⁺ (Kir) channels play an important role in reestablishing the resting membrane state of the action potential of excitable cells in humans. KirBac1.1 is a prokaryotic Kir channel with a high degree of homology to human Kir channels and can be isotopically labeled in NMR quantities for structural studies. Functional assays and NMR assignments confirm that KirBac1.1 is in a constitutively conductive state. Solid-state NMR assignments further reveal alternate conformations at key sites in the protein that are well conserved through human Kir channels, hinting at a possible allosteric network between channels. These underlying sequential and structural motifs could explain abnormal conductive properties of these channels fundamental to their native gating processes.

The conformational changes required for activation and K⁺ conduction in inward-rectifier K⁺ (Kir) channels are still debated. These structural changes are brought about by lipid binding. It is unclear how this process relates to fast gating or if the intracellular and extracellular regions of the protein are coupled. Here, we examine the structural details of KirBac1.1 reconstituted into both POPC and an activating lipid mixture of 3:2 POPC:POPG (wt/wt). KirBac1.1 is a prokaryotic Kir channel that shares homology with human Kir channels. We establish that KirBac1.1 is in a constitutively active state in POPC:POPG bilayers through the use of real-time fluorescence quenching assays and Förster resonance energy transfer (FRET) distance measurements. Multidimensional solid-state NMR (SSNMR) spectroscopy experiments reveal two different conformers within the transmembrane regions of the protein in this activating lipid environment, which are distinct from the conformation of the channel in POPC bilayers. The differences between these three distinct channel states highlight conformational changes associated with an open activation gate and suggest a unique allosteric pathway that ties the selectivity filter to the activation gate through interactions between both transmembrane helices, the turret, selectivity filter loop, and the pore helix. We also identify specific residues involved in this conformational exchange that are highly conserved among human Kir channels.

Life with Bacterial Mechanosensitive Channels, from Discovery to Physiology to Pharmacological Target

General principles in biology have often been elucidated from the study of bacteria. This is true for the bacterial mechanosensitive channel of large conductance, MscL, the channel highlighted in this review. This channel functions as a last-ditch emergency release valve discharging cytoplasmic solutes upon decreases in osmotic environment. Opening the largest gated pore, MscL passes molecules up to 30 Å in diameter; exaggerated conformational changes yield advantages for study, including in vivo assays. MscL contains structural/functional themes that recur in higher organisms and help elucidate how other, structurally more complex, channels function. These features of MscL include (i) the ability to directly sense, and respond to, biophysical changes in the membrane, (ii) an α helix ("slide helix") or series of charges ("knot in a rope") at the cytoplasmic membrane boundary to guide transmembrane movements, and (iii) important subunit interfaces that, when disrupted, appear to cause the channel to gate inappropriately. MscL may also have medical applications: the modality of the MscL channel can be changed, suggesting its use as a triggered nanovalve in nanodevices, including those for drug targeting. In addition, recent studies have shown that the antibiotic streptomycin opens MscL and uses it as one of the primary paths to the cytoplasm. Moreover, the recent identification and study of novel specific agonist compounds demonstrate that the channel is a valid drug target. Such compounds may serve as novel-acting antibiotics and adjuvants, a way of permeabilizing the bacterial cell membrane and, thus, increasing the potency of commonly used antibiotics.

Novel compounds that specifically bind and modulate MscL: insights into channel gating mechanisms

The bacterial mechanosensitive channel of large conductance (MscL) normally functions as an emergency release valve discharging cytoplasmic solutes upon osmotic stress. Opening the large pore of MscL inappropriately is detrimental to the cell, and thus it has been speculated to be a potential antibiotic target. Although MscL is one of the best studied mechanosensitive channels, no chemical that influenced bacterial growth by modulating MscL is known. We therefore used a high-throughput screen to identify compounds that slowed growth in an MscL-dependent manner. We characterized 2 novel sulfonamide compounds identified in the screen. We demonstrated that, although both increase MscL gating, one of these compounds does not work through the folate pathway, as other antimicrobial sulfonamides; indeed, the sulfonamide portion of the compound is not needed for activity. The only mode of action appears to be MscL activation. The binding pocket is where an α-helix runs along the cytoplasmic membrane and interacts with a neighboring subunit; analogous motifs have been observed in several prokaryotic and eukaryotic channels. The data not only demonstrate that MscL is a viable antibiotic target, but also give insight into the gating mechanisms of MscL, and they may have implications for developing agonists for other channels.-Wray, R., Iscla, I., Kovacs, Z., Wang, J., Blount, P. Novel compounds that specifically bind and modulate MscL: insights into channel gating mechanisms.

Hydrophobic pore gates regulate ion permeation in polycystic kidney disease 2 and 2L1 channels

PKD2 and PKD1 genes are mutated in human autosomal dominant polycystic kidney disease. PKD2 can form either a homomeric cation channel or a heteromeric complex with the PKD1 receptor, presumed to respond to ligand(s) and/or mechanical stimuli. Here, we identify a two-residue hydrophobic gate in PKD2L1, and a single-residue hydrophobic gate in PKD2. We find that a PKD2 gain-of-function gate mutant effectively rescues PKD2 knockdown-induced phenotypes in embryonic zebrafish. The structure of a PKD2 activating mutant F604P by cryo-electron microscopy reveals a π- to α-helix transition within the pore-lining helix S6 that leads to repositioning of the gate residue and channel activation. Overall the results identify hydrophobic gates and a gating mechanism of PKD2 and PKD2L1.

Mutations in the cation channel PKD2 cause human autosomal dominant polycystic kidney disease but its channel function and gating mechanism are poorly understood. Here authors study PKD2 using electrophysiology and cryo-EM, which identifies hydrophobic gates and proposes a gating mechanism for PKD2.

Identification of Intrahelical Bifurcated H-Bonds as a New Type of Gate in K+ Channels

Gating of ion channels is based on structural transitions between open and closed states. To uncover the chemical basis of individual gates, we performed a comparative experimental and computational analysis between two K⁺ channels, Kcv_S and Kcv_NTS. These small viral encoded K⁺ channel proteins, with a monomer size of only 82 amino acids, resemble the pore module of all complex K⁺ channels in terms of structure and function. Even though both proteins share about 90% amino acid sequence identity, they exhibit different open probabilities with ca. 90% in Kcv_NTS and 40% in Kcv_S. Single channel analysis, mutational studies and molecular dynamics simulations show that the difference in open probability is caused by one long closed state in Kcv_S. This state is structurally created in the tetrameric channel by a transient, Ser mediated, intrahelical hydrogen bond. The resulting kink in the inner transmembrane domain swings the aromatic rings from downstream Phes in the cavity of the channel, which blocks ion flux. The frequent occurrence of Ser or Thr based helical kinks in membrane proteins suggests that a similar mechanism could also occur in the gating of other ion channels.

A Gate Hinge Controls the Epithelial Calcium Channel TRPV5

TRPV5 is unique within the large TRP channel family for displaying a high Ca²⁺ selectivity together with Ca²⁺-dependent inactivation. Our study aims to uncover novel insights into channel gating through in-depth structure-function analysis. We identify an exceptional tryptophan (W583) at the terminus of the intracellular pore that is unique for TRPV5 (and TRPV6). A combination of site-directed mutagenesis, biochemical and electrophysiological analysis, together with homology modeling, demonstrates that W583 is part of the gate for Ca²⁺ permeation. The W583 mutants show increased cell death due to profoundly enhanced Ca²⁺ influx, resulting from altered channel function. A glycine residue above W583 might act as flexible linker to rearrange the tryptophan gate. Furthermore, we hypothesize functional crosstalk between the pore region and carboxy terminus, involved in Ca²⁺-calmodulin-mediated inactivation. This study proposes a unique channel gating mechanism and delivers detailed molecular insight into the Ca²⁺ permeation pathway that can be extrapolated to other Ca²⁺-selective channels.

Structural Basis for Differences in Dynamics Induced by Leu Versus Ile Residues in the CD Loop of Kir Channels

The effect of the conserved Leu/Ile site in the CD loop on the gating dynamics of Kir channels and corresponding micro-structural mechanism remains unclear. Molecular dynamics simulations were performed to investigate the structural mechanism of chicken Kir2.2. Compared to WT, the I223L mutant channel bound to PIP₂ more strongly, was activated more rapidly, and maintained the activation state more stably after PIP₂ dissociation. Cellular electrophysiology assays of mouse Kir2.1 and human Kir2.2 indicated that, consistent with simulations, the Leu residue increased the channel responses to PIP₂ through increased binding affinity and faster activation kinetics, and the deactivation kinetics decreased upon PIP₂ inhibition. The Ile residue induced the opposite responses. This difference was attributed to the distinct hydrophobic side chain symmetries of Leu and Ile; switching between these residues caused the interaction network to redistribute and offered effective conformation transduction in the Leu systems, which had more rigid and independent subunits.

Three pairs of weak interactions precisely regulate the G-loop gate of Kir2.1 channel

Kir2.1 (also known as IRK1) plays key roles in regulation of resting membrane potential and cell excitability. To achieve its physiological roles, Kir2.1 performs a series of conformational transition, named as gating. However, the structural basis of gating is still obscure. Here, we combined site-directed mutation, two-electrode voltage clamp with molecular dynamics simulations and determined that H221 regulates the gating process of Kir2.1 by involving a weak interaction network. Our data show that the H221R mutant accelerates the rundown kinetics and decelerates the reactivation kinetics of Kir2.1. Compared with the WT channel, the H221R mutation strengthens the interaction between the CD- and G-loops (E303-R221) which stabilizes the close state of the G-loop gate and weakens the interactions between C-linker and CD-loop (R221-R189) and the adjacent G-loops (E303-R312) which destabilizes the open state of G-loop gate. Our data indicate that the three pairs of interactions (E303-H221, H221-R189 and E303-R312) precisely regulate the G-loop gate by controlling the conformation of G-loop. Proteins 2016; 84:1929-1937. © 2016 Wiley Periodicals, Inc.

Structural dynamics of potassium-channel gating revealed by single-molecule FRET

Crystallography has provided invaluable insights to ion channel selectivity and gating, but to advance understanding to a new level, dynamic views of channel structures within membranes are essential. We labeled tetrameric KirBac1.1 potassium channels with single donor and acceptor fluorophores at different sites, and examined structural dynamics within lipid membranes by single molecule FRET. We found that the extracellular region is structurally rigid in both closed and open states, whereas the N-terminal slide helix undergoes marked conformational fluctuations. The cytoplasmic C-terminal domain fluctuates between two major structural states both of which become less dynamic and move away from the pore axis and away from the membrane in closed channels. Our results reveal mobile and rigid conformations of functionally relevant KirBac1.1 channel motifs, implying similar dynamics for similar motifs in eukaryotic Kir channels and for cation channels in general.

Two-Dimensional Crystallization Procedure, from Protein Expression to Sample Preparation

Membrane proteins play important roles for living cells. Structural studies of membrane proteins provide deeper understanding of their mechanisms and further aid in drug design. As compared to other methods, electron microscopy is uniquely suitable for analysis of a broad range of specimens, from small proteins to large complexes. Of various electron microscopic methods, electron crystallography is particularly well-suited to study membrane proteins which are reconstituted into two-dimensional crystals in lipid environments. In this review, we discuss the steps and parameters for obtaining large and well-ordered two-dimensional crystals. A general description of the principle in each step is provided since this information can also be applied to other biochemical and biophysical methods. The examples are taken from our own studies and published results with related proteins. Our purpose is to give readers a more general idea of electron crystallography and to share our experiences in obtaining suitable crystals for data collection.

Identification of the Conformational transition pathway in PIP2 Opening Kir Channels

The gating of Kir channels depends critically on phosphatidylinositol 4,5-bisphosphate (PIP2), but the detailed mechanism by which PIP2 regulates Kir channels remains obscure. Here, we performed a series of Targeted molecular dynamics simulations on the full-length Kir2.1 channel and, for the first time, were able to achieve the transition from the closed to the open state. Our data show that with the upward motion of the cytoplasmic domain (CTD) the structure of the C-Linker changes from a loop to a helix. The twisting of the C-linker triggers the rotation of the CTD, which induces a small downward movement of the CTD and an upward motion of the slide helix toward the membrane that pulls the inner helix gate open. At the same time, the rotation of the CTD breaks the interaction between the CD- and G-loops thus releasing the G-loop. The G-loop then bounces away from the CD-loop, which leads to the opening of the G-loop gate and the full opening of the pore. We identified a series of interaction networks, between the N-terminus, CD loop, C linker and G loop one by one, which exquisitely regulates the global conformational changes during the opening of Kir channels by PIP2.

Growth of large and highly ordered 2D crystals of a K⁺ channel, structural role of lipidic environment

2D crystallography has proven to be an excellent technique to determine the 3D structure of membrane proteins. Compared to 3D crystallography, it has the advantage of visualizing the protein in an environment closer to the native one. However, producing good 2D crystals is still a challenge and little statistical knowledge can be gained from literature. Here, we present a thorough screening of 2D crystallization conditions for a prokaryotic inwardly rectifying potassium channel (>130 different conditions). Key parameters leading to very large and well-organized 2D crystals are discussed. In addition, the problem of formation of multilayers during the growth of 2D crystals is also addressed. An intermediate resolution projection map of KirBac3.1 at 6 Å is presented, which sheds (to our knowledge) new light on the structure of this channel in a lipid environment.

Conformational changes underlying pore dilation in the cytoplasmic domain of mammalian inward rectifier K+ channels

The cytoplasmic domain of inward rectifier K⁺ (Kir) channels associates with cytoplasmic ligands and undergoes conformational change to control the gate present in its transmembrane domain. Ligand-operated activation appears to cause dilation of the pore at the cytoplasmic domain. However, it is still unclear how the cytoplasmic domain supports pore dilation and how alterations to this domain affect channel activity. In the present study, we focused on 2 spatially adjacent residues, i.e., Glu236 and Met313, of the G protein-gated Kir channel subunit Kir3.2. In the closed state, these pore-facing residues are present on adjacent βD and βH strands, respectively. We mutated both residues, expressed them with the m₂-muscarinic receptor in Xenopus oocytes, and measured the acetylcholine-dependent K⁺ currents. The dose-response curves of the Glu236 mutants tended to be shifted to the right. In comparison, the slopes of the concentration-dependent curves were reduced and the single-channel properties were altered in the Met313 mutants. The introduction of arginine at position 236 conferred constitutive activity and caused a leftward shift in the conductance-voltage relationship. The crystal structure of the cytoplasmic domain of the mutant showed that the arginine contacts the main chains of the βH and βI strands of the adjacent subunit. Because the βH strand forms a β sheet with the βI and βD strands, the immobilization of the pore-forming β sheet appears to confer unique properties to the mutant. These results suggest that the G protein association triggers pore dilation at the cytoplasmic domain in functional channels, and the pore-constituting structural elements contribute differently to these conformational changes.

Membrane channels as integrators of G-protein-mediated signaling

A variety of extracellular stimuli regulate cellular responses via membrane receptors. A well-known group of seven-transmembrane domain-containing proteins referred to as G protein-coupled receptors, directly couple with the intracellular GTP-binding proteins (G proteins) across cell membranes and trigger various cellular responses by regulating the activity of several enzymes as well as ion channels. Many specific populations of ion channels are directly controlled by G proteins; however, indirect modulation of some channels by G protein-dependent phosphorylation events and lipid metabolism is also observed. G protein-mediated diverse modifications affect the ion channel activities and spatio-temporally regulate membrane potentials as well as of intracellular Ca(2+) concentrations in both excitatory and non-excitatory cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Regulation of CRAC channels by protein interactions and post-translational modification

Store-operated Ca(2+) entry (SOCE) is a widespread mechanism to elevate the intracellular Ca(2+) concentrations and stimulate downstream signaling pathways affecting proliferation, secretion, differentiation and death in different cell types. In immune cells, immune receptor stimulation induces intracellular Ca(2+) store depletion that subsequently activates Ca(2+)-release-activated-Ca(2+) (CRAC) channels, a prototype of store-operated Ca(2+) (SOC) channels. CRAC channel opening leads to activation of diverse downstream signaling pathways affecting proliferation, differentiation, cytokine production and cell death. Recent identification of STIM1 as the endoplasmic reticulum Ca(2+) sensor and Orai1 as the pore subunit of CRAC channels has provided the much-needed molecular tools to dissect the mechanism of activation and regulation of CRAC channels. In this review, we discuss the recent advances in understanding the associating partners and posttranslational modifications of Orai1 and STIM1 proteins that regulate diverse aspects of CRAC channel function.

Tetrameric structure of SUR2B revealed by electron microscopy of oriented single particles

The ATP-sensitive potassium (K_ATP) channel is a hetero-octameric complex that links cell metabolism to membrane electrical activity in many cells, thereby controlling physiological functions such as insulin release, muscle contraction and neuronal activity. It consists of four pore-forming Kir6.2 and four regulatory sulfonylurea receptor (SUR) subunits. SUR2B serves as the regulatory subunit in smooth muscle and some neurones. An integrative approach, combining electron microscopy and homology modelling, has been used to obtain information on the structure of this large (megadalton) membrane protein complex. Single-particle electron microscopy of purified SUR2B tethered to a lipid monolayer revealed that it assembles as a tetramer of four SUR2B subunits surrounding a central hole. In the absence of an X-ray structure, a homology model for SUR2B based on the X-ray structure of the related ABC transporter Sav1866 was used to fit the experimental images. The model indicates that the central hole can readily accommodate the transmembrane domains of the Kir tetramer, suggests a location for the first transmembrane domains of SUR2B (which are absent in Sav1866) and suggests the relative orientation of the SUR and Kir6.2 subunits.

Multi-ion distributions in the cytoplasmic domain of inward rectifier potassium channels

Inward rectifier potassium (Kir) channels act as cellular diodes, allowing unrestricted flow of potassium (K(+)) into the cell while preventing currents of large magnitude in the outward direction. The rectification mechanism by which this occurs involves a coupling between K(+) and intracellular blockers-magnesium (Mg(2+)) or polyamines-that simultaneously occupy the permeation pathway. In addition to the transmembrane pore, Kirs possess a large cytoplasmic domain (CD) that provides a favorable electronegative environment for cations. Electrophysiological experiments have shown that the CD is a key regulator of both conductance and rectification. In this study, we calculate and compare averaged equilibrium probability densities of K(+) and Cl(-) in open-pore models of the CDs of a weak (Kir1.1-ROMK) and a strong (Kir2.1-IRK) rectifier through explicit-solvent molecular-dynamics simulations in ~1 M KCl. The CD of both channels concentrates K(+) ions greater than threefold inside the cytoplasmic pore while IRK shows an additional K(+) accumulation region near the cytoplasmic entrance. Simulations carried out with Mg(2+) or spermine (SPM(4+)) show that these ions interact with pore-lining residues, shielding the surface charge and reducing K(+) in both channels. The results also show that SPM(4+) behaves differently inside these two channels. Although SPM(4+) remains inside the CD of ROMK, it diffuses around the entire volume of the pore. In contrast, this polyatomic cation finds long-lived conformational states inside the IRK pore, interacting with residues E224, D259, and E299. The strong rectifier CD is also capable of sequestering an additional SPM(4+) at the cytoplasmic entrance near a cluster of negative residues D249, D274, E275, and D276. Although understanding the actual mechanism of rectification blockade will require high-resolution structural information of the blocked state, these simulations provide insight into how sequence variation in the CD can affect the multi-ion distributions that underlie the mechanisms of conduction, rectification affinity, and kinetics.

Structural rearrangements underlying ligand-gating in Kir channels

Inward rectifier potassium (Kir) channels are physiologically regulated by a wide range of ligands that all act on a common gate, although structural details of gating are unclear. Here we show, using small molecule fluorescent probes attached to introduced cysteines, the molecular motions associated with gating of KirBac1.1 channels. The accessibility of the probes indicates a major barrier to fluorophore entry to the inner cavity. Changes in FRET between fluorophores attached to KirBac1.1 tetramers show that PIP₂-induced closure involves tilting and rotational motions of secondary structural elements of the cytoplasmic domain that couple ligand binding to a narrowing of the cytoplasmic vestibule. The observed ligand-dependent conformational changes in KirBac1.1 provide a general model for ligand-induced Kir channel gating at the molecular level.

Structure of a KirBac potassium channel with an open bundle crossing indicates a mechanism of channel gating

KirBac channels are prokaryotic homologs of mammalian inwardly rectifying (Kir) potassium channels, and recent crystal structures of both Kir and KirBac channels have provided major insight into their unique structural architecture. However, all of the available structures are closed at the helix bundle crossing, and therefore the structural mechanisms that control opening of their primary activation gate remain unknown. In this study, we engineered the inner pore-lining helix (TM2) of KirBac3.1 to trap the bundle crossing in an apparently open conformation and determined the crystal structure of this mutant channel to 3.05 Å resolution. Contrary to previous speculation, this new structure suggests a mechanistic model in which rotational 'twist' of the cytoplasmic domain is coupled to opening of the bundle-crossing gate through a network of inter- and intrasubunit interactions that involve the TM2 C-linker, slide helix, G-loop and the CD loop.

Interactions of cations with the cytoplasmic pores of inward rectifier K(+) channels in the closed state

Ion channels gate at membrane-embedded domains by changing their conformation along the ion conduction pathway. Inward rectifier K(+) (Kir) channels possess a unique extramembrane cytoplasmic domain that extends this pathway. However, the relevance and contribution of this domain to ion permeation remain unclear. By qualitative x-ray crystallographic analysis, we found that the pore in the cytoplasmic domain of Kir3.2 binds cations in a valency-dependent manner and does not allow the displacement of Mg(2+) by monovalent cations or spermine. Electrophysiological analyses revealed that the cytoplasmic pore of Kir3.2 selectively binds positively charged molecules and has a higher affinity for Mg(2+) when it has a low probability of being open. The selective blocking of chemical modification of the side chain of pore-facing residues by Mg(2+) indicates that the mode of binding of Mg(2+) is likely to be similar to that observed in the crystal structure. These results indicate that the Kir3.2 crystal structure has a closed conformation with a negative electrostatic field potential at the cytoplasmic pore, the potential of which may be controlled by conformational changes in the cytoplasmic domain to regulate ion diffusion along the pore.

Gating at the selectivity filter of ion channels that conduct Na+ and K+ ions

The NaK channel is a cation selective channel with similar permeability for K(+) and Na(+). The available crystallographic structure of wild-type (WT) NaK is usually associated with a conductive state of the channel. Here, potential of mean force for complete conduction events of Na(+) and K(+) ions through NaK show that: i), large energy barriers prevent the passage of ions through the WT NaK structure, ii), the barriers are correlated to the presence of a hydrogen bond between Asp-66 and Asn-68, and iii), the structure of NaK mutated to mimic cyclic nucleotide-gated channels conducts Na(+) and K(+). These results support the hypothesis that the filter of cation selective channels can adopt at least two different structures: a conductive one, represented by the x-ray structures of the NaK-CNG chimeras, and a closed one, represented by the x-ray structures of the WT NaK.

Cholesterol regulates prokaryotic Kir channel by direct binding to channel protein

Cholesterol is a major regulator of a variety of ion channels but the mechanisms underlying cholesterol sensitivity of ion channels are still poorly understood. The key question is whether cholesterol regulates ion channels by direct binding to the channel protein or by altering the physical environment of lipid bilayer. In this study, we provide the first direct evidence that cholesterol binds to prokaryotic Kir channels, KirBac1.1, and that cholesterol binding is essential for its regulatory effect. Specifically, we show that cholesterol is eluted together with the KirBac1.1 protein when separated on an affinity column and that the amount of bound cholesterol is proportional to the amount of the protein. We also show that cholesterol binding to KirBac1.1 is saturable with a K(D) of 390μM. Moreover, there is clear competition between radioactive and non-radioactive cholesterol for the binding site. There is no competition, however, between cholesterol and 5-Androsten 3β-17 β-diol, a sterol that we showed previously to have no effect on KirBac1.1 function. Finally, we show that cholesterol-KirBac1.1 binding is significantly inhibited by trifluoperazine, known to inhibit cholesterol binding to other proteins, and that inhibition of cholesterol-KirBac1.1 binding results in full recovery of the channel activity. Collectively, results from this study indicate that cholesterol-induced suppression of KirBac1.1 activity is mediated by direct interaction between cholesterol and the channel protein.

Forced gating motions by a substituted titratable side chain at the bundle crossing of a potassium channel

Numerous inwardly rectifying potassium (Kir) channels possess an aromatic residue in the helix bundle crossing region, forming the narrowest pore constriction in crystal structures. However, the role of the Kir channel bundle crossing as a functional gate remains uncertain. We report a unique phenotype of Kir6.2 channels mutated to encode glutamate at this position (F168E). Despite a prediction of four glutamates in close proximity, Kir6.2(F168E) channels are predominantly closed at physiological pH, whereas alkalization causes rapid and reversible channel activation. These findings suggest that F168E glutamates are uncharged at physiological pH but become deprotonated at alkaline pH, forcing channel opening due to mutual repulsion of nearby negatively charged side chains. The potassium channel pore scaffold likely brings these glutamates close together, causing a significant pK(a) shift relative to the free side chain (as seen in the KcsA selectivity filter). Alkalization also shifts the apparent ATP sensitivity of the channel, indicating that forced motion of the bundle crossing is coupled to the ATP-binding site and may resemble conformational changes involved in wild-type Kir6.2 gating. The study demonstrates a novel mechanism for engineering extrinsic control of channel gating by pH and shows that conformational changes in the bundle crossing region are involved in ligand-dependent gating of Kir channels.

Role of amino-terminal half of the S4-S5 linker in type 1 ryanodine receptor (RyR1) channel gating

The type 1 ryanodine receptor (RyR1) is a Ca(2+) release channel found in the sarcoplasmic reticulum of skeletal muscle and plays a pivotal role in excitation-contraction coupling. The RyR1 channel is activated by a conformational change of the dihydropyridine receptor upon depolarization of the transverse tubule, or by Ca(2+) itself, i.e. Ca(2+)-induced Ca(2+) release (CICR). The molecular events transmitting such signals to the ion gate of the channel are unknown. The S4-S5 linker, a cytosolic loop connecting the S4 and S5 transmembrane segments in six-transmembrane type channels, forms an α-helical structure and mediates signal transmission in a wide variety of channels. To address the role of the S4-S5 linker in RyR1 channel gating, we performed alanine substitution scan of N-terminal half of the putative S4-S5 linker (Thr(4825)-Ser(4829)) that exhibits high helix probability. The mutant RyR1 was expressed in HEK cells, and CICR activity was investigated by caffeine-induced Ca(2+) release, single-channel current recordings, and [(3)H]ryanodine binding. Four mutants (T4825A, I4826A, S4828A, and S4829A) had reduced CICR activity without changing Ca(2+) sensitivity, whereas the L4827A mutant formed a constitutive active channel. T4825I, a disease-associated mutation for malignant hyperthermia, exhibited enhanced CICR activity. An α-helical wheel representation of the N-terminal S4-S5 linker provides a rational explanation to the observed activities of the mutants. These results suggest that N-terminal half of the S4-S5 linker may form an α-helical structure and play an important role in RyR1 channel gating.

Identification of a novel bacterial K(+) channel

In an attempt to explore unknown K(+) channels in mammalian cells, especially ATP-sensitive K(+) (K(ATP)) channels, we compared the sequence homology of Kir6.1 and Kir6.2, two pore-forming subunits of mammalian K(ATP) channel genes, with bacterial genes that code for selective proteins with confirmed or putative ion transport properties. BLAST analysis revealed that a prokaryotic gene (ydfJ) expressed in Escherichia coli K12 strain shared 8.6% homology with Kir6.1 and 8.3% with Kir6.2 genes. Subsequently, we cloned and sequenced ydfJ gene from E. coli K12 and heterologously expressed it in mammalian HEK-293 cells. The whole-cell patch-clamp technique was used to record ion channel currents generated by ydfJ-encoded protein. Heterologous expression of ydfJ gene in HEK-293 cells yielded a novel K(+) channel current that was inwardly rectified and had a reversal potential close to K(+) equilibrium potential. The expressed ydfJ channel was blocked reversibly by low concentration of barium in a dose-dependent fashion. Specific K(ATP) channel openers or blockers did not alter the K(+) current generated by ydfJ expression alone or ydfJ coexpressed with rvSUR1 or rvSUR2B subunits of K(ATP) channel complex. Furthermore, this coexpressed ydfJ/rvSUR1 channels were not inhibited by ATP dialysis. On the other hand, ydfJ K(+) currents were inhibited by protopine (a nonspecific K(+) channel blocker) but not by dofetilide (a HERG channel blocker). In summary, heterologously expressed prokaryotic ydfJ gene formed a novel functional K(+) channel in mammalian cells.

Cholesterol sensitivity of KIR2.1 is controlled by a belt of residues around the cytosolic pore

Kir channels play an important role in setting the resting membrane potential and modulating membrane excitability. A common feature of several Kir channels is that they are regulated by cholesterol. Yet, the mechanism by which cholesterol affects channel function is unclear. We recently showed that the cholesterol sensitivity of Kir2 channels depends on several CD-loop residues. Here we show that this cytosolic loop is part of a regulatory site that also includes residues in the G-loop, the N-terminus, and the connecting segment between the C-terminus and the inner transmembrane helix. Together, these residues form a cytosolic belt that surrounds the pore of the channel close to its interface with the transmembrane domain, and modulate the cholesterol sensitivity of the channel. Furthermore, we show that residues in this cluster are correlated with residues located in the most flexible region of the G-loop, the major cytosolic gate of Kir2.1, implying that the importance of these residues extends beyond their effect on the channel's cholesterol sensitivity. We suggest that the residues of the cholesterol sensitivity belt are critical for channel gating.

Molecular basis and structural insight of vascular K(ATP) channel gating by S-glutathionylation

The vascular ATP-sensitive K(+) (K(ATP)) channel is targeted by a variety of vasoactive substances, playing an important role in vascular tone regulation. Our recent studies indicate that the vascular K(ATP) channel is inhibited in oxidative stress via S-glutathionylation. Here we show evidence for the molecular basis of the S-glutathionylation and its structural impact on channel gating. By comparing the oxidant responses of the Kir6.1/SUR2B channel with the Kir6.2/SUR2B channel, we found that the Kir6.1 subunit was responsible for oxidant sensitivity. Oxidant screening of Kir6.1-Kir6.2 chimeras demonstrated that the N terminus and transmembrane domains of Kir6.1 were crucial. Systematic mutational analysis revealed three cysteine residues in these domains: Cys(43), Cys(120), and Cys(176). Among them, Cys(176) was prominent, contributing to >80% of the oxidant sensitivity. The Kir6.1-C176A/SUR2B mutant channel, however, remained sensitive to both channel opener and inhibitor, which indicated that Cys(176) is not a general gating site in Kir6.1, in contrast to its counterpart (Cys(166)) in Kir6.2. A protein pull-down assay with biotinylated glutathione ethyl ester showed that mutation of Cys(176) impaired oxidant-induced incorporation of glutathione (GSH) into the Kir6.1 subunit. In contrast to Cys(176), Cys(43) had only a modest contribution to S-glutathionylation, and Cys(120) was modulated by extracellular oxidants but not intracellular GSSG. Simulation modeling of Kir6.1 S-glutathionylation suggested that after incorporation to residue 176, the GSH moiety occupied a space between the slide helix and two transmembrane helices. This prevented the inner transmembrane helix from undergoing conformational changes necessary for channel gating, retaining the channel in its closed state.

Polyamine block of inwardly rectifying potassium channels

Polyamine blockade of inwardly rectifying potassium (Kir) channels underlies their steep voltage--dependence observed in native cells. The structural determinants of polyamine blockade and the structure-activity profile of endogenous polyamines requires specialized methodology for characterizing polyamine interactions with Kir channels. Recent identification and growing interest in the structure and function of prokaryotic Kir channels (KirBacs) has driven the development of new techniques for measuring ion channel activity. Several methods for measuring polyamine interactions with prokaryotic and eukaryotic Kir channels are discussed.

Functional complementation and genetic deletion studies of KirBac channels: activatory mutations highlight gating-sensitive domains

The superfamily of prokaryotic inwardly rectifying (KirBac) potassium channels is homologous to mammalian Kir channels. However, relatively little is known about their regulation or about their physiological role in vivo. In this study, we have used random mutagenesis and genetic complementation in K(+)-auxotrophic Escherichia coli and Saccharomyces cerevisiae to identify activatory mutations in a range of different KirBac channels. We also show that the KirBac6.1 gene (slr5078) is necessary for normal growth of the cyanobacterium Synechocystis PCC6803. Functional analysis and molecular dynamics simulations of selected activatory mutations identified regions within the slide helix, transmembrane helices, and C terminus that function as important regulators of KirBac channel activity, as well as a region close to the selectivity filter of KirBac3.1 that may have an effect on gating. In particular, the mutations identified in TM2 favor a model of KirBac channel gating in which opening of the pore at the helix-bundle crossing plays a far more important role than has recently been proposed.

Gating of a G protein-sensitive mammalian Kir3.1 prokaryotic Kir channel chimera in planar lipid bilayers

Kir3 channels control heart rate and neuronal excitability through GTP-binding (G) protein and phosphoinositide signaling pathways. These channels were the first characterized effectors of the βγ subunits of G proteins. Because we currently lack structures of complexes between G proteins and Kir3 channels, their interactions leading to modulation of channel function are not well understood. The recent crystal structure of a chimera between the cytosolic domain of a mammalian Kir3.1 and the transmembrane region of a prokaryotic KirBac1.3 (Kir3.1 chimera) has provided invaluable structural insight. However, it was not known whether this chimera could form functional K(+) channels. Here, we achieved the functional reconstitution of purified Kir3.1 chimera in planar lipid bilayers. The chimera behaved like a bona fide Kir channel displaying an absolute requirement for PIP(2) and Mg(2+)-dependent inward rectification. The channel could also be blocked by external tertiapin Q. The three-dimensional reconstruction of the chimera by single particle electron microscopy revealed a structure consistent with the crystal structure. Channel activity could be stimulated by ethanol and activated G proteins. Remarkably, the presence of both activated Gα and Gβγ subunits was required for gating of the channel. These results confirm the Kir3.1 chimera as a valid structural and functional model of Kir3 channels.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Conformational changes during the gating of a potassium channel revealed by structural mass spectrometry

Potassium channels are dynamic proteins that undergo large conformational changes to regulate the flow of K(+) ions across the cell membrane. Understanding the gating mechanism of these channels therefore requires methods for probing channel structure in both their open and closed conformations. Radiolytic footprinting is used to study the gating mechanism of the inwardly-rectifying potassium channel KirBac3.1. The purified protein stabilized in either open or closed conformations was exposed to focused synchrotron X-ray beams on millisecond timescales to modify solvent accessible amino acid side chains. These modifications were identified and quantified using high-resolution mass spectrometry. The differences observed between the closed and open states were then used to reveal local conformational changes that occur during channel gating. The results provide support for a proposed gating mechanism of the Kir channel and demonstrate a method of probing the dynamic gating mechanism of other integral membrane proteins and ion channels.

The human breast cancer resistance protein (BCRP/ABCG2) shows conformational changes with mitoxantrone

BCRP/ABCG2 mediates efflux of drugs and xenobiotics. BCRP was expressed in Pichia pastoris, purified to > 90% homogeneity, and subjected to two-dimensional (2D) crystallization. The 2D crystals showed a p12(1) symmetry and projection maps were determined at 5 A resolution by cryo-electron microscopy. Two crystal forms with and without mitoxantrone were observed with unit cell dimensions of a = 55.4 A, b = 81.4 A, gamma = 89.8 degrees , and a = 57.3 A, b = 88.0 A, gamma = 89.7 degrees , respectively. The projection map without mitoxantrone revealed an asymmetric structure with ring-shaped density features probably corresponding to a bundle of transmembrane alpha helices, and appeared more open and less symmetric than the map with mitroxantrone. The open and closed inward-facing forms of BCRP were generated by homology modeling, representing the substrate-free and substrate-bound conformations in the absence of nucleotide, respectively. These models are consistent with the experimentally observed conformational change upon substrate binding.

Visualizing water molecules in transmembrane proteins using radiolytic labeling methods

Essential to cells and their organelles, water is both shuttled to where it is needed and trapped within cellular compartments and structures. Moreover, ordered waters within protein structures often colocalize with strategically placed polar or charged groups critical for protein function, yet it is unclear if these ordered water molecules provide structural stabilization, mediate conformational changes in signaling, neutralize charged residues, or carry out a combination of all these functions. Structures of many integral membrane proteins, including G protein-coupled receptors (GPCRs), reveal the presence of ordered water molecules that may act like prosthetic groups in a manner quite unlike bulk water. Identification of "ordered" waters within a crystalline protein structure requires sufficient occupancy of water to enable its detection in the protein's X-ray diffraction pattern, and thus, the observed waters likely represent a subset of tightly bound functional waters. In this review, we highlight recent studies that suggest the structures of ordered waters within GPCRs are as conserved (and thus as important) as conserved side chains. In addition, methods of radiolysis, coupled to structural mass spectrometry (protein footprinting), reveal dynamic changes in water structure that mediate transmembrane signaling. The idea of water as a prosthetic group mediating chemical reaction dynamics is not new in fields such as catalysis. However, the concept of water as a mediator of conformational dynamics in signaling is just emerging, because of advances in both crystallographic structure determination and new methods of protein footprinting. Although oil and water do not mix, understanding the roles of water is essential to understanding the function of membrane proteins.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Computational studies on the interactions of inhalational anesthetics with proteins

Despite the widespread clinical use of anesthetics since the 19th century, a clear understanding of the mechanism of anesthetic action has yet to emerge. On the basis of early experiments by Meyer, Overton, and subsequent researchers, the cell's lipid membrane was generally concluded to be the primary site of action of anesthetics. However, later experiments with lipid-free globular proteins, such as luciferase and apoferritin, shifted the focus of anesthetic action to proteins. Recent experimental studies, such as photoaffinity labeling and mutagenesis on membrane proteins, have suggested specific binding sites for anesthetic molecules, further strengthening the proteocentric view of anesthetic mechanism. With the increased availability of high-resolution crystal structures of ion channels and other integral membrane proteins, as well as the availability of powerful computers, the structure-function relationship of anesthetic-protein interactions can now be investigated in atomic detail. In this Account, we review recent experiments and related computer simulation studies involving interactions of inhalational anesthetics and proteins, with a particular focus on membrane proteins. Globular proteins have long been used as models for understanding the role of protein-anesthetic interactions and are accordingly examined in this Account. Using selected examples of membrane proteins, such as nicotinic acetyl choline receptor (nAChR) and potassium channels, we address the issues of anesthetic binding pockets in proteins, the role of conformation in anesthetic effects, and the modulation of local as well as global dynamics of proteins by inhaled anesthetics. In the case of nicotinic receptors, inhalational anesthetic halothane binds to the hydrophobic cavity close to the M2-M3 loop. This binding modulates the dynamics of the M2-M3 loop, which is implicated in allosterically transmitting the effects to the channel gate, thus altering the function of the protein. In potassium channels, anesthetic molecules preferentially potentiate the open conformation by quenching the motion of the aromatic residues implicated in the gating of the channel. These simulations suggest that low-affinity drugs (such as inhalational anesthetics) modulate the protein function by influencing local as well as global dynamics of proteins. Because of intrinsic experimental limitations, computational approaches represent an important avenue for exploring the mode of action of anesthetics. Molecular dynamics simulations-a computational technique frequently used in the general study of proteins-offer particular insight in the study of the interaction of inhalational anesthetics with membrane proteins.

Characterization and functional restoration of a potassium channel Kir6.2 pore mutation identified in congenital hyperinsulinism

The inwardly rectifying potassium channel Kir6.2 assembles with sulfonylurea receptor 1 to form the ATP-sensitive potassium (K(ATP)) channels that regulate insulin secretion in pancreatic beta-cells. Mutations in K(ATP) channels underlie insulin secretion disease. Here, we report the characterization of a heterozygous missense Kir6.2 mutation, G156R, identified in congenital hyperinsulinism. Homomeric mutant channels reconstituted in COS cells show similar surface expression as wild-type channels but fail to conduct potassium currents. The mutated glycine is in the pore-lining transmembrane helix of Kir6.2; an equivalent glycine in other potassium channels has been proposed to serve as a hinge to allow helix bending during gating. We found that mutation of an adjacent asparagine, Asn-160, to aspartate, which converts the channel from a weak to a strong inward rectifier, on the G156R background restored ion conduction in the mutant channel. Unlike N160D channels, however, G156R/N160D channels are not blocked by intracellular polyamines at positive membrane potential and exhibit wild-type-like nucleotide sensitivities, suggesting the aspartate introduced at position 160 interacts with arginine at 156 to restore ion conduction and gating. Using tandem Kir6.2 tetramers containing G156R and/or N160D in designated positions, we show that one mutant subunit in the tetramer is insufficient to abolish conductance and that G156R and N160D can interact in the same or adjacent subunits to restore conduction. We conclude that the glycine at 156 is not essential for K(ATP) channel gating and that the Kir6.2 gating defect caused by the G156R mutation could be rescued by manipulating chemical interactions between pore residues.

Structural and regulatory evolution of cellular electrophysiological systems

Cellular electrophysiological systems, like developmental systems, appear to evolve primarily by means of regulatory evolution. It is suggested that electrophysiological systems share two key features with developmental systems that account for this dependence on regulatory evolution. For both systems, structural evolution has the potential to create significant problems of pleiotropy and both systems are predominantly computational in nature. It is concluded that the relative balance of physical and computational tasks that a biological system has to perform, combined with the probability that these tasks may have to change significantly during the course of evolution, will be major factors in determining the relative mix of regulatory and structural evolution that is observed for a given system. Physiological systems that directly interface with the environment will almost always perform some low-level physical task. In the majority of cases this will require evolution of protein function in order for the tasks themselves to evolve. For complex physiological systems a large fraction of their function will be devoted to high-level control functions that are predominantly computational in nature. In most cases regulatory evolution will be sufficient in order for these computational tasks to evolve.

The gates of ion channels and enzymes

Protein dynamics are essential for virtually all protein functions, certainly for gating mechanisms of ion channels and regulation of enzyme catalysis. Ion channels usually feature a gate in the channel pore that prevents ion permeation in the closed state. Some bifunctional enzymes with two distant active sites use a tunnel to transport intermediate products; a gate can help prevent premature leakage. Enzymes with a buried active site also require a tunnel for substrate entrance; a gate along the tunnel can contribute to selectivity. The gates in these different contexts show distinct characteristics in sequence, structure and dynamics, but they also have common features. In particular, aromatic residues often appear to serve as gates, probably because of their ability, through side chain rotation, to effect large changes in cross section.

Elucidation of the gating of the GIRK channel using a spectroscopic approach

The traditional view of G protein-coupled receptor (GPCR)-mediated signalling puts the players in this signalling cascade, namely the GPCR, the G protein and its effector, as individual components in space, where the signalling specificity is obtained mainly by the interaction of the GPCR and the Galpha subunits of the G protein. A question is then raised as to how fidelity in receptor signalling is achieved, given that many systems use the same components of the G protein signalling machinery. One possible mechanism for obtaining the specific flow of the downstream signals, from the activated G protein to its specific effector target, in a timely manner, is compartmentalization, a spatial arrangement of the complex in a rather restricted space. Here we review our recent findings related to these issues, using the G protein-coupled potassium channel (GIRK) as a model effector and fluorescence-based approaches to reveal how the signalling complex is arranged and how the G protein exerts its action to activate the GIRK channel in intact cells.