One-shot design elevates functional expression levels of a voltage-gated potassium channel

Membrane proteins play critical physiological roles as receptors, channels, pumps, and transporters. Despite their importance, however, low expression levels often hamper the experimental characterization of membrane proteins. We present an automated and web-accessible design algorithm called mPROSS (https://mPROSS.weizmann.ac.il), which uses phylogenetic analysis and an atomistic potential, including an empirical lipophilicity scale, to improve native-state energy. As a stringent test, we apply mPROSS to the Kv1.2-Kv2.1 paddle chimera voltage-gated potassium channel. Four designs, encoding 9-26 mutations relative to the parental channel, were functional and maintained potassium-selective permeation and voltage dependence in Xenopus oocytes with up to 14-fold increase in whole-cell current densities. Additionally, single-channel recordings reveal no significant change in the channel-opening probability nor in unitary conductance, indicating that functional expression levels increase without impacting the activity profile of individual channels. Our results suggest that the expression levels of other dynamic channels and receptors may be enhanced through one-shot design calculations.

Store-operated Ca2+ entry regulatory factor (SARAF) alters murine metabolic state in an age-dependent manner via hypothalamic pathways

Store-Operated Calcium Entry (SOCE) is a vital process aimed at refilling cellular internal Ca2+ stores and a primary cellular-signaling driver for transcription factors’ entry to the nucleus. SARAF (SOCE-associated regulatory factor)/TMEM66 is an endoplasmic reticulum (ER) resident transmembrane protein that promotes SOCE inactivation and prevents Ca2+ overfilling of the cell. Here we demonstrate that mice deficient in SARAF develop age-dependent sarcopenic obesity with decreased energy expenditure, lean mass, and locomotion without affecting food consumption. Moreover, SARAF ablation reduces hippocampal proliferation, modulates the activity of the hypothalamus-pituitary-adrenal (HPA) axis, and mediates changes in anxiety-related behaviors. Interestingly, selective SARAF ablation in the hypothalamus’s paraventricular nucleus (PVN) neurons reduces old age-induced obesity and preserves locomotor activity, lean mass, and energy expenditure, suggesting a possible central control with a site-specific role for SARAF. At the cellular level, SARAF ablation in hepatocytes leads to elevated SOCE, elevated vasopressin-induced Ca2+ oscillations, and an increased mitochondrial spare respiratory capacity, thus providing insights into the cellular mechanisms that may affect the global phenotypes. These effects may be mediated via the liver X receptor (LXR), and IL-1 signaling metabolic regulators explicitly altered in SARAF ablated cells. In short, our work supports both central and peripheral roles of SARAF in regulating metabolic, behavioral, and cellular responses.

Modern venomics-Current insights, novel methods, and future perspectives in biological and applied animal venom research

Venoms have evolved >100 times in all major animal groups, and their components, known as toxins, have been fine-tuned over millions of years into highly effective biochemical weapons. There are many outstanding questions on the evolution of toxin arsenals, such as how venom genes originate, how venom contributes to the fitness of venomous species, and which modifications at the genomic, transcriptomic, and protein level drive their evolution. These questions have received particularly little attention outside of snakes, cone snails, spiders, and scorpions. Venom compounds have further become a source of inspiration for translational research using their diverse bioactivities for various applications. We highlight here recent advances and new strategies in modern venomics and discuss how recent technological innovations and multi-omic methods dramatically improve research on venomous animals. The study of genomes and their modifications through CRISPR and knockdown technologies will increase our understanding of how toxins evolve and which functions they have in the different ontogenetic stages during the development of venomous animals. Mass spectrometry imaging combined with spatial transcriptomics, in situ hybridization techniques, and modern computer tomography gives us further insights into the spatial distribution of toxins in the venom system and the function of the venom apparatus. All these evolutionary and biological insights contribute to more efficiently identify venom compounds, which can then be synthesized or produced in adapted expression systems to test their bioactivity. Finally, we critically discuss recent agrochemical, pharmaceutical, therapeutic, and diagnostic (so-called translational) aspects of venoms from which humans benefit.

Ion channel auxiliary subunit: does one size fit all?

Ion channels can tailor their activity to the particular cellular context by incorporating auxiliary subunits that are channel-type specific. In this issue of Cell, Ávalos Prado et al. now find that a well-characterized voltage-gated K⁺ channel auxiliary subunit can also modulate the gating of Ca²⁺-activated Cl^- channels.

Shaker-IR K+ channel gating in heavy water: Role of structural water molecules in inactivation

It has been reported earlier that the slow (C-type) inactivated conformation in K_v channels is stabilized by a multipoint hydrogen-bond network behind the selectivity filter. Furthermore, MD simulations revealed that structural water molecules are also involved in the formation of this network locking the selectivity filter in its inactive conformation. We found that the application of an extracellular, but not intracellular, solution based on heavy water (D₂O) dramatically slowed entry into the slow inactivated state in Shaker-IR mutants (T449A, T449A/I470A, and T449K/I470C, displaying a wide range of inactivation kinetics), consistent with the proposed effect of the dynamics of structural water molecules on the conformational stability of the selectivity filter. Alternative hypotheses capable of explaining the observed effects of D₂O were examined. Increased viscosity of the external solution mimicked by the addition of glycerol had a negligible effect on the rate of inactivation. In addition, the inactivation time constants of K⁺ currents in the outward and the inward directions in asymmetric solutions were not affected by a H₂O/D₂O exchange, negating an indirect effect of D₂O on the rate of K⁺ rehydration. The elimination of the nonspecific effects of D₂O on our macroscopic current measurements supports the hypothesis that the rate of structural water exchange at the region behind the selectivity filter determines the rate of slow inactivation, as proposed by molecular modeling.

A Molecular Lid Mechanism of K+ Channel Blocker Action Revealed by a Cone Peptide

Many venomous organisms carry in their arsenal short polypeptides that block K⁺ channels in a highly selective manner. These toxins may compete with the permeating ions directly via a "plug" mechanism or indirectly via a "pore-collapse" mechanism. An alternative "lid" mechanism was proposed but remained poorly defined. Here we study the Drosophila Shaker channel block by Conkunitzin-S1 and Conkunitzin-C3, two highly similar toxins derived from cone venom. Despite their similarity, the two peptides exhibited differences in their binding poses and biophysical assays, implying discrete action modes. We show that while Conkunitzin-S1 binds tightly to the channel turret and acts via a "pore-collapse" mechanism, Conkunitzin-C3 does not contact this region. Instead, Conk-C3 uses a non-conserved Arg to divert the permeant ions and trap them in off-axis cryptic sites above the SF, a mechanism we term a "molecular-lid". Our study provides an atomic description of the "lid" K⁺ blocking mode and offers valuable insights for the design of therapeutics based on venom peptides.

Production of recombinant venom peptides as tools for ion channel research

Animal venom is a rich source for peptide toxins that bind and modulate the function of ion channels. Owing to their ability to bind receptor sites on the channel protein with high affinity and specificity, peptide neurotoxins have become an indispensable tool for ion channel research. Recent breakthroughs in structural biology and advances in computer simulations of biomolecules have sparked a new interest in animal toxins as probes of channel protein structure and function. Here, we focus on methods used to produce animal toxins for research purposes using recombinant expression. The specific challenges associated with heterologous production of venom peptides are discussed, and several methods targeting these issues are presented with an emphasis on E. coli based systems. An efficient protocol for the bacterial expression, folding, and purification of recombinant venom peptides is described.

Quinone Methide-Based Organophosphate Hydrolases Inhibitors: Trans Proximity Labelers versus Cis Labeling Activity-Based Probes

Quinone methide (QM) chemistry is widely applied including in enzyme inhibitors. Typically, enzyme-mediated bond breaking releases a phenol product that rearranges into an electrophilic QM that in turn covalently modifies protein side chains. However, the factors that govern the reactivity of QM-based inhibitors and their mode of inhibition have not been systematically explored. Foremost, enzyme inactivation might occur in cis, whereby a QM molecule inactivates the very same enzyme molecule that released it, or by trans if the released QMs diffuse away and inactivate other enzyme molecules. We examined QM-based inhibitors for enzymes exhibiting phosphoester hydrolase activity. We tested different phenolic substituents and benzylic leaving groups, thereby modulating the rates of enzymatic hydrolysis, phenolate-to-QM rearrangement, and the electrophilicity of the resulting QM. By developing assays that distinguish between cis and trans inhibition, we have identified certain combinations of leaving groups and phenyl substituents that lead to inhibition in the cis mode, while other combinations gave trans inhibition. Our results suggest that cis-acting QM-based substrates could be used as activity-based probes to identify various phospho- and phosphono-ester hydrolases, and potentially other hydrolases.

Reduced activity of GIRK1-containing heterotetramers is sufficient to affect neuronal functions, including synaptic plasticity and spatial learning and memory

KEY POINTS

G-protein inwardly rectifying K⁺ (GIRK) channels consist of four homologous subunits (GIRK1-4) and are essential regulators of electrical excitability in the nervous system. GIRK2-null mice have been widely investigated for their distinct behaviour and altered depotentiation following long-term potentiation (LTP), whereas GIRK1 mice are less well characterized. Here we utilize a novel knockin mouse strain in which the GIRK1 subunit is fluorescently tagged with yellow fluorescent protein (YFP-GIRK1) and the GIRK1-null mouse line to investigate the role of GIRK1 in neuronal processes such as spatial learning and memory, locomotion and depotentiation following LTP. Neurons dissected from YFP-GIRK1 mice had significantly reduced potassium currents and this mouse line phenotypically resembled GIRK1-null mice, making it a 'functional knockdown' model of GIRK1-containing channels. YFP-GIRK1 and GIRK1-null mice had increased locomotion, reduced spatial learning and memory and blunted depotentiation following LTP.

ABSTRACT

GIRK channels are essential for the slow inhibition of electrical activity in the nervous system and heart rate regulation via the parasympathetic system. The implications of individual GIRK isoforms in specific physiological activities are based primarily on studies conducted with GIRK-null mouse lines. Here we utilize a novel knockin mouse line in which YFP was fused in-frame to the N-terminus of GIRK1 (YFP-GIRK1) to correlate GIRK1 spatial distribution with physiological activities. These mice, however, displayed spontaneous seizure-like activity and thus were investigated for the origin of such activity. We show that GIRK tetramers containing YFP-GIRK1 are correctly assembled and trafficked to the plasma membrane, but are functionally impaired. A battery of behavioural assays conducted on YFP-GIRK1 and GIRK1-null (GIRK1^-/- ) mice revealed similar phenotypes, including impaired nociception, reduced anxiety and hyperactivity in an unfamiliar environment. However, YFP-GIRK1 mice exhibited increased home-cage locomotion while GIRK1^-/- mice did not. In addition, we show that the GIRK1 subunit is essential for intact spatial learning and memory and synaptic plasticity in hippocampal brain slices. This study expands our knowledge regarding the role of GIRK1 in neuronal processes and underlines the importance of GIRK1-containing heterotetramers.

Non-sedating antihistamines block G-protein-gated inwardly rectifying K+ channels

BACKGROUND AND PURPOSE

A second-generation antihistamine, terfenadine, is known to induce arrhythmia by blocking hERG channels. In this study, we have shown that terfenadine also inhibits the activity of G-protein-gated inwardly rectifying K⁺ (GIRK) channels, which regulate the excitability of neurons and cardiomyocytes. To clarify the underlying mechanism(s), we examined the effects of several antihistamines on GIRK channels and identified the structural determinant for the inhibition.

EXPERIMENTAL APPROACH

Electrophysiological recordings were made in Xenopus oocytes and rat atrial myocytes to analyse the effects of antihistamines on various GIRK subunits (K_ir 3.x). Mutagenesis analyses identified the residues critical for inhibition by terfenadine and the regulation of ion selectivity. The potential docking site of terfenadine was analysed by molecular docking.

KEY RESULTS

GIRK channels containing K_ir 3.1 subunits heterologously expressed in oocytes and native GIRK channels in atrial myocytes were inhibited by terfenadine and other non-sedating antihistamines. In K_ir 3.1 subunits, mutation of Phe137, located in the centre of the pore helix, to the corresponding Ser in K_ir 3.2 subunits reduced the inhibition by terfenadine. Introduction of an amino acid with a large side chain in K_ir 3.2 subunits at Ser148 increased the inhibition. When this residue was mutated to a non-polar amino acid, the channel became permeable to Na⁺ . Phosphoinositide-mediated activity was also decreased by terfenadine.

CONCLUSION AND IMPLICATIONS

The Phe137 residue in K_ir 3.1 subunits is critical for inhibition by terfenadine. This study provides novel insights into the regulation of GIRK channels by the pore helix and information for drug design.

Tracking Conformational Changes in Calmodulin in vitro, in Cell Extract, and in Cells by Electron Paramagnetic Resonance Distance Measurements

It is an open question whether the conformations of proteins sampled in dilute solutions are the same as in the cellular environment. Here we address this question by double electron-electron resonance (DEER) distance measurements with Gd(III) spin labels to probe the conformations of calmodulin (CaM) in vitro, in cell extract, and in human HeLa cells. Using the CaM mutants N53C/T110C and T34C/T117C labeled with maleimide-DOTA-Gd(III) in the N- and C-terminal domains, we observed broad and varied interdomain distance distributions. The in vitro distance distributions of apo-CaM and holo-CaM in the presence and absence of the IQ target peptide can be described by combinations of closed, open, and collapsed conformations. In cell extract, apo- and holo-CaM bind to target proteins in a similar way as apo- and holo-CaM bind to IQ peptide in vitro. In HeLa cells, however, in the presence or absence of elevated in-cell Ca²⁺ levels CaM unexpectedly produced more open conformations and very broad distance distributions indicative of many different interactions with in-cell components. These results show-case the importance of in-cell analyses of protein structures.

SARAF Luminal Domain Structure Reveals a Novel Domain-Swapped β-Sandwich Fold Important for SOCE Modulation

Store-Operated Calcium Entry (SOCE) plays key roles in cell proliferation, muscle contraction, immune responses, and memory formation. The coordinated interactions of a number of proteins from the plasma and endoplasmic reticulum membranes control SOCE to replenish internal Ca²⁺ stores and generate intracellular Ca²⁺ signals. SARAF, an endoplasmic reticulum resident component of the SOCE pathway having no homology to any characterized protein, serves as an important brake on SOCE. Here, we describe the X-ray crystal structure of the SARAF luminal domain, SARAF_L. This domain forms a novel 10-stranded β-sandwich fold that includes a set of three conserved disulfide bonds, denoted the "SARAF-fold." The structure reveals a domain-swapped dimer in which the last two β-strands (β9 and β10) are exchanged forming a region denoted the "SARAF luminal switch" that is essential for dimerization. Sequence comparisons reveal that the SARAF-fold is highly conserved in vertebrates and in a variety of pathologic fungi. Förster resonance energy transfer experiments using full-length SARAF validate the formation of the domain-swapped dimer in cells and demonstrate that dimerization is reversible. A designed variant lacking the SARAF luminal switch shows that the domain swapping is essential to function and indicates that the SARAF dimer accelerates SOCE inactivation.

Tethered protein display identifies a novel Kir3.2 (GIRK2) regulator from protein scaffold libraries

Use of randomized peptide libraries to evolve molecules with new functions provides a means for developing novel regulators of protein activity. Despite the demonstrated power of such approaches for soluble targets, application of this strategy to membrane systems, such as ion channels, remains challenging. Here, we have combined libraries of a tethered protein scaffold with functional selection in yeast to develop a novel activator of the G-protein-coupled mammalian inwardly rectifying potassium channel Kir3.2 (GIRK2). We show that the novel regulator, denoted N5, increases Kir3.2 (GIRK2) basal activity by inhibiting clearance of the channel from the cellular surface rather than affecting the core biophysical properties of the channel. These studies establish the tethered protein display strategy as a means to create new channel modulators and highlight the power of approaches that couple randomized libraries with direct selections for functional effects. Our results further underscore the possibility for the development of modulators that influence channel function by altering cell surface expression densities rather than by direct action on channel biophysical parameters. The use of tethered library selection strategies coupled with functional selection bypasses the need for a purified target and is likely to be applicable to a range of membrane protein systems.

Nonenzymatic rapid control of GIRK channel function by a G protein-coupled receptor kinase

G protein-coupled receptors (GPCRs) respond to agonists to activate downstream enzymatic pathways or to gate ion channel function. Turning off GPCR signaling is known to involve phosphorylation of the GPCR by GPCR kinases (GRKs) to initiate their internalization. The process, however, is relatively slow and cannot account for the faster desensitization responses required to regulate channel gating. Here, we show that GRKs enable rapid desensitization of the G protein-coupled potassium channel (GIRK/Kir3.x) through a mechanism independent of their kinase activity. On GPCR activation, GRKs translocate to the membrane and quench channel activation by competitively binding and titrating G protein βγ subunits away from the channel. Of interest, the ability of GRKs to effect this rapid desensitization depends on the receptor type. The findings thus reveal a stimulus-specific, phosphorylation-independent mechanism for rapidly downregulating GPCR activity at the effector level.

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.

The use of FRET microscopy to elucidate steady state channel conformational rearrangements and G protein interaction with the GIRK channels

while X-ray crystallography provides extremely high-resolution snapshot of protein structure, it lacks the ability to provide dynamic information on the processes involving conformational rearrangements of the protein. Methods to record protein conformational dynamics are present, in particular those that are based on fluorescence measurements, and are now more and more utilized in studying proteins in their natural environment. Here we describe the use of fluorescence resonance energy transfer (FRET) technique to monitor the conformational rearrangements associated with the gating of the G protein-coupled potassium channel (GIRK/Kir3.x), and its relation with the G protein subunits. The FRET technique is combined with total internal fluorescence (TIRF) microscopy, and allows the dissection of the signal originating from channel proteins that reside exclusively in the plasma membrane. Since most of the components associated with GIRK channel gating are intracellular, that involve various biochemical steps, proteins were labeled with genetically encoded variants of the green fluorescence protein and signals were acquired from live cells in culture. Using these methodologies we were able to show that gating conformational rearrangements, i.e. the opening of the channel, involve the rotation and expansion of the channel subunits cytosolic termini, along the channel's central axis. In addition, the G proteins that trigger this process reside very close to the channel, to ensure high signaling specificity and to provide temporal precision of the gating process.

PIP2--the master key

The function of inwardly rectifying K+ (Kir) channels is highly diverse and therefore is tightly regulated by various environmental factors. In their article in this issue of Neuron, Rapedius et al. recognize a conserved structural mechanism for Kir channels gating by both pH and PIP2. In light of these findings and accumulated knowledge, PIP2 is suggested to have a common coregulatory role in the gating of Kir channels by all their soluble modulators.

GIRK Channel Activation Involves a Local Rearrangement of a Preformed G Protein Channel Complex

G protein-coupled signaling is one of the major mechanisms for controlling cellular excitability. One of the main targets for this control at postsynaptic membranes is the G protein-coupled potassium channels (GIRK/Kir3), which generate slow inhibitory postsynaptic potentials following the activation of Pertussis toxin-sensitive G protein-coupled receptors. Using total internal reflection fluorescence (TIRF) microscopy combined with fluorescence resonance energy transfer (FRET), in intact cells, we provide evidence for the existence of a trimeric G protein-channel complex at rest. We show that activation of the channel via the receptor induces a local conformational switch of the G protein to induce channel opening. The presence of such a complex thus provides the means for a precise temporal and highly selective activation of the channel, which is required for fine tuning of neuronal excitability.

Modulation of basal and receptor-induced GIRK potassium channel activity and neuronal excitability by the mammalian PINS homolog LGN

G protein-activated inwardly rectifying potassium (GIRK) channels mediate slow synaptic inhibition and control neuronal excitability. It is unknown whether GIRK channels are subject to regulation by guanine dissociation inhibitor (GDI) proteins like LGN, a mammalian homolog of Drosophila Partner of Inscuteable (mPINS). Here we report that LGN increases basal GIRK current but reduces GIRK activation by metabotropic transmitter receptors coupled to Gi or Go, but not Gs. Moreover, expression of its N-terminal, TPR-containing protein interaction domains mimics the effects of LGN in mammalian cells, probably by releasing sequestered endogenous LGN. In hippocampal neurons, expression of LGN, or LGN fragments that mimic or enhance LGN activity, hyperpolarizes the resting potential due to increased basal GIRK activity and reduces excitability. Using Lenti virus for LGN RNAi to reduce endogenous LGN levels in hippocampal neurons, we further show an essential role of LGN for maintaining basal GIRK channel activity and for harnessing neuronal excitability.

The pore helix dipole has a minor role in inward rectifier channel function

Ion channels lower the energetic barrier for ion passage across cell membranes and enable the generation of bioelectricity. Electrostatic interactions between permeant ions and channel pore helix dipoles have been proposed as a general mechanism for facilitating ion passage. Here, using genetic selections to probe interactions of an exemplar potassium channel blocker, barium, with the inward rectifier Kir2.1, we identify mutants bearing positively charged residues in the potassium channel signature sequence at the pore helix C terminus. We show that these channels are functional, selective, resistant to barium block, and have minimally altered conductance properties. Both the experimental data and model calculations indicate that barium resistance originates from electrostatics. We demonstrate that potassium channel function is remarkably unperturbed when positive charges occur near the permeant ions at a location that should counteract pore helix electrostatic effects. Thus, contrary to accepted models, the pore helix dipole seems to be a minor factor in potassium channel permeation.

Gating of GIRK channels: details of an intricate, membrane-delimited signaling complex

G protein-coupled inwardly rectifying potassium channels (GIRK/Kir3) are important elements in controlling cellular excitability. In recent years, tremendous progress has been made toward understanding various components involved in channel activation, modulation, and signaling specificity. In this review, we summarize these recent findings and attempt to put them in context with recently available structural data.

The pore helix is involved in stabilizing the open state of inwardly rectifying K+ channels

Ion channels can be gated by various extrinsic cues, such as voltage, pH, and second messengers. However, most ion channels display extrinsic cue-independent transitions as well. These events represent spontaneous conformational changes of the channel protein. The molecular basis for spontaneous gating and its relation to the mechanism by which channels undergo activation gating by extrinsic cue stimulation is not well understood. Here we show that the proximal pore helix of inwardly rectifying (Kir) channels is partially responsible for determining spontaneous gating characteristics, affecting the open state of the channel by stabilizing intraburst openings as well as the bursting state itself without affecting K(+) ion-channel interactions. The effect of the pore helix on the open state of the channel is qualitatively similar to that of two well-characterized mutations at the second transmembrane domain (TM2), which stabilize the channel in its activated state. However, the effects of the pore helix and the TM2 mutations on gating were additive and independent of each other. Moreover, in sharp contrast to the two TM2 mutations, the pore helix mutation did not affect the functionality of the agonist-responsive gate. Our results suggest that in Kir channels, the bottom of the pore helix and agonist-induced conformational transitions at the TM2 ultimately stabilize via different pathways the open conformation of the same gate.

Conformational Rearrangements Associated with the Gating of the G Protein-Coupled Potassium Channel Revealed by FRET Microscopy

G protein-coupled potassium channels (GIRK/Kir3.x) are key determinants that translate inhibitory chemical neurotransmission into changes in cellular excitability. To understand the mechanism of channel activation by G proteins, it is necessary to define the structural rearrangements in the channel that result from interaction with Gbetagamma subunits. In this study we used a combination of fluorescence spectroscopy and through-the-objective total internal reflection microscopy to monitor the conformational rearrangements associated with the activation of GIRK channels in single intact cells. We detect activation-induced changes in FRET consistent with a rotation and expansion of the termini along the central axis of the channel. We propose that this rotation and expansion of the termini drives the channel to open by bending and possibly rotating the second transmembrane segment.

Casein kinase 2 specifically binds to and phosphorylates the carboxy termini of ENaC subunits

A number of findings have suggested the involvement of protein phosphorylation in the regulation of the epithelial Na+ channel (ENaC). A recent study has demonstrated that the C tails of the beta and gamma subunits of ENaC are subject to phosphorylation by at least three protein kinases [Shi, H., Asher, C., Chigaev, A., Yung, Y., Reuveny, E., Seger, R. & Garty, H. (2002) J. Biol. Chem. 277, 13539-13547]. One of them was identified as ERK which phosphorylates betaT613 and gammaT623 and affects the channel interaction with Nedd4. The current study identifies a second protein kinase as casein kinase 2 (CK2), or CK-2-like kinase. It phosphorylates betaS631, a well-conserved serine on the beta subunit. Such phosphorylation is observed both in vitro using glutathione-S-transferase-ENaC fusion proteins and in vivo in ENaC-expressing Xenopus oocytes. The gamma subunit is weakly phosphorylated by this protein kinase on another residue (gammaT599), and the C tail of alpha is not significantly phosphorylated by this kinase. Thus, CK2 may be involved in the regulation of the epithelial Na+ channel.

Trapping the sensor

Voltage-gated ion channels open in response to a change in membrane potential. The "sensor," or the channel's molecular entity responsible for the detection of voltage change, is formed by a transmembrane element, rich with basic residues, called the "voltage sensor" or the "S4 domain." The movement of the S4 drives a global conformational change leading to the opening of the permeation pathway and ion conduction. In this issue of Neuron, Schönherr and colleagues show that physical constrains of the "gating canal," or the crevice through which the S4 moves, determines whether voltage-gated potassium channels open quickly or slowly.

Graded contribution of the Gbeta gamma binding domains to GIRK channel activation

G protein coupled inwardly rectifying K(+) channels (GIRK/Kir3.x) are mainly activated by a direct interaction with Gbetagamma subunits, released upon the activation of inhibitory neurotransmitter receptors. Although Gbetagamma binding domains on all four subunits have been found, the relative contribution of each of these binding sites to channel gating has not yet been defined. It is also not known whether GIRK channels open once all Gbetagamma sites are occupied, or whether gating is a graded process. We used a tandem tetrameric approach to enable the selective elimination of specific Gbetagamma binding domains in the tetrameric context. Here, we show that tandem tetramers are fully operational. Tetramers with only one wild-type channel subunit showed receptor-independent high constitutive activity. The presence of two or three wild-type subunits reconstituted receptor activation gradually. Furthermore, a tetramer with no GIRK1 Gbetagamma binding domain displayed slower kinetics of activation. The slowdown in activation was found to be independent of regulator of G protein signaling or receptor coupling, but this slowdown could be reversed once only one Gbetagamma binding domain of GIRK1 was added. These results suggest that partial activation can occur under low Gbetagamma occupancy and that full activation can be accomplished by the interaction with three Gbetagamma binding subunits.

Interactions of beta and gamma ENaC with Nedd4 can be facilitated by an ERK-mediated phosphorylation

Phosphorylation of the epithelial Na(+) channel (ENaC) has been suggested to play a role in its regulation. Here we demonstrate that phosphorylating the carboxyl termini of the beta and gamma subunits facilitates their interactions with the ubiquitin ligase Nedd4 and inhibits channel activity. Three protein kinases, which phosphorylate the carboxyl termini of beta and gammaENaC, have been identified by an in vitro assay. One of these phosphorylates betaThr-613 and gammaThr-623, well-conserved C-tail threonines in the immediate vicinity of the PY motifs. Phosphorylation of gammaThr-623 has also been demonstrated in vivo in channels expressed in Xenopus oocytes, and mutating betaThr-613 and gammaThr-623 into alanine increased the channel activity by 3.5-fold. Effects of the above phosphorylations on interactions between ENaC and Nedd4 have been studied using surface plasmon resonance. Peptides having phospho-threonine at positions beta613 or gamma623 bind the WW domains of Nedd4 two to three times better than the non-phosphorylated analogues, due to higher association rate constants. Using a number of different approaches it was demonstrated that the protein kinase acting on betaThr-613 and gammaThr-623 is the extracellular regulated kinase (ERK). It is suggested that an ERK-mediated phosphorylation of betaThr-613 and gammaThr-623 down-regulates the channel by facilitating its interaction with Nedd4.

tBID Homooligomerizes in the mitochondrial membrane to induce apoptosis

Activation of the tumor necrosis factor R1/Fas receptor results in the cleavage of cytosolic BID to truncated tBID. tBID translocates to the mitochondria to induce the oligomerization of BAX or BAK, resulting in the release of cytochrome c (Cyt c). Here we demonstrate that in tumor necrosis factor alpha-activated FL5.12 cells, tBID becomes part of a 45-kDa cross-linkable mitochondrial complex that does not include BAX or BAK. Using fluorescence resonance energy transfer analysis and co-immunoprecipitation, we demonstrate that tBID-tBID interactions occur in the mitochondria of living cells. Cross-linking experiments using a tBID-GST chimera indicated that tBID forms homotrimers in the mitochondrial membrane. To test the functional consequence of tBID oligomerization, we expressed a chimeric FKBP-tBID molecule. Enforced dimerization of FKBP-tBID by the bivalent ligand FK1012 resulted in Cyt c release, caspase activation, and apoptosis. Surprisingly, enforced dimerization of tBID did not result in the dimerization of either BAX or BAK. Moreover, a tBID BH3 mutant (G94E), which does not interact with or induce the dimerization of either BAX or BAK, formed the 45-kDa complex and induced both Cyt c release and apoptosis. Thus, tBID oligomerization may represent an alternative mechanism for inducing mitochondrial dysfunction and apoptosis.

Redox-dependent gating of G protein-coupled inwardly rectifying K+ channels

G protein-coupled inwardly rectifying K(+) channels (GIRK) play a major role in inhibitory signaling in excitable and endocrine tissues. The gating mechanism of these channels is mediated by a direct interaction of the Gbetagamma subunits of G protein, which are released upon inhibitory neurotransmitter receptor activation. This gating mechanism is further manifested by intracellular factors such as anionic phospholipids and Na(+) and Mg(2+) ions. In addition to the essential role of these components for channel function, phosphorylation events can also modulate channel activity. In this study we explored the involvement of redox modulation on GIRK channel function. Extracellular application of the reducing agent dithiothreitol (DTT), but not reduced glutathione, activated GIRK channels without affecting their permeation or rectification properties. The DTT-dependent activation was found to mimic receptor activation and to act directly on the channel in a membrane delimited fashion. A critical cysteine residue located in the N-terminal cytoplasmic domain was found to be essential for DTT-dependent activation in hetero- and homotetrameric contexts. Interestingly, when mutating this cysteine residue, DTT-dependent activation was abolished, but receptor-mediated channel activation was not affected. These results suggest that intracellular redox potential can play a major role in tuning GIRK channel activity in a receptor-independent manner. This sort of redox modulation can be part of an important cellular protective mechanism against ischemic or hypoxic insults.

Mechanism of Ba(2+) block of a mouse inwardly rectifying K+ channel: differential contribution by two discrete residues

1. The block of the IRK1/Kir2.1 inwardly rectifying K+ channel by a Ba(2+) ion is highly voltage dependent, where the ion binds approximately half-way within the membrane electrical field. The mechanism by which two distinct mutations, E125N and T141A, affect Ba(2+) block of Kir2.1 was investigated using heterologous expression in Xenopus oocytes. 2. Analysis of the blocking kinetics showed that E125 and T141 affect the entry and binding of Ba(2+) to the channel, respectively. Replacing the glutamate at position 125 with an asparagine greatly decreased the rate at which the Ba(2+) ions enter and leave the pore. In contrast, replacing the polar threonine at position 141 with an alanine affected the entry rate of the Ba(2+) ions while leaving the exit rate unchanged. 3. Acidification of the extracellular solution slowed the exit rate of the Ba(2+) from the wild-type channel, but had no such effect on the Kir2.1(E125N) mutant. 4. These results thus reveal two unique roles for the amino acids at positions 125 and 141 in aiding the interaction of Ba(2+) with the channel. Their possible roles in K+ permeation are discussed.

In vitro phosphorylation of COOH termini of the epithelial Na+ channel and its effects on channel activity inXenopus oocytes

Recent findings have suggested the involvement of protein phosphorylation in the regulation of the epithelial Na+ channel (ENaC). This study reports the in vitro phosphorylation of the COOH termini of ENaC subunits expressed as glutathione S-transferase fusion proteins. Channel subunits were specifically phosphorylated by kinase-enriched cytosolic fractions derived from rat colon. The phosphorylation observed was not mediated by the serum- and glucocorticoid-regulated kinase sgk. For the γ-subunit, phosphorylation occurred on a single, well-conserved threonine residue located in the immediate vicinity of the PY motif (T630). The analogous residue on β(S620) was phosphorylated as well. The possible role of γT630 and βS620 in channel function was studied in Xenopus laevis oocytes. Mutating these residues to alanine had no effect on the basal channel-mediated current. They do, however, inhibit the sgk-induced increase in channel activity but only in oocytes that were preincubated in low Na+ and had a high basal Na+ current. Thus mutating γT630 or βS620 may limit the maximal channel activity achieved by a combination of sgk and low Na+.

Coupling Gbetagamma-dependent activation to channel opening via pore elements in inwardly rectifying potassium channels

G protein-coupled inwardly rectifying potassium channels, GIRK/Kir3.x, are gated by the Gbetagamma subunits of the G protein. The molecular mechanism of gating was investigated by employing a novel yeast-based random mutagenesis approach that selected for channel mutants that are active in the absence of Gbetagamma. Mutations in TM2 were found that mimicked the Gbetagamma-activated state. The activity of these channel mutants was independent of receptor stimulation and of the availability of heterologously expressed Gbetagamma subunits but depended on PtdIns(4,5)P(2). The results suggest that the TM2 region plays a key role in channel gating following Gbetagamma binding in a phospholipid-dependent manner. This mechanism of gating in inwardly rectifying K+ channels may be similar to the involvement of the homologous region in prokaryotic KcsA potassium channel and, thus, suggests evolutionary conservation of the gating structure.

Regulation of sgk by aldosterone and its effects on the epithelial Na(+) channel

Aldosterone is the major corticosteroid regulating Na(+) absorption in tight epithelia and acts primarily by activating the epithelial Na(+) channel (ENaC) through unknown induced proteins. Recently, it has been reported that aldosterone induces the serum- and glucocorticoid-dependent kinase sgk and that coexpressing ENaC with this kinase in Xenopus laevis oocytes increases the amiloride-sensitive Na(+) current (Chen SY, Bhargava A, Mastroberardino L, Meijer OC, Wang J, Buse P, Firestone GL, Verrey F, and Pearce D. Proc Natl Acad Sci USA 96: 2514-2519, 1999). The present study was done to further characterize regulation of sgk by aldosterone in native mammalian epithelia and to examine its effect on ENaC. With both in vivo and in vitro protocols, an almost fivefold increase in the abundance of sgk mRNA has been demonstrated in rat kidney and colon but not in lung. Induction of sgk by aldosterone was detected in kidney cortex and medulla, whereas the papilla expressed a constitutively high level of the kinase. The increase in sgk mRNA was detected as early as 30 min after the hormonal application and was independent of de novo protein synthesis. The observed aldosterone dose-response relationships suggest that the response is mediated, at least in part, by occupancy of the mineralocorticoid receptor. Coexpressing sgk and ENaC in Xenopus oocytes evoked a fourfold increase in the amiloride-blockable Na(+) channel activity. A point mutation in the beta-subunit known to impair regulation of the channel by Nedd4 (Y618A) had no significant effect on the response to sgk.

Fast inactivation of a brain K+ channel composed of Kv1.1 and Kvbeta1.1 subunits modulated by G protein beta gamma subunits

Modulation of A-type voltage-gated K+ channels can produce plastic changes in neuronal signaling. It was shown that the delayed-rectifier Kv1.1 channel can be converted to A-type upon association with Kvbeta1.1 subunits; the conversion is only partial and is modulated by phosphorylation and microfilaments. Here we show that, in Xenopus oocytes, expression of Gbeta1gamma2 subunits concomitantly with the channel (composed of Kv1.1 and Kvbeta1.1 subunits), but not after the channel's expression in the plasma membrane, increases the extent of conversion to A-type. Conversely, scavenging endogenous Gbetagamma by co-expression of the C-terminal fragment of the beta-adrenergic receptor kinase reduces the extent of conversion to A-type. The effect of Gbetagamma co-expression is occluded by treatment with dihydrocytochalasin B, a microfilament-disrupting agent shown previously by us to enhance the extent of conversion to A-type, and by overexpression of Kvbeta1.1. Gbeta1gamma2 subunits interact directly with GST fusion fragments of Kv1.1 and Kvbeta1.1. Co-expression of Gbeta1gamma2 causes co-immunoprecipitation with Kv1.1 of more Kvbeta1.1 subunits. Thus, we suggest that Gbeta1gamma2 directly affects the interaction between Kv1.1 and Kvbeta1.1 during channel assembly which, in turn, disrupts the ability of the channel to interact with microfilaments, resulting in an increased extent of A-type conversion.

Molecular basis for interactions of G protein betagamma subunits with effectors

Both the alpha and betagamma subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins) communicate signals from receptors to effectors. Gbetagamma subunits can regulate a diverse array of effectors, including ion channels and enzymes. Galpha subunits bound to guanine diphosphate (Galpha-GDP) inhibit signal transduction through Gbetagamma subunits, suggesting a common interface on Gbetagamma subunits for Galpha binding and effector interaction. The molecular basis for interaction of Gbetagamma with effectors was characterized by mutational analysis of Gbeta residues that make contact with Galpha-GDP. Analysis of the ability of these mutants to regulate the activity of calcium and potassium channels, adenylyl cyclase 2, phospholipase C-beta2, and beta-adrenergic receptor kinase revealed the Gbeta residues required for activation of each effector and provides evidence for partially overlapping domains on Gbeta for regulation of these effectors. This organization of interaction regions on Gbeta for different effectors and Galpha explains why subunit dissociation is crucial for signal transmission through Gbetagamma subunits.

Probing the G-protein regulation of GIRK1 and GIRK4, the two subunits of the KACh channel, using functional homomeric mutants

In heart, G-protein-activated channels are complexes of two homologous proteins, GIRK1 and GIRK4. Expression of either protein alone results in barely active or non-active channels, making it difficult to assess the individual contribution of each subunit to the channel complex. The residue Phe137, located within the H5 region of GIRK1, is critical to the synergy between GIRK1 and GIRK4 (Chan, K. W., Sui, J. L., Vivaudou, M., and Logothetis, D. E. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 14193-14198). By modifying this residue or the matching residue of GIRK4, Ser143, we have been able to generate mutant proteins that produced large inwardly rectifying, G-protein-modulated currents when expressed alone in Xenopus oocytes. The enhanced activity of the heterologous expression of each of two active mutants, GIRK1(F137S) and GIRK4(S143T), was not caused by association with an endogenous oocyte channel subunit, and these mutants did not display apparent differences in the ability to localize to the cell surface compared with their wild-type counterparts. When these functional mutant channels were compared individually with wild-type heteromeric channels, they responded with only small differences to a number of maneuvers involving coexpression with muscarinic receptors, G-protein betagamma subunits, wild-type or mutated G-protein alpha subunits, and active protomers of pertussis toxin. These experiments, which confirmed the crucial, though not exclusive, role of Gbetagamma in regulating channel activity, demonstrated that GIRK1(F137S) and GIRK4(S143T), and by extrapolation their wild-type counterparts, interact in a qualitatively similar way with G-protein subunits. These findings suggest that functionally important sites of interaction with G-proteins are likely to be located within the homologous regions of GIRK1 and GIRK4 rather than within the divergent terminal regions. They also raise the question of the functional advantage of a heteromeric over homomeric design for G-protein-gated channels.

Contributions of a negatively charged residue in the hydrophobic domain of the IRK1 inwardly rectifying K+ channel to K(+)-selective permeation

Inwardly rectifying K+ channels are highly selective for K+ ions and show strong interaction with ions in the pore. Both features are important for the physiological functions of these channels and pose intriguing mechanistic questions of ion permeation. The aspartate residue in the second putative transmembrane segment of the IRK1 inwardly rectifying K+ channel, previously implicated in inward rectification gating due to cytoplasmic Mg2+ and polyamine block, is found in this study to be crucial for the channel's ability to distinguish between K+ and Rb+ ions. Mutation of this residue also perturbs the interaction between the channel pore and the Sr2+ blocking ion. Our studies suggest that this aspartate residue contributes to a selectivity filter near the cytoplasmic end of the pore.

Identification of structural elements involved in G protein gating of the GIRK1 potassium channel

Chimeras of GIRK1 and IRK1, a G protein-insensitive inward rectifier, are activated by coexpression of G beta gamma if they contain either the N-terminal or part of the C-terminal hydrophilic domain of GIRK1. The N-terminal domain of GIRK1 also facilitates the fast rates of activation and deactivation following m2 muscarinic receptor stimulation. The hydrophobic core of GIRK1 (M1-H5-M2) is important for determining the brief single-channel open times typical of GIRK1 but not important for determining G beta gamma sensitivity. Coexpression with CIR revealed that the gating properties associated with different GIRK1 domains could not have arisen from altered ability to form heteromultimers. These results implicate specific regions of GIRK1 in G protein activation and suggest that GIRK1 may be closely linked to the m2 muscarinic receptor-G protein complex.