Biology subjects

Grabe, M.

Publications and source records attributed to Grabe, M..

At least 19 recordsLinked to original sources

bioRxiv2026

Sequence adaptations satisfy the constraints of mitochondrial membrane protein evolution

Inner mitochondrial membrane proteins must be sufficiently hydrophilic to withstand aqueous exposure during translation and transit to the mitochondria. Meanwhile, their transmembrane segments must be sufficiently hydrophobic to stably embed in the lipid membrane. We hypothesized that sequence-level adaptations evolved to balance these constraints. Here, we integrate structure-informed evolutionary analyses of mitochondrial proteins with atomistic simulations and cell-based experiments to identify aliphatic-to-threonine substitutions (ATS) as a potential solution to these constraints. With high statistical confidence, this transmembrane segment-specific adaptation is recurrently and convergently observed throughout mitochondrial evolution. Conformational analyses show that threonine interacts with both water and the transmembrane helix backbone, thereby lowering hydrophobicity without destabilizing secondary structure. In the extremely hydrophobic ATP6 protein, reverting threonines to aliphatic residues disrupts mitochondrial targeting, while introducing threonines into a poorly targeted variant improves its mitochondrial localization. These findings have implications for mitochondrial genome evolution, the rational design of membrane proteins, and potentially mitochondrial gene therapy.

biochemistry↗

bioRxiv2026

Hydrophobic mismatch induces lipid sorting based on tail unsaturation

Biological membranes contain a diverse set of membrane proteins surrounded by many different lipids, and the lateral organization and function of these molecules are closely intertwined. Here, we use coarse-grained molecular dynamics (MD) simulations to explore how hydrophobic mismatch between the length of transmembrane (TM) proteins and the thickness of the surrounding lipid membrane impacts the spatial distribution of the lipids. We constructed idealized cylindrically symmetric proteins, inspired by the "Mattress Model" developed in the 1980s, and simulated these model proteins in different lipid compositions. We found that unsaturated lipids were attracted to short TM proteins that thinned the membrane, while fully saturated lipids were attracted to long TM proteins that induced membrane extension. A simple mechanical description of the membrane deformation energy coupled to a lipid mixing model accurately predicted the enrichment/depletion, which was up to 33% in some cases. Our simulations also highlight that lipid sorting behavior is sensitive to protein tilt and protein surface roughness. By teasing out the fundamental physical principles in these simple models, our results provide a foundational understanding of how proteins and lipids form complex and transient assemblies, which we believe will be important for interpreting lipid-protein interactions for a host of membrane proteins that regulate cellular membranes and cell function. SIGNIFICANCETransmembrane (TM) proteins in the eukaryotic plasma membrane are solvated by hundreds of lipid species with different head group chemistries and tail configurations. This work explores how the number of double C-C bonds in the lipid acyl chains (unsaturations) affects the enrichment and depletion of lipid species around TM proteins that exhibit hydrophobic mismatch: i.e., they are either too long or too short for the membrane. We use molecular simulation and elastic modeling to show that short TM proteins attract unsaturated lipids, while long TM proteins attract saturated ones. This finding unlocks a currently understudied aspect of protein-lipid interactions that has important implications for our fundamental understanding of membrane proteins.

biophysics↗

bioRxiv2026

Inhibiting the interaction between the mitochondrial receptor Tom70 and SARS CoV 2 Orf9b with small molecules

The SARS CoV 2 accessory protein Orf9b is in a complex monomer-dimer equilibrium that influences its interactions with the host mitochondrial receptor Tom70. This interaction is critical for viral suppression of a Type-1 interferon response during infection. Modulating this equilibrium with a small molecule, either by stabilizing the Orf9b dimer or blocking its interaction with Tom70, represents a promising strategy for restoring interferon signaling and the antiviral response. To build tool molecules that could test this concept, we performed two screens: a crystallographic fragment screen against the Orf9b homodimer and a high-throughput fluorescence polarization screen for competitors of an Orf9b-derived peptide binding to Tom70. Fragment screening revealed two binding sites with potential to be developed into an inhibitor: one located at the peripheral dimer interface and the other just outside the lipid-binding channel that defines the central dimer interface. Functionalization of the fragments outside of the lipid-binding channel with hydrophobic moieties stabilized the Orf9b dimer thereby indirectly inhibiting association with Tom70. In parallel, the high throughput screen for competitive inhibitors of the Tom70:Orf9b interaction discovered a separate series of molecules. These molecules display dynamic structure activity relationship (SAR) and could be improved in the future to modulate the interaction between Tom70 and potentially a wide range of substrates. Collectively, these results demonstrate the feasibility of two distinct strategies to manipulate the Orf9b-Tom70 equilibrium, which is critical to the host response to SARS CoV 2 infection.

biophysics↗

bioRxiv2026

Conformational changes upon pore blocker removal reveal conductive states of TMEM16A

TMEM16A is a Ca2+-activated anion channel that provides direct electrical feedback to the plasma membrane in response to intracellular Ca2+. Its conductive state remains unresolved, leaving questions about gating, Cl- permeation, and modulation by Ca2+, depolarization, and lipids. To investigate the open state, we performed molecular dynamics simulations of TMEM16A bound to the putative open-state blocker 1PBC. After inhibitor removal, the putative, pore-lining helix TM4 developed kinks at two sites: an upper site that opens the pore for Cl- permeation, and a deeper site causing constriction. A conserved hydrophobic network between TM3 and TM4 persisted in most open structures but separated during extreme dilation, allowing lipids to transiently block the pore. Patch-clamp recordings indicated that the intact network promotes activation. Further simulations yielded >60 Cl- permeation events and a single-channel conductance matching experiments. Additional electrostatic and kinetic modeling indicated that TMEM16As transition from outward-rectification to Ohmic conductance with increasing Ca2+ results from a weak voltage dependence of Ca2+ binding, which act cooperatively to open the pore. HIGHLIGHTSO_LIRemoval of the inhibitor 1PBC from TMEM16A induces spontaneous transitions to ion conductive and non-conductive states via bending in TM4 at two different locations. C_LIO_LISimulations suggest that the open state is stabilized by a small hydrophobic network between TM3 and 4 and disrupting this network biases channel closure in electrophysiological recordings. C_LIO_LIA kinetic model of conduction based on energetics from the all-atom MD simulations coupled to continuum calculations gives a linear current-voltage curve consistent with the fully open conformation and removal of 1 Ca2+ switches to an outwardly rectifying state via electrostatic influence on the Cl- energy profile. However, the rectification is too weak to match experiment, but a cooperative model of Ca2+ binding with weak voltage-dependence does match experiment. C_LI

biophysics↗

bioRxiv2025

Comparing Multislice Simulations of MD Simulations with CryoEM Exposes Membrane Prediction Errors

Cryo-electron microscopy (cryoEM) is a powerful tool for atomic- and molecular-resolution structure determination, while molecular dynamics (MD) simulations are similarly powerful tools for predicting molecular trajectories. Given the challenges in estimating biomolecule dynamics with cryoEM alone, MD simulations are employed to forecast molecular motions and to interpret cryoEM reconstructions. Few methods, however, can evaluate MD predictions directly. Here, we use multislice wave propagation to project sampled snapshots of MD trajectories, either coarse-grained (CG) or all-atom (AA), into simulated cryoEM 3D reconstructions. We compared simulated and experimental images of low- and high-curvature membranes to show that MD simulations qualitatively reflect the fluidity and thus the contrast of biological membranes observed by cryoEM. MD simulations also correctly predicted bilayer dimensions for single component flat bilayers observed in cryoEM images. However, Martini3 CG-MD simulations failed to predict changes in membrane thickness induced by high curvature and with heterogeneous lipid compositions. We pinpointed the misbehavior of polyunsaturated lipid tails and cholesterol in Martini3 simulations as the main error sources contributing to inaccurate bilayer thicknesses. Our comparisons also explain membrane structure discrepancies between cryoEM and small angle X-ray scattering (SAXS). Further testing of MD predictions by direct comparisons between simulated and experimental cryoEM images should lead to the development of more accurate MD force fields. Statement of SignificanceMolecular dynamics (MD) simulations are frequently employed to predict the dynamics of biological macromolecules and assemblies, but these predictions remain difficult to validate experimentally. Cryo-electron microscopy (cryoEM) can be used to directly image the conformational ensemble of macromolecules, but images of Coulombic potential cannot be easily compared to snapshots of atoms from MD simulations. Here, we show that a physics-based multislice image projection algorithm accurately converts MD trajectories to simulated cryoEM 2D images and 3D reconstructions. Using this approach, we identify consistencies and discrepancies between MD simulations and cryoEM experiments. Notably, coarse-grained MD performs poorly compared to all-atom MD when simulating membranes composed of mixtures of lipids that include cholesterol and polyunsaturated lipids, providing observables for MD force field improvement.

biophysics↗

bioRxiv2025

Mechanism of phosphoinositide regulation of lysosomal pH via inhibition of CLC-7

AbstractLysosomes process cellular waste and coordinate responses to metabolic challenge. Central to lysosomal homeostasis are phosphoinositide lipids, key signaling molecules which establish organelle identity, regulate membrane dynamics and are tightly linked to the pathophysiology and therapy of lysosomal storage disorders, neurodegeneration, and cancer. Phosphatidylinositol 3,5- bisphosphate (PI(3,5)P2) interacts with multiple lysosomal membrane proteins and plays a critical role in regulating lysosomal pH by directly inhibiting the chloride/proton antiporter ClC-7, though the molecular mechanism of this inhibition remains unclear. Here, using a combination of functional, structural, and computational analysis, we demonstrate that PI(3,5)P2 binding dramatically remodels the structure of ClC-7 by inducing close association between cytosolic and transmembrane domains. Disease-causing mutations show increased transport activity through loss of PI(3,5)P2 binding and subsequent inhibition. Conversely, ClC-7 activation is correlated with dissociation and increased disorder of the cytoplasmic domain along with novel transmembrane domain conformations, revealing a mechanistic link between specific lysosomal lipids, transporter regulation, and the enigmatic basis of the ClC-7 slow gate.

biochemistry↗

bioRxiv2025

Detection of Cytosolic Ion Concentrations by the Trans-Golgi Network/Early Endosome is Important for Salt Tolerance

Plant survival requires cellular sensing systems that detect and respond to nutrient and ion fluctuations. The ability to monitor and adjust solute concentrations is essential for managing environmental stresses, particularly salt stress, yet the molecular mechanisms underlying cellular ion sensing remain largely unknown. Here, we combine luminal pH measurements with mathematical modelling of ion transport to investigate the role of the Trans-Golgi Network/Early Endosome (TGN/EE) for cellular ion homeostasis. Our results demonstrate that cytosolic concentrations of Cl-, Na+ and K+ directly impact ion transport activity at the TGN/EE, thereby influencing luminal pH dynamics. Specifically, we find that the TGN/EE lumen becomes more alkaline when plants are exposed to elevated NaCl concentrations, indicating a previously unrecognised role for endomembrane pH regulation in salt stress responses. Furthermore, we demonstrate that TGN/EE ion transport mutants that fail to exhibit NaCl-induced alkalisation display hypersensitivity to Na+. These findings indicate that the translation of elevated cytosolic ion concentrations into TGN/EE luminal alkalisation, represents an important mechanism for conferring Na+ tolerance in plant cells.

plant biology↗

bioRxiv2025

A cell-based scrambling assay reveals phospholipid headgroup preference of TMEM16F on the plasma membrane

The asymmetric resting distribution of the three major phospholipid classes on the mammalian plasma membrane, with phosphatidylserine and phosphatidylethanolamine mostly on the inner leaflet, and phosphatidylcholine mostly on the outer leaflet, is maintained by ATP-dependent flippases and floppases that exhibit headgroup selectivity. Upon signaling cues, this asymmetry can be dissipated by various phospholipid scramblases, allowing cells to respond to stimuli and adapt to different physiological contexts. The prevailing view in the field is that phospholipid scramblases on the plasma membrane act without headgroup preference. Here we report contrary experimental evidence based on a phospholipid scrambling assay that quantifies the fluorescence polarization of nitrobenzoxadiazole-labeled phospholipids for kinetic monitoring of phospholipid scrambling on the plasma membrane of living cells. Our experiments reveal that the plasma membrane-residing calcium-activated phospholipid scramblase TMEM16F preferentially acts on phosphatidylserine and phosphatidylcholine over phosphatidylethanolamine. Significance StatementPhospholipid scramblases on the mammalian plasma membrane are thought to act promiscuously without preference for headgroup. Thoroughly addressing this question, however, requires the development of new methodologies. We devised a cell-based phospholipid scrambling assay that utilizes the fluorescence polarization of nitrobenzoxadiazole (NBD)-labeled phospholipids, allowing the monitoring of their scrambling in a native environment. We discovered that the plasma membrane-residing calcium-activated phospholipid scramblase TMEM16F preferentially acts on phosphatidylserine and phosphatidylcholine over phosphatidylethanolamine. Major Classification: Biological Sciences; Minor Classification: Cell biology, Biophysics and Computational Biology

cell biology↗

bioRxiv2025

Coupled equilibria of dimerization and lipid binding modulate SARS Cov 2 Orf9b interactions and interferon response

Open Reading Frame 9b (Orf9b), an accessory protein of SARS-CoV and -2, is involved in innate immune suppression through its binding to the mitochondrial receptor Translocase of Outer Membrane 70 (Tom70). Previous structural studies of Orf9b in isolation revealed a {beta}-sheet-rich homodimer, however, structures of Orf9b in complex with Tom70 revealed a monomeric helical fold. Here, we developed a biophysical model that quantifies how Orf9b switches between these conformations and binds to Tom70, a requirement for suppressing the type 1 interferon response. We used this model to characterize the effect of lipid binding and mutations in variants of concern to the Orf9b:Tom70 equilibrium. We found that the binding of a lipid to the Orf9b homodimer biases the Orf9b monomer:dimer equilibrium towards the dimer by reducing the dimer dissociation rate [~]100-fold. We also found that mutations in variants of concern can alter different microscopic rate constants without significantly affecting binding to Tom70. Together our results highlight how perturbations to different steps in these coupled equilibria can affect the apparent affinity of Orf9b to Tom70, with potential downstream implications for interferon signaling in coronavirus infection.

biochemistry↗

bioRxiv2025

A Cooperative Model for Symmetric Ligand Binding to Protein Fibrils

A hallmark of neurodegenerative diseases like Alzheimers Disease (AD) and chronic traumatic encephalopathy (CTE) is the presence of toxic protein aggregates in neurons. In AD and CTE specifically, the protein tau forms insoluble fibrils that are hundreds of nanometers in length. Intriguingly, recent experimental structures suggest that tau ligands like the disaggregator EGCG and positron emission tomography (PET) tracers like GTP-1 and MK-6240 bind to tau fibrils in long stacks reflecting the symmetry of the protein across many binding sites. In these stacks, each ligand makes more contact with its symmetry mates than it does with the protein. To interpret the binding of these molecules and new ligands, we must understand the effects of the cooperativity between sites and the entropy coming from the number of sites. Here, we investigate a nearest-neighbors model of cooperativity and use statistical mechanics to derive binding isotherms for saturation and competition experiments. This model allows us to relate measured EC50 so IC50 and so values to the intrinsic binding affinity to a single site and to cooperativity across sites in ways resembling the Cheng-Prusoff Equation. Depending on the degree of cooperativity between molecular species, this model permits solutions that lack the steep binding curves expected from cooperative systems and even solutions resembling 2-site systems. We finally consider conditions for a fibrils detection in a PET scan and practical matters of fitting this models parameters to data.

biophysics↗

bioRxiv2024

Robust lipid scrambling by TMEM16 proteins requires an open groove and thin membrane

Biological membranes are complex and dynamic structures with different populations of lipids in their inner and outer leaflets. The Ca2+-activated TMEM16 family of membrane proteins plays an important role in collapsing this asymmetric lipid distribution by spontaneously, and bidirectionally, scrambling phospholipids between the two leaflets, which can initiate signaling and alter the physical properties of the membrane. While evidence shows that lipid scrambling can occur via an open hydrophilic pathway ("groove") that spans the membrane, it remains unclear if all family members facilitate lipid movement in this manner. Here we present a comprehensive computational study of lipid scrambling by all TMEM16 members with experimentally solved structures. We performed coarse-grained molecular dynamics (MD) simulations of 27 structures from five different family members solved under activating and non-activating conditions, and we captured over 700 scrambling events in aggregate. This enabled us to directly compare scrambling rates, mechanisms, and protein-lipid interactions for fungal and mammalian TMEM16s, in both open (Ca2+-bound) and closed (Ca2+-free) conformations with statistical rigor. We show that all TMEM16 structures thin the membrane and that the majority of scrambling (>90%) occurs at the groove only when TM4 and TM6 have sufficiently separated. Surprisingly, we also observed 60 scrambling events that occurred outside the canonical groove, over 90% of which took place at the dimer-dimer interface in mammalian TMEM16s. This new site suggests an alternative mechanism for lipid scrambling in the absence of an open groove. Impact StatementThe majority of TMEM16 lipid scrambling occurs in the open groove associated with Ca2+-activation, but limited scrambling also occurs in the dimer interface independent of Ca2+.

biophysics↗

bioRxiv2024

Structure of the human K2P13.1(THIK-1) channel reveals a novel hydrophilic pore restriction and lipid cofactor site

The halothane-inhibited K2P leak potassium channel K2P13.1 (THIK-1)1-3 is found in diverse cells1,4 including neurons1,5 and microglia6-8 where it affects surveillance6, synaptic pruning7, phagocytosis7, and inflammasome-mediated interleukin-1{beta} release6,8,9. As with many K2Ps1,5,10-14 and other voltage-gated ion channel (VGIC) superfamily members3,15,16, polyunsaturated fatty acid (PUFA) lipids modulate K2P13.1 (THIK-1)1,5,14,17 via a poorly understood mechanism. Here, we present cryo-electronmicroscopy (cryo-EM) structures of human K2P13.1 (THIK-1) and mutants in lipid nanodiscs and detergent. These reveal that, unlike other K2Ps13,18-24, K2P13.1 (THIK-1) has a two-chamber aqueous inner cavity obstructed by a M4 transmembrane helix tyrosine (Tyr273, the flow restrictor). This hydrophilic barrier can be opened by an activatory mutation, S136P25, at natural break in the M2 transmembrane helix and by intrinsic channel dynamics. The structures also reveal a buried lipid in the P1/M4 intersubunit interface at a location, the PUFA site, that coincides with the TREK subfamily K2P modulator pocket for small molecule agonists18,26,27. This overlap, together with the effects of mutation on K2P13.1 (THIK-1) PUFA responses, indicates that the PUFA site lipids are K2P13.1 (THIK-1) cofactors. Comparison with the PUFA-responsive VGIC Kv7.1 (KCNQ1)28-31 reveals a shared role for the equivalent pore domain intersubunit interface in lipid modulation, providing a framework for dissecting the effects of PUFAs on the VGIC superfamily. Our findings reveal the unique architecture underlying K2P13.1 (THIK-1) function, highlight the importance of the P1/M4 interface in control of K2Ps by both natural and synthetic agents, and should aid development of THIK subfamily modulators for diseases such as neuroinflammation6,32 and autism6.

biophysics↗

bioRxiv2022

Allosteric Regulation of a Synaptic Vesicle Glutamate Transporter

Concentration of neurotransmitter inside synaptic vesicles (SVs) underlies the quantal nature of synaptic transmission. In contrast to many transporters, SV uptake of the principal excitatory neurotransmitter glutamate is driven by membrane potential. To prevent nonquantal efflux of glutamate after SV exocytosis, the vesicular glutamate transporters (VGLUTs) are allosterically inhibited by the neutral pH of the synaptic cleft. We have now determined high-resolution structures of VGLUT2 with a cyclic analog of glutamate bound that defines the mechanism of substrate recognition, a positively charged cytoplasmic vestibule that electrostatically attracts the negatively charged substrate, and modification by palmitoylation that promotes retrieval of the transporter after exocytosis. The structure also incorporates an extensive, cytoplasmic network of electrostatic interactions that acts as a gate. Functional analysis shows how this cytoplasmic gate confers the allosteric requirement for lumenal H+ required to restrict VGLUT activity to SVs.

biochemistry↗

bioRxiv2022

Membrane curvature sensing and symmetry breaking of the M2 proton channel from Influenza A

The M2 proton channel aids in the exit of mature influenza viral particles from the host plasma membrane through its ability to stabilize regions of high negative gaussian curvature (NGC) that occur at the neck of budding virions. The channels are homo-tetramers that contain a cytoplasm-facing amphipathic helix (AH) that is necessary and sufficient for NGC generation; however, constructs containing the transmembrane spanning helix, which facilitates tetramerization, exhibit enhanced curvature generation. Here we used all-atom molecular dynamics (MD) simulations to explore the conformational dynamics of M2 channels in lipid bilayers revealing that the AH is dynamic, quickly breaking the 4-fold symmetry observed in most structures. Next, we carried out MD simulations with the protein restrained in 4-fold and 2-fold symmetric conformations to determine the impact on the membrane shape. While each pattern was distinct, all configurations induced pronounced curvature in the outer leaflet with rather subtle lipid tilt, while conversely, the inner leaflets adjacent to the AHs showed minimal curvature and significant lipid tilt. The MD-generated profiles at the protein-membrane interface were then extracted and used as boundary conditions in a continuum elastic membrane model to calculate the membrane bending energy of each conformation embedded in different membrane surfaces characteristic of a budding virus. The calculations show that all three M2 conformations are stabilized in concave spherical caps and destabilized in convex spherical caps, the latter reminiscent of a budding virus. Only C2-broken symmetry conformations are stabilized in NGC surfaces, by 1-3 kBT depending on the AH domain arrangement. The most favored conformation is stabilized in saddles with curvatures corresponding to 33 nm radii. In total, our work provides atomistic insight into the curvature sensing capabilities of M2 channels and how enrichment in the nascent viral particle depends on protein shape and membrane geometry.

biophysics↗

bioRxiv2021

Membrane Constriction by ESCRT-III Proteins Induces Structural Lipid Bilayer Asymmetries

Lipids in biological membranes are thought to be functionally organized, but few experimental tools can probe nanoscale membrane structure. Using brominated lipids as contrast probes for cryo-EM and a model ESCRT-III membrane remodeling system, we observed leaflet-level and protein-localized lipid structural patterns within highly constricted and thinned membrane nanotubes. These nanotubes differed markedly from protein-free, flat bilayers in leaflet thickness, lipid diffusion rates, and lipid compositional and conformational asymmetries. Simulations and cryo-EM imaging of brominated SDPC showed how a pair of phenylalanine residues scored the outer leaflet with a helical hydrophobic defect where polyunsaturated docosahexaenoyl (DHA) tails accumulated at the bilayer surface. Combining cryo-EM of halogenated lipids with molecular dynamics thus enables new characterizations of the composition and structure of membranes on molecular length scales. One-Sentence SummaryCryo-EM imaging of halogenated lipids and MD simulations provide molecular-scale insight into constricted bilayer structure.

biophysics↗

bioRxiv2020

Elucidation of the molecular interactions that enable stable assembly and structural diversity in multicomponent immune receptors

Multi-component immune receptors are essential complexes in which distinct ligand-recognition and signaling subunits are held together by interactions between acidic and basic residues of their transmembrane helices. A 2-to-1 acidic to basic motif in the transmembrane domains of the subunits is necessary and sufficient to assemble these receptor complexes. Here, we study a prototype for these receptors, a DAP12-NKG2C 2:1 heterotrimeric complex, in which the two DAP12 subunits each contribute a single transmembrane Asp residue, and the NKG2C subunit contributes a Lys to form the complex. DAP12 can also associate with 20 other subunits using a similar motif. Here we use molecular dynamics simulations to understand the basis for the high affinity and diversity of interactions in this group of receptors. Simulations of the transmembrane helices with differing protonation states of the Asp-Asp-Lys triad identified a structurally stable interaction in which a singly protonated Asp-Asp pair forms a hydrogen-bonded carboxyl-carboxylate clamp that clasps onto a charged Lys sidechain. This polar motif was also supported by density functional theory and a Protein Data Bank-wide search. In contrast, the helices are dynamic at sites distal to the stable carboxyl-carboxylate clamp motif. Such a locally stable but globally dynamic structure is well-suited to accommodate the sequence and structural variations in the transmembrane helices of multi-component receptors, which mix and match subunits to create combinatorial functional diversity from a limited number of subunits. It also supports a signaling mechanism based on multi-subunit clustering rather than propagation of rigid conformational changes through the membrane. SIGNIFICANCEReceptors that separate ligand recognition and intracellular signaling into separate protein subunits are ubiquitous in immunity. These subunits mix and match to create combinatorial functional diversity. The transmembrane domains of these receptors assemble through the interaction between two acidic and one basic residue on different helices. Using computational methods to study the DAP12-NKG2C receptor complex we identified a polar motif in which a singly protonated Asp-Asp pair forms a carboxyl-carboxylate clamp that clasps a charged Lys sidechain in the membrane. This local interaction allows dynamic variations in other regions of the helices that tolerate sequence diversity of the interacting subunits in this class of receptors, which signal through multi-subunit clustering rather than propagation of rigid conformational changes through the membrane.

biophysics↗

bioRxiv2020

Genetically encoded ratiometric biosensor for probing lysosomal pH in mammalian cells and C. elegans

Lysosomes are important sites for macromolecular degradation, defined by an acidic lumenal pH of [~]4.5. To better understand lysosomal pH, we designed a novel, genetically encoded, fluorescent protein (FP) based pH biosensor called FIRE-pHLy (Fluorescence Indicator REporting pH in Lysosomes). This biosensor was targeted to lysosomes with lysosomal-associated membrane protein 1 (LAMP1) and reported lumenal pH between 3.5 and 6.0 with monomeric teal fluorescent protein 1 (mTFP1), a bright cyan pH sensitive FP variant with a pKa of 4.3. Ratiometric quantification was enabled with cytosolically oriented mCherry using high-content quantitative imaging. We expressed FIRE-pHLy in several cellular models and quantified the alkalinizing response to bafilomycin A1, a specific V-ATPase inhibitor. In summary, we have engineered FIRE-pHLy, a specific, robust and versatile lysosomal pH biosensor that has broad applications for investigating pH dynamics in aging and lysosome-related diseases, as well as in lysosome-based drug discovery.

cell biology↗

bioRxiv2020

K2P channel C-type gating involves asymmetric selectivity filter order-disorder transitions

K2P channels regulate nervous, cardiovascular, and immune system functions1,2 through the action of their selectivity filter (C-type) gate3-6. Although structural studies show K2P conformations that impact activity7-13, no selectivity filter conformational changes have been observed. Here, combining K2P2.1 (TREK-1) X-ray crystallography in different potassium concentrations, potassium anomalous scattering, molecular dynamics, and functional studies, we uncover the unprecedented, asymmetric, potassium-dependent conformational changes underlying K2P C-type gating. Low potassium concentrations evoke conformational changes in selectivity filter strand 1 (SF1), selectivity filter strand 2 (SF2), and the SF2-transmembrane helix 4 loop (SF2-M4 loop) that destroy the S1 and S2 ion binding sites and are suppressed by C-type gate activator ML335. Shortening the uniquely long SF2-M4 loop to match the canonical length found in other potassium channels or disrupting the conserved Glu234 hydrogen bond network supporting this loop blunts C-type gate response to various physical and chemical stimuli. Glu234 network destabilization also compromises ion selectivity, but can be reversed by channel activation, indicating that the ion binding site loss reduces selectivity similar to other channels14. Together, our data establish that C-type gating occurs through potassium-dependent order-disorder transitions in the selectivity filter and adjacent loops that respond to gating cues relayed through the SF2-M4 loop. These findings underscore the potential for targeting the SF2-M4 loop for the development of new, selective K2P channel modulators.

biophysics↗