Kenton Jon Swartz

Nanobodies are powerful tools for modulating ion channels for mechanistic investigations and developing new therapeutics. The Kv1.3 channel is highly expressed in T-lymphocytes where it promotes sustained T-cell activation, its expression is elevated in autoimmune disorders and inhibitory nanobodies are immunosuppressive. The A019400G09 nanobody (NB1.3) binds to the external surface of Kv1.3 and inhibits the channel by promoting slow C-type inactivation of the ion selectivity filter. Here we explore the mechanism by which NB1.3 promotes inactivation by determining a series of cryo-EM structures of Kv1.3 and mutating the interface between NB1.3 and the channel. Our results reveal that interaction of NB1.3 with both the S1-S4 voltage-sensing domain and the turret within the pore domain are required to promote inactivation. We also identify a network of interacting hydrophobic residues linking the turret to the ion selectivity filter that stabilizes the conducting state and mediate the actions of NB1.3. These findings provide a foundation for developing therapeutics targeting Kv1.3 channels and exploring how nanobodies can interact with other tetrameric cation channels to modulate their activity.

Extracellular adenosine 5′-triphosphate (ATP) activates P2X receptor channels (P2XRs) that serve important roles in the immune and nervous systems. Available structures of P2XRs in detergents reveal that ATP binding to the extracellular domain leads to severing of subunit interfaces within transmembrane regions as the pore opens. Here we report cryo–electron microscopy structures of the human P2X2R in lipid nanodiscs in an apo closed state, with ATP ⁴⁻ , Mg-ATP ²⁻ , and suramin bound. We find that a unique Arg residue interacts with the γ-PO ₄ of ATP ⁴⁻ in P2X2R and underlies the requirement of this subtype for ATP ⁴⁻ . Channel opening and desensitization occur when ATP ⁴⁻ binds, whereas the channel remains closed when Mg-ATP ²⁻ binds. A continuous belt of partially resolved lipids in the outer leaflet stabilizes the closed state, and the presence of lipids prevents the severing of subunit interfaces as the channel opens. These findings establish key mechanistic principles of gating for P2X2R in a membrane-like environment, providing a framework for future mechanistic studies and therapeutic development.

Extracellular ATP activates P2X receptor channels (P2XRs) that serve important roles in the immune and nervous systems. Available structures of P2XRs in detergents reveal that ATP binding to the extracellular domain leads to severing of subunit interfaces within transmembrane regions as the pore opens. Here we report cryo-EM structures of the human P2X2R in lipid nanodiscs in an apo closed state, with ATP4-, Mg-ATP2- and suramin bound. We find that a unique Arg residue coordinates the γ-PO4 of ATP4- in P2X2R and underlies the requirement of this subtype for ATP4-. Channel opening and desensitization occur when ATP4- binds, whereas the channel remains closed when Mg-ATP2- binds. A continuous belt of partially resolved lipids in the outer leaflet stabilizes the closed state, and the presence of lipids prevents the severing of subunit interfaces as the channel opens. These findings establish key mechanistic principles of gating for P2X2R in a membrane-like environment, providing a framework for future mechanistic studies and therapeutic development.

Action potentials are generated by opening of voltage-activated sodium (Na_v) and potassium (K_v) channels¹, which can rapidly inactivate to shape the nerve impulse and contribute to synaptic facilitation and short-term memory^1–4. The mechanism of fast inactivation was proposed to involve an intracellular domain that blocks the internal pore in both Na_v ^5,6 and K_v ^7–9 channels; however, recent studies in Na_v ^10,11 and K_v ^12,13 channels support a mechanism in which the internal pore closes during inactivation. Here we investigate the mechanism of fast inactivation in the Shaker K_v channel using cryo-electron microscopy, mass spectrometry and electrophysiology. We resolved structures of a fully inactivated state in which the non-polar end of the N terminus plugs the internal pore in an extended conformation. The N-terminal methionine is deleted, leaving an alanine that is acetylated and interacts with a pore-lining isoleucine residue where RNA editing regulates fast inactivation¹⁴. Opening of the internal activation gate is required for fast inactivation because it enables the plug domain to block the pore and repositions gate residues to interact with and stabilize that domain. We also show that external K⁺ destabilizes the inactivated state by altering the conformation of the ion selectivity filter rather than by electrostatic repulsion. These findings establish the mechanism of fast inactivation in K_v channels, revealing how it is regulated by RNA editing and N-terminal acetylation, and providing a framework for understanding related mechanisms in other voltage-activated channels.

Transient receptor potential (TRP) channels are a large and diverse family of tetrameric cation-selective channels that are activated by many different types of stimuli, including noxious heat or cold, organic ligands such as vanilloids or cooling agents, or intracellular Ca²⁺. Structures available for all subtypes of TRP channels reveal that the transmembrane domains are closely related despite their unique sensitivity to activating stimuli. Here, we use computational and electrophysiological approaches to explore the conservation of the cooling agent binding pocket identified within the S1–S4 domain of the Melastatin subfamily member TRPM8, the mammalian sensor of noxious cold, with other TRPM channel subtypes. We find that a subset of TRPM channels, including TRPM2, TRPM4, and TRPM5, contain pockets very similar to the cooling agent binding pocket in TRPM8. We then show how the cooling agent icilin modulates activation of mouse TRPM4 to intracellular Ca²⁺, enhancing the sensitivity of the channel to Ca²⁺ and diminishing outward-rectification to promote opening at negative voltages. Mutations known to promote or diminish activation of TRPM8 by cooling agents similarly alter activation of TRPM4 by icilin, suggesting that icilin binds to the cooling agent binding pocket to promote opening of the channel. These findings demonstrate that TRPM4 and TRPM8 channels share related ligand binding pockets that are allosterically coupled to opening of the pore.

Transient Receptor Potential (TRP) channels are a large and diverse family of tetrameric cation selective channels that are activated by many different types of stimuli, including noxious heat or cold, organic ligands such as vanilloids or cooling agents, or intracellular Ca2+. Structures available for all subtypes of TRP channels reveal that the transmembrane domains are closely related despite their unique sensitivity to activating stimuli. Here we use computational and electrophysiological approaches to explore the conservation of the cooling agent binding pocket identified within the S1-S4 domain of the Melastatin subfamily member TRPM8, the mammalian sensor of noxious cold, with other TRPM channel subtypes. We find that a subset of TRPM channels, including TRPM2, TRPM4 and TRPM5, contain pockets very similar to the cooling agent binding pocket in TRPM8. We then show how the cooling agent icilin modulates activation of TRPM4 to intracellular Ca2+, enhancing the sensitivity of the channel to Ca2+ and diminishing outward-rectification to promote opening at negative voltages. Mutations known to promote or diminish activation of TRPM8 by cooling agents similarly alter activation of TRPM4 by icilin, suggesting that icilin binds to the cooling agent binding pocket to promote opening of the channel. These findings demonstrate that TRPM4 and TRPM8 channels share related ligand binding pockets that are allosterically coupled to opening of the pore.

Transient Receptor Potential (TRP) channels are a large and diverse family of tetrameric cation selective channels that are activated by many different types of stimuli, including noxious heat or cold, organic ligands such as vanilloids or cooling agents, or intracellular Ca ²⁺ . Structures available for all subtypes of TRP channels reveal that the transmembrane domains are closely related despite their unique sensitivity to activating stimuli. Here we use computational and electrophysiological approaches to explore the conservation of the cooling agent binding pocket identified within the S1-S4 domain of the Melastatin subfamily member TRPM8, the mammalian sensor of noxious cold, with other TRPM channel subtypes. We find that a subset of TRPM channels, including TRPM2, TRPM4 and TRPM5, contain pockets very similar to the cooling agent binding pocket in TRPM8. We then show how the cooling agent icilin modulates activation of TRPM4 to intracellular Ca ²⁺ , enhancing the sensitivity of the channel to Ca ²⁺ and diminishing outward-rectification to promote opening at negative voltages. Mutations known to promote or diminish activation of TRPM8 by cooling agents similarly alter activation of TRPM4 by icilin, suggesting that icilin binds to the cooling agent binding pocket to promote opening of the channel. These findings demonstrate that TRPM4 and TRPM8 channels share related ligand binding pockets that are allosterically coupled to opening of the pore.

Eukaryotic voltage-gated K ⁺ channels have been extensively studied, but the structural bases for some of their most salient functional features remain to be established. C-type inactivation, for example, is an auto-inhibitory mechanism that confers temporal resolution to their signal-firing activity. In a recent breakthrough, studies of a mutant of Shaker that is prone to inactivate indicated that this process entails a dilation of the selectivity filter, the narrowest part of the ion conduction pathway. Here, we report an atomic-resolution cryo–electron microscopy structure that demonstrates that the wild-type channel can also adopt this dilated state. All-atom simulations corroborate this conformation is congruent with the electrophysiological characteristics of the C-type inactivated state, namely, residual K ⁺ conductance and altered ion specificity, and help rationalize why inactivation is accelerated or impeded by certain mutations. In summary, this study establishes the molecular basis for an important self-regulatory mechanism in eukaryotic K ⁺ channels, laying a solid foundation for further studies.

The Kv2.1 voltage-activated potassium (Kv) channel is a prominent delayed-rectifier Kv channel in the mammalian central nervous system, where its mechanisms of activation and inactivation are critical for regulating intrinsic neuronal excitability^1,2. Here we present structures of the Kv2.1 channel in a lipid environment using cryo-electron microscopy to provide a framework for exploring its functional mechanisms and how mutations causing epileptic encephalopathies^3–7 alter channel activity. By studying a series of disease-causing mutations, we identified one that illuminates a hydrophobic coupling nexus near the internal end of the pore that is critical for inactivation. Both functional and structural studies reveal that inactivation in Kv2.1 results from dynamic alterations in electromechanical coupling to reposition pore-lining S6 helices and close the internal pore. Consideration of these findings along with available structures for other Kv channels, as well as voltage-activated sodium and calcium channels, suggests that related mechanisms of inactivation are conserved in voltage-activated cation channels and likely to be engaged by widely used therapeutics to achieve state-dependent regulation of channel activity.