Regulation of the heat- and capsaicin-activated Transient Receptor Potential Vanilloid 1 (TRPV1)
channel by phosphoinositides is complex and controversial. In the most recent TRPV1 cryo-EM structure,
endogenous phosphatidylinositol (PtdIns) was detected in the vanilloid binding site, and
phosphoinositides were proposed to act as competitive vanilloid antagonists. This model is difficult
to reconcile with phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] being a well-established
positive regulator of TRPV1. Here we show that in the presence of PtdIns(4,5)P2 in excised patches,
PtdIns, but not PtdIns(4)P, partially inhibited TRPV1 activity at low, but not at high capsaicin
concentrations. This is consistent with PtdIns acting as a competitive vanilloid antagonist. However,
in the absence of PtdIns(4,5)P2, PtdIns partially stimulated TRPV1 activity. We computationally
identified residues, which are in contact with PtdIns, but not with capsaicin in the vanilloid
binding site. The I703A mutant of TRPV1 showed increased sensitivity to capsaicin, as expected
when removing the effect of an endogenous competitive antagonist. I703A was not inhibited by PtdIns
in the presence of PtdIns(4,5)P2, but it was still activated by PtdIns in the absence of PtdIns(4,5)P2
indicating that inhibition, but not activation by PtdIns proceeds via the vanilloid binding site. In
molecular dynamics simulations PtdIns was more stable than PtdIns(4,5)P2 in the inhibitory site, while
PtdIns(4,5)P2 was more stable than PtdIns in a previously identified, non-overlapping, putative
activating binding site. Our data indicate that phosphoinositides regulate channel activity via
functionally distinct binding sites, which may explain some of the complexities of the effects of
these lipids on TRPV1.
Polyamines such as spermidine and spermine are found in nearly all cells, at concentrations ranging up to 0.5 mM. These cations are endogenous regulators of cellular K+ efflux, binding tightly in the pores of inwardly rectifying K+ (Kir) channels in a voltage-dependent manner. Although the voltage dependence of Kir channel polyamine blockade is thought to arise at least partially from the energetically coupled movements of polyamine and K+ ions through the pore, the nature of physical interactions between these molecules is unclear. Here we analyze the polyamine-blocking mechanism in the model K+ channel MthK, using a combination of electrophysiology and computation. Spermidine (SPD3+) and spermine (SPM4+) each blocked current through MthK channels in a voltage-dependent manner, and blockade by these polyamines was described by a three-state kinetic scheme over a wide range of polyamine concentrations. In the context of the scheme, both SPD3+ and SPM4+ access a blocking site with similar effective gating valences (0.84 ± 0.03 e0 for SPD3+ and 0.99 ± 0.04 e0 for SPM4+), whereas SPM4+ binds in the blocked state with an ∼20-fold higher affinity than SPD3+ (Kd = 28.1 ± 3.1 µM for SPD3+ and 1.28 ± 0.20 µM for SPM4+), consistent with a free energy difference of 1.8 kcal/mol. Molecular simulations of the MthK pore in complex with either SPD3+ or SPM4+ are consistent with the leading amine interacting with the hydroxyl groups of T59, at the selectivity filter threshold, with access to this site governed by outward movement of K+ ions. These coupled movements can account for a large fraction of the voltage dependence of blockade. In contrast, differences in binding energetics between SPD3+ and SPM4+ may arise from distinct electrostatic interactions between the polyamines and carboxylate oxygens on the side chains of E92 and E96, located in the pore-lining helix.
We used coarse-grained molecular dynamics simulations to characterize the global and local mechanical properties of a DNA origami triangle nanostructure. The structure presents two metastable conformations separated by a free energy barrier that is lowered upon omission of four specific DNA staples (defect). In contrast, only one stable conformation is present upon removing eight staples. The metastability is explained in terms of the intrinsic conformations of the three trapezoidal substructures. We computationally modeled the local accessibility to endonucleases, to predict the reactivity of twenty sites, and found good agreement with the experimental data. We showed that global fluctuations affect local reactivity: the removal of the DNA staples increased the computed accessibility to a restriction enzyme, at sites as distant as 40 nm, due to an increase in global fluctuation. These results raise the intriguing possibility of the rational engineering of allosterically modulated DNA origami.
The nonselective cation channel TRPV1 is responsible for transducing noxious stimuli into action potentials propagating through peripheral nerves. It is activated by temperatures greater than 43 °C, while remaining completely nonconductive at temperatures lower than this threshold. The origin of this sharp response, which makes TRPV1 a biological temperature sensor, is not understood. Here we used molecular dynamics simulations and free energy calculations to characterize the molecular determinants of the transition between nonconductive and conductive states. We found that hydration of the pore and thus ion permeation depends critically on the polar character of its molecular surface: in this narrow hydrophobic enclosure, the motion of a polar side-chain is sufficient to stabilize either the dry or wet state. The conformation of this side-chain is in turn coupled to the hydration state of four peripheral cavities, which undergo a dewetting transition at the activation temperature.
Hv1 is a transmembrane four-helix bundle that transports protons in a voltage-controlled manner. Its crucial role in many pathological conditions, including cancer and ischemic brain damage, makes Hv1 a promising drug target. Starting from the recently solved crystal structure of Hv1, we used structural modeling and molecular dynamics simulations to characterize the channel’s most relevant conformations along the activation cycle. We then performed computational docking of known Hv1 inhibitors, 2-guanidinobenzimidazole (2GBI) and analogs. Although salt-bridge patterns and electrostatic potential profiles are well-defined and distinctive features of activated versus nonactivated states, the water distribution along the channel lumen is dynamic and reflects a conformational heterogeneity inherent to each state. In fact, pore waters assemble into intermittent hydrogen-bonded clusters that are replaced by the inhibitor moieties upon ligand binding. The entropic gain resulting from releasing these conformationally restrained waters to the bulk solvent is likely a major contributor to the binding free energy. Accordingly, we mapped the water density fluctuations inside the pore of the channel and identified the regions of maximum fluctuation within putative binding sites. Two sites appear as outstanding: One is the already known binding pocket of 2GBI, which is accessible to ligands from the intracellular side; the other is a site located at the exit of the proton permeation pathway. Our analysis of the waters confined in the hydrophobic cavities of Hv1 suggests a general strategy for drug discovery that can be applied to any ion channel.