Mechanism of ligand activation of a eukaryotic cyclic nucleotide−gated channel

Abstract

Cyclic nucleotide-gated (CNG) channels convert cyclic nucleotide binding and unbinding into electrical signals in sensory receptors and neurons. The molecular conformational changes underpinning ligand activation are largely undefined. We report both closed- and open-state atomic cryo-EM structures of a full-length C. elegans cGMP-activated channel TAX-4 reconstituted in lipid nanodiscs. These structures, together with computational and functional analyses and a mutant channel structure, reveal a double-barrier hydrophobic gate formed by two S6 amino acids in the central cavity. cGMP binding produces global conformational changes that open the cavity gate located ~52 Å away but do not alter the structure of the selectivity filter – the commonly presumed activation gate. Our work provides mechanistic insights into the allosteric gating and regulation of cyclic nucleotide-gated and -modulated channels and CNG channel-related channelopathies.

Prolyl isomerization controls activation kinetics of a cyclic nucleotide-gated ion channel

“…8d ). Although the resolution of this conformation is low, the conformational changes in the C-linker/CNBD domains and the increase in radius at the intracellular gate are structural features of an open-activated cyclic nucleotide-modulated channel 18 , 49 and raise the possibility that this minor conformation represents an open state.…”

The Complex Story of Plant Cyclic Nucleotide-Gated Channels

“…The overall model CNGC subunit has six transmembrane domains (S1–S6; Figure 2 ) with a pore region (P loop) between S5 and S6 that permits ion transport [ 13 ]. Animal and bacterial cation channel subunits that contain a single P loop form tetramers; this includes animal CNGs, with clear evidence from cryo-electron microscopy showing tetramer formation in a lipid environment [ 94 ]. Triplet amino acid residues in the P loop that could act as selectivity filters (AGN, AND, GNL, GQG, GQN, GQS) vary between the Arabidopsis CNGCs [ 95 ], with AND or GQs thought to confer some level of Ca 2+ selectivity [ 4 , 5 , 6 , 30 , 35 , 45 , 78 ].…”

“…By comparison, many plant species harbour more cyclic nucleotide-gated channel subunits than animals. Vertebrates (including mammals) and invertebrates have only six CNGs [ 94 , 98 , 135 ]. Subunits of the hyperpolarisation activated cyclic nucleotide-gated (HCN) cation channels (that operate in cardiac cells) are also present in low numbers (three in invertebrates, four in mammals and four to six in other vertebrates) [ 136 ].…”

A generalized kinetic model describes ion-permeation mechanisms in various ion channels

“…In addition, the guanidinium moiety of the optical molecule 424 may also play a role to bring about these changes, possibly through interacting with 425 the positively charged ions upon switching its locations under the different light 426 where the ES and FT states describe the channel abilities for the inward and outward 434 conductions, respectively. The cryo-EM structures reveal that F403 and V407 form 435 the hydrophobic gate inside the channel cavity that they block the permeation 436 pathway during the closed state but pave the way for ion permeation by rotating aside 437 upon switching to the open state (Zheng et al, 2020). Mutations to valine and alanine 438 certainly increase the central space in the cavity that makes the channel a bit "leaky" 439 in a sense (Zheng et al, 2020).…”

“…The cryo-EM structures reveal that F403 and V407 form 435 the hydrophobic gate inside the channel cavity that they block the permeation 436 pathway during the closed state but pave the way for ion permeation by rotating aside 437 upon switching to the open state (Zheng et al, 2020). Mutations to valine and alanine 438 certainly increase the central space in the cavity that makes the channel a bit "leaky" 439 in a sense (Zheng et al, 2020). Comparing the model parameters, the mutations 440 increase the conduction rates in both directions (kʹ′ 1, wt = 0.054 nA, kʹ′ 1, mutant = 0.31 nA, 441 and kʹ′ 2, wt = 0.18 nA, kʹ′ 2, mutant = 0.46 nA), conforming the "leaky" property of the 442 mutant channel.…”