Exocytotic fusion pores are composed of both lipids and proteins.

During exocytosis, fusion pores form the first aqueous connection that allows escape of neurotransmitters and hormones from secretory vesicles. Although it is well established that SNARE proteins catalyze fusion, the structure and composition of fusion pores remain unknown. Here, we exploited the rigid framework and defined size of nanodiscs to interrogate the properties of reconstituted fusion pores, using the neurotransmitter glutamate as a content-mixing marker. Efficient Ca(2+)-stimulated bilayer fusion, and glutamate release, occurred with approximately two molecules of mouse synaptobrevin 2 reconstituted into ∼6-nm nanodiscs. The transmembrane domains of SNARE proteins assumed distinct roles in lipid mixing versus content release and were exposed to polar solvent during fusion. Additionally, tryptophan substitutions at specific positions in these transmembrane domains decreased glutamate flux. Together, these findings indicate that the fusion pore is a hybrid structure composed of both lipids and proteins.We thank Gerhard Wagner for providing the MSP∆1D1H4-H6 plasmid. This study was supported by a grant from the US National Institutes of Health (MH061876). H.B. is supported by a postdoctoral fellowship from Human Frontier Science Program. B.C. and M.P.G are supported by funding from the US National Institutes of Health (R01 GM084140). P.J. is supported by Kidney Research UK. J.M.E. is supported by the Biotechnology and Biological Sciences Research Council (BB/J018236/1) and Kidney Research UK. E.R.C. is supported as an Investigator of the Howard Hughes Medical Institute.This is the author accepted manuscript. The final version is available from NPG via http://dx.doi.org/10.1038/nsmb.314

Loop G in the GABA_A receptor α1 subunit influences gating efficacy

KEY POINTS: The functional importance of residues in loop G of the GABA(A) receptor has not been investigated. D43 and T47 in the α1 subunit are of particular significance as their structural modification inhibits activation by GABA. While the T47C substitution had no significant effect, non‐conservative substitution of either residue (D43C or T47R) reduced the apparent potency of GABA. Propofol potentiated maximal GABA‐evoked currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors. Non‐stationary variance analysis revealed a reduction in maximal GABA‐evoked P (open), suggesting impaired agonist efficacy. Further analysis of α1(T47R)β2γ2 receptors revealed that the efficacy of the partial agonist THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridine‐3‐ol) relative to GABA was impaired. GABA‐, THIP‐ and propofol‐evoked currents mediated by α1(T47R)β2γ2 receptors deactivated faster than those mediated by α1β2γ2 receptors, indicating that the mutation impairs agonist‐evoked gating. Spontaneous gating caused by the β2(L285R) mutation was also reduced in α1(T47R)β2(L285R)γ2 compared to α1β2(L285R)γ2 receptors, confirming that α1(T47R) impairs gating independently of agonist activation. ABSTRACT: The modification of cysteine residues (substituted for D43 and T47) by 2‐aminoethyl methanethiosulfonate in the GABA(A) α1 subunit loop G is known to impair activation of α1β2γ2 receptors by GABA and propofol. While the T47C substitution had no significant effect, non‐conservative substitution of either residue (D43C or T47R) reduced the apparent potency of GABA. Propofol (1 μm), which potentiates sub‐maximal but not maximal GABA‐evoked currents mediated by α1β2γ2 receptors, also potentiated maximal currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors. Furthermore, the peak open probabilities of α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors were reduced. The kinetics of macroscopic currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors were characterised by slower desensitisation and faster deactivation. Similar changes in macroscopic current kinetics, together with a slower activation rate, were observed with the loop D α1(F64C) substitution, known to impair both efficacy and agonist binding, and when the partial agonist THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridine‐3‐ol) was used to activate WT or α1(T47R)β2γ2 receptors. Propofol‐evoked currents mediated by α1(T47R)β2γ2 and α1(F64C)β2γ2 receptors also exhibited faster deactivation than their WT counterparts, revealing that these substitutions impair gating through a mechanism independent of orthosteric binding. Spontaneous gating caused by the introduction of the β2(L285R) mutation was also reduced in α1(T47R)β2(L285R)γ2 compared to α1β2(L285R)γ2 receptors, confirming that α1(T47R) impairs gating independently of activation by any agonist. These findings implicate movement of the GABA(A) receptor α1 subunit's β1 strand during agonist‐dependent and spontaneous gating. Immobilisation of the β1 strand may provide a mechanism for the inhibition of gating by inverse agonists such as bicuculline

Structural model of the open-closed-inactivated cycle of prokaryotic voltage-gated sodium channels

In excitable cells, the initiation of the action potential results from the opening of voltage-gated sodium channels. These channels undergo a series of conformational changes between open, closed, and inactivated states. Many models have been proposed for the structural transitions that result in these different functional states. Here, we compare the crystal structures of prokaryotic sodium channels captured in the different conformational forms and use them as the basis for examining molecular models for the activation, slow inactivation, and recovery processes. We compare structural similarities and differences in the pore domains, specifically in the transmembrane helices, the constrictions within the pore cavity, the activation gate at the cytoplasmic end of the last transmembrane helix, the C-terminal domain, and the selectivity filter. We discuss the observed differences in the context of previous models for opening, closing, and inactivation, and present a new structure-based model for the functional transitions. Our proposed prokaryotic channel activation mechanism is then compared with the activation transition in eukaryotic sodium channels

Benzodiazepine Modulation of GABAA Receptors: A Mechanistic Perspective

Benzodiazepines (BZDs) are a class of widely prescribed psychotropic drugs that target GABAA receptors (GABAARs) to tune inhibitory synaptic signaling throughout the central nervous system. Despite knowing their molecular target for over 40 years, we still do not fully understand the mechanism of modulation at the level of the channel protein. Nonetheless, functional studies, together with recent cryo-EM structures of GABAA(α1)2(βX)2(γ2)1 receptors in complex with BZDs, provide a wealth of information to aid in addressing this gap in knowledge. Here, mechanistic interpretations of functional and structural evidence for the action of BZDs at GABAA(α1)2(βX)2(γ2)1 receptors are reviewed. The goal is not to describe each of the many studies that are relevant to this discussion nor to dissect in detail all the effects of individual mutations or perturbations but rather to highlight general mechanistic principles in the context of recent structural information