August 2011 featured article
Raising a glass to GLIC
The pentameric ligand-gated ion channels (pLGICs) are known to be affected by alcohol in eukaryotes. Analyses of the prokaryotic GLIC suggest that it may be a good structural model for the effects of n-alcohols on pLGICs.
The effects of alcohol on prokaryotic GLIC were examined.
Alcohol has obvious physiological effects on Homo sapiens, but the molecular mechanisms underlying these effects are still being elucidated. Given the effects of alcohol, ion channels are obvious candidates for being affected, and it is already known that alcohols of different chain lengths do affect ion channel function in vitro. Howard et al. 1 have now examined the effect of alcohol on GLIC, a prokaryotic pentameric ligand-gated ion channel (pLGIC) and thus a member of a family known to be affected by alcohol in eukaryotes. As there is published structural data examining GLIC, this represents a tractable system for examining the effects of alcohol.
To test whether this is a suitable system that provides relevant information on the eukaryotic channels, the authors first examined the effect of alcohols of varying chain lengths on GLIC. GLIC was affected in a similar way to nicotinic acetylcholine receptor (nAChR) by short versus long chain alcohols, with high concentrations of the short chain alcohols methanol and ethanol potentiating, and those longer than ethanol inhibiting it. The size cut off for inhibition is nonanol, as is also true for nAChR as well as the GlyRs and GABAARs.
The authors next tested which residues might be important for alcohol modulation and focused on a region previously implicated in effects on eukaryotic channels, the C-terminal region of transmembrane helix M2. Here they used scanning mutagenesis, replacing each residue with a cysteine, and found that two residues, Phe14′ and Leu17′, in particular enhanced the effect of ethanol. In the F(14′)C mutation especially, the concentration of alcohol required for an effect was brought down into a range more closely approximating the responses of eukaryotic pLGICs. This mutant may therefore provide a closer prokaryotic model for the effects of alcohol on the eukaryotic pLGICs. In addition to testing the F(14′)C mutation, the authors found that a smaller side chain enhanced the effect of ethanol even further, also indicating that the basis of the observed effect is not linked to disulfide bond formation. Mutation to smaller side chains at position 14 also caused longer-chain alcohols to enhance rather than inhibit GLIC activity, and is consistent with the presence of smaller side chain volumes at this position in sensitive eukaryotic pLGICs.
In contrast with some previous work, the authors found through mutagenesis and molecular modeling that potentiating alcohols probably occupy a pore-facing cavity in the transmembrane region. The results further support the idea that analysis of structurally and biochemically tractable prokaryotic GLIC may give mechanistic insight into the effects of alcohol on eukaryotic channels and already provide a model for the dynamics around the alcohol-binding cavity for further testing.