acids in the putative alcohol-binding pocket of these channels eliminates the modulation by EtOH3 or isofluorane30. By contrast, decreasing the size of amino acids in the same region of the decanol-insensitive GABA ρ1 receptors now enables potentiation by decanol3,6. Taken together with our findings, these studies support a model that physical pockets of defined dimensions can be probed with mutations that change the dimension of the alcohol-binding sites. Using the IRK1-MPD structure as a guide, we estimate the volume of the hydrophobic alcohol binding pocket of GIRK channels to be ∼250 Å3, which is large enough to accommodate bulky alcohols like MPD (∼130 Å3, Fig. 8b). A Trp mutation in the pocket would decrease the volume that could potentially occlude larger alcohols (Fig. 8b). In addition, the sensitivity to activation may be different between GIRK2 and GIRK4* channels. For example, Trp substitution in GIRK2 eliminated MPD activation, revealing inhibition of current. In GIRK4*, Trp substitution eliminated 1-PrOH activation but showed only a significant decrease in MPD activation when comparing Ala with Trp substitutions. One possible explanation is that MPD fits differently in the alcohol pocket of GIRK4*, which could be revealed in a complex of MPD and GIRK4 structure.
