Patch-Clamp Recordings of the KcsA K+ Channel in Unilamellar Blisters

Proteoliposomes reconstituted with human aquaporin-1 reveal novel single-ion-channel properties

Human aquaporin 1 (hAQP1) forms homotetrameric channels that facilitate fluxes of water and small solutes across cell membranes. In addition to water channel activity, hAQP1 displays non-selective monovalent cation-channel activity gated by intracellular cyclic GMP. Dual water and ion-channel activity of hAQP1, thought to regulate cell shape and volume, could offer a target for novel therapeutics relevant to controlling cancer cell invasiveness. This study probed properties of hAQP1 ion channels using proteoliposomes, which, unlike conventional cell-based systems such as Xenopus laevis oocytes, are relatively free of background ion channels. Histidine-tagged recombinant hAQP1 protein was synthesized and purified from the methylotrophic yeast, Pichia pastoris, and reconstituted into proteoliposomes for biophysical analyses. Osmotic water channel activity confirmed correct folding and channel assembly. Ion-channel activity of hAQP1-Myc-His6 was recorded by patch-clamp electrophysiology with excised patches. In symmetrical potassium, the hAQP1-Myc-His6 channels displayed coordinated gating, a single-channel conductance of approximately 75 pS, and multiple subconductance states. Applicability of this method for structure-function analyses was tested using hAQP1-Myc-His6 D48A/D185A channels modified by site-directed mutations of charged Asp residues estimated to be adjacent to the central ion-conducting pore of the tetramer. No differences in conductance were detected between mutant and wild-type constructs, suggesting the open-state conformation could differ substantially from expectations based on crystal structures. Nonetheless, the method pioneered here for AQP1 demonstrates feasibility for future work defining structure-function relationships, screening pharmacological inhibitors, and testing other classes in the broad family of aquaporins for previously undiscovered ion-conducting capabilities.

Correlating ion channel structure and function

Recent developments in cryogenic electron microscopy (cryo-EM) led to an exponential increase in high-resolution structures of membrane proteins, and in particular ion channels. However, structures alone can only provide limited information about the workings of these proteins. In order to understand ion channel function and regulation in molecular detail, the obtained structural data need to be correlated to functional states of the same protein. Here, we describe several techniques that can be employed to study ion channel structure and function in vitro and under defined, similar conditions. Lipid nanodiscs provide a native-like environment for membrane proteins and have become a valuable tool in membrane protein structural biology and biophysics. Combined with liposome-based flux assays for the kinetic analysis of ion channel activity as well as electrophysiological recordings, researchers now have access to an array of experimental techniques allowing for detailed structure-function correlations using purified components. Two examples are presented where we put emphasis on the lipid environment and time-resolved techniques together with mutations and protein engineering to interpret structural data obtained from single particle cryo-EM on cyclic nucleotide-gated or Ca²⁺-gated K⁺ channels. Furthermore, we provide short protocols for all the assays used in our work so that others can adapt these techniques to their experimental needs. Comprehensive structure-function correlations are essential in order to pharmacologically target channelopathies.

Direct binding of phosphatidylglycerol at specific sites modulates desensitization of a ligand-gated ion channel

Pentameric ligand-gated ion channels (pLGICs) are essential determinants of synaptic transmission, and are modulated by specific lipids including anionic phospholipids. The exact modulatory effect of anionic phospholipids in pLGICs and the mechanism of this effect are not well understood. Using native mass spectrometry, coarse-grained molecular dynamics simulations and functional assays, we show that the anionic phospholipid, 1-palmitoyl-2-oleoyl phosphatidylglycerol (POPG), preferentially binds to and stabilizes the pLGIC, Erwinia ligand-gated ion channel (ELIC), and decreases ELIC desensitization. Mutations of five arginines located in the interfacial regions of the transmembrane domain (TMD) reduce POPG binding, and a subset of these mutations increase ELIC desensitization. In contrast, a mutation that decreases ELIC desensitization, increases POPG binding. The results support a mechanism by which POPG stabilizes the open state of ELIC relative to the desensitized state by direct binding at specific sites.