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Islam MS, Gaston JP, Baker MAB. Fluorescence Approaches for Characterizing Ion Channels in Synthetic Bilayers. MEMBRANES 2021; 11:857. [PMID: 34832086 PMCID: PMC8619978 DOI: 10.3390/membranes11110857] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 10/11/2021] [Revised: 10/25/2021] [Accepted: 10/28/2021] [Indexed: 12/12/2022]
Abstract
Ion channels are membrane proteins that play important roles in a wide range of fundamental cellular processes. Studying membrane proteins at a molecular level becomes challenging in complex cellular environments. Instead, many studies focus on the isolation and reconstitution of the membrane proteins into model lipid membranes. Such simpler, in vitro, systems offer the advantage of control over the membrane and protein composition and the lipid environment. Rhodopsin and rhodopsin-like ion channels are widely studied due to their light-interacting properties and are a natural candidate for investigation with fluorescence methods. Here we review techniques for synthesizing liposomes and for reconstituting membrane proteins into lipid bilayers. We then summarize fluorescence assays which can be used to verify the functionality of reconstituted membrane proteins in synthetic liposomes.
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Affiliation(s)
- Md. Sirajul Islam
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Kensington, NSW 2052, Australia; (M.S.I.); (J.P.G.)
| | - James P. Gaston
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Kensington, NSW 2052, Australia; (M.S.I.); (J.P.G.)
| | - Matthew A. B. Baker
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Kensington, NSW 2052, Australia; (M.S.I.); (J.P.G.)
- CSIRO Synthetic Biology Future Science Platform, GPO Box 2583, Brisbane, QLD 4001, Australia
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Zhao C, Hong L, Galpin JD, Riahi S, Lim VT, Webster PD, Tobias DJ, Ahern CA, Tombola F. HIFs: New arginine mimic inhibitors of the Hv1 channel with improved VSD-ligand interactions. J Gen Physiol 2021; 153:212451. [PMID: 34228044 PMCID: PMC8263924 DOI: 10.1085/jgp.202012832] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2020] [Accepted: 06/14/2021] [Indexed: 12/14/2022] Open
Abstract
The human voltage-gated proton channel Hv1 is a drug target for cancer, ischemic stroke, and neuroinflammation. It resides on the plasma membrane and endocytic compartments of a variety of cell types, where it mediates outward proton movement and regulates the activity of NOX enzymes. Its voltage-sensing domain (VSD) contains a gated and proton-selective conduction pathway, which can be blocked by aromatic guanidine derivatives such as 2-guanidinobenzimidazole (2GBI). Mutation of Hv1 residue F150 to alanine (F150A) was previously found to increase 2GBI apparent binding affinity more than two orders of magnitude. Here, we explore the contribution of aromatic interactions between the inhibitor and the channel in the presence and absence of the F150A mutation, using a combination of electrophysiological recordings, classic mutagenesis, and site-specific incorporation of fluorinated phenylalanines via nonsense suppression methodology. Our data suggest that the increase in apparent binding affinity is due to a rearrangement of the binding site allowed by the smaller residue at position 150. We used this information to design new arginine mimics with improved affinity for the nonrearranged binding site of the wild-type channel. The new compounds, named “Hv1 Inhibitor Flexibles” (HIFs), consist of two “prongs,” an aminoimidazole ring, and an aromatic group connected by extended flexible linkers. Some HIF compounds display inhibitory properties that are superior to those of 2GBI, thus providing a promising scaffold for further development of high-affinity Hv1 inhibitors.
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Affiliation(s)
- Chang Zhao
- Department of Physiology and Biophysics, University of California, Irvine, Irvine, CA.,Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA
| | - Liang Hong
- Department of Physiology and Biophysics, University of California, Irvine, Irvine, CA
| | - Jason D Galpin
- Department of Molecular Physiology and Biophysics, University of Iowa, Iowa City, IA
| | - Saleh Riahi
- Department of Chemistry, University of California, Irvine, Irvine, CA
| | - Victoria T Lim
- Department of Chemistry, University of California, Irvine, Irvine, CA
| | - Parker D Webster
- Department of Physiology and Biophysics, University of California, Irvine, Irvine, CA
| | - Douglas J Tobias
- Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA.,Department of Chemistry, University of California, Irvine, Irvine, CA
| | - Christopher A Ahern
- Department of Molecular Physiology and Biophysics, University of Iowa, Iowa City, IA
| | - Francesco Tombola
- Department of Physiology and Biophysics, University of California, Irvine, Irvine, CA.,Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA
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Kühnel RM, Grifell-Junyent M, Jørgensen IL, Kemmer GC, Schiller J, Palmgren M, Justesen BH, Günther Pomorski T. Short-chain lipid-conjugated pH sensors for imaging of transporter activities in reconstituted systems and living cells. Analyst 2019; 144:3030-3037. [PMID: 30901016 DOI: 10.1039/c8an02161a] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The design of ion sensors has gained importance for the study of ion dynamics in cells, with fluorescent proton nanosensors attracting particular interest because of their applicability in monitoring pH gradients in biological microcompartments and reconstituted membrane systems. In this work, we describe the improved synthesis, photophysical properties and applications of pH sensors based on amine-reactive pHrodo esters and short-chain lipid derivatives of phosphoethanolamine. The major features of these novel probes include strong fluorescence under acidic conditions, efficient partitioning into membranes, and extractability by back exchange to albumin. These features allow for the selective labeling of the inner liposomal leaflet in reconstituted membrane systems for studying proton pumping activities in a quantitative fashion, as demonstrated by assaying the activity of a plant plasma membrane H+-ATPase. Furthermore, the short-chain lipid-conjugated pH sensors enable the monitoring of pH changes from neutral to acidic conditions in the endocytic pathway of living cells. Collectively, our results demonstrate the applicability of short-chain lipid-conjugated sensors for in vivo and in vitro studies and thus pave the way for the design of lipid-conjugated sensors selective to other biologically relevant ions, e.g. calcium and sodium.
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Affiliation(s)
- Ronja Marie Kühnel
- Faculty of Chemistry and Biochemistry, Department of Molecular Biochemistry, Ruhr University Bochum, Universitätstrasse 150, D-44780 Bochum, Germany.
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