A family of fluoride-specific ion channels with dual-topology architecture

Fluoride ion, ubiquitous in soil, water, and marine environments, is a chronic threat to microorganisms. Many prokaryotes, archea, unicellular eukaryotes, and plants use a recently discovered family of F− exporter proteins to lower cytoplasmic F− levels to counteract the anion’s toxicity. We show here that these ‘Fluc’ proteins, purified and reconstituted in liposomes and planar phospholipid bilayers, form constitutively open anion channels with extreme selectivity for F− over Cl−. The active channel is a dimer of identical or homologous subunits arranged in antiparallel transmembrane orientation. This dual-topology assembly has not previously been seen in ion channels but is known in multidrug transporters of the SMR family, and is suggestive of an evolutionary antecedent of the inverted repeats found within the subunits of many membrane transport proteins.

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Ion homeostasis, channels, and transporters: an update on cellular mechanisms

AJP Advances in Physiology Education ◽

10.1152/advan.00046.2004 ◽

2004 ◽

Vol 28 (4) ◽

pp. 143-154 ◽

Cited By ~ 64

Author(s):

George R. Dubyak

Keyword(s):

Ion Channels ◽

Subcellular Localization ◽

Ion Transport ◽

Membrane Transport ◽

Cell Biology ◽

Functional Characterization ◽

Transport Proteins ◽

Transport Protein ◽

Membrane Transport Proteins ◽

Functional Analyses

The steady-state maintenance of highly asymmetric concentrations of the major inorganic cations and anions is a major function of both plasma membranes and the membranes of intracellular organelles. Homeostatic regulation of these ionic gradients is critical for most functions. Due to their charge, the movements of ions across biological membranes necessarily involves facilitation by intrinsic membrane transport proteins. The functional characterization and categorization of membrane transport proteins was a major focus of cell physiological research from the 1950s through the 1980s. On the basis of these functional analyses, ion transport proteins were broadly divided into two classes: channels and carrier-type transporters (which include exchangers, cotransporters, and ATP-driven ion pumps). Beginning in the mid-1980s, these functional analyses of ion transport and homeostasis were complemented by the cloning of genes encoding many ion channels and transporter proteins. Comparison of the predicted primary amino acid sequences and structures of functionally similar ion transport proteins facilitated their grouping within families and superfamilies of structurally related membrane proteins. Postgenomics research in ion transport biology increasingly involves two powerful approaches. One involves elucidation of the molecular structures, at the atomic level in some cases, of model ion transport proteins. The second uses the tools of cell biology to explore the cell-specific function or subcellular localization of ion transport proteins. This review will describe how these approaches have provided new, and sometimes surprising, insights regarding four major questions in current ion transporter research. 1) What are the fundamental differences between ion channels and ion transporters? 2) How does the interaction of an ion transport protein with so-called adapter proteins affect its subcellular localization or regulation by various intracellular signal transduction pathways? 3) How does the specific lipid composition of the local membrane microenvironment modulate the function of an ion transport protein? 4) How can the basic functional properties of a ubiquitously expressed ion transport protein vary depending on the cell type in which it is expressed?

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CLC-0 and CFTR: Chloride Channels Evolved From Transporters

Physiological Reviews ◽

10.1152/physrev.00058.2006 ◽

2008 ◽

Vol 88 (2) ◽

pp. 351-387 ◽

Cited By ~ 95

Author(s):

Tsung-Yu Chen ◽

Tzyh-Chang Hwang

Keyword(s):

Ion Channels ◽

Membrane Transport ◽

Chloride Channels ◽

Transport Proteins ◽

Transmembrane Conductance Regulator ◽

Protein Families ◽

Transmembrane Conductance ◽

Functional Studies ◽

Membrane Transport Proteins ◽

Cystic Fibrosis Transmembrane

CLC-0 and cystic fibrosis transmembrane conductance regulator (CFTR) Cl− channels play important roles in Cl− transport across cell membranes. These two proteins belong to, respectively, the CLC and ABC transport protein families whose members encompass both ion channels and transporters. Defective function of members in these two protein families causes various hereditary human diseases. Ion channels and transporters were traditionally viewed as distinct entities in membrane transport physiology, but recent discoveries have blurred the line between these two classes of membrane transport proteins. CLC-0 and CFTR can be considered operationally as ligand-gated channels, though binding of the activating ligands appears to be coupled to an irreversible gating cycle driven by an input of free energy. High-resolution crystallographic structures of bacterial CLC proteins and ABC transporters have led us to a better understanding of the gating properties for CLC and CFTR Cl− channels. Furthermore, the joined force between structural and functional studies of these two protein families has offered a unique opportunity to peek into the evolutionary link between ion channels and transporters. A promising byproduct of this exercise is a deeper mechanistic insight into how different transport proteins work at a fundamental level.

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A facile approach for the in vitro assembly of multimeric membrane transport proteins

eLife ◽

10.7554/elife.36478 ◽

2018 ◽

Vol 7 ◽

Cited By ~ 6

Author(s):

Erika A Riederer ◽

Paul J Focke ◽

Elka R Georgieva ◽

Nurunisa Akyuz ◽

Kimberly Matulef ◽

...

Keyword(s):

Ion Channels ◽

Membrane Proteins ◽

Glutamate Transporter ◽

Spectroscopic Studies ◽

Transport Proteins ◽

Native State ◽

General Applicability ◽

Membrane Transport Proteins ◽

In Vitro Assembly

Membrane proteins such as ion channels and transporters are frequently homomeric. The homomeric nature raises important questions regarding coupling between subunits and complicates the application of techniques such as FRET or DEER spectroscopy. These challenges can be overcome if the subunits of a homomeric protein can be independently modified for functional or spectroscopic studies. Here, we describe a general approach for in vitro assembly that can be used for the generation of heteromeric variants of homomeric membrane proteins. We establish the approach using GltPh, a glutamate transporter homolog that is trimeric in the native state. We use heteromeric GltPh transporters to directly demonstrate the lack of coupling in substrate binding and demonstrate how heteromeric transporters considerably simplify the application of DEER spectroscopy. Further, we demonstrate the general applicability of this approach by carrying out the in vitro assembly of VcINDY, a Na+-coupled succinate transporter and CLC-ec1, a Cl-/H+ antiporter.

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