Anion exchanger family
| Anion Exchanger, bicarbonate transporter family | |
|---|---|
| Identifiers | |
| Symbol | HCO3_cotransp |
TCDB | 2.A.31 |
The anion exchanger family (TC# 2.A.31, also named bicarbonate transporter family) is a member of the large APC superfamily of secondary carriers.[1] Members of the AE family are generally responsible for the transport of anions across cellular barriers, although their functions may vary. All of them exchange bicarbonate. Characterized protein members of the AE family are found in plants, animals, insects and yeast. Uncharacterized AE homologues may be present in bacteria (e.g., in Enterococcus faecium, 372 aas; gi 22992757; 29% identity in 90 residues). Animal AE proteins consist of homodimeric complexes of integral membrane proteins that vary in size from about 900 amino acyl residues to about 1250 residues. Their N-terminal hydrophilic domains may interact with cytoskeletal proteins and therefore play a cell structural role. Some of the currently characterized members of the AE family can be found in the Transporter Classification Database.
Family overview
| Bicarbonate transporter, C-terminal domain | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| |||||||||
| Band 3 cytoplasmic domain | |||||||||
|---|---|---|---|---|---|---|---|---|---|
TCDB 2.A.31 | | ||||||||
| OPM superfamily | 284 | ||||||||
| OPM protein | 1btq | ||||||||
| |||||||||
Bicarbonate (HCO3 −) transport mechanisms are the principal regulators of pH in animal cells. Such transport also plays a vital role in acid-base movements in the stomach, pancreas, intestine, kidney, reproductive organs and the central nervous system. Functional studies have suggested different HCO3 − transport modes.
- Na+/HCO3 −
A conserved domain is found at the C terminus of many bicarbonate transport proteins. It is also found in some
The
Anion exchangers in humans
In humans, anion exchangers fall under the solute carrier family 4 (SLC4) family, which is composed of 10 paralogous members (SLC4A1-5; SLC4A7-11). Nine encode proteins that transport HCO−
3. Functionally, eight of these proteins fall into two major groups: three Cl-HCO−
3 exchangers (AE1-3) and five Na+-coupled HCO−
3 transporters (NBCe1, NBCe2, NBCn1, NBCn2, NDCBE). Two of the Na+-coupled transporters (NBCe1, NBCe2) are electrogenic; the other three Na+-coupled HCO−
3 transporters and all three AEs are electroneutral.
3 by epithelia (e.g. absorption of HCO−
3 in the renal proximal tubule, secretion of HCO−
3 in the pancreatic duct), as well as the regulation of cell volume and intracellular pH.[8]
Based on their hydropathy plots all SLC4 proteins are hypothesized to share a similar topology in the cell membrane. They have relatively long cytoplasmic N-terminal domains composed of a few hundred to several hundred residues, followed by 10-14 transmembrane (TM) domains, and end with relatively short cytoplasmic C-terminal domains composed of ~30 to ~90 residues. Although the C-terminal domain comprises a small percentage of the size of the protein, this domain in some cases, has (i) binding motifs that may be important for protein-protein interactions (e.g., AE1, AE2, and NBCn1), (ii) is important for trafficking to the cell membrane (e.g., AE1 and NBCe1), and (iii) may provide sites for regulation of transporter function via protein kinase A phosphorylation (e.g., NBCe1).[9]
The SLC4 family comprises the following proteins.
Anion exchanger 1
The human anion exchanger 1 (AE1 or
AE1 in human red blood cells has been shown to transport a variety of inorganic and organic anions. Divalent anions may be symported with H+. Additionally, it catalyzes flipping of several anionic amphipathic molecules such as sodium dodecyl sulfate (SDS) and phosphatidic acid from one monolayer of the phospholipid bilayer to the other monolayer. The rate of flipping is sufficiently rapid to suggest that this AE1-catalyzed process is physiologically important in red blood cells and possibly in other animal tissues as well. Anionic phospholipids and fatty acids are likely to be natural substrates. However, the mere presence of TMSs enhances the rates of lipid flip-flop.[14][15]
Structure
The crystal structure of AE1 (CTD) at 3.5 angstroms has been determined.
There are several crystal structures available for the AE1 protein in RCSB (links are also available in TCDB).
- AE1: 1BH7, 1BNX, 1BTQ, 1BTR, 1BTS, 1BTT, 1BZK, 2BTA, 1HYN, 2BTB, 3BTB, 2BTA, 2BTB, 3BTB, 4KY9, PDB: 4YZF, 1HYN, 5A16
Other members
3 cotransporters have been found to be members of the AE family. They catalyze the reabsorption of HCO−
3 in the renal proximal tubule in an electrogenic process that is inhibited by typical stilbene inhibitors of AE such as DIDS and SITS. They are also found in many other body tissues. At least two genes encode these symporters in any one mammal. A 10 TMS model has been presented,[17] but this model conflicts with the 14 TMS model proposed for AE1. The transmembrane topology of the human pancreatic electrogenic Na+:HO−
3 transporter, NBC1, has been studied.[18] A TMS topology with N- and C-termini in the cytoplasm has been suggested. An extracellular loop determines the stoichiometry of Na+-HCO−
3 cotransporters.[19]
In addition to the Na+-independent anion exchangers (AE1-3) and the Na+:HCO−
3 cotransporters (NBCs) (which may be either electroneutral or electrogenic), a Na+-driven HCO−
3/Cl− exchanger (NCBE) has been sequenced and characterized.[20] It transports Na+ + HCO−
3 preferentially in the inward direction and H+ + Cl− in the outward direction. This NCBE is widespread in mammalian tissues where it plays an important role in cytoplasmic alkalinization. For example, in pancreatic β-cells, it mediates a glucose-dependent rise in pH related to insulin secretion.
Animal cells in tissue culture expressing the gene-encoding the ABC-type chloride channel protein CFTR (TC# 3.A.1.202.1) in the plasma membrane have been reported to exhibit cyclic AMP-dependent stimulation of AE activity. Regulation was independent of the Cl− conductance function of CFTR, and mutations in the nucleotide-binding domain #2 of CFTR altered regulation independently of their effects on chloride channel activity. These observations may explain impaired HCO−
3 secretion in cystic fibrosis patients.
Anion exchangers in plants and fungi
Plants and yeast have anion transporters that in both the pericycle cells of plants and the plasma membrane of yeast cells export borate or boric acid (pKa = 9.2).[5] In A. thaliana, boron is exported from pericycle cells into the root stellar apoplasm against a concentration gradient for uptake into the shoots. In S. cerevisiae, export is also against a concentration gradient. The yeast transporter recognizes HCO−
3, I−, Br−, NO−
3 and Cl−, which may be substrates. Tolerance to boron toxicity in cereals is known to be associated with reduced tissue accumulation of boron. Expression of genes from roots of boron-tolerant wheat and barley with high similarity to efflux transporters from Arabidopsis and rice lowered boron concentrations due to an efflux mechanism.[21] The mechanism of energy coupling is not known, nor is it known if borate or boric acid is the substrate. Several possibilities (uniport, anion:anion exchange and anion:cation exchange) can account for the data.[5]
Transport reactions
The physiologically relevant transport reaction catalyzed by anion exchangers of the AE family is:[9]
- Cl− (in) + HCO−
3 (out) ⇌ Cl− (out) + HCO−
3 (in).
That for the Na+:HCO3- cotransporters is:
- Na+ (out) + nHCO−
3 (out) → Na+ (in) + nHCO−
3 (in).
That for the Na+/HCO−
3:H+/Cl− exchanger is:
- Na+ (out) + HCO−
3 (out) + H+ (in) + Cl− (in) ⇌ Na+ (in) + HCO−
3 (in) + H+ (out) + Cl− (out).
That for the boron efflux protein of plants and yeast is:
- Boron (in) → Boron (out)
See also
References
As of 28 January 2016, this article is derived in whole or in part from
- PMID 25043943.
- PMID 2289848.
- PMID 9261985.
- PMID 9235899.
- ^ S2CID 4418856.
- PMID 11049968.
- S2CID 808344.
- ^ PMID 23506864.
- ^ a b c Saier, MH Jr. "2.A.31 The Anion Exchanger (AE) Family". Transporter Classification Database. Saier Lab Bioinformatics Group @ UCSD / SDSC.
- PMID 11606574.
- PMID 12813056.
- PMID 12446737.
- PMID 21257764.
- PMID 11523991.
- PMID 20666375.
- S2CID 5331647.
- PMID 10099707.
- PMID 12534288.
- PMID 21224233.
- PMID 10993873.
- PMID 18003669.
