Anion Transport Mechanisms in Erythrocyte Membranes

Erythrocyte membranes maintain gas and acid–base balance through specialised anion transport systems. The principal mediator, band 3 (anion exchanger 1, AE1), catalyses the electroneutral exchange of chloride and bicarbonate, facilitating CO₂ uptake in tissues and its release in the lungs. This process relies on alternating-access conformational changes in a dimeric assembly. Complementary transporters, including anion exchanger 2 (AE2) and sodium-coupled acid–base transporters, further modulate intracellular pH, ion gradients and CO₂/bicarbonate homeostasis. High-resolution structural studies have revealed a conserved 7 + 7 transmembrane-segment core with an independently mobile substrate-binding domain and a scaffold dimerisation domain. Interactions with cytosolic carbonic anhydrase and the red blood cell cytoskeleton refine transport kinetics and provide local buffering. Functional models propose an elevator-type mechanism in which a substrate-binding domain moves relative to a static dimerisation framework. Advances in molecular dynamics and lipid–protein interaction analyses highlight the influence of the bilayer environment and accessory proteins such as glycophorin A on transport efficiency. These insights underpin our understanding of red cell physiology, hereditary anaemias and potential pharmacological modulation of anion transport.

Research from Nature Portfolio

Recent structural work on anion exchanger 2 (AE2) has elucidated five major cryo-EM states, revealing inward-facing, outward-facing and intermediate conformations that define the pH-sensitive exchange of bicarbonate and chloride. Detailed analysis of N-terminal and C-terminal interdomain interactions uncovers a self-inhibitory loop that gates transport and a domain-interlock mechanism that senses intracellular pH changes. Complementary studies on the sodium-driven chloride/bicarbonate exchanger NDCBE (SLC4A8) have resolved its outward-facing cryo-EM structure with bound sodium and carbonate ions, clarifying how electrostatic determinants and extracellular loop organisation dictate its electroneutral transport mode. Computational modelling coupled with functional assays has identified key residues in the substrate-binding pocket that differentiate NDCBE from other SLC4 family members and govern its unique coupling of sodium and carbonate exchange.

Anion Transport Mechanisms in Erythrocyte Membranes publication trend

The graph below shows the total number of articles in anion transport mechanisms in erythrocyte membranes across all publications each year (not limited to Nature Index journals).

Technical terms

Anion exchanger: Membrane protein that facilitates the one-for-one exchange of negatively charged ions across a lipid bilayer.

Cryo-EM: Cryogenic electron microscopy, a technique that images proteins in vitrified samples to determine high-resolution structures.

Alternating-access mechanism: Model of transport in which a substrate-binding site alternately faces one side of the membrane then the other.

Elevator-type mechanism: Form of alternating-access in which a substrate-binding domain moves across the membrane relative to a static scaffold domain.

References

1. The structural basis of the pH-homeostasis mediated by the Cl−/HCO3− exchanger, AE2. Nature Communications (2023).
2. Structural and functional properties of the transporter SLC26A6 reveal mechanism of coupled anion exchange. eLife (2023).
3. Cryo-EM structures of the human band 3 transporter indicate a transport mechanism involving the coupled movement of chloride and bicarbonate ions. PLOS Biology (2024).
4. Cryo-EM structure of the sodium-driven chloride/bicarbonate exchanger NDCBE. Nature Communications (2021).
5. A Transport Metabolon FUNCTIONAL INTERACTION OF CARBONIC ANHYDRASE II AND CHLORIDE/BICARBONATE EXCHANGERS*. Journal of Biological Chemistry (2001).
6. Interaction of the human erythrocyte Band 3 anion exchanger 1 (AE1, SLC4A1) with lipids and glycophorin A: Molecular organization of the Wright (Wr) blood group antigen. PLOS Computational Biology (2018).
7. Asymmetry of inverted-topology repeats in the AE1 anion exchanger suggests an elevator-like mechanism. Journal of General Physiology (2017).
8. Structural basis for functional interactions in dimers of SLC26 transporters. Nature Communications (2019).
9. Interactions between protein 4.1 and band 3. An alternative binding site for an element of the membrane skeleton.. Journal of Biological Chemistry (1985).