Anion exchange protein 3 (Anion_exchange_3)
Type: Family

Bicarbonate (HCO
3
-
) 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 four different HCO
3
-
transport modes. Anion exchanger proteins exchange HCO
3
-
for Cl
-
in a reversible, electroneutral manner . Na
+
/HCO
3
-
co-transport proteins mediate the coupled movement of Na
+
and HCO
3
-
across plasma membranes, often in an electrogenic manner . Na
+
driven Cl
-
/HCO
3
-
exchange and K
+
/HCO
3
-
exchange activities have also been detected in certain cell types, although the molecular identities of the proteins responsible remain to be determined.

Sequence analysis of the two families of HCO
3
-
transporters that have been cloned to date (the anion exchangers and Na
+
/HCO
3
-
co-transporters) reveals that they are homologous. This is not entirely unexpected, given that they both transport HCO
3
-
and are inhibited by a class of pharmacological agents called disulphonic stilbenes . They share around ~25-30% sequence identity, which is distributed along their entire sequence length, and have similar predicted membrane topologies, suggesting they have ~10 transmembrane (TM) domains.

Anion exchange proteins participate in pH and cell volume regulation. They are glycosylated, plasma-membrane transport proteins that exchange hydrogen carbonate (HCO
3
-
) for chloride (Cl
-
) in a reversible, electroneutral manner . To date three anion exchanger isoforms have been identified (AE1-3), AE1 being the previously-characterised erythrocyte band 3 protein. They share a predicted topology of 12-14 transmembrane (TM) domains, but have differing distribution patterns and cellular localisation. The best characterised isoform, AE1, is known to be the most abundant membrane protein in mature erythrocytes. It has a molecular mass of ~95kDa and consists of two major domains. The N-terminal 390 residues form a water-soluble, highly elongated domain that serves as an attachment site for the binding of the membrane skeleton and other cytoplasmic proteins. The remainder of the protein is a 55kDa hydrophobic domain that is responsible for catalysing anion exchange. The function of the analogous domains of AE2 and AE3 remains to be determined .

AE3 is an anion exchanger that is primarily expressed in the brain and
heart. Several tissue-specific variants have been identified, which arise
due to both alternative promoter and exon usage. Two AE3-encoding cDNAs have
been isolated from human heart. These clones share long portions of common
sequence but have different 5' ends, therefore encoding distinct N-terminal
amino acid sequences. The longer AE3 polypeptide (1232 amino acids) displays
~96% amino acid sequence identity to the rat and mouse AE3 'brain isoforms'.
The shorter polypeptide (1034 amino acids) corresponds to the rat AE3
'cardiac isoform'. Studies of Cl-transport suggest that both isoforms are capable of anion exchange . Mutations in AE3 has been related to epilepsy and different retinal diseases .