2.A.81 The Aspartate:Alanine Exchanger (AAEx) Family
A single functionally characterized protein, the aspartate:alanine exchanger (AspT) of the Gram-positive lactic acid bacterium, Tetragenococcus halophila D10 serves to characterize the AAEx family (Abe et al., 2002). This organism takes up L-aspartate, decarboxylates it to L-alanine and CO2 in the cytoplasm, catalyzed by L-aspartate β-decarboxylase (AspD), and exports the L-alanine in a 1:1 exchange reaction with L-aspartate. AspT is a hydrophobic protein of 543 aas and 10 putative TMSs with two TrkA-C domains between TMSs 5 and 6. This protein has many Gram-negative and Gram-positive bacterial homologues of unknown function, and possibly one very distant homologue in the archaeon, Halobacterium sp. strain NRC-1. This protein (384 aas; 10-12 putative TMSs; AAC82885) includes a 54-residue region (residues 6-60) that shows 35% identity and 51% similarity with the ammonium transporter, Amt of Corynebacterium glutamicum (spP54146). These proteins exhibit an internal duplication with 5 or 6 TMSs per repeat element.
Because one more negative charge is brought in (aspartate) that is exported (alanine), the exchange transport process results in net charge movement, creating a membrane potential, negative inside. Further, decarboxylation of aspartate consumes a scalar proton and thus generates a pH gradient (basic inside). The resultant pmf can drive ATP synthesis via the F-type ATPase (TC #3.A.2). Other such exchangers generating a pmf are the prototypical oxalate/formate exchanger of the MFS (TC #3.A.1) as well as glutamate/γ-amino butyrate, malate/lactate, citrate/lactate and histidine/histamine exchangers (for references see Abe et al., 2002).
AspT has 10 transmembrane helices (TMS), a large hydrophilic cytoplasmic loop (about 180 amino acids) between TM5 and TM6, N and C termini that face the periplasm, and a positively charged residue (arginine 76) within TM3 (Nanatani et al., 2007). The hydrophilic cytoplasmic loop of AspT possesses a sequence divergent TrkA_C domain.
The generalized transport reaction catalyzed by AspT is:
L-aspartate (out) + L-alanine (in) ⇌ L aspartate (in) + L-alanine (out)