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).

 


 

References:

Abe, K., F. Ohnishi, K. Yagi, T. Nakajima, T. Higuchi, M. Sano, M. Machida, R.I. Sarker, and P.C. Maloney. (2002). Plasmid-encoded asp operon confers a proton motive metabolic cycle catalyzed by an aspartate-alanine exchange reaction. J. Bacteriol. 184: 2906-2913.
Nanatani, K., T. Fujiki, K. Kanou, M. Takeda-Shitaka, H. Umeyama, L. Ye, X. Wang, T. Nakajima, T. Uchida, P.C. Maloney, and K. Abe. (2007). Topology of AspT, the aspartate:alanine antiporter of Tetragenococcus halophilus, determined by site-directed fluorescence labeling. J. Bacteriol. 189: 7089-7097.
Rodionov, D.A., P. Hebbeln, A. Eudes, J. ter Beek, I.A. Rodionova, G.B. Erkens, D.J. Slotboom, M.S. Gelfand, A.L. Osterman, A.D. Hanson, and T. Eitinger. (2009). A novel class of modular transporters for vitamins in prokaryotes. J. Bacteriol. 191: 42-51.
Examples:

TC#NameOrganismal TypeExample
2.A.81.1.1The L-aspartate:L-alanine exchanger, AspTBacteria and archaeaAspT of Tetragenococcus halophila sp. D10 (Q8L3K8)
 
2.A.81.1.2

The putative cobalt porter, CbtD (Rodionov et al., 2009)

Bacteroidetes

CbtD of Bacteroides fragilis (Q5LCC7)

 
Examples:

TC#NameOrganismal TypeExample
2.A.81.2.1

Homologue of AspT (384 aas; 10 putative TMSs; with potential membrane embedded loops between TMSs 4 and 5 and TMSs 9 and 10 (the two halves are internally duplicated)). The central hydrophilic domain in 2.A.81.1.1 is absent in this homologue.

Archaea

AspT homologue of Halobacterium sp. NRC-1 (AAC82885)