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)



This family belongs to the CPA Superfamily.

 

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.

Fujiki, T., K. Nanatani, K. Nishitani, K. Yagi, F. Ohnishi, H. Yoneyama, T. Uchida, T. Nakajima, and K. Abea. (2007). Membrane topology of aspartate:alanine antiporter AspT from Comamonas testosteroni. J Biochem 141: 85-91.

Fukui, K., C. Koseki, Y. Yamamoto, J. Nakamura, A. Sasahara, R. Yuji, K. Hashiguchi, Y. Usuda, K. Matsui, H. Kojima, and K. Abe. (2011). Identification of succinate exporter in Corynebacterium glutamicum and its physiological roles under anaerobic conditions. J Biotechnol 154: 25-34.

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., A.G. Vitreschak, A.A. Mironov, and M.S. Gelfand. (2003). Comparative genomics of the vitamin B12 metabolism and regulation in prokaryotes. J. Biol. Chem. 278: 41148-41159.

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.

Suzuki, S., K. Nanatani, and K. Abe. (2016). R76 in transmembrane domain 3 of the aspartate:alanine transporter AspT is involved in substrate transport. Biosci. Biotechnol. Biochem. 80: 744-747.

Examples:

TC#NameOrganismal TypeExample
2.A.81.1.1

The L-aspartate:L-alanine exchanger, AspT.  A mutant, R76K, has higher activity than the AspT-WT (R76), whereas R76D and R76E have lower activity than AspT-WT. Thus, R76 is involved in AspT substrate transport (Suzuki et al. 2016).

Bacteria and archaea

AspT of Tetragenococcus halophila sp. D10 (Q8L3K8)

 
2.A.81.1.2

The putative cobalt porter, CbtD (Rodionov et al. 2003)

Bacteroidetes

CbtD of Bacteroides fragilis (Q5LCC7)

 
2.A.81.1.3

Succinate exporter, SucE1. Induced under microaerobic and anaerobic conditions when succinate is produced from glucose via the reductive tricarboxylic acid cycle. Exhibits succinate counterflow (self exchange) (Fukui et al., 2011).

Bacteria

SucE1 of Corynebacterium glutamicum (Q8NNI8)

 
2.A.81.1.4

Putative transporter, YbjL

Bacteria

YbjL of E. coli

 
2.A.81.1.5Putative transport protein YidE

Bacteria

YidE of Escherichia coli

 
2.A.81.1.6

Aspartate:alanine antiporter, AspT, of 561 aas. AspT has 7 TMSs, a large cytoplasmic loop containing approximately 200 aas between TMS4 and TMS5, a cytoplasmic N-terminus, and a periplasmic C-terminus (Fujiki et al. 2007).

AspT of Pseudomonas dacunhae (Comamonas testosteroni)

 
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)

 
2.A.81.2.2

YidE/YbjL duplication protein

Thermotogae

YidE homologue of Thermosipho melanesiensis