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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 associated with 2.A.81 family:

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. 12003930
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. 17158863
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. 21420450
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. 17660287
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. 12869542
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. 18931129
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. 26849958