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2.A.85 The Aromatic Acid Exporter (ArAE) Family

The ArAE family consists of bacterial, archaeal and eukaryotic members for exampele, from plants, yeast and protozoans. The bacterial proteins are of 655 to 755 aas and exhibit a repeat sequence due to an internal gene duplication event with residue positions 1-180 exhibiting 6 putative TMSs, residue positions, 181-320 being hydrophilic, residue positions, 320-460 exhibiting another 6 putative TMSs, and residue positions 460-660 being hydrophilic in an average hydropathy plot. There are four E. coli homologues as well as one from H. influenzae and one from Synechocystis in TCDB. At least two ArAE family members are encoded within operons that also encode membrane fusion proteins (MFP; TC #8.A.1). This provides the basis for suggesting that these proteins catalyze efflux (Harley and Saier, 2000).

The plant proteins are of 506-560 residues and exhibit only 6 putative TMSs (residue positions 60-270 in the average hydropathy plot) followed by a long hydrophilic domain (residue positions 271-650). The P. falciparum and S. pombe proteins are 669 and 977 residues in length. The S. pombe protein has a topology resembling the bacterial proteins although it clusters phylogenetically with the eukaryotic proteins. The P. falciparum protein exhibits repeats of the hydrophilic domain but may not be a transporter. None of these eukaryotic proteins is functionally characterized.

A single bacterial member of the ArAE family has been functionally characterized (Van Dyk et al., 2004). This protein is YhcP of E. coli which depends on a membrane fusion protein (MFP family; TC #8.A.1), YhcQ, for activity. This protein proves to be a pmf-dependent para-hydroxybenzoic acid (pHBA) efflux pump (Van Dyk et al., 2004). Only a few aromatic carboxylic acids of hundreds of compounds tested proved to be substrates of the YhcQP (AaeAB) efflux pump. It may function as a 'metabolic relief valve' to relieve the toxic effects of unbalanced metabolism. Half-sized homologues are also found in the NCBI and TCDB databases, although these have not been characterized biochemically. One such protein is YqjA of Bacillus subtilis (322 aas). It has 5 or 6 TMSs (residues 17-141) followed by a 180 residue hydrophilic domain (TC #2.A.85.5.1), and is very distantly related to the full-length proteins.

The aluminium-activated malate transporters (ALMTs, see TC# 2.A.85.2) comprise a membrane protein family that demonstrates various physiological functions in plants, such as tolerance to environmental Al3+ and the regulation of stomatal movement. Dabravolski and Isayenkov 2023 summarized the knowledge about this transporter family and assess their involvement in diverse physiological processes and comprehensive regulatory mechanisms. They have conducted a thorough bioinformatic analysis to decipher the functional importance of conserved residues, structural components, and domains. Phylogenetic analyses have provided insights into the molecular evolution of ALMT family proteins, expanding their scope beyond the plant kingdom (Dabravolski and Isayenkov 2023). 

This family belongs to the: Major Facilitator (MFS) Superfamily.

References associated with 2.A.85 family:

Dabravolski, S.A. and S.V. Isayenkov. (2023). Recent Updates on ALMT Transporters'' Physiology, Regulation, and Molecular Evolution in Plants. Plants (Basel) 12:. 37687416
Dreyer, I., J.L. Gomez-Porras, D.M. Riaño-Pachón, R. Hedrich, and D. Geiger. (2012). Molecular Evolution of Slow and Quick Anion Channels (SLACs and QUACs/ALMTs). Front Plant Sci 3: 263. 23226151
Harley, K.T. and M.H. Saier, Jr. (2000). A novel ubiquitous family of putative efflux transporters. J. Mol. Microbiol. Biotechnol. 2: 195-198. 10939244
Hoekenga, O.A., L.G. Maron, M.A. Piñeros, G.M. Cançado, J. Shaff, Y. Kobayashi, P.R. Ryan, B. Dong, E. Delhaize, T. Sasaki, H. Matsumoto, Y. Yamamoto, H. Koyama, and L.V. Kochian. (2006). AtALMT1, which encodes a malate transporter, is identified as one of several genes critical for aluminum tolerance in Arabidopsis. Proc. Natl. Acad. Sci. U.S.A. 103: 9738-9743. 16740662
Kovermann, P., S. Meyer, S. Hörtensteiner, C. Picco, J. Scholz-Starke, S. Ravera, Y. Lee, and E. Martinoia. (2007). The Arabidopsis vacuolar malate channel is a member of the ALMT family. Plant J. 52: 1169-1180. 18005230
Li, C., L. Dougherty, A.E. Coluccio, D. Meng, I. El-Sharkawy, E. Borejsza-Wysocka, D. Liang, M.A. Piñeros, K. Xu, and L. Cheng. (2020). Apple ALMT9 Requires a Conserved C-Terminal Domain for Malate Transport Underlying Fruit Acidity. Plant Physiol. 182: 992-1006. 31772076
Meyer, S., P. Mumm, D. Imes, A. Endler, B. Weder, K.A. Al-Rasheid, D. Geiger, I. Marten, E. Martinoia, and R. Hedrich. (2010). AtALMT12 represents an R-type anion channel required for stomatal movement in Arabidopsis guard cells. Plant J. 63: 1054-1062. 20626656
Motoda, H., T. Sasaki, Y. Kano, P.R. Ryan, E. Delhaize, H. Matsumoto, and Y. Yamamoto. (2007). The Membrane Topology of ALMT1, an Aluminum-Activated Malate Transport Protein in Wheat (Triticum aestivum). Plant Signal Behav 2: 467-472. 19517008
Mumm P., Imes D., Martinoia E., Al-Rasheid KA., Geiger D., Marten I. and Hedrich R. (2013). C-terminus-mediated voltage gating of Arabidopsis guard cell anion channel QUAC1. Mol Plant. 6(5):1550-63. 23314055
Paulsen, I.T., J.H. Park, P.S. Choi, and M.H. Saier, Jr. (1997). A family of Gram-negative bacterial outer membrane factors that function in the export of proteins, carbohydrates, drugs and heavy metals from Gram-negative bacteria. FEMS Microbiol. Lett. 156: 1-8. 9368353
Piñeros, M.A., G.M. Cançado, L.G. Maron, S.M. Lyi, M. Menossi, and L.V. Kochian. (2008). Not all ALMT1-type transporters mediate aluminum-activated organic acid responses: the case of ZmALMT1 - an anion-selective transporter. Plant J. 53: 352-367. 18069943
Piñeros, M.A., G.M. Cançado, and L.V. Kochian. (2008). Novel properties of the wheat aluminum tolerance organic acid transporter (TaALMT1) revealed by electrophysiological characterization in Xenopus Oocytes: functional and structural implications. Plant Physiol. 147: 2131-2146. 18550686
Qin, L., L.H. Tang, J.S. Xu, X.H. Zhang, Y. Zhu, C.R. Zhang, M.H. Wang, X.L. Liu, F. Li, F. Sun, M. Su, Y. Zhai, and Y.H. Chen. (2022). Cryo-EM structure and electrophysiological characterization of ALMT from reveal a previously uncharacterized class of anion channels. Sci Adv 8: eabm3238. 35235352
Ramesh, S.A., M. Kamran, W. Sullivan, L. Chirkova, M. Okamoto, F. Degryse, M. McLaughlin, M. Gilliham, and S.D. Tyerman. (2018). Aluminum-Activated Malate Transporters Can Facilitate GABA Transport. Plant Cell 30: 1147-1164. 29618628
Ramesh, S.A., Y. Long, A. Dashtbani-Roozbehani, M. Gilliham, M.H. Brown, and S.D. Tyerman. (2022). Picrotoxin Delineates Different Transport Configurations for Malate and γ Aminobutyric Acid through TaALMT1. Biology (Basel) 11:. 36009788
Ryan, P.R., S.D. Tyerman, T. Sasaki, T. Furuichi, Y. Yamamoto, W.H. Zhang, and E. Delhaize. (2011). The identification of aluminium-resistance genes provides opportunities for enhancing crop production on acid soils. J Exp Bot 62: 9-20. 20847099
Sasaki T., Mori IC., Furuichi T., Munemasa S., Toyooka K., Matsuoka K., Murata Y. and Yamamoto Y. (2010). Closing plant stomata requires a homolog of an aluminum-activated malate transporter. Plant Cell Physiol. 51(3):354-65. 20154005
Sasaki T., Tsuchiya Y., Ariyoshi M., Ryan PR., Furuichi T. and Yamamoto Y. (2014). A domain-based approach for analyzing the function of aluminum-activated malate transporters from wheat (Triticum aestivum) and Arabidopsis thaliana in Xenopus oocytes. Plant Cell Physiol. 55(12):2126-38. 25311199
Sulavik, M.C., C. Houseweart, C. Cramer, N. Jiwani, N. Murgolo, J. Greene, B. DiDomenico, K.J. Shaw, G.H. Miller, R. Hare, and G. Shimer. (2001). Antibiotic susceptibility profiles of Escherichia coli strains lacking multidrug efflux pump genes. Antimicrob. Agents Chemother. 45: 1126-1136. 11257026
Van Dyk, T.K., L.J. Templeton, K.A. Cantera, P.L. Sharpe, and F.S. Sariaslani. (2004). Characterization of the Escherichia coli AaeAB efflux pump: a metabolic relief valve? J. Bacteriol. 186: 7196-7204. 15489430
Wang, J., X. Yu, Z.J. Ding, X. Zhang, Y. Luo, X. Xu, Y. Xie, X. Li, T. Yuan, S.J. Zheng, W. Yang, and J. Guo. (2021). Structural basis of ALMT1-mediated aluminum resistance in Arabidopsis. Cell Res. [Epub: Ahead of Print] 34799726
Zhang J., Baetz U., Krugel U., Martinoia E. and De Angeli A. (2013). Identification of a probable pore-forming domain in the multimeric vacuolar anion channel AtALMT9. Plant Physiol. 163(2):830-43. 23918900
Zhang, W.H., P.R. Ryan, T. Sasaki, Y. Yamamoto, W. Sullivan, and S.D. Tyerman. (2008). Characterization of the TaALMT1 protein as an Al3+-activated anion channel in transformed tobacco (Nicotiana tabacum L.) cells. Plant Cell Physiol. 49: 1316-1330. 18676980
Zhou, H., Z. Hu, Y. Luo, C. Feng, and Y. Long. (2022). Multiple ALMT subunits combine to form functional anion channels: A case study for rice ALMT7. Front Plant Sci 13: 1012578. 36452104