2.A.85 The Aromatic Acid Exporter (ArAE) Family The ArAE family consists of bacterial and eukaryotic members from plants, yeast and protozoans. The bacterial proteins are of 655 to 755 amino acyl residues 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. 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 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 database, 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.
References:
Harley, K.T. and M.H. Saier, Jr. (2000). A novel ubiquitous family of putative efflux transporters. J. Mol. Microbiol. Biotechnol. 2: 195-198.
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.
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.
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.
Sasaki, T., I.C. Mori, T. Furuichi, S. Munemasa, K. Toyooka, K. Matsuoka, Y. Murata, and Y. Yamamoto. (2010). Closing plant stomata requires a homolog of an aluminum-activated malate transporter. Plant Cell Physiol. [Epub: Ahead of Print]
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.
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.
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.
Examples:
TC#	Name	Organismal Type	Example
2.A.85.1.1	YccS of unknown specificity	Bacteria	*YccS of E. coli* (720 aas) (P75870)**

2.A.85.1.2	*p-hydroxybenzoate efflux carrier, AaeB (Van Dyk et al., 2004).*	Gram-negative bacteria	*AaeB (YhcP) of E. coli* (655 aas) (P46481)**

2.A.85.1.3	YhfK of unknown specificity	Bacteria	*YhfK of E. coli* (P45537)**

Examples:
TC#	Name	Organismal Type	Example
2.A.85.2.1	*An inorganic anion (Cl^-/NO₃^-) transporter, ALMT12, incapable of transporting organic anions; involved in stomatal closure (Sasaki et al., 2010).*	Plants	*Orf1 of Arabidopsis thaliana* (560 aas) (O49696)**

2.A.85.2.2	Putative protein	Plants	*Orf5 of Arabidopsis thaliana* (533 aas) (Q9SX23)**

2.A.85.2.3	*The root aluminum-activated malate efflux transporter, ALMT1 (required for aluminum tolerance) (Hoekenga et al., 2006)*	Plants	*ALMT1 of Arabidopsis thaliana* (Q15EV0)**

2.A.85.2.4	*The anion-selective transporter ALMT1 (transports anions) (35% identical to 2.A.85.2.3) (Pineros et al., 2008)*	Plants	*ALMT1 of Zea mays* (A1XGH3)**

2.A.85.2.5	*Aluminum-activated anion (Malate >> NO₃^- > Cl^-; Malate/Cl^- ≈ 20) channel (Zhang et al., 2008) (67% identical to 2.A.85.2.4)*	Plants	*ALMT1 of Triticum aestivum* (Q76LB2)**

Examples:
TC#	Name	Organismal Type	Example
2.A.85.3.1	Hypothetical protein	Yeast	*Ydg8 of Schizosaccharomyces pombe* (977 aas) (Q10495)**

Examples:
TC#	Name	Organismal Type	Example
2.A.85.4.1	Asparagine-rich protein (clone 25C4)	Protozoa	*Clone 25C4 of Plasmodium falciparum* (669 aas) (Q25771)**

Examples:
TC#	Name	Organismal Type	Example
2.A.85.5.1	5 or 6 TMS 'half sized', YqjA	Bacteria	*YqjA of Bacillus subtilis* (322 aas) (P54538)**

Examples:
TC#	Name	Organismal Type	Example
2.A.85.6.1	MdtO (YjcQ), Multidrug resistance protein (involved in resistance to puromycin, acriflavin and tetraphenyl arsonium chloride; acts with MdtN (TC# 8.A.1.1.3) and MdtP (TC# 1.B.17.3.9)) (Sulavik et al., 2001).	Bacteria	*MdtO of E. coli* (P32715)**