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).