2.A.18 The Amino Acid/Auxin Permease (AAAP) Family
The AAAP family includes hundreds of proteins from plants, animals, yeast and fungi. Individual permeases of the AAAP family transport auxin (indole-3-acetic acid), a single amino acid or multiple amino acids. Some of these permeases exhibit very broad specificities transporting all twenty amino acids naturally found in proteins. Some also transport D-amino acids. There are 7 AAAP paralogues in Saccharomyces cerevisiae, at least 9 in Arabidopsis thaliana and at least 5 in Caenorhabditis elegans. Six AAPs in A. thaliana transport neutral and charged amino acids with varying specificities and affinities (Fischer et al., 2002). All transport neutral amino acids and some acidic amino acids, always with just one proton. AAP3 and AAP5 are the only ones transporting basic amino acids, and only AAP6 transports aspartate (Fischer et al., 2002). The analysis of genes encoding AAPs in apple has been reported (Feng et al. 2022).
AAAP family proteins, all from eukaryotes, vary from 376 to 713 amino acyl residues in length, but most are of 400-500 residues. Most of the size variation occurs as a result of the presence of long N-terminal hydrophilic extensions in some of the proteins. Some of the yeast proteins are particularly long. Variation in the loops and the C-termini also occurs. These proteins exhibit 11 (or 10) putative transmembrane α-helical spanners (TMSs). One homologue, AAP1 of A. thaliana (TC #2.A.18.2.1), has 11 established TMSs (Chang and Bush, 1997). Members of the family have been tabulated (sometimes with functions) for foxtail millet (Setaria italica) (Yang et al. 2021). Extranuclear auxin signaling as well as intronuclear auxin signaling has been documented (Pérez-Henríquez and Yang 2023).
Among animal AAAP family members are numerous growth regulating System A and System N isoforms, each exhibiting distinctive tissue and subcellular localizations. The different isoforms also exhibit different relative affinities for the amino acid substrates. Some catalyze H+ antiport and can function bidirectionally. Since Systems A are electrogenic although Systems N are not, the amino acid:cation stoichiometries may differ (Chaudhry et al., 2001, 2002; Varoqui et al., 2000).
Six auxin/amino acid permeases (AAAPs) from Arabidopsis mediate transport of a wide spectrum of amino acids (Fischer et al., 2002). AAAPs are distantly related to plasma membrane amino acid transport systems N and A and to vesicular transporters such as VGAT from mammals. Although capable of recognizing and transporting a wide spectrum of amino acids, individual AAAPs differ with respect to specificity. Apparent substrate affinities are influenced by structure and net charge and vary by three orders of magnitude (Fischer et al., 2002). AAAPs mediate cotransport of neutral amino acids with one proton, and uncharged forms of acidic and basic amino acids are cotransported with one proton. Since all AAAPs are differentially expressed, different tissues may be supplied with a different spectrum of amino acids.
Amino acids increase the activity of the microenvironmental sensor mechanistic Target of Rapamycin Complex 1 (mTORC1) to promote cellular growth and anabolic processes. They can be brought into cells by the closely related Proton-assisted Amino acid Transporter (PAT or SLC36) subfamily, and the Sodium-coupled Neutral Amino acid Transporter (SNAT or SLC38) subfamily, both members of the AAAP family. Members of both families can act as amino acid-stimulated receptors, or so-called 'transceptors,' connecting amino acids to mTORC1 activation (Fan and Goberdhan 2018). PATs and SNATs at the surfaces of multiple intracellular compartments are linked to the recruitment and activation of different pools of mTORC1. Late endosomes and lysosomes are mTORC1 regulatory hubs, but a Golgi-localized PAT is also required for mTORC1 activation. PATs and SNATs can also traffic between the cell surface and intracellular compartments, with regulation of this movement providing a means of controlling their mTORC1 regulatory activity (Fan and Goberdhan 2018).
The generalized transport reaction catalyzed by the proteins of the AAAP family is:
Substrate (out) + nH+ (out) → Substrate (in) + nH+ (in)