The Amino Acid/Auxin Permease (AAAP) Family (TC# 2.A.18) is a family of secondary carrier proteins, a member of the APC Superfamily that includes hundreds of proteins from plants, animals, fungi and lower eukaryotes. There are 7 AAAP paralogues in Saccharomyces cerevisiae, at least 9 in Arabidopsis thaliana and at least 5 in Caenorhabditis elegans. Six AAAPs in A. thaliana are well characterized and transport neutral and charged amino acids with varying specificities and affinities.
Contents
Function
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.
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 which Systems N are not, the amino acid:cation stoichiometries may differ.
Six auxin/amino acid permeases (AAAPs) from Arabidopsis mediate transport of a wide spectrum of amino acids. AAAPs are distantly related to plasma membrane amino acid transport systems N and A and to vesicular transporters such as Vesicular GABA transporter (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. 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.
Transport Reaction
The generalized transport reaction catalyzed by the proteins of the AAAP family is:
Substrate (out) + nH+ (out) → Substrate (in) + nH+ (in)Structure
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. One homologue, AAP1 of A. thaliana (TC #2.A.18.2.1), has 11 established TMSs.