2.A.69 The Auxin Efflux Carrier (AEC) Family

Plants possess tissue-specific, pmf-driven, cellular, auxin efflux systems. These carriers are saturable, auxin-specific, and localized to the basal ends of auxin transport-competent cells. They may be found in various plant tissues including vascular tissues and roots. They are responsible for the polar (downwards) transport of auxins from the leaves to the roots. They also function in gravitropism. In fact, gravity-dependent relocation of auxin efflux carriers has been demonstrated (Ottenschläger et al., 2003). A single plant such as Arabidopsis thaliana possesses at least six such systems. Two isoforms in A. thaliana, one in vascular tissue (PIN1) and one in roots (REH1 or EIR1) have been functionally characterized as have homologues from Oryza sativa. These plant proteins are 600-700 amino acyl residues long and exhibit 8-12 transmembrane spanners. Expression analysis of PIN family genes in Chinese hickory revealed their potential roles during grafting and salt stress (Yang et al. 2022). The directional movement of auxin between cells is largely facilitated by the canonical PIN-FORMED (PIN) proteins in the plasma membrane. In contrast, noncanonical PIN and the PIN-LIKES (PILS) proteins appear to reside mainly in the endoplasmic reticulum (ER) (Ung et al. 2023).These systems appear to use an elevator mechanism, catalyzing pmf-driven uniport (Ung et al. 2023).  Li et al. 2023 provided evidence for mechanistic insight on leaf hyponasty, which might facilitate the manipulation of the shade avoidance response in crops.

The rate of auxin transport across the plasma membrane is regulated by the Auxin Binding Protein 1, ABP1, which influcences PIN activity at the plasma membrane (Čovanová et al. 2013).  This highlights the relevance of ABP1 for the formation of developmentally important, PIN-dependent auxin gradients. Genome-wide analysis of the PIN gene family in common wheat (Triticum aestivum) identified 44 TaPIN genes and characterized them to understand their structures, functions, and distribution across various tissues (Kumar et al. 2021). Phylogenetic analyses led to the classification of TaPIN genes into seven groups, providing evidence of an evolutionary relationship with Arabidopsis thaliana and Oryza sativa. The physical and biochemical properties, conserved motifs, chromosomal, subcellular localization, transmembrane domains, and 3D structures were also examined. Expression patterns of the TaPIN genes were different in different tissues and developmental stages. Several members of the TaPIN family were induced during biotic and abiotic stress (Kumar et al. 2021).

Morphogenesis and adaptive tropic growth in plants depends on gradients of the phytohormone auxin, mediated by PIN auxin transporters. PINs localize to a particular side of the plasma membrane (PM) or to the endoplasmic reticulum (ER) to directionally transport auxin and maintain intercellular and intracellular auxin homeostasis, respectively. Zhang et al. 2020 swapped the domains between ER- and PM-localized PIN proteins, as well as between apical- and basal PM-localized PINs from Arabidopsis thaliana, to shed light on why PIN family members with similar topological structures reside at different membrane compartments within cells. The N- and C-terminal TMSs and central hydrophilic loop contribute to their differential subcellular localizations and cellular polarities, but the pairwise-matched N- and C-terminal TMSs, resulting from intramolecular domain-domain co-evolution, are also crucial for their divergent patterns of localization (Zhang et al. 2020).

Homologues of the AEC family are found in bacteria (E. coli, Klebsiella pneumoniae, Synechocystis, Aquifex aeolicus, Bacillus subtilis and Rickettsia prowazekii) as well as in archaea (Methanococcus jannaschii and Methanobacterium thermoautotrophicum.) The K. pneumoniae homologues (MdcF, 319 aas) has been implicated in malonate uptake. The O. oeni homologue, MleP, is a malate permease. The bacterial proteins are 300-400 aas in length (Young et al. 1999).

Yeast also possess homologues of the AEC family. Saccharomyces cerevisiae has three functionally uncharacterized AEC members (YL52, spP54072, 64.0 kDa; YNJ5, spP53930, 71.2 kDa; and YB8B, spP38355, 47.5 kDa), and Schizosaccharomyces pombe also has a sequenced homologue. It is thus clear that members of the AEC family are widespread, being found in Gram-negative, Gram-positive and cyanobacteria, in archaea, and in both fungi and plants. C. elegans, however, appears to lack identifiable homologues of the AEC family (Young et al. 1999).

Members of the AEC family are homologous to members of the BART superfamily (Mansour et al. 2007). Interestingly, the first halves of BASS family (TC# 2.A.28) members show extensive similarity with the second halves of AEC family members but not vice versa. Repeats of the basic 5 TMS element have not yet been demonstrated in members of the AEC family. 

The transport reaction probably catalyzed by the auxin efflux carrier is:

Auxin (in)  nH⁺ (out) → Auxin (out) nH⁺ (in)