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, one in vascular tissue (PIN1) and one in roots (REH1) have been functionally characterized as has a homologue from Oryza satira. These plant proteins are 600-700 amino acyl residues long and exhibit 8-12 transmembrane spanners. 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 (MdeF, 319 aas) has been implicated in malonate uptake. The bacterial proteins are 300-400 aas in length. 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. Based on PSI-BLAST results, the AEC family may be distantly related to the bile acid:Na⁺ symporter (BASS) family (TC #2.A.28) and the divalent anion:Na⁺ symporter (DASS) family which include members found in animals. It is therefore a constituent of the ion transporter (IT) superfamily. The transport reaction probably catalyzed by the auxin efflux carrier is: Auxin (in) + nH⁺ (out) → Auxin (out) + nH⁺ (in).
References:
Friml, J., A. Vieten, M. Sauer, D. Weijers, H. Schwarz, T. Hamann, R. Offringa, and G. Jürgens. (2003). Efflux-dependent auxin gradients establish the apical-basal axis of Arabisopsis. Nature 426: 147-153.
Gälweiler, L., C. Guan, A. Müller, E. Wisman, K. Mendgen, A. Yephremov, and K. Palme. (1998). Regulation of polar auxin transport by AtPIN1 in Arabidopsis vascular tissue. Science 282: 2226-2230.
Hoenke, S., M. Schmid, and P. Dimroth. (1997). Sequence of a gene cluster from Klebsiella pneumoniae encoding malonate decarboxylase and expression of the enzyme in Escherichia coli. Eur. J. Biochem. 246: 530-538.
Labarre, C., C. Divies, and J. Guzzo. (1996). Genetic organization of the mle locus and identification of a mleR-like gene from Leuconostoc oenos. Appl. Env. Microbiol. 62: 4493-4498.
Luschnig, C., R.A. Gaxiola, P. Grisafi, and G.R. Fink. (1998). EIR1, a root-specific protein involved in auxin transport, is required for gravitropism in Arabidopsis thaliana. Genes Dev. 12: 2175-2187.
Ottenschläger, I., P. Wolff, C. Wolverton, R.P. Bhalerao, G. Sandberg, H. Ishikawa, M. Evans, and K. Palme. (2003). Gravity-regulated differential auxin transport from columella to lateral root cap cells. Proc. Natl. Acad. Sci. USA 100: 2987-2991.
Reinhardt, D., E.-R. Pesce, P. Stieger, T. Mandel, K. Baltensperger, M. Bennett, J. Traas, J. Friml, and C. Kuhlemeier. (2003). Regulation of phyllotaxis by polar auxin transport. Nature 426: 255-260.
Examples:
TC#	Name	Organismal Type	Example
2.A.69.1.1	Auxin efflux carrier, PIN1 (Reinhardt et al., 2003)	Plants	PIN1 of Arabidopsis thaliana

2.A.69.1.2	Auxin transporter, EIR1	Plants	EIR1 of Arabidopsis thaliana

2.A.69.1.3	Auxin efflux carrier, PIN7 (promotes embryonic axis formation) (Friml et al., 2003)	Plants	*PIN7 of Arabidopsis thaliana* (NP_849923)**

2.A.69.2.1	Putative malonate transporter, MdcF	Bacteria	MdcF of Klebsiella pneumoniae

2.A.69.3.1	Malate permease, MleP	Bacteria	MleP of Oenococcus (Leuconostoc) oeni