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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.

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.

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).


This family belongs to the: BART Superfamily.

References associated with 2.A.69 family:

Čovanová, M., M. Sauer, J. Rychtář, J. Friml, J. Petrášek, and E. Zažímalová. (2013). Overexpression of the auxin binding protein1 modulates PIN-dependent auxin transport in tobacco cells. PLoS One 8: e70050. 23894588
Carraro, N., T.E. Tisdale-Orr, R.M. Clouse, A.S. Knöller, and R. Spicer. (2012). Diversification and Expression of the PIN, AUX/LAX, and ABCB Families of Putative Auxin Transporters in Populus. Front Plant Sci 3: 17. 22645571
Costantini, A., E. Vaudano, K. Rantsiou, L. Cocolin, and E. Garcia-Moruno. (2011). Quantitative expression analysis of mleP gene and two genes involved in the ABC transport system in Oenococcus oeni during rehydration. Appl. Microbiol. Biotechnol. 91: 1601-1609. 21814807
Dueñas, E., C.R. Vazquez de Aldana, T. de Cos, C. Castro, and M. Henar Valdivieso. (1999). Generation of null alleles for the functional analysis of six genes from the right arm of Saccharomyces cerevisiae chromosome II. Yeast 15: 615-623. 10341424
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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. 9208947
Labarre, C., C. Divies, and J. Guzzo. (1996a). Genetic organization of the mle locus and identification of a mleR-like gene from Leuconostoc oenos. Appl. Env. Microbiol. 62: 4493-4498. 8953720
Labarre, C., J. Guzzo, J.F. Cavin, and C. Diviès. (1996). Cloning and characterization of the genes encoding the malolactic enzyme and the malate permease of Leuconostoc oenos. Appl. Environ. Microbiol. 62: 1274-1282. 8919788
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Mansour, N.M., M. Sawhney, D.G. Tamang, C. Vogl, and M.H. Saier, Jr. (2007). The bile/arsenite/riboflavin transporter (BART) superfamily. FEBS J. 274: 612-629. 17288550
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Nodzyński, T., S. Vanneste, M. Zwiewka, M. Pernisová, J. Hejátko, and J. Friml. (2016). Enquiry into the topology of plasma membrane localized PIN auxin transport components. Mol Plant. [Epub: Ahead of Print] 27622590
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. 12594336
Petrasek, J., J. Mravec, R. Bouchard, J.J. Blakeslee, M. Abas, D. Seifertova, J. Wisniewska, Z. Tadele, M. Kubes, M. Covanova, P. Dhonukshe, P. Skupa, E. Benkova, L. Perry, P. Krecek, O.R. Lee, G.R. Fink, M. Geisler, A.S. Murphy, C. Luschnig, E. Zazimalova, and J. Friml. (2006). PIN proteins perform a rate-limiting function in cellular auxin efflux. Science 312: 914-918. 16601150
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. 14628043
Wang, P., T. Cheng, S. Wu, F. Zhao, G. Wang, L. Yang, M. Lu, J. Chen, and J. Shi. (2014). Phylogeny and Molecular Evolution Analysis of PIN-FORMED 1 in Angiosperm. PLoS One 9: e89289. 24586663
Watson, M.D. (2001). Disruption and basic phenotypic analysis of six novel genes from the right arm of chromosome XII of Saccharomyces cerevisiae. Yeast 18: 473-480. 11255256
Young, G.B., D.L. Jack, D.W. Smith, and M.H. Saier, Jr. (1999). The amino acid/auxin:proton symport permease family. Biochim. Biophys. Acta. 1415: 306-322. 9889387
Zhou, C., L. Han, and Z.Y. Wang. (2011). Potential but limited redundant roles of MtPIN4, MtPIN5 and MtPIN10/SLM1 in the development of Medicago truncatula. Plant Signal Behav 6: 1834-1836. 22057323