References

166 T. Müller, W. Koch, D. Wipf
cells, and it has been speculated that the corresponding plant homologs
might fulfil similar functions (Wipf et al. 2002). Quite recently a mechanism
similar to the vesicular secretion of neurotransmitters has been shown
for yeasts. A membrane protein localized to internal membranes is responsible
for secretion of AA to the surrounding medium in yeasts (Velasco
et al. 2004). The recent analysis of an Arabidopsis mutant hypersecreting
glutaminefromhydathodesledtotheidentificationofanewclassofmembrane
proteins possibly involved in this process (Pilot et al. 2004). These
proteins, which contain only a single transmembrane domain, might help
to identify the mechanism for AA export from plant cells. However, the
high number of proteins putatively involved in AA transport in plants (up
to 53; Wipf et al. 2002) as well as in animals, and the different mechanisms
prevented significant progress. To better understand the mechanisms underlying
the loading and unloading of AA from phloem and xylem and
the exchange between the two systems, it will be essential to analyse each
member individually.
Acknowledgements. We thank Felicity de Courcy for critical reading of
the manuscript. Our work has been funded by grants from the Deutsche
Forschungsgemeinsschaft.
References
Albers A, Broer A, Wagner CA, Setiawan I, Lang PA, Kranz EU, Lang F, Broer S (2001)
Na + transport by the neural glutamine transporter ATA1. Pflugers Arch Eur J Physiol
443:92–101
Arriza JL, Eliasof S, Kavanaugh MP, Amara SG (1997) Excitatory amino acid transporter 5,
a retinal glutamate transporter coupled to a chloride conductance. Proc Natl Acad Sci
USA 94:4155–4160
Atkins CA (2000) Biochemical aspects of assimilate transfers along the phloem path: Nsolutes
in lupins. Aust J Plant Physiol 27:531–537
Bai L, Xu H, Collins JF, Ghishan FK (2001) Molecular and functional analysis of a novel
neuronal vesicular glutamate transporter. J Biol Chem 276:36764–36769
Bellocchio EE, Reimer RJ, Fremeau RTJ, Edwards RH (2000) Uptake of glutamate into
synaptic vesicles by an inorganic phosphate transporter. Science 289:957–960
Blakely RD, Bauman AL (2000) Biogenic amine transporters: regulation in flux. Curr Opin
Neurobiol 10:328–336
Boll M, Daniel H, Gasnier B (2004) The SLC36 family: proton-coupled transporters for
the absorption of selected amino acids from extracellular and intracellular proteolysis.
Pflugers Arch Eur J Physiol 447:776–779
Broer A, Albers A, Setiawan I, Edwards RH, Chaudhry FA, Lang F, Wagner CA, Broer S
(2002) Regulation of the glutamine transporter SN1 by extracellular pH and intracellular
sodium ions. J Physiol 539:3–14
Broer A, Wagner C, Lang F, Broer S (2000) Neutral amino acid transporter ASCT2 displays
substrate-induced Na + exchange and a substrate-gated anion conductance. Biochem J
346:705–710