References

11 AA transport in plant versus neutrotransmission 159
It is important to mention that further AA transporters exist in animals.
TAT1, a SLC16 (monocarboxylate transporter family) member specifically
is an aromatic AA transporter (Kim et al. 2001). Members of the mitochondrial
carrier family (SLC 25) potentially involved in AA transport are
present in animals and plants (Wipf et al. 2002).
11.3
Amino Acid Transport in Plants
For the distribution of AA in the plant several transport steps across membranes
are necessary. AA have to cross the plasma membrane when taken
up from the soil into root cells. In mycorrhized plants AA that pass the
fungal layer or are produced within the fungus must be taken up from the
plantrootcells.AAsynthesizedinroottissuehavetobeexportedtothe
shoot via xylem. Since mature xylem elements are dead cells, AA need to
cross the plasma membrane to enter the xylem, potentially via facilitators,
exchangers or antiporters. From the site of biosynthesis within the plant,
mainly in the plastids of the leaf or root, AA have to pass several membranes
to enter the long-distance traffic routes. Plastids are surrounded by
two membranes, an outer and an inner envelope. AA cross the outer envelope
of plastids via outer envelope proteins 16 and 14, which form channels
permeable to amines and AA (Pohlmeyer et al. 1997, 1998). For the inner
envelope, no AA transporters have been described so far. To leave the cytoplasm
of the cell, AA have to pass the plasma membrane. The concentration
of the AA in mesophyll cytoplasm and in the phloem is significantly higher
compared with that in the apoplasm, suggesting an active transport of AA
into the phloem (Lohaus et al. 1995). In feeding experiments it was shown
that large parts of AA fed directly to the xylem sap appear unchanged in
thephloemsap.ThisindicatesthatAAcanbeexchangedbetweenxylem
and phloem (Atkins 2000; Pate and Sharkey 1975) and can cycle within the
plant passing the membranes of multiple cells via transporters.
11.3.1
Amino Acid–Polyamine–Choline Transporter Family
Uptake of AA is well characterized in yeasts (Fischer et al. 1998), where
24 transporters of the AA–polyamine–choline (APC) family (pfam00324)
have been functionally characterized. The members of the APC family
cluster in subgroups reflecting the three kingdoms (yeast, plant and animal)
(Fig. 11.2) (Wipf et al. 2002). Mammalian CATs are representatives
of system y + ,mediatingNa + -independent uptake of cationic AA (Closs