References

160 T. Müller, W. Koch, D. Wipf
et al. 1993), and are characterized by a high affinity for cationic AA. The
plant membranes maintain a proton gradient over the plasma membrane
via plasma membrane ATPases, and, correspondingly, the first plant memberoftheAPCfamily,AtCAT1,
mediated the accumulation of histidine
in a pH-dependent manner (Frommer et al. 1995). However, it remains
unclear whether AtCAT1 functions as a uniporter or an exchanger, or as
a secondary active proton cotransporter. Recent studies of a related protein,
AtCAT5, showed that the transport of basic AA mediated by AtCAT5 is
sensitive to protonophores, indicating that at least AtCAT5 is a proton symporter.
AtCAT5 is a high-affinity transporter for arginin with an apparent
Km value of 12 µM (Su et al. 2004). Other basic AA were good competitors
for 14 C arginine uptake into yeast cells expressing AtCAT5, while neutral or
acidic AA like aspartate, proline, glutamate and glutamine were less effective
in uptake inhibition. The transport selectivity of AtCAT5 is similar to
that of AtCAT1, which also transports basic AA preferentially.
For other members of the CAT-family in Arabidopsis no direct transport
activity was demonstrated, nevertheless AtCAT3 and AtCAT6 expressed
in yeast cells increased toxicity to the toxic analogue of glutamine, (S)-2
amino-6-diazo-5-oxo-L-norleucine, but not to the arginine analog canavanine
(Su et al. 2004). The fact that arginine is not a substrate for AtCAT3
and AtCAT6 already shows that not all members of the CAT family in
Arabidopsis canbesupposedtobecationicAAtransporters.
In animals, the APC-type AA transporters require an additional subunit
for functional expression at the plasma membrane (Sect. 11.2.3). Members
of the mammalian CAT family essentially transport cationic AA by facilitateddiffusion,whiletheHATsareincontrastquitediverseintermsof
substrate selectivity and function as obligatory exchangers. In plants no homologs
of secondary subunits have yet been identified, and one subbranch
of the APC family, the LAT (Fig. 11.2), have still not been functionally
characterized in plants.
11.3.2
Amino Acid Transporter Family 1
Members of the ATF1 family were first described in plants as functional
AA transporters. Structurally related proteins were identified in yeasts
and animals (Fischer et al. 1998). The superfamily contains plant-specific
subbranches, and branches that are more structurally related to mammalian
transporters (Fig. 11.3). Within the plant-specific subbranches,
the best-characterized members are the Arabidopsis AA permeases (AAP;
Figs. 11.3, 11.4). Several AAP have been isolated from Arabidopsis by complementation
of yeast transport mutants with plant cDNAs (Fischer et al.