Amino Acid Transport in Plants and Transport of Neurotransmitters in Animals: a CommonMechanism? - Communication in Plants: Neuronal Aspects of Plant Life - page 160

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 mem-
beroftheAPCfamily, At CAT1, mediated the accumulation of histidine
in a pH-dependent manner (Frommer et al. 1995). However, it remains
unclear whether At CAT1 functions as a uniporter or an exchanger, or as
a secondary active proton cotransporter. Recent studies of a related pro-
tein, At CAT5, showed that the transport of basic AA mediated by At CAT5 is
sensitive to protonophores, indicating that at least At CAT5 is a proton sym-
porter. At CAT5 is a high-affinity transporter for arginin with an apparent
K m value of 12 µM (Su et al. 2004). Other basic AA were good competitors
for 14 C arginine uptake into yeast cells expressing At CAT5, while neutral or
acidic AA like aspartate, proline, glutamate and glutamine were less effec-
tive in uptake inhibition. The transport selectivity of At CAT5 is similar to
that of At CAT1, which also transports basic AA preferentially.
For other members of the CAT-family in Arabidopsis no direct transport
activity was demonstrated, nevertheless At CAT3 and At CAT6 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 canava-
nine (Su et al. 2004). The fact that arginine is not a substrate for At CAT3
and At CAT6 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 facil-
itateddiffusion,whiletheHATsareincontrastquitediverseintermsof
substrate selectivity and function as obligatory exchangers. In plants no ho-
mologs 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 mam-
malian 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 com-
plementation of yeast transport mutants with plant cDNAs (Fischer et al.