References

12 GABA and GHB Neurotransmitters in Plants and Animals 175
12.2.3
GABA Transporters
There are two main types of GABA transport systems in living organisms:
cell membrane GABA transporters (GATs) and vesicular GABA transporters(vGATs).AlistofGABAtransportersindifferentorganismsisgiven
in Fig. 12.2. Four complementary DNAs (cDNAs) encoding high-affinity
plasma membrane GABA transporters (GATs) have been isolated in rodent
and human nervous systems (Borden 1996; Conti et al. 2004). The highly
homologous GATs (GAT-1, GAT-2, GAT-3, and BGT-12) exhibit different
ionic dependencies and inhibitor sensitivities, and are differentially distributed
within the central nervous system (Conti et al. 2004). A vGAT
was first isolated from the nematode C. elegans,andwastermedUNC-47.
It is a multipass transmembrane protein with weak sequence homology
to plasma membrane amino acid transporters. It is expressed in GABA
neurons and is located in the membrane of synaptic vesicles. In addition,
the rat homolog of UNC-47 is capable of transporting GABA into vesicular
compartments of PC12 cells with the same kinetics of transport that was
shown for synaptic vesicles purified from rat brain (McIntire et al. 1997).
Moreover, there is a plant homolog of UNC-47 (Fig. 12.2), but no plant homolog
was found of any of the GATs. Our bioinformatics search for a GABA
transporter in the Arabidopsis genome revealed one putative transporter,
which resembles the yeast GABA-specific transporter UGA4 (Fig. 12.2).
When Saccharomyces cerevisiae GABAtransportwascharacterized,three
different GABA permeases were found: the general amino acid permease
(GAP1), the proline permease (PUT4), and a specific GABA permease
(UGA4). Triple mutant cells (gap1, put4, uga4)growverypoorlyonGABA
asthesolenitrogensource:theirGABAuptakerateisverylowunderall
growth conditions. Expression of one of the three genes is sufficient to
restore growth on GABA (Jauniaux et al. 1987). Importantly, the GABAspecific
UGA4 permease gene is activated only when yeast cells are exposed
to GABA (Jauniaux et al. 1987; Vissers et al. 1989).
UGA4 is a 62-kDa protein, with 9–12 putative transmembrane regions,
which shares significant sequence similarity with the yeast choline transporter
(CTR gene), exhibiting but limited similarity to the previously
reported GABA transporters, i.e., the yeast GAP1 and PUT4 permeases and
theratbrainGATs.InductionofUGA4inthepresenceofGABAisexertedat
the level of UGA4 messenger RNA accumulation, most probably at the level
of transcription itself (Andre et al. 1993; Bermudez Moretti et al. 1995).
Arabidopsis proteins capable of transporting GABA were identified by
heterologous complementation of a GABA transport-deficient yeast mutant
(gap1, put4, uga4) (Breitkreuz et al. 1999). Expression of these Arabidopsis
transporters allowed the growth of the GABA transport-deficient yeast