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Fig. 12.2.
GABA transporters in different organisms. Vesicular and cell-membrane GABA
transporters from different organisms are listed in
rows
according to their type and amino
acid sequence homologies. Amino acid similarities (%) and identities (%) are presented for
Arabidopsis
proteins in comparison with UNC-47 (vesicular transpoters) and UGA4 (cell
membrane transporters)
mutant (
gap1
,
put4
,
uga4
) on GABA as a sole nitrogen source (Breitkreuz
et al. 1999). Two plant amino acid transporters, amino acid permease 3
(AAP3) (Fischer et al. 1995) and the proline transporter 2 (ProT2) (Rentsch
et al. 1996), effectively mediated GABA transport in yeast. In addition,
growth of
Arabidopsis
on a medium supplemented with GABA as the sole
nitrogen source provided evidence that plants possess proteins capable
of transporting GABA. Recently the subcellular localization and tissue-
specific expression of the AtProT protein family was reported (Grallath
et al. 2005). Transient expression of green fluorescent protein fused to
individualAtProTsintobaccoprotoplastsrevealedthatallthreeAtProTs
were localized at the plasma membrane. Expression and functional analysis
in a yeast mutant (
gap1
,
put4
,
uga4
) demonstrated that the affinity of all
three AtProTs was highest for glycine betaine, lower for Pro, and lowest
for GABA. Also, it has been shown that the expression patterns of the
three AtProTs are complementary. AtProT1 expression was found in the
phloem, or in phloem parenchyma cells, throughout the plant, indicative
of a role in long-distance transport of compatible solutes. In contrast,
AtProT2 expression was restricted to the epidermis and to the cortex cells
in roots, whereas in leaves expression could be demonstrated only after
wounding. AtProT3 expression was restricted to the aboveground parts of
the plant and could be localized to the epidermal cells in leaves.
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