Amino Acid Transport

and in stomata. Overall, ART1 belongs to
favors aromatic amino acids and, perhaps,
Chen and D. R. Bush, unpublished data).
Two proline transporters have also
a new class of amino acid transporter that
to other aromatic compounds in plants (L.
been recently cloned, ProTl and Pro72
(Rentsch et al. 1996). These transporters were identified by complementing a Shr3- yeast
strain that is unable to target yeast amino acid transporters to the plasma membrane.
Significantly, the plant amino acid transporters are not affected by this processing mutation;
therefore, several amino acid transporters (many of which were previously identified)
complemented this yeast strain. A survey of different amino acids for evidence of
transport through these porters showed that they are proline translocators. However, the
rate of proline transport in yeast cells expressing these porters is 300 times slower than
proline transport by AAP6 expressed in the same cell line. It is not known if this is a
result of lower expression levels, or if it is an intrinsic property of these transporters.
ProTl and ProT2 are closely related genes that encode typical membrane proteins that
contain 442 and 439 amino acids, and have 10 putative transmembrane domains. Both
genes are widely expressed in Arabidopsis, with ProTl strongest in roots, flowers and
stems. Significantly, Pro22 expression was enhanced under conditions of water or salt
stress, which is consistent with proline’s putative role in water and salt tolerance (Delauney
and Verma 1993).
Molecular descriptions of amino acid transport have included the isolation of several
transport mutants in Arabidupsis. Two mutants that lack a low-affinity basic amino
acid transport activity were isolated based on their resistance to lysine plus threonine or
to ~-2-~inoethyl-~-cysteine, respectively (Heremans et al. 1997). The specificity of the
lost transport activities were resolved based on apparent K, and sensitivity to transport
inhibitors. Although both mutations have been mapped to chromosome 1, it is not clear
if they are allelic. In a similar approach, the raz1 mutant of Arabidopsis was selected
based on its resistance to the toxic proline analogue, azetidine-2-carboxylic acid (Verbruggen
et al. 1996). Likewise, we have isolated two T-DNA-tagged lines that are resistant
to high concentrations of valine in their growth medium (Chen and Bush 1994).
Exogenous valine is a powerful plant growth inhibitor because it is a negative regulator
of acetohy~ozyacid synthase (also known as acetolactate synthase), an enzyme in the
valine biosynthetic pathway. Once down-regulated in the presence of excess valine,
growth is inhibited because the cells starve for other amino acids, the precursors of which
also pass through acetohy~oxyacid synthase. We were able to show that our lines are
transport mutants versus regulation mutants because they are also resistant to azaserine,
a toxic amino acid analogue that targets other enzymes in the cell. Thus, resistance to
azaserine is consistent with loss of transport activity. We are currently attempting to
identify the disrupted gene(s).
One of the unexpected observations emerging from recent molecular descriptions
of plant transporter genes is that there are large families of related porters that function
in assimilate partitioning (Bush et al. 1996). Examples include the AHA family of protonpumping
ATPases (Harper et al. 1990), the major facilitator superfamily of sugar transporters
(Griffith et al. 1992; Bush et al. 1996), and the amino acid transporter families
reviewed here. Although it is easy to imagine how these porters fulfill complementary
functions in different organs and cells, it is clear that a major challenge in this field is
to determine the unique contributions of the various transporters and, ultimately, to integrate
their combined activities across the plant as a multicellular organism.