Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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K. Gasic and S.S. Korban
completely and specifically reversed by adding copper, and increased sensitivity to
growth inhibition by the copper-specific chelator bathocuproine disulfonic acid.
4.3.2. CDF – Type Family
Cation diffusion facilitator (CDF) proteins have been recently discovered, and these
belong to a family of cation efflux transporters that likely play essential roles in metal
homeostasis and tolerance. CDFs have six transmembrane domains, long cytoplasmic
C-terminal tail domain, and histidine-rich region (Guerinot, 2000). Major substrates for
these mechanistically poorly understood transporters are Zn 2+ and Cd 2+ . CDFs have
been reported to confer tolerance for Zn 2+ , Co 2+ , or Cd 2+ ions in bacteria (Nies 1992),
yeast (Clemens et al., 2002; Borrelly et al., 2002), animals and plants (Blaudez et al.,
2003).
Identification, characterization, and localization of PtdMTP1,a member of the
CDF family in the hybrid poplar Populus trichocarpa x Populus deltoids, have been
described by Blaudez et al. (2003). However, PtdMTP1 is constitutively and ubiquitously
expressed at low levels. Heterologous expression of PtMTP1 in yeast has
demonstrated its ability to complement the hypersensitivity of mutant strains to Zn, but
not to other metals, such as Cd, Co, Mn, and Ni. In both yeast and in plant cells,
PtdMTP1 fused to a green fluorescent protein (GFP) was localized to the vacuolar
membrane. This is consistent with its role in zinc sequestration. In fact, overexpression
of PtdMTP1 in Arabidopsis has conferred Zn tolerance. When expressed in yeast and
Arabidopsis, PtdMTP1 forms homooligomers, a novel feature of CDF members. Oligomer
formation is disrupted by reducing agents, thus indicating a possible disulfide
bridge formation.
The yeast Saccharomyces cerevisiae ShMTP1 encodes membrane-bound
proteins of the CDF family involved in Mn 2+ tolerance. When expressed in Arabidopsis,
it conferred Mn 2+ tolerance through internal sequestration (Delhaize et al., 2003). The
ShMTP1 proteinfused to a GFP was localized to the tonoplast of Arabidopsis cells, but
it was localized in the endoplasmic reticulum of yeast.
4.3.3. Other Transporters
Among other transporters that have been identified so far, nine oligotransporter (OPT)
orthologs (AtOPT1 to AtOPT9) were identified in Arabidopsis (Koh et al., 2002). These
had distinct tissue-specific expression patterns suggesting different functional roles.
Complementary expression studies in yeast, S. cerevisiae, provided new evidence for
presence of multiple peptide transporter systems in Arabidopsis. This also suggested
important physiological roles for those small peptides in plants.