Full Journal - Journal of Cell and Molecular Biology - Haliç Üniversitesi

52 Gülriz Bayçu
plants and fungi (Cobbett et al., 1998; Vatamaniuk et
al., 1999). Experiments with CAD1 mutants of
A. thaliana conducted by Howden et al. (1995)
proved the function of PCs in inactivation of heavy
metals. It was found that these CAD1 mutants, which
are unable to synthesize PCs, exhibited extreme
sensitivity to Cd (Piechalak et al., 2002). As indicated
by the hypersensitivity of PC-deficient Arabidopsis
CAD1 mutants to Cd (Howden et al., 1995), PC
synthase genes conribute most markedly to Cd
detoxification in planta (Cobbett et al., 1998;
Vatamaniuk et al., 1999).
In another investigation with A. thaliana including
a PC-deficient mutant, CAD1-3, and the wild type
plants, the potential effect of prior exposure to
different Cd concentrations on Cd uptake and
accumulation is tested. Plants were grown for 1 week
in nutrient solution containing different Cd
concentrations (0.05-1.0 µM Cd(NO3)2), and
thereafter they were subjected to 0.5 µM Cd labelled
with 109 Cd for 2 h. According to these results, it has
been shown that the PC-deficient mutant CAD1-3,
accumulates less Cd than the wild type (Larsson et al.,
2002). The possibility that the differences in Cd
accumulation in mutant and wild-type lines may be
due to the cytosolic Cd regulation, which is inhibited
by the complexation of Cd by PCs (Speiser et al.,
1992).
Can PCs have a role in Cd detoxification at levels
of Cd exposure relevant to plants in a natural
environment? It has been estimated that solutions of
non-polluted soils contain Cd concentrations of less
than 0.3 µM (Wagner, 1993). Nonetheless, the
sensitivity of the Arabidopsis CAD1-3 mutant to
concentrations of Cd as low as 0.6 µM (Howden et
al., 1995) suggests that PCs may have a role in
heavy-metal detoxification in an unpolluted
environment (Cobbett, 2000).
PC synthase genes and Cd-detoxification in
animals
Cd, which is an environmental pollutant with
well-known mutagenic, carcinogenic, and teratogenic
effects is known to accumulate in the human kidney
for a relatively long time and at high doses and it is
also known to have harmful effects on the respiratory
system and has been associated with bone disease. At
the molecular and cellular levels, a lot of studies on
apoptosis and stress kinase activation of Cd have been
performed (Takagi et al., 2002).
Although PCs have not yet been detected in
animal species, unexpectedly genes with similar
sequences to those encoding PC synthase, for
example a gene similar to AtPCS1 found in
A. thaliana have been identified in the nematode,
Caenorhabditis elegans (Cobbett, 2000; Takagi et al.,
2002). The amino-terminal region of the predicted
gene product is equally similar to the plant and yeast
proteins. In contrast, the carboxy-terminal domain has
little obvious similarity to the plant or yeast
gene-products, except that it contains multiple pairs
of Cys residues. The existence of PC-synthase-like
genes in animals suggests that PCs play a wider role
in heavy-metal detoxification than previously
expected (Cobbett, 2000).
Mammalian cells can not synthesize PC because
of their lack of the key enzymes for PC biosynthesis,
PC synthase. To determine whether or not PC can be
synthesized in mammalian cells with the introduction
of the PC synthase gene (AtPCS1) from A. thaliana
the gene AtPCS1 gene was amplified by PCR. The
apoptotic cell death caused by Cd were examined and
the effects of PC on the detoxification of Cd were
studied by attempting to express these plant-specific
peptides (PCs) in mammalian cells by transfecting
Jurkat cells with AtPCS1 (Takagi et al., 2002). With
the addition of the chemically synthesized PC7 to the
culture medium containing 10-100 µM CdCl2 the
survival ratio of Jurkat CAGGS cells clearly
recovered. In the case of Jurkat PCS cells treated with
20, PC synthesis corresponding to PC2-PC6 could be
clearly observed indicating that the PC synthase gene
from A. thaliana could catalyze PC synthesis even in
Jurkat cells. Indeed, the activity of the PC synthase in
these cells was also stimulated by Cd. Therefore, not
only the catalytic activity but also the activation
mechanism of the plant PC synthase was functional in
the mammalian cells. It has been found interesting
that the plant AtPCS1 gene can be utilized to enhance
the resistance to Cd toxicity of mammalian cells
(Takagi et al., 2002).
Conclusions
Higher plants like other organisms are believed to
possess intracellular metal buffer systems, i.e. metal
chelating subtances, which serve to keep the