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

Heavy Metal Stress
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4.2. Chaperones
In recent years a family of soluble metal receptor proteins, known as “metallochaperones”,
that are active in intercellular metal trafficking has emerged. These metal receptors are
not detoxification proteins as they clearly function in a “chaperone-like” manner, but
are involved in guiding and protecting the metal ion while facilitating its delivery to
appropriate receptors.
Genetic studies in prokaryotic organisms and yeast have identified membraneassociated
proteins that mediate either the uptake or export of copper from cells. Within
these cells, small cytosolic proteins, designated as copper chaperones, have been
identified. These bind copper ions and deliver them tospecific compartments and to
copper requiring proteins (Peña et al., 1999). Three distinct copper trafficking pathways
have been identified; these include delivery of copper to mitochondria, the ATX1 pathway
of copper delivery to Golgi, and copper delivery to cytosolic superoxide dismutase (see
review by Halloran and Culotta, 2000).
A copper chaperone (CCH) and a responsive to antagonist 1 (RAN1), both
identified in A. thaliana, were the first Cu delivery systems identified in plant cells
(Himelblau and Amasino, 2000; Fig. 1; Table 1). The CCH, an Arabidopsis homolog to
yeast copper chaperone antioxidant 1 (ATX1), is upregulated in senescing tissue
(Himelblau et al., 1998), and it is involved in long-distance mobilization of Cu from
senescing tissues via phloem (Mira et al., 2001). While the RAN1, an Arabidopsis
homolog to yeast CCC2, involved in Cu trafficking, is involved in Cu delivery to ethylene
receptors in post-Golgi vesicles (Hirayama et al., 1999; Fig. 1; Table 1).
Orthologues of the three copper chaperones characterized in yeast, ATX1,
CCS and COX17, have been found in A. thaliana (Wintz and Vulpe, 2002). Genome
analysis of Arabidopsis has revealed that CCH belongs to a family consisting of 31
genes coding for proteins that possess heavy metal associated (HMA) domains similar
to the ATX1 HMA domain. Analysis of ATX1 homologues has suggested that these
homologues play a variety of roles in intracellular and intercellular trafficking of Cu, and
possibly of other metals, including transport of metals into chloroplast, detoxification,
export of metals from cells, and possibly regulation of gene expression.
A plant homologue of CCS, the yeast chaperone to Cu/Zn superoxide
dismutase, has been identified in tomato (Zhu et al., 2000) and Arabidopsis (Wintz and
Vulpe, 2002). Wintz and Vulpe (2002) have concluded that the encoded protein has a
potential chloroplast-targeting sequence indicating that it is a chloroplast protein, and
it is most likely involved in maturation of the chloroplast Cu/Zn superoxide dismutase
apoprotein. They have also identified two Arabidopsis homologues of COX17, including
AtCOX17-1 and AtCOX17-2, localized on chromosomes 3 and 1, respectively. The
two COX17 proteins are predominantly expressed in roots, thus suggesting that their
function may not be restricted to copper delivery to mitochondria, but that they may be
also involved in copper transport in roots.