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

More documents

Recommendations

Info

1990). Heavy metal toxicity can elicit adaptive and constitutive responses in plants (Sanità di Toppi and Gabbrielli, 1999) and plants possess a range of potential cellular mechanisms that may be involved in the detoxification of heavy metal ions and thus tolerance to metal stress. A regulated network of metal transport including metal-binding to cell walls and reduced transport across cell membrane, trafficking and sequestration activities such as active efflux, vacuolar compartmentalization and chelation functions to provide the uptake, distribution and detoxification of metal ions (Steffens, 1990; Neumann et al., 1994). Plants appear to accumulate metal-chelating compounds upon exposure to excessive metal availability levels, such as amino acids and amino acid derivatives, citric acid, malic acid, peptides, polypeptides and phytochelatins (Cobbett, 2000). Heavy metals can induce the synthesis of thiol-rich metal-chelating peptides (Grill et al., 1985; Gekeler et al., 1989; Rauser, 1990; Steffens, 1990). The physiologically relevant aspect of metal binding is only evident with complexes to liberate the constituent metal-free polypeptides. In 1957 a cysteine-rich, Cd-binding protein devoid of aromatic amino acids was isolated from equine kidney (Margoshes and Vallee, 1957; Nussbaum et al., 1988) and subsequently called "metallothionein". Induction of these cysteine-rich, 4 to 8 kDa small metal-binding peptides confer tolerance to a broad range of metals in mammals, Drosophila, Neurospora crassa, Saccharomyces cerevisiae and they also appear to be involved in both metal detoxification and homeostasis (Speiser et al., 1992; Vatamaniuk et al., 2000). Cd-detoxification and Cd-binding polypeptides: Phytochelatins Higher plants, algae and certain organisms of the kingdom fungi produce class III metallothioneins, which are atypical, nontranslationally synthesized metal thiolate polypeptides (Grill et al., 1985; Rauser, 1990; Steffens, 1990; Vatamaniuk et al., 2000). A variety of metals including Cd, Cu, Zn, Pb, Hg, Ni, Bi, Ag and Au; in addition, multiatomic anions including SeO4 -2 , SeO4 -3 and AsO4 -3 also induce their synthesis (Grill et al., 1991; Rauser, 1991). Among Phytochelatin and cadmium 47 the common metals, Cd is a strong inducer, whereas Zn appears to be a weak inducer, requiring high external levels for induction (Steffens, 1990). These cysteine-rich peptides capable of binding heavy metal ions via thiolate coordination (Grill et al., 1989; Leopold et al., 1999) have been shown to bind Cd and Cu directly and are believed to bind Pb and Hg by competition with Cd (Speiser et al., 1992) which mostly contribute to Cd detoxification and to Cu homeostasis (Grill et al., 1988, 1989; Tukendorf and Rauser, 1990). With regard to Zn it is difficult to demonstrate binding to these polypeptides because of the low affinity of the ligand for Zn ions (Grill et al., 1989). This inducible Cd-binding compounds show properties distinctly different from metallothioneins (Grill et al., 1985). Similarities were high cysteine content and comparable circular dichroism spectra, indicating the lack of aromatic residues and the possible binding of heavy metals by mercaptide complexes (Nussbaum et al., 1988). This type of biological response to heavy metal stress involving the synthesis of small heavy metal complexing peptides in higher plants, algae and in some fungi termed as "phytochelatins" (PCs) (Grill et al., 1985; Rauser, 1990; Steffens, 1990; Cobbett, 2001). They are also called as cadystins, Cd-binding peptides, γ-glutamyl metal-binding peptides or (γ-EC)nG 1 ( 1 E, L-glutamic acid; C, L-cysteine; G, glycine). The primary structure of these polypeptides show a series of molecules of poly (γ-glutamyl-cysteinyl)-glycine (Figure 1) and consists of the repeated dipeptide γ-Glu-Cys attached to a carboxy-terminal glycine, with a structure of (γ-Glu-Cys)n-Gly, where n=2 Figure 1: Structure of phytochelatins (from Manunza et al., 1997).
48 Gülriz Bayçu Figure 2: Optimized structure of the Cd(PC2)2 complex. Only the S atoms enter the coordination sphere of Cd (from Manunza et al., 1997). through 11 depending on the source (Rauser, 1995). These complexes generally contain predominantly Cd, a range of cysteine-rich-polypeptides, and acid-labile sulfide (Figure 2). Glutamic acid found in the composition is linked to each sulfur containing cysteine by a γ-peptide linkage and because of the repetitive γ-glutamic acid bonds, PCs can not be regarded as primary gene products (Speiser et al., 1992). In a few members of the order Fabales, such as in Vicia faba, PCs are substituted by a peptide family containing a ß-alanine carboxy terminus instead of the glycine. These peptides are termed homo-phytochelatins, h-PCn or (γ-Glu-Cys)n-ß-Ala (n=2-7) (Gekeler et al., 1989). PCs are secondary metabolites synthesized enzymatically from glutathione (GSH) by γ-glutamyl-cysteine dipeptidyl transpeptidase (PC synthase, GCS), a 25 kDa protein that removes a γ-glutamyl-cysteine moiety from one molecule of GSH and couples it to another GSH. PC biosynthesis can be induced very rapidly in roots and tissue culture cells and is accompanied by a fall in the concentration of GSH during early PC accumulation, following the addition of Cd or other heavy metals (Grill et al., 1989). Studies indicate GSH as a substrate for PC synthesis. Kinetic data for plant cells show that synthesis of PC2 (i.e., n=2) is faster than that of PC3, which is faster than that of PC4, as if the shorter PC is the precursor to the longer PC (Tukendorf and Rauser, 1990). The number of repeating units varies with the conditions of Cd exposure (Speiser et al., 1992). Cd activates the PC synthase to form PCs and synthesis stops when free Cd is no longer present (Cobbett, 2000) which suggests that the structure of PC consisting thiol and carboxyl groups are essential for the formation of tight PC-Cd complexes (Satofuka et al., 2001). In laboratory conditions, PC complexes elute as broad peaks from gel permeation columns. The molecular weight (Mr) PC-Cd complexes were found about 3 to 10 kDa depending upon ionic strength . The lower Mr observed at high ionic strength suggests that complexes possess a trimeric or tetrameric peptide stoichiometry. The high Mr observed at low ionic strength has been suggested to result both from electrostatic repulsion of the negatively charged free Glu-carboxylates of the polypeptides and from complex aggregation (Steffens, 1990). Owing to the high content of cysteine, PCs are able to create complex compounds with toxic ions of metals. These complexes transport heavy metal into the vacuole by the ABC transporter which is localized in the tonoplast (Ortiz et al., 1995), thus separating them from cell metabolism. PCs bind Cd with high affinity and localize together with Cd to the vacuole of intact cells (Vögeli-Lange and Wagner, 1990; Vatamaniuk et al., 1999). A vacuolar HMT1 transporter catalyses MgATP-dependent uptake of both PC-Cd complexes and h-PCs. Activation of the detoxicative-PC system in the cytosol (Rauser, 1995; Zenk, 1996; Sanità di Toppi and Gabbrielli, 1999; Cobbett, 2000) may show that phytochelatins play an important part in detoxification and homeostatis (Cobbett, 2001; Piechalak et al., 2002). Some possible molecular bases of mechanisms of metal detoxification and tolerance involving PCs are as follows: a. Increased activity of PC biosynthesis, such as PC synthase in metal-tolerant plants, b. Increased activity of enzymes responsible to S 2- saturation of metal-PC complexes, c. Modified chloroplastic/extrachloroplastic compartmentation of one of the components; PC, S 2- , or metal, d. Modified transport of metal-PC complexes into the vacuole, e. Modified rates of PC turnover. Modification of some process not directly related to metal accumulation by PC, but which allows cell
Page 1 and 2: STANBUL 1998 VOLUME 1 • NO. 2 •
Page 3 and 4: Journal of Cell and Molecular Biolo
Page 5: 46 Gülriz Bayçu Cd Contamination
Page 9 and 10: 50 Gülriz Bayçu PC production was
Page 11 and 12: 52 Gülriz Bayçu plants and fungi
Page 13 and 14: 54 Gülriz Bayçu References Abraha
Page 17 and 18: cells (cancer), inflammatory sites
Page 19 and 20: 1991, Russel and Synder, 1968; Pegg
Page 21 and 22: Figure 3: Several sources of PAs ar
Page 23 and 24: chemotherapy and lead to a very lar
Page 25 and 26: Thomas T and Thomas T. Polyamines i
Page 27 and 28: 70 Tülin Aktaç and Elvan Bakar Ma
Page 29 and 30: 72 Tülin Aktaç and Elvan Bakar Ak
Page 31 and 32: 74 Seyhan Altun et al. Altun (1996)
Page 33 and 34: 76 Seyhan Altun et al. increased a
Page 35 and 36: 78 Seyhan Altun et al. PH ratio and
Page 39 and 40: epirubicin + tamoxifen were added t
Page 41 and 42: Murphy B and Muss HB. Hormonal ther
Page 43 and 44: 88 Gülruh Ulako¤lu et al., 1993).
Page 45 and 46: 90 Gülruh Ulako¤lu According to t
Page 49 and 50: pad3 locus from the Col-pad3 mutant
Page 51 and 52: Table 2: Phenotypic interaction bet
Page 53 and 54: locus recognise distinct Peronospor
Page 55 and 56: 102 Narçin Palavan-Ünsal et al. I
Page 57 and 58:
104 Narçin Palavan-Ünsal et al. F
Page 59 and 60:
106 Narçin Palavan-Ünsal et al. F
Page 61 and 62:
108 Narçin Palavan-Ünsal et al. c
Page 63 and 64:
110 Ayfle TABAKO⁄LU-O⁄UZ, Anima
Page 65 and 66:
112 by superscript small letters an
Page 67 and 68:
114 Volume 1, No. 2 Review articles
Page 69:
Journal of Cell and Molecular Biolo
show all

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

Create successful ePaper yourself

Delete template?

Save as template?