Metal-binding proteins tolerance

In this chapter we examine the relationship between metal homeostasis (i.e. essential metal metabolism and protection from metal toxicity) and metal tolerance, and the role that metal-binding proteins and metal-regulated gene expression play in these processes. 

The differences in sensitivity to Cd toxicity between phytoplankton species may be related to differences in their ability to detoxify the metal [43,44]. Cd-induced phyto-chelatin production is the most common detoxification mechanism in phytoplankton (see Section 5.1), but other thiol-containing peptides or proteins may also be involved. For example, a Cd-tolerant phytoplankton species, Isochrysis galbana, produces a metal-binding protein rich in cysteine [54] the cyanobacterium Synechococcus sp. produces a metallothionein-like protein to complex metals [55]. In addition, some species may induce efflux systems to remove intracellular Cd [47,56,57], or sequester Cd into the vacuole to reduce the cytosohc Cd concentration [58]. 

Zinc efflux is mediated by a zinc exporter known as ZntA (Zn + transport or tolerance), a membrane protein which was identified through studies of bacterial strains that were hypersensitive to zinc and cadmium. Sequence inspection revealed that ZntA was a member of the family of cation transport P-type ATPases, a major family of ion-translocating membrane proteins in which ATPase activity in one portion of the protein is used to phophorylate an aspartate within a highly conserved amino acid sequence, DKTG, in another portion of the protein. The cysteine rich N-terminus of these soft metal transport proteins contains several metal-binding sites. How the chemical energy released by ATP hydrolysis results in metal ion transport is not yet known, in part because there is only partial information about the structures of these proteins. The bacterial zinc exporter also pumps cadmium and lead and is therefore also involved in protection from heavy metal toxicity (see Metal Ion Toxicity). 

Suzuki, K.T., H. Sunaga, S. Hatakeyama, Y. Sumi, and T. Suzuki. 1989. Differential binding of cadmium and copper to the same protein in a heavy metal tolerant species of mayfly (Baetis thermicus) larvae. Comp. Biochem. Physiol. 94C 99-103. 

Tolerance to metal stress relies on plant capacity to detoxify metals having entered cells. The postulated mechanisms involve biochemical detoxification, for example by binding to organic acids (especially citrate) or proteins like ferritin, metallothioneins and phytochelatins, and finally compart-mentalization of the metal within the cell. In most plant cells the vacuole comprises more than 80-90% of the cell volume and is acting as a central storage compartment for ions (Briat et al. 1999). 

Studies on mutagenesis by metal compounds may be complicated by inducible tolerance mechanisms in some cells. Metallothioneins (MT) are small cysteine-rich proteins which bind a number of metals with high affinity. The reader is referred to Chap. 5 in this volume for more detail. Besides cadmium, zinc, and copper salts, a number of other metal salts have also been shown to induce MT synthesis and/or to bind to the MT protein. Hg(II), Co(II), and Ni(II), but not Pb(II), induce MT synthesis in primary cultures of rat hepatocytes (Bracken and Klaassen 1987), and may do so in the cultured cells used for mammalian mutagenesis experiments. Thus, under some experimental protocols, the metal salt being assayed may itself induce metallothionein, or the results of mutagenicity assays may be confounded by components of the medium or serum which affect the levels of metallothionein in the cell (Rossman and Goncharova, unpublished). 

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