Metal tolerance protein

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. 

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. 

Cellular responses to metal ions include changes of patterns of gene expression. Many of these changes are non-specific shock effects for example, subsets of the heat shock proteins are synthesised in response to Cd (Czarnecka et al., 1984 Lin et al., 1984 Delhaize et al., 1989). However, some mRNAs are specifically induced by Cd (Delhaize et al., 1989). Thus, by examining metal-regulated gene expression, whether specific or non-specific, it may be possible to determine the relative roles of proteins and polypeptides specific to metal homeostasis and metal tolerance, as opposed to functions involved in general stress responses. 

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). 

The bZIP protein family is the third prominent group of TFs playing a vital role in PDR. Of the several YAP genes in yeast, the basic leucine zipper transcription factors Yapl, Yap2, and Yap8 are linked to oxidative stress response [98,120], vacuolar detoxification, and heavy metal tolerance. 

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]. 

This ability to grow in polluted soils and withstand high heavy metal concentrations rests on complex mechanisms involving both avoidance through exclusion of metal ions from the cytoplasm and tolerance of high internal metal concentrations (126), this being often dependent on the induction of specific genes and proteins (126,127). 

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