Toward a property/function relationship for metallothioneins: Histidine coordination and unusual cluster composition in a zinc‐metallothionein from plants

Abstract

Early cysteine labeled (E(C)) proteins are plant metallothioneins, which were first identified in wheat embryos and are thought to be seed-specific. An exhaustive analysis of expressed sequence tag (EST) entries reveals that homologs are expressed in embryos of both classes of flowering plants (monocotyledons and dicotyledons), but also occur in conifers (gymnosperms) and seed-free spike moss (lycophyta). Mass spectrometric and elemental analysis results indicate that, contrary to the widely propagated number of five, E(C) binds predominantly six zinc ions in at least two zinc-thiolate clusters. 1H and 111Cd NMR experiments suggest that, in contrast to the majority of previously characterized metallothioneins, two conserved histidine residues participate in metal binding. The collected data is consistent with the presence of clusters unprecedented in metallothioneins so far. This novel cluster composition is accompanied by metal-binding properties that are substantially different from other metallothioneins; thus wheat E(C) binds zinc less strongly than either mammalian or cyanobacterial MTs. The unique biochemical properties of wheat E(C) render it ideally suited for a role in zinc donation to nascent proteins during seed development, a role that has been suggested based on the fact that E(C) is induced by the plant hormone abscisic acid, but not by heavy metals. Our results provide a step further toward developing a property/function relationship for metallothioneins.

Examining the Specific Contributions of Individual Arabidopsis Metallothioneins to Copper Distribution and Metal Tolerance

“…The Cyssulfhydryl groups allow a wide range of both essential and nonessential metals to bind to MT proteins (Hamer, 1986). The arrangement of Cys residues is crucial in determining the metal-binding properties of MT proteins and their functions (Klaassen et al, 1999;Domenech et al, 2006;Leszczyszyn et al, 2007). In mammalian MTs, the positions of Cys residues are highly conserved.…”

“…Unlike other plant MTs, type-4 MTs contain three Cys-rich domains and two conserved His residues in the central Cys-rich domain (Cobbett and Goldsbrough, 2002). Results from NMR experiments indicate that the two conserved His residues in the wheat Ec protein participate with the Cys residues in metal binding or coordination, especially for Zn (Leszczyszyn et al, 2007). His is frequently used for Zn coordination in other metalloproteins, including those that contain Zn-finger structures (Blindauer and Sadler, 2005).…”

Structural features specific to plant metallothioneins

“…Taken together, it is more than legitimate to say that such an arrangement of the three Cys-rich regions would have been more than difficult to predict with theoretical methods, especially given the novelty of the structural motifs. Nevertheless, the basic motif of a two-domain protein formed by the Nterminal Cys-rich region and the combined central and C-terminal Cys-rich regions was already predicted based on proteolytic digestion experiments as the main method [56] and also contemplated while studying pH-dependent Zn II release from the full-length protein with ESI-MS [58].…”

Characteristic and Expression Analysis of a Metallothionein Gene, OsMT2b, Down-Regulated by Cytokinin Suggests Functions in Root Development and Seed Embryo Germination of Rice

“…For plant, MTs can efficiently bind metals (Murphy et al, 1997;Leszczyszyn et al, 2007) and some of them are transcriptionally regulated by metals; they are also thought to play important roles in metal tolerance, detoxification, and homeostasis in plants (Cobbett and Goldsbrough, 2002;Hall, 2002;Hassinen et al, 2007). On the other hand, there were also intensive studies for the responses of MTs to stress conditions, such as draft and reactive oxidant species (Akashi et al, 2004;Wong et al, 2004).…”