Metal-binding characteristics of the protein which shows the highest histidine content in the Arabidopsis genome

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It is well documented that metal-binding peptides, such as phytochelatins and metallothioneins, are involved in metal homeostasis and tolerance in plants. These peptides bind metals by means of the thiol groups of cysteine residues. Histidine is also known to be a metal-binding residue. It has been demonstrated that microorganisms and mammals possess histidine-rich metal-binding peptides for the storage and homeostasis of metals. In plants, however, only several examples which describe the characteristics of the histidine-rich metal binding peptides have been reported. We therefore searched for histidine-rich peptides in the Arabidopsis database. Here, we describe a candidate gene designated Arabidopsis thaliana histidine-rich peptide 1 (AtHIRP1). AtHIRP1, which belongs to a small auxin-up RNA (SAUR) family in Arabidopsis, shows the highest histidine content (19.7% of total amino acid residues) in the Arabidopsis genome. The recombinant AtHIRP1 apparently bound to Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, but weakly to Cd²⁺. In the case of the AtHIRP1-Zn²⁺ binding, the dissociation constant was 0.58 μM and the maximum binding capacity was 12 mol Zn²⁺ per 1 mol AtHIRP1. The accumulation of AtHIRP1 transcripts increased by drought stresses. These results suggest that AtHIRP1 is a metal-binding peptide which may function in plants exposed to abiotic stresses.