Effects of Fe3+ and Zn2+ on the structural and thermodynamic properties of a soybean ASR protein

Abscisic acid-, stress-, and ripening-induced (ASR) protein play important roles in protecting plants from abiotic stress. The functions of some ASR proteins are known to be modulated by binding to metal ions. In this study, we demonstrated that the non-tagged full-length soybean (Glycine max) ASR protein (GmASR) can bind Fe(3+), Ni(2+), Cu(2+), and Zn(2+). The direct binding properties of GmASR to Fe(3+) and Zn(2+) were further confirmed by intrinsic fluorescence assays. The GmASR protein was found to have three Fe(3+) binding sites but only two Zn(2+) binding sites. Natively disordered in aqueous solution, GmASR remained disordered in the presence of Fe(3+), but was found to aggregate in the presence of Zn(2+). The aggregated GmASR protein was partially resolubilized after Zn(2+) was chelated by EDTA. GmASR exhibited Fe(3+)-binding-dependent antioxidant activity in vitro. We speculate that GmASR thus protects against oxidation damage by buffering metal ions, thus alleviating metal toxicity in plant cells under stressed conditions.