In-sights into the effect of heavy metal stress on the endogenous mustard cystatin

Abstract

Phytocystatins have been ascribed several protective roles against abiotic and biotic stress conditions. It was, therefore, thought worthwhile to document the effect of heavy metal stress on the endogenous plant cystatin. The mustard cystatin, purified from Brassica juncea (B. juncea) seeds retained its functional property of cysteine proteinase inhibition, despite exposure to high concentrations of metal ions, Cd2+ and Ni2+. An increase in inhibitory activity, ∼26% for Ni2+ and ∼16% for Cd2+ was observed, suggesting changes in protein conformation upon metal ion interaction. Isothermal calorimetric (ITC) studies show formation of a 1:1 binary complex on interaction with both metal ions but suggest a higher affinity for Ni2+. Fluorescence quenching data suggest a static quenching mechanism of interaction. Various spectroscopic analyses, namely, synchronous fluorescence, ANS fluorescence, far UV CD and ATR-FTIR spectroscopy show that the native mustard cystatin acquires a more ordered conformation upon interaction with metal ions. Differential Scanning Calorimetry indicates that the thermo-stability of the Ni2+ bound protein (Tm=109.4°C) is greater than both, the Cd2+ bound (Tm=104.5°C) and the native (Tm=99.5°C) forms. The B. juncea seed cystatin, is thus, identified as a potent and resilient member of the phytocystatin family with considerable inhibitory capacity despite exposure to heavy metal stress.