Zinc-binding and structural properties of the histidine-rich loop of Arabidopsis thaliana vacuolar membrane zinc transporter MTP1

The vacuolar Zn2+/H+ antiporter of Arabidopsis thaliana, AtMTP1, has a cytosolic histidine-rich loop (His-loop). We characterized the structures and Zn2+-binding properties of the His-loop and other domains. Circular dichroism analyses revealed that the His-loop partly consists of a polyproline type II structure and that its conformational change is induced by Zn2+ as well as the C-terminal domain. Isothermal titration calorimetry of the His-loop revealed a binding number of four Zn2+ per molecule. Numbers of Ni and Co associated with the His-loop were approximately one ion per molecule and the thermodynamic parameters of the association with these ions were different from that of Zn2+. These results suggest the involvement of the His-loop in sensing cytosolic Zn2+ and in the regulation of zinc transport activity through Zn2+-induced structural change.