Growth inhibitory factor/metallothionein-3 is a sulfane sulfur-binding protein

Cysteine-bound sulfane sulfur atoms in proteins have received much attention as key factors in cellular redox homeostasis. However, the role of sulfane sulfur in zinc regulation has been overlooked. We report here that cysteine-bound sulfane sulfur atoms serve as ligands to hold and release zinc ions in growth inhibitory factor (GIF)/metallothionein-3 (MT3) with an unexpected C–S–S–Zn structure. Oxidation of such a zinc/persulfide cluster in Zn₇GIF/MT3 results in the release of zinc ions, and intramolecular tetrasulfide bridges in apo-GIF/MT3 efficiently undergo S–S bond cleavage by thioredoxin to regenerate Zn₇GIF/MT3. Three-dimensional molecular modeling confirmed the critical role of the persulfide group in the thermostability and Zn-binding affinity of GIF/MT3. The present discovery raises the fascinating possibility that the function of other Zn-binding proteins is controlled by sulfane sulfur.