Cell 157, 1203–1215 (2014)

The redox state of a cell is thought to be under circadian control, but the exact impact of redox regulation on the circadian clock transcriptional program remains unclear. Members of this program, Period (PER) and Cryptochrome (CRY) proteins, form a complex that represses the CLOCK/BMAL1–mediated activation of clock genes. Schmalen et al. determined the crystal structure of the photolyase domain of mouse CRY1 with a C-terminal fragment of PER2. Among the dimer interfaces observed, the authors identified a three-cysteine and one-histidine arrangement that coordinates a zinc ion. Interestingly, the previously reported apo-CRY1 crystal structure contained a disulfide bridge in the vicinity of the zinc interface, and CRY1 undergoes oxidation in cells. This bridge then undergoes reduction upon CRY1–PER2 complex formation. Mutation of the CRY1 residues involved in the zinc interface resulted in decreased PER2 binding, whereas loss of disulfide bond formation slightly increased PER2 binding, suggesting that the transition from the disulfide bond to zinc coordination ensures PER2–CRY1 complex formation. Consistent with this, decreased PER2–CRY1 binding resulting from the loss of the zinc interface can be rescued by preventing disulfide bond formation. Finally, zinc directly interacted with and enhanced CRY1–PER2 complex formation and was dependent on the presence of the zinc-coordinating residues for binding. The release of zinc promoted the oxidation of CRY1 and restoration of the disulfide bonds. These results provide an intriguing link between redox regulation and circadian clock activity.