The Human Circadian Clock Protein Cryptochrome-1 Responds to Photochemistry by Conformational Changes.

The human clock protein cryptochrome-1 (HsCRY1) can be targeted by small molecular modulators to shift the 24 h rhythm, opening up possibilities for treating circadian disorders. GO1323 is a photopharmacological compound that selectively binds to HsCRY1 and enables light-controlled manipulation of the circadian rhythm via photoisomerization. We synthesized isotope-labeled and unlabeled GO1323 to investigate conformational changes in full-length HsCRY1 that are caused by isomerization of GO1323 using infrared difference spectroscopy. Activation of GO1323 induces specific changes in secondary structure including a rearrangement of the β-sheet, which are surprisingly similar to those observed in the activation of evolutionarily separated plant cryptochromes by flavin photoreduction. Accordingly, we investigated the response of HsCRY1 to flavin photochemistry despite the reported low affinity to oxidized flavin. We revealed changes in secondary structure only during the transition from the flavin neutral radical to the fully reduced flavin, which requires the sequential absorption of two photons. Difference spectra suggest a similar HsCRY1 activation by GO1323 isomerization and by flavin photoreduction. Therefore, we present a model in which HsCRY1 acts as a bifunctional protein, responding to interaction partners and flavin photoreduction as a high light receptor.