Structure of European robin cryptochrome 1 reveals a role in circadian rhythms, not magnetoreception.

Cryptochromes (CRYs) play critical roles in regulating diverse physiological functions, including circadian rhythms and neuronal firing in light-dependent or -independent fashions. Structural studies of CRYs have highlighted common features, such as the photolyase homology region (PHR), but they also reveal key differences, particularly in the binding of the flavin adenine dinucleotide (FAD) cofactor, leading to a long-standing debate, namely, whether Type I CRYs can function as FAD-dependent photosensors. This study solves the first crystal structure of a Type II CRY from a migratory songbird, namely, the European robin (Erithacus rubecula) CRY1. Structural, biochemical, and computational analyses of erCRY1 reveal that, unlike light-activated Type I and IV CRYs, Type II CRYs do not bind FAD and employ an open primary pocket for protein-protein interactions. These findings offer new insights into the structural basis of CRY function and suggest that migratory song-bird Type II CRYs function as circadian regulators, not magnetoreceptors.