Animal Cryptochromes: Divergent Roles in Light Perception, Circadian Timekeeping and Beyond

Abstract

Cryptochromes are evolutionarily related to the light-dependent DNA repair enzyme photolyase, serving as major regulators of circadian rhythms in insects and vertebrate animals. There are two types of cryptochromes in the animal kingdom: Drosophila-like CRYs that act as nonvisual photopigments linking circadian rhythms to the environmental light/dark cycle, and vertebrate-like CRYs that do not appear to sense light directly, but control the generation of circadian rhythms by acting as transcriptional repressors. Some animals have both types of CRYs, while others possess only one. Cryptochromes have two domains, the photolyase homology region (PHR) and an extended, intrinsically disordered C-terminus. While all animal CRYs share a high degree of sequence and structural homology in their PHR domains, the C-termini are divergent in both length and sequence identity. Recently, cryptochrome function has been shown to extend beyond its pivotal role in circadian clocks, participating in regulation of the DNA damage response, cancer progression and glucocorticoid signaling, as well as being implicated as possible magnetoreceptors. In this review, we provide a historical perspective on the discovery of animal cryptochromes, examine similarities and differences of the two types of animal cryptochromes and explore some of the divergent roles for this class of proteins.

Figure 1. A model for the dCRY C-terminal tail in the Drosophila clock

(a) Schematic model for the light-responsive degradation of dCRY and TIM. Light stimulates a structural change in dCRY that exposes the binding interface for TIM and JETLAG. This interaction results in the polyubiquitination (depicted as orange circles) and proteasomal degradation of TIM. The same light-induced conformational change in dCRY also renders it sensitive to polyubiquitination by BRWD3. (b) Structure of full-length dCRY (PDB: 4GU5) in the dark-adapted state. A hydrophobic motif in the C-terminal extension (gray) docks onto the PHR domain (purple) in close proximity to the flavin (pink).

Figure 2. Comparison of C-terminal tail extensions in representative photolyase and cryptochrome proteins

(a) Schematic representation of the domains present in E. coli photolyase, Drosophila CRY, honeybee CRY2, and mouse CRY1. The overall structure and organization of the PHR domain remains relatively unchanged between the different proteins, but the C-terminal tails vary in length. (b) Sequences of the unstructured C-terminal extensions of Drosophila CRY, honeybee CRY2, and mouse CRY1. Amino acids are colored according to their physicochemical properties using the Jalview Zappo coloring scheme (138): pink, aliphatic/hydrophobic; gold, aromatic; purple, positive; red, negative; green, hydrophilic; light purple, conformationally special; yellow, cysteine.

Figure 3. Roles of CRY outside of CLOCK:BMAL1 regulation

In mammals, CRYs negatively regulate CLOCK:BMAL1 activity to generate a ~24 hour clock that regulates ~40% of the genome (88). CRY is also reported to regulate GPCR signaling and downstream metabolism through interaction with the G_sα subunit to block glucagon-stimulated increases in intracellular cAMP (top left). CRY negatively regulates the glucocorticoid receptor to maintain glucose homeostasis, partly through regulation of Pck1 expression (top right). Interaction of CRY with components of the ATR-mediated DNA damage checkpoint control phase shifting of the clock in response to DNA damage (bottom left). While ablation of the SCN increases tumor formation in mouse models, deletion of cryptochromes extends lifespan after ionizing radiation in a p53 null background (bottom right).