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Migratory birds have a light-dependent magnetic compass, the mechanism of which is thought to involve radical pairs formed photochemically in cryptochrome proteins in the retina. Theoretical descriptions of this compass have thus far been unable to account for the high precision with which birds are able to detect the direction of the Earth’s magnetic field. Here we use coherent spin dynamics simulations to explore the behavior of realistic models of cryptochrome-based radical pairs. We show that when the spin coherence persists for longer than a few microseconds, the output of the sensor contains a sharp feature, referred to as a spike. The spike arises from avoided crossings of the quantum mechanical spin energy-levels of radicals formed in cryptochromes. Such a feature could deliver a heading precision sufficient to explain the navigational behavior of migratory birds in the wild. Our results (i) afford new insights into radical pair magnetoreception, (ii) suggest ways in which the performance of the compass could have been optimized by evolution, (iii) may provide the beginnings of an explanation for the magnetic disorientation of migratory birds exposed to anthropogenic electromagnetic noise, and (iv) suggest that radical pair magnetoreception may be more of a quantum biology phenomenon than previously realized.

Cryptochromes are blue-light receptors that regulate development and the circadian clock in plants and animals. We found that Arabidopsis cryptochrome 2 (CRY2) undergoes blue light-dependent homodimerization to become physiologically active. We identified BIC1 (blue-light inhibitor of cryptochromes 1) as an inhibitor of plant cryptochromes that binds to CRY2 to suppress the blue light-dependent dimerization, photobody formation, phosphorylation, degradation, and physiological activities of CRY2. We hypothesize that regulated dimerization governs homeostasis of the active cryptochromes in plants and other evolutionary lineages.

A modular optogenetic method for higher-order protein oligomerization uses a single cryptochrome 2-based fusion for rapid, reversible and tunable oligomerization in response to blue light. Inducible aggregation can be used to specifically activate different signaling pathways. We report an optogenetic method based on Arabidopsis thaliana cryptochrome 2 for rapid and reversible protein oligomerization in response to blue light. We demonstrated its utility by photoactivating the β-catenin pathway, achieving a transcriptional response higher than that obtained with the natural ligand Wnt3a. We also demonstrated the modularity of this approach by photoactivating RhoA with high spatiotemporal resolution, thereby suggesting a previously unknown mode of activation for this Rho GTPase.

Protein modules that dimerize rapidly upon exposure to light are reported. They permit light-induced control of dimerization of fused protein targets and can be manipulated with two-photon illumination for experiments in thick samples and in vivo . Dimerizers allowing inducible control of protein-protein interactions are powerful tools for manipulating biological processes. Here we describe genetically encoded light-inducible protein-interaction modules based on Arabidopsis thaliana cryptochrome 2 and CIB1 that require no exogenous ligands and dimerize on blue-light exposure with subsecond time resolution and subcellular spatial resolution. We demonstrate the utility of this system by inducing protein translocation, transcription and Cre recombinase–mediated DNA recombination using light.

Cryptochromes are highly conserved blue-light-absorbing flavoproteins which have been linked to the perception of electromagnetic stimuli in numerous organisms. These include sensing the direction of the earth’s magnetic field in migratory birds and the intensity of magnetic fields in insects and plants. When exposed to light, cryptochromes undergo flavin reduction/reoxidation redox cycles leading to biological activation which generate radical pairs thought to be the basis for magnetic sensitivity. However, the nature of the magnetically sensitive radical pairs and the steps at which they act during the cryptochrome redox cycle are currently a matter of debate. Here, we investigate the response of Arabidopsis cryptochrome-1 in vivo to a static magnetic field of 500 µT (10 × earth’s field) using both plant growth and light-dependent phosphorylation as an assay. Cryptochrome responses to light were enhanced by the magnetic field, as indicated by increased inhibition of hypocotyl elongation and increased cryptochrome phosphorylation. However, when light and dark intervals were given intermittently, a plant response to the magnetic field was observed even when the magnetic field was given exclusively during the dark intervals between light exposures. This indicates that the magnetically sensitive reaction step in the cryptochrome photocycle must occur during flavin reoxidation, and likely involves the formation of reactive oxygen species.

Cryptochromes (CRY) are flavoproteins that direct a diverse array of developmental processes in response to blue light in plants. Conformational changes in CRY are induced by the absorption of photons and result in the propagation of light signals to downstream components. In Arabidopsis, CRY1 and CRY2 serve both distinct and partially overlapping functions in regulating photomorphogenic responses and photoperiodic flowering. For example, both CRY1 and CRY2 regulate the abundance of transcription factors by directly reversing the activity of E3 ubiquitin ligase on CONSTITUTIVE PHOTOMORPHOGENIC 1 and SUPPRESSOR OF PHYA-105 1 complexes in a blue light-dependent manner. CRY2 also specifically governs a photoperiodic flowering mechanism by directly interacting with a transcription factor called CRYPTOCHROME-INTERACTING BASIC-HELIX-LOOP-HELIX. Recently, structure/function analysis of CRY1 revealed that the CONSTITUTIVE PHOTOMORPHOGENIC 1 independent pathway is also involved in CRY1-mediated inhibition of hypocotyl elongation. CRY1 and CRY2 thus not only share a common pathway but also relay light signals through distinct pathways, which may lead to altered developmental programs in plants.

Issue Title: Including the JPR Symposium on The Cutting Edge of Photoresponse Mechanisms: Photoreceptor and Signaling Mechanism

Drosophila CRYPTOCHROME (dCRY) mediates electrophysiological depolarization and circadian clock resetting in response to blue or ultraviolet (UV) light. These light-evoked biological responses operate at different timescales and possibly through different mechanisms. Whether electron transfer down a conserved chain of tryptophan residues underlies biological responses following dCRY light activation has been controversial. To examine these issues in in vivo and in ex vivo whole-brain preparations, we generated transgenic flies expressing tryptophan mutant dCRYs in the conserved electron transfer chain and then measured neuronal electrophysiological phototransduction and behavioral responses to light. Electrophysiological-evoked potential analysis shows that dCRY mediates UV and blue-light–evoked depolarizations that are long lasting, persisting for nearly a minute. Surprisingly, dCRY appears to mediate red-light–evoked depolarization in wild-type flies, absent in both cry-null flies, and following acute treatment with the flavin-specific inhibitor diphenyleneiodonium in wild-type flies. This suggests a previously unsuspected functional signaling role for a neutral semiquinone flavin state (FADH•) for dCRY. The W420 tryptophan residue located closest to the FAD-dCRY interaction site is critical for blue- and UV-light–evoked electrophysiological responses, while other tryptophan residues within electron transfer distance to W420 do not appear to be required for light-evoked electrophysiological responses. Mutation of the dCRY tryptophan residue W342, more distant from the FAD interaction site, mimics the cry-null behavioral light response to constant light exposure. These data indicate that light-evoked dCRY electrical depolarization and clock resetting are mediated by distinct mechanisms.

Cryptochromes are flavoproteins that drive diverse developmental light-responses in plants and participate in the circadian clock in animals. Plant cryptochromes have found application as photoswitches in optogenetics. We have studied effects of pH and ATP on the functionally relevant photoreduction of the oxidized FAD cofactor to the semi-reduced FADH · radical in isolated Arabidopsis cryptochrome 1 by transient absorption spectroscopy on nanosecond to millisecond timescales. In the absence of ATP, the yield of light-induced radicals strongly decreased with increasing pH from 6.5 to 8.5. With ATP present, these yields were significantly higher and virtually pH-independent up to pH 9. Analysis of our data in light of the crystallographic structure suggests that ATP-binding shifts the p K a of aspartic acid D396, the putative proton donor to FAD· − , from ~7.4 to >9 and favours a reaction pathway yielding long-lived aspartate D396 − . Its negative charge could trigger conformational changes necessary for signal transduction.

Cryptochromes (CRYs) are an ubiquitously occurring class of photoreceptors, which are important for regulating the circadian rhythm of animals via a time-delayed transcription-translation feedback loop (TTFL). Due to their protein architecture and common FAD chromophore, they belong to the same superfamily as photolyases (PHLs), an enzyme class that repairs UV-induced DNA lesions upon blue light absorption. Apart from their different functions the only prominent structural difference between CRY and PHL is the highly variable C-terminal extension (CTE) of the former. The nature of the CTE is still unclear and highly speculated. In this study, we show by hydrogen/deuterium exchange and subsequent mass-spectrometric analysis that the CTE of the animal-like cryptochrome from the green algae Chlamydomonas reinhardtii ( Cr aCRY) binds to the surface of the photolyase homology region, which flanks the DNA binding site. We also compared the fully oxidized and fully reduced states of the flavoprotein and designed a tool, so called light chamber, for automated HDX-MS measurements of photoreceptors in defined photostates. We could observe some striking differences between the two photostates and propose a model for light-dependent switching of this bifunctional cryptochrome.