Rapid Oxidation Following Photoreduction in the Avian Cryptochrome4 Photocycle

Possibility of two-dimensional ordering of cryptochrome 4a from European robin

Cryptochrome (Cry) in some species could act as a quantum senser to detect the inclination angle of geomagnetic field, the function of which attributes the magnetic sensitivity of spins of unpaired electrons in radical pair (RP) in CRY generated by blue light irradiation. However, the effect of blue light on the structure and molecular behavior of Cry has not been well investigated. We conducted the size exclusion chromatography (SEC) and small-angle X-ray scattering (SAXS) analyses to inspect the molecular structure and behavior of cryptochrome 4a (ErCry4a) from European robin, a representative magnetosensory animal. The results indicated that ErCry4a could form flat-shape oligomers. Moreover, blue light irradiation induced the contraction of the ErCry4a molecule at the monomer scale and simultaneously accelerated the two-dimensional oligomerization of ErCry4a. This oligomerization may enhance the regularity of the two-dimensional arrangement of ErCry4a molecules, providing a positive effect for detecting the inclination angle.

Structural Rearrangements of Pigeon Cryptochrome 4 Undergoing a Complete Redox Cycle

Cryptochrome is currently the major contender of a protein to underpin magnetoreception, the ability to sense the Earth's magnetic field. Among various types of cryptochromes, cryptochrome 4 has been identified as the likely magnetoreceptor in migratory birds. All-atom molecular dynamics (MD) studies have offered first insights into the structural dynamics of cryptochrome but are limited to a short time scale due to large computational demands. Here, we employ coarse-grained MD simulations to investigate the emergence of long-lived states and conformational changes in pigeon cryptochrome 4. Our coarse-grained simulations complete the picture by permitting observation on a significantly longer time scale. We observe conformational transitions in the phosphate-binding loop of pigeon cryptochrome 4 upon activation and identify prominent motions in residues 440-460, suggesting a possible role as a signaling state of the protein or as a gated interaction site for forming protein complexes that might facilitate downstream processes. The findings highlight the importance of considering longer time scales in studying cryptochrome dynamics and magnetoreception. Coarse-grained MD simulations offer a valuable tool to unravel the complex behavior of cryptochrome proteins and shed new light on the mechanisms underlying their role in magnetoreception. Further exploration of these conformational changes and their functional implications may contribute to a deeper understanding of the molecular mechanisms of magnetoreception in birds.

Effects of inter-radical interactions and scavenging radicals on magnetosensitivity: spin dynamics simulations of proposed radical pairs

Although the magnetosensitivity to weak magnetic fields, such as the geomagnetic field, which was exhibited by radical pairs that are potentially responsible for avian navigation, has been previously investigated by spin dynamics simulations, understanding this behavior for proposed radical pairs in other species is limited. These include, for example, radical pairs formed in the single-cell green alga Chlamydomonas reinhardtii (CraCRY) and in Columba livia (ClCRY4). In addition, the radical pair of FADH^• with the one-electron reduced cyclobutane thymine dimer that was shown to be sensitive to weak magnetic fields has been of interest. In this work, we investigated the directional magnetosensitivity of these radical pairs to a weak magnetic field by spin dynamics simulations. We find significant reduction in the magnetosensitivity by inclusion of dipolar and exchange interactions, which can be mitigated by a scavenging radical, as demonstrated for the [FAD^•- TyrD^•] radical pair in CraCRY, but not for the [FADH^• T□T^•-] radical pair because of the large exchange coupling. The directional magnetosensitivity of the ClCRY4 [FAD^•- TyrE^•] radical pair can survive this adverse effect even without the scavenging reaction, possibly motivating further experimental exploration.

Hypomagnetic Fields and Their Multilevel Effects on Living Organisms

The Earth’s magnetic field is one of the basic abiotic factors in all environments, and organisms had to adapt to it during evolution. On some occasions, organisms can be confronted with a significant reduction in a magnetic field, termed a “hypomagnetic field—HMF”, for example, in buildings with steel reinforcement or during interplanetary flight. However, the effects of HMFs on living organisms are still largely unclear. Experimental studies have mostly focused on the human and rodent models. Due to the small number of publications, the effects of HMFs are mostly random, although we detected some similarities. Likely, HMFs can modify cell signalling by affecting the contents of ions (e.g., calcium) or the ROS level, which participate in cell signal transduction. Additionally, HMFs have different effects on the growth or functions of organ systems in different organisms, but negative effects on embryonal development have been shown. Embryonal development is strictly regulated to avoid developmental abnormalities, which have often been observed when exposed to a HMF. Only a few studies have addressed the effects of HMFs on the survival of microorganisms. Studying the magnetoreception of microorganisms could be useful to understand the physical aspects of the magnetoreception of the HMF.

Midnight/midday-synchronized expression of cryptochrome genes in the eyes of three teleost species, zebrafish, goldfish, and medaka

Photoperiodic responses are observed in many organisms living in the temperate zones. The circadian clock is involved in photoperiodic time measurement; however, the underlying molecular mechanism for detection of the day length remains unknown. We previously compared the expression profiles of the Cryptochrome(Cry) genes in the zebrafish eye and reported that Cry1ab has a double peak with variable expression duration depending on the photoperiod. In this study, to understand commonalities and differences in the photoperiodic responses of ocular Cry genes, we identified Cryptochrome genes in two other teleost species, goldfish and medaka, living in temperate zones, and measured ocular Cry mRNA levels in all of the three species, under different photoperiods (long-day [14 h light: 10 h dark] and short-day [10 h light: 14 h dark] and in constant darkness. Cry1ab mRNA levels did not show dual peaks in goldfish or medaka under the examined conditions; however, the mRNA expression profiles of many Crys were altered in all three species, depending on the day length and light condition. Based on their expression profiles, Cry mRNA peaks were classified into three groups that better synchronize to sunrise (light-on), midnight/midday (middle points of the dark/light periods), or sunset (light-off). These results suggest the presence of multiple oscillators that oscillate independently or a complex oscillator in which Cry expression cycles change in a photoperiod-dependent manner in the eye.

Cryptochrome magnetoreception: four tryptophans could be better than three

The biophysical mechanism of the magnetic compass sensor in migratory songbirds is thought to involve photo-induced radical pairs formed in cryptochrome (Cry) flavoproteins located in photoreceptor cells in the eyes. In Cry4a-the most likely of the six known avian Crys to have a magnetic sensing function-four radical pair states are formed sequentially by the stepwise transfer of an electron along a chain of four tryptophan residues to the photo-excited flavin. In purified Cry4a from the migratory European robin, the third of these flavin-tryptophan radical pairs is more magnetically sensitive than the fourth, consistent with the smaller separation of the radicals in the former. Here, we explore the idea that these two radical pair states of Cry4a could exist in rapid dynamic equilibrium such that the key magnetic and kinetic properties are weighted averages. Spin dynamics simulations suggest that the third radical pair is largely responsible for magnetic sensing while the fourth may be better placed to initiate magnetic signalling particularly if the terminal tryptophan radical can be reduced by a nearby tyrosine. Such an arrangement could have allowed independent optimization of the essential sensing and signalling functions of the protein. It might also rationalize why avian Cry4a has four tryptophans while Crys from plants have only three.