Human cryptochrome-1 confers light independent biological activity in transgenic Drosophila correlated with flavin radical stability.

Cryptochromes are conserved flavoprotein receptors found throughout the biological kingdom with diversified roles in plant development and entrainment of the circadian clock in animals. Light perception is proposed to occur through flavin radical formation that correlates with biological activity in vivo in both plants and Drosophila. By contrast, mammalian (Type II) cryptochromes regulate the circadian clock independently of light, raising the fundamental question of whether mammalian cryptochromes have evolved entirely distinct signaling mechanisms. Here we show by developmental and transcriptome analysis that Homo sapiens cryptochrome--1 (HsCRY1) confers biological activity in transgenic expressing Drosophila in darkness, that can in some cases be further stimulated by light. In contrast to all other cryptochromes, purified recombinant HsCRY1 protein was stably isolated in the anionic radical flavin state, containing only a small proportion of oxidized flavin which could be reduced by illumination. We conclude that animal Type I and Type II cryptochromes may both have signaling mechanisms involving formation of a flavin radical signaling state, and that light independent activity of Type II cryptochromes is a consequence of dark accumulation of this redox form in vivo rather than of a fundamental difference in signaling mechanism.

The cryptochromes: blue light photoreceptors in plants and animals.

Cryptochromes are flavoprotein photoreceptors first identified in Arabidopsis thaliana, where they play key roles in growth and development. Subsequently identified in prokaryotes, archaea, and many eukaryotes, cryptochromes function in the animal circadian clock and are proposed as magnetoreceptors in migratory birds. Cryptochromes are closely structurally related to photolyases, evolutionarily ancient flavoproteins that catalyze light-dependent DNA repair. Here, we review the structural, photochemical, and molecular properties of cry-DASH, plant, and animal cryptochromes in relation to biological signaling mechanisms and uncover common features that may contribute to better understanding the function of cryptochromes in diverse systems including in man.

Conformational change induced by ATP binding correlates with enhanced biological function of Arabidopsis cryptochrome.

Cryptochromes are widely distributed blue light photoreceptors involved in numerous signaling functions in plants and animals. Both plant and animal-type cryptochromes are found to bind ATP and display intrinsic autokinase activity; however the functional significance of this activity remains a matter of speculation. Here we show in purified preparations of Arabidopsis cry1 that ATP binding induces conformational change independently of light and increases the amount and stability of light-induced flavin radical formation. Nucleotide binding may thereby provide a mechanism whereby light responsivity in organisms can be regulated through modulation of cryptochrome photoreceptor conformation.

Human and Drosophila cryptochromes are light activated by flavin photoreduction in living cells.

Cryptochromes are a class of flavoprotein blue-light signaling receptors found in plants, animals, and humans that control plant development and the entrainment of circadian rhythms. In plant cryptochromes, light activation is proposed to result from photoreduction of a protein-bound flavin chromophore through intramolecular electron transfer. However, although similar in structure to plant cryptochromes, the light-response mechanism of animal cryptochromes remains entirely unknown. To complicate matters further, there is currently a debate on whether mammalian cryptochromes respond to light at all or are instead activated by non-light-dependent mechanisms. To resolve these questions, we have expressed both human and Drosophila cryptochrome proteins to high levels in living Sf21 insect cells using a baculovirus-derived expression system. Intact cells are irradiated with blue light, and the resulting cryptochrome photoconversion is monitored by fluorescence and electron paramagnetic resonance spectroscopic techniques. We demonstrate that light induces a change in the redox state of flavin bound to the receptor in both human and Drosophila cryptochromes. Photoreduction from oxidized flavin and subsequent accumulation of a semiquinone intermediate signaling state occurs by a conserved mechanism that has been previously identified for plant cryptochromes. These results provide the first evidence of how animal-type cryptochromes are activated by light in living cells. Furthermore, human cryptochrome is also shown to undergo this light response. Therefore, human cryptochromes in exposed peripheral and/or visual tissues may have novel light-sensing roles that remain to be elucidated.

Evidence of a light-sensing role for folate in Arabidopsis cryptochrome blue-light receptors.

Arabidopsis cryptochromes cry1 and cry2 are blue-light signalling molecules with significant structural similarity to photolyases--a class of blue-light-sensing DNA repair enzymes. Like photolyases, purified plant cryptochromes have been shown to bind both flavin and pterin chromophores. The flavin functions as a light sensor and undergoes reduction in response to blue light that initiates the signalling cascade. However, the role of the pterin in plant cryptochromes has until now been unknown. Here, we show that the action spectrum for light-dependent degradation of cry2 has a significant peak of activity at 380 nm, consistent with absorption by a pterin cofactor. We further show that cry1 protein expressed in living insect cells responds with greater sensitivity to 380 nm light than to 450 nm, consistent with a light-harvesting antenna pigment that transfers excitation energy to the oxidized flavin of cry1. The pterin biosynthesis inhibitor DHAP selectively reduces cryptochrome responsivity at 380 nm but not 450 nm blue light in these cell cultures, indicating that the antenna pigment is a folate cofactor similar to that of photolyases.