Cellular Metabolites Enhance the Light Sensitivity of Arabidopsis Cryptochrome through Alternate Electron Transfer Pathways |
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Authors: | Christopher Engelhard Xuecong Wang David Robles Julia Moldt Lars-Oliver Essen Alfred Batschauer Robert Bittl Margaret Ahmad |
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Affiliation: | aFachbereich Physik, Free University, 14195 Berlin, Germany;bUniversity of Paris VI, UMR 8256 (B2A), IBPS, 75005 Paris, France;cDepartment of Plant Physiology and Photobiology, Faculty of Biology, Philipps-University, 35032 Marburg, Germany;dBiomedical Research Centre/Faculty of Chemistry, Philipps-University, 35032 Marburg, Germany;eXavier University, Cincinatti, Ohio 45207 |
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Abstract: | Cryptochromes are blue light receptors with multiple signaling roles in plants and animals. Plant cryptochrome (cry1 and cry2) biological activity has been linked to flavin photoreduction via an electron transport chain comprising three evolutionarily conserved tryptophan residues known as the Trp triad. Recently, it has been reported that cry2 Trp triad mutants, which fail to undergo photoreduction in vitro, nonetheless show biological activity in vivo, raising the possibility of alternate signaling pathways. Here, we show that Arabidopsis thaliana cry2 proteins containing Trp triad mutations indeed undergo robust photoreduction in living cultured insect cells. UV/Vis and electron paramagnetic resonance spectroscopy resolves the discrepancy between in vivo and in vitro photochemical activity, as small metabolites, including NADPH, NADH, and ATP, were found to promote cry photoreduction even in mutants lacking the classic Trp triad electron transfer chain. These metabolites facilitate alternate electron transfer pathways and increase light-induced radical pair formation. We conclude that cryptochrome activation is consistent with a mechanism of light-induced electron transfer followed by flavin photoreduction in vivo. We further conclude that in vivo modulation by cellular compounds represents a feature of the cryptochrome signaling mechanism that has important consequences for light responsivity and activation. |
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