A Protein's Magnetic Personality

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Science  13 Apr 2012:
Vol. 336, Issue 6078, pp. 132
DOI: 10.1126/science.336.6078.132-c

Cryptochromes, identified in plants as bluelight photoreceptors, may have yet another function to add to their résumé. The proteins are known to help regulate circadian rhythm and development and are found in plants, insects, and animals. Sequence similarities also link cryptochromes to bacterial DNA photolyases. Maeda et al. have now analyzed how redox signaling through the cofactor flavin adenine dinucleotide (FAD) can make these proteins function as magnetoreceptors. Studying a cryptochrome from Arabidopsis and a DNA photolyase from Escherichia coli, the authors spectroscopically measured microsecond radical-pair kinetics. In response to light, electron transfer from tryptophan residues generates a FAD radical anion. Imposition of a 28-mT magnetic field promoted back electron transfer (presumably by influencing spin state interconversion rates), with a consequent drop in conversion of the initial radical pair to a longer-lived intermediate via tryptophan deprotonation. Effects were apparent in magnetic fields as low as 1 mT. Although this is somewhat stronger than Earth's magnetic field, these particular Arabidopsis and E. coli proteins are not normally known for perceiving magnetic fields. The authors speculate about how cryptochrome proteins would need to be tethered and immobilized to function more efficiently as magnetic compasses in birds.

Proc. Natl. Acad. Sci. U.S.A. 109, 4774 (2012).

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