Spectroscopic Characterization of Radical Pair Photochemistry in Nonmigratory Avian Cryptochromes: Magnetic Field Effects in Cry4a.

The magnetic compass sensor in night-migratory songbirds is thought to be a flavin-tryptophan radical pair formed by blue-light excitation of the protein cryptochrome-4a (Cry4a) localized in photoreceptor cells in the birds' retinas. The effects of applied magnetic fields on the photochemistry of purified Cry4a from the migratory European robin are well characterized, but it is less clear what, if anything, distinguishes the magnetic responses of the Cry4a proteins from migratory and nonmigratory species. We present here a detailed study of the magnetic sensitivity of Cry4a from the nonmigratory chicken. The wild-type protein is compared with two mutants in which either Arg317 or Glu320, both close to the tryptophan radical, were replaced by the amino acids Cys and Lys, respectively, found in Cry4a from robins and other night-migratory passerines. These sites had previously been identified as probably facilitating the evolution of an optimized magnetic sensor for nocturnal orientation in songbirds. Neither of these mutations was found to affect the reaction kinetics or magnetic sensitivity of the radical pairs, suggesting that any differences in Cry4a between robin and chicken must stem from their ability to transmit magnetic information, for example via protein-protein interactions. In contrast, a Trp → Phe mutation at the end of the tryptophan-tetrad electron transfer chain in both cryptochromes led to a large increase in magnetic sensitivity, suggesting different sensing and signaling roles for the third and fourth tryptophans.