Cryptochrome 4b protein is likely irrelevant for the radical pair based magnetoreception in the European robin

Avian cryptochrome 4 (Cry4) protein is a putative magnetosensitive molecule facilitating precise long-distance navigation in migratory birds. Two splice variants of Cry4 were reported in European robin (Erithacus rubecula), namely ErCry4a and ErCry4b. It is known that ErCry4a protein exhibits electron transfer between the flavin adenine dinucleotide (FAD) cofactor and tryptophan residues that generates magnetically sensitive radical pairs for magnetoreception. However, little is known about the ErCry4b isoform. We therefore characterized the properties of ErCry4b to see whether it fulfills prerequisites to be a radical pair magnetic sensor molecule. Our results show that ErCry4b protein does not bind FAD in vitro. Computational structure simulations revealed that the FAD non-binding in ErCry4b is likely due to protein structure dynamics. Furthermore, ErCry4b protein abundance in the robin retina, cerebellum and liver is below the detection limit of immunoprecipitation assays coupled with mass spectrometry. Meanwhile, transcript analyses show that ErCRY4b mRNA abundance is 10 times less than ErCRY4b in the retina. In conclusion, ErCry4b does not fulfill the prerequisites to be a radical pair based magnetic sensing molecule due to the lack of FAD binding, and it might not even be expressed as a functional protein in the European robin.