European Robin Cryptochrome-4a Associates with Lipid Bilayers in an Ordered Manner, Fulfilling a Molecular-Level Condition for Magnetoreception

Since the middle of the 20th century, long-distance avian migration has been known to rely partly on the geomagnetic field. However, the underlying sensory mechanism is still not fully understood. Cryptochrome 4a (ErCry4a), found in European Robin ( Erithacus rubecula ), a night-migratory songbird has been suggested to be a magnetic sensory molecule. It is sensitive to external magnetic fields via the so-called radical-pair mechanism. ErCry4a is primarily located in the outer segments of the double cone photoreceptor cells in the eye, which contain stacked and highly ordered membranes that could facilitate the anisotropic attachment of ErCry4a needed for magnetic compass sensing. Here, we investigate possible interactions of ErCry4a with a model membrane that mimics the lipid composition of outer segments of vertebrate photoreceptor cells by using experimental and computational approaches. Experimental results show that the attachment of ErCry4a to the membrane could be controlled by the physical state of lipid molecules (average area per lipid) in the outer leaflet of the lipid bilayer. Furthermore, polarization modulation infrared reflection absorption spectroscopy allowed us to determine the conformation, motional freedom, and average orientation of the α- helices in ErCry4a in a membrane-associated state. Atomistic molecular dynamics studies supported the experimental results. A ∼1000 kcal mol ⁻¹ decrease in the interaction energy as a result of ErCry4a membrane binding was determined compared to cases where no protein binding to the membrane occurred. At the molecular level, the binding seems to involve negatively charged carboxylate groups of the phosphoserine lipids and the C-terminal residues of ErCry4a. Our study reveals a potential direct interaction of ErCry4a with the lipid membrane and discusses how this binding could be an essential step for ErCry4a to propagate a magnetic signal further and thus fulfill a role as a magnetoreceptor.