Editors' ChoiceBiochemistry

In search of a biocompass

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Sci. Signal.  24 Nov 2015:
Vol. 8, Issue 404, pp. ec348
DOI: 10.1126/scisignal.aad9092

Animals use the Earth’s magnetic field to navigate and orient themselves in different geographical habitats. The Earth’s magnetic field is very weak and using it as a means of navigation or orientation requires that the animal can detect all of the parameters of the field—strength, inclination, and direction at a particular geographical location. Using a bioinformatic genome-wide screening approach, Qin et al. identified candidate proteins in Drosophila that would be predicted to bind both iron and Cry, which is a cryptochrome type of flavoprotein implicated in sensing magnetic fields and in responding to light, and were abundant in the fly head. Only one of the candidate proteins meeting these criteria, CG8198, which is an iron-sulfur binding protein, bound to Cry when tested in vitro in the presence of light and a magnetic field. In vitro, homologs of CG8198 from species representing mammals, insects, and birds bound a species-matched homolog of Cry in the presence of both light and a magnetic field. Analysis of the pigeon CG8198 and Cry4 complex with size-exclusion chromatography suggested that the complex existed in several oligomeric states, and the fluorescence emission spectrum of the purified protein complex indicated the presence of the iron-binding cofactor flavin adenine dinucleotide (FAD). Purified pigeon CG8198 and Cry4 protein complexes analyzed with electron microscopy (EM) revealed that the isolated complex appeared as rod-like shapes and disc-like shapes. The most commonly viewed structure consisted of an outer layer of CRY proteins surrounding an inner core of a rod-like structure formed by CG8198. Homology modeling of the pigeon CG8198 protein based on the bacterial homolog IscA, an iron-sulfur cluster protein, and molecular docking analysis combined with the EM data suggested that the rod-like and disk-like shapes represented the same complex viewed from different planes. EM imaging revealed that approximately half of the rod-like protein complexes oriented parallel to an applied external magnetic field. Protein crystals, representing crystals of the Cry4 and pigeon CG8198 complex that had been isolated from cells expressing the two proteins using iron beads, realigned when the magnetic field was inverted. Although the complex may exist in vivo—in the pigeon retina, CRY and CG8198 colocalized by immunohistochemical analysis—whether the complex reorients in response to its position relative to the Earth’s magnetic field in vivo remains to be shown.

S. Qin, H. Yin, C. Yang, Y. Dou, Z. Liu, P. Zhang, H. Yu, Y. Huang, J. Feng, J. Hao, J. Hao, L. Deng, X. Yan, X. Dong, Z. Zhao, T. Jiang, H.-W. Wang, S.-J. Luo, C. Xie, A magnetic protein biocompass. Nat. Mater. 10.1038/NMAT4484 (2015). [PubMed]

D. Cyranoski, Long-sought 'biocompass' discovery claimed. Nature 527, 283–284 (2015). [Online Journal]