In the cells of fruit flies, Chinese scientists say that they have found abiological compass needle: a rod-shaped complex of proteins that can alignwith Earth’s weak magnetic field.
The biocompass — whose constituent proteins exist in related forms in otherspecies, including humans — could explain a long-standing puzzle: howanimals such as birds and insects sense magnetism. It might also become aninvaluable tool for using magnetic fields to control cells, reportresearchers led by biophysicist Xie Can at Peking University in Beijing, ina paper published on 16 November in Nature Materials (S. Qin et al. NatureMater. http://dx.doi.org/10.1038/nmat4484; 2015).
“It’s an extraordinary paper,” says Peter Hore, a biochemist at theUniversity of Oxford, UK. But Xie’s team has not shown that the complexactually behaves as a biocompass inside living cells, nor explained exactlyhow it senses magnetism. “It’s either a very important paper or totallywrong. I strongly suspect the latter,” says David Keays, a neuroscientistwho studies magnetoreception at the Institute of Molecular Pathology inVienna.Related stories
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Many organisms — ranging from whales to butterflies, and termites topigeons — use Earth’s magnetic field to navigate or orient themselves inspace. But the molecular mechanism behind this ability, termed magneto-reception, is unclear.
Some researchers have pointed to magnetically sensitive proteins called ‘
cryptochromes’, or ‘Cry’. Fruit flies lacking the proteins lose theirsensitivity to magnetic fields, for example. But the Cry proteins alonecannot act as a compass, says Xie, because they cannot sense the polarity (north–south orientation) of magnetic fields.
Others have suggested that iron-based minerals might be responsible.Magnetite, a form of iron oxide, has been found in the beak cells of homingpigeons. Yet studies suggest that magnetite plays no part in pigeonmagnetoreception.
Xie says that he has found a protein in fruit flies that both binds to ironand interacts with Cry. Known as CG8198, it binds iron and sulfur atoms andis involved in fruit-fly circadian rhythms. Together with Cry, it forms ananoscale ‘needle’: a rod-like core of CG8198 polymers with an outer layerof Cry proteins that twists around the core (see ''Protein biocompass'').
Credit: S. Qin et al. Nature Mater. http://dx.doi.org/10.1038/nmat4484 (2015).
Using an electron microscope, Xie’s team saw assemblies of these rodsorienting themselves in a weak magnetic field in the same way as compassneedles. Xie gave CG8198 the new name of MagR, for magnetic receptor.
The discovery offers scientists the prospect of using magnetic fields tocontrol cells. Over the past decade, scientists have commandeered the light-sensing capacity of some proteins to manipulate neurons, usually byinserting a fibre-optic cable directly into the brain — a tool calledoptogenetics. But magnetosensing proteins have the advantage that they couldbe manipulated by magnetic fields outside the brain.
Zhang Sheng-jia, a neuroscientist at Tsinghua University in Beijing, claimsto have already demonstrated this ‘magnetogenetic’ capability. InSeptember, he provided a surprise preview of Xie’s work when he published apaper reporting use of the biocompass to manipulate neurons in worms (X.Long et al. Sci. Bull. http://doi.org/883; 2015). Xie and others complained that Zhang’s early publication violated a collaboration agreement between the two researchers — the details of which are disputed — and asked for it to be retracted. In October, Zhang was fired from his university, a decision that he is contesting.
Xie says that in April, he submitted a Chinese patent application thatincludes the use of magnetogenetics and the protein’s magnetic capacity tomanipulate large molecules. He is also starting to look at the structure ofMagR proteins in other animals, including humans. Variants in the humanversion of MagR might even relate to differences in people’s sense ofdirection, he suggests.Sceptical voices
Other scientists are not convinced that the biological needles function likecompasses in living organisms. Xie’s team has shown that MagR and Cry areproduced in the same cells in pigeon retinas — the birds’ proposedmagnetoreception centre — but MagR and Cry are found in many cells, saysKeays. “With such a small amount of iron, one has to ask whether in vivo,at physiological temperatures, MagR is capable of possessing magneticproperties at all,” he says. “If MagR is the real magnetoreceptor, I’lleat my hat.”
“If MagR is the real magnetoreceptor, I’ll eat my hat.”
Xie hopes that others will strengthen his case with further experiments,such as inactivating the gene for MagR in certain fruit-fly tissues to seewhether it affects the animals’ sense of direction. He published withoutdoing this work, he says, because he just wanted to report the findings,which he has been working on for six years.
The lack of an exact mechanism for how the protein complex senses magnetism,or how any signal it sends might be processed by the brain, gives someresearchers pause. MagR’s biocompass activity might simply be the result ofexperimental contamination, says Michael Winklhofer, a magnetism specialistand Earth scientist at Ludwig Maximilian University of Munich in Germany.He is planning experiments to follow up on Xie’s team’s findings. If itholds up, says Winklhofer, then the discovery of MagR “appears to be amajor step forward towards unravelling the molecular basis ofmagnetoreception”.
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