Why do birds migrate at night?

It was a puzzle about birds.

Migratory birds are known to rely on the Earth's magnetic field to help them navigate the world. And it was suspected that a protein called cryptochrome, sensitive to blue light, allowed birds to do it.

Yet, many of these animals also migrate at night when there is not much light available. It was therefore difficult to know how cryptochrome would work under these conditions in birds.

A new study conducted by the UT Southwestern Medical Center in collaboration with the SMU (Southern Methodist University), however, may have found the solution to this problem.

The researchers found that cryptochromes of migratory birds have developed a mechanism that improves their ability to react to light, which can enable them to detect and react to magnetic fields.

"We have been able to show that protein cryptochrome is extremely effective in collecting and reacting to low levels of light," said EMS chemist Brian D. Zoltowski, one of the lead authors of a new study on the results. "The result of this research is that we now understand how vertebral cryptochromes can respond to very low light intensities and operate in night-time conditions."

The study was published in the journal PNAS in September.

Cryptochromes are present in plants and animals and are responsible for circadian rhythms in various species. In birds, scientists have dedicated themselves specifically to finding out about an unusual ocular protein called CRY4, which is part of a class of cryptochromes.

The laboratory of Joseph Takahashi, an expert in circadian rhythms at the UT Southwestern Medical Center, worked with other Southwestern UT scientists to purify and resolve the crystal structure of the protein – the first atomic structure of one. photoactive cryptochrome molecule derived from a vertebrate. Brian Zoltowski's laboratory, a specialist in blue light photoreceptors, investigated the efficacy of light-induced reactions by identifying a CRY4 protein-specific pathway that facilitates function in low-light conditions.

"Although in plants and insects, cryptochromes are known to be photoactive, they react to the sun, much less is known of vertebrates, and the majority of vertebrate cryptochromes do not appear to be photoactive," said Takahashi. Chairman of the Neuroscience Council of the UT. South West and researcher at the Howard Hughes Medical Institute. "This photosensitivity and the possibility that CRY4 is affected by the magnetic field make this specific cryptochrome a very interesting molecule."

The researchers took a sample of CRY4 from a pigeon and grew crystals of the protein. They then exposed the crystals to X-rays, allowing them to map the location of all the atoms in the protein.

And while pigeons are not nocturnal migratory songbirds, the sequences of their CRY4 proteins are very similar, noted the study.

"These structures allow us to visualize at the atomic scale the functioning of these proteins and to understand how they can use blue light to detect magnetic fields," said Zoltowski, associate professor of chemistry at Dedman College of Humanities & Science of SMU. "The new structures also provide the first detail at the atomic level of the functioning of these proteins, opening the door to more detailed studies of cryptochromes in migratory organisms."

In this study, researchers uncovered unusual changes in key regions of the protein structure that could improve their ability to capture light from their environment.

"Cryptochromes work by absorbing a photon of light, which causes an electron to move in an amino acid sequence." These amino acids usually consist of a chain of 3 or 4 sites that act as a wire that electrons can cross, "explained Zoltowski. . "But in pigeons, it has been identified that this chain could be expanded to contain 5 sites."

This mutation of the electronic chain in pigeons makes cryptochrome less dependent on the fact that the environment of the bird is very enlightened for the protein to be activated.

"The birds have come up with a mechanism to improve efficiency, so even when there is very little light, they have enough signal generated to migrate," Zoltowski said.