A multidisciplinary team has shed new light on the mechanisms behind circadian rhythms, offering hope for treating sleep disorders such as jet lag and insomnia. Using cryo-electron microscopy techniques, the researchers identified the structure of the circadian rhythm photosensor and its target in fruit flies, a major organism used to study circadian rhythms. The research, published in Nature, focused on fruit fly cryptochromes, which are key components of the circadian clocks of plants and animals, including humans. Activated by blue light, cryptochromes serve as the primary light sensors for setting circadian rhythms in flies and other insects. The target of the cryptochrome photosensor, known as “Timeless” (TIM), is a large, complex protein that could not previously be imaged, making its interactions with the cryptochrome not well understood.
The researchers found that the TIM protein, along with its partner, the Period (PER) protein, act together to inhibit the genes responsible for their own production via genetic feedback loops. An oscillation in protein levels is established with suitable delays between the events of gene expression and repression, representing the “ticking of the clock” unique to the circadian rhythm. Blue light changes the chemistry and structure of cryptochrome’s flavin cofactor, allowing the protein to bind the TIM protein and inhibit its ability to repress gene expression and reset the oscillation.
To study the complex of cryptochrome-TIM, which is a large, unwieldy protein, the researchers had to figure out how to produce it. First author Changfan Lin modified the cryptochrome protein to improve the stability of the cryptochrome-TIM complex and used innovative techniques to purify the samples, making them suitable for high-resolution imaging. The study not only deepened the understanding of circadian rhythm regulation but also opened up new possibilities for developing therapies targeting related processes.
The study sheds light on how DNA damage is repaired in a cell. Cryptochromes are closely related to a family of enzymes involved in repairing damage to DNA, called photolyases. The research explains why these families of proteins are closely related to each other. The study also offers an explanation for the genetic variation of flies that allows them to adapt to higher latitudes. These flies have more of a certain genetic variant that involves a change in the TIM protein. The researchers found that the variation reduces the affinity of TIM for the cryptochrome, modulating the interaction between the proteins and changing the ability of light to reset the oscillation, altering the circadian clock and extending the period of the fly’s dormancy, helping it survive the winter.
The study may help understand key interactions between components that regulate sleep behavior in people, such as how the critical delays in the basic timing mechanism get built into the system. The researchers also discovered an important structural area in TIM called the “groove,” which helps explain how TIM enters the cell nucleus, providing a clearer understanding of this phenomenon.
Source: Cornell University