Research uncovers crucial molecular interaction that regulates our circadian rhythms timing.

The molecular clocks in our cells synchronize our bodies with the day-night cycle, direct us to sleep and wake, and drive circadian cycles in nearly every aspect of our physiology. Scientists studying the molecular mechanisms of our biological clocks have now identified a key event that controls the timing of the clock.

The new findings, published May 18 in molecular cell, reveal important details about the molecular interactions that are disrupted in people with a genetic sleep disorder called familial advanced sleep phase syndrome (FASP). The syndrome is caused by a genetic mutation that shortens the timing of the clock, causing people to be extreme “morning larks” because their internal clocks run on a 20-hour cycle instead of being in sync with our planet’s 24-hour cycle.

“It’s like perpetual jet lag, because their internal clock is not affected by the length of the day,” said corresponding author Carrie Partch, a professor of chemistry and biochemistry at the University of California, Santa Cruz. “The FASP mutation was discovered 20 years ago, and we knew it had a big impact, but we didn’t know how or why.”

The FASP mutation affects one of the basic clock proteins, called a period, and changes one amino acid in the protein’s structure. The new study shows how this alteration disrupts the interactions of the period protein with an enzyme kinase (casein kinase 1), reducing the stability of the period protein and shortening an important step in the clock cycle.

First author Jonathan Philpott, a postdoctoral researcher in Bartsch’s lab at UCLA, explained that kinases regulate period by binding to phosphate groups (a process called phosphorylation), and there are two different parts of the protein where it can do this. Phosphorylation in the ‘degron’ region marks the period protein for degradation, while phosphorylation in the FASP region stabilizes it. The balance between degradation and stability determines the length of the clock cycle, and a FASP mutation tilts the balance toward period degradation and cycle shortening.

“There’s a shortening of about four hours when you have a FASP surge,” Philpott said.

An important finding of the new study is that the phosphorylated FASP region inactivates the kinase. This periodic feedback inhibition mechanism enables active upregulation of its regulator, slowing degron region phosphorylation and prolonging the cycle. “We need this pause button to slow down what can be very fast biochemistry,” Partch said.

The authors show that inhibition results from binding of the phosphorylated region of FASP to a specific site on the kinase, which a drug can target.

“We can start to think of this as an adaptable system,” said Philpott. “We have identified regions on the kinase that are likely to be targeted to adjust its activity for therapeutic applications.”

Partch noted that most drugs that target kinases work by blocking the enzyme’s active site. “This is basically a hammer that stops kinase activity,” she said. “But with the discovery of new pockets unique to this kinase, we can target those pockets to modulate its activity in a more controlled way.”

This can help not only people with familial advanced sleep phase syndrome, but also people whose sleep cycles have been disrupted by shift work, jet lag, and other challenges of the modern world.

Another surprising finding in the new study is that the feedback inhibition of kinase by period protein also occurs in fruit flies, although the phosphorylation sites are different.

“It turns out that the short-cycle mutation in Drosophila, discovered in 1970, does the same thing as the short-cycle FASP mutation in humans,” Bartsch said. “This mechanism has likely been present throughout the evolution of multicellular animals. The fact that it is rooted in place for so long indicates that it is fundamental to making the biological clocks on Earth have a 24-hour cycle.”

Partch and Philpott said their collaboration with multiple labs at other institutions enabled them to go beyond their experimental observations to study the clock’s mechanisms from a variety of angles. The study involved the use of nuclear magnetic resonance spectroscopy, molecular dynamics simulations, and transgenic human cell lines, as well as characterization of the same molecular mechanisms in humans and Drosophila fruit flies. “It was a great collaborative team,” Partch said.