"Biological clocks are the cellular basis of the commonly known circadian rhythms that determine many of our body's functions, including when we go to sleep and wake up. Slow resetting by the clock is the underlying cause of jet lag, and clock malfunction has been linked to seasonal affective disorder and various sleep and mental disorders," explain researchers Jay Dunlap, PhD, professor of biochemistry, and Jennifer Loros, PhD, associate professor of biochemistry.
Their findings, published in the May 2 issue of Science with Susan Crosthwaite, postdoctoral fellow, detail the actions of two clock proteins, White Collar-1 and White Collar-2. These results suggest a link in the evolutionary spectrum from light perception to time keeping that paves the way for detailing the gears of the modern clock in many organisms.
The work was supported by the National Science Foundation, the National Institutes of Health and the U.S. Air Force.
Though they dictate diverse functions, all circadian clocks share common characteristics, including their 24-hour light-dark cycle. Given the ubiquity, similarity and cellular basis of these biological clocks, findings in lower forms are likely to apply to humans.
Visible light is a major cue for the internal circadian rhythms that time behavior and metabolism in most plants and animals. Research by Loros and Dunlap, who for years have pieced together clocks in one of the best-known model systems, has helped explain how light resets all biological clocks. Exploring cellular timekeepers that tell the bread mold, Neurospora, when to send out spores, the investigators were the first to demonstrate how light resets the rhythm.
Now they have found that the two white collar proteins, known to play a role in light regulation, are also essential to the circadian clock, or oscillator and that they work in the dark without light stimulation.
"Previous activities ascribed to the proteins were associated only with light signals, so their the involvement time keeping came as a complete surprise, " said Dunlap.
The dual function of the proteins opens an evolutionary window on the origin of biological timers from primitive proteins that rely on light. The white collar proteins are similar to others involved either in light responses in plants or separately in time keeping in insects. The sequence of amino acids comprising the proteins indicates ties to photoreception proteins in bacteria and plants and also to time keeping proteins in the fruit fly, Drosophila, evidence that all biological clocks may share common components, say the researchers.
"This provides a clear connection between molecules involved in light perception and in circadian rhythmicity in an organism, and strongly suggests an evolutionary link connecting ancient proteins involved in photoreception with modern proteins required for the assembly of circadian clocks in organisms ranging from fungi through mammals," Loros said.
The clockwork cycle is an intricate loop where products feed back to shut off activity, based on light relays. If clocks operated solely on a negative feedback delay, they would wind down quickly. The white collar proteins, particularly White Collar-2, are proteins with a known biochemical function that are involved in the timekeeping loop; they provide the positive feedback in the loop that was anticipated, but not previously found.
White Collar-1 is a clock-associated protein, required for sustained rhythmicity in the dark, but outside the feedback loop. White Collar-2, however, appears to be a clock component required to complete the loop. Unlike other clock components only associated with keeping time, White Collar-2 plays a distinct role in light response and is also necessary to operate the clock in constant darkness.
All organisms have exhibited a correlation between the ability to respond to light and the ability to keep time, and vertebrate tissues that can respond to light also have internal biological clocks.