In what has to be one of the most bizarre-sounding biological experiments ever, scientists have demonstrated that Viagra, the erectile dysfunction drug, can ward off jet lag in hamsters. The exact mode of action remains unknown and whether or not the drug could lead to oversubscription of the human mile-high club is unclear. Nevertheless, the work demonstrates just how complicated are circadian rhythms.

In unconnected work, new insights into the workings of circadian rhythms at the cellular level have been obtained using X-ray diffraction of key components of the control mechanisms. These components respond to differences in light levels between night and day and so allow organisms to anticipate changes in the environment by pacing their metabolism to the daily cycle.

Circadian rhythms are not the preserve of mammals, though, despite the assertions of hamsters, they play a role in floral blooming, fungal spore release, as well as controlling sleep. The disruption of circadian rhythms is well known in causing jet lag, stress, and even some forms of mental illness. It could also have a role in cancer, although that area of medicine is not without controversy.

"These clocks are highly conserved in all organisms, and in organisms separated by hundreds of millions of years of evolution," explains Brian Crane of Cornell University. Crane and his colleagues at Dartmouth Medical School, Hanover, New Hampshire, have studied the filamentous fungus, Neurospora crassa and revealed how circadian light sensors control production of carotenoids in this fungus. The carotenoids act as the organism's natural sunscreen during the day.

The team looked at Vivid, a chromophore-containing protein that traps light energy, changes conformation and initiates a cascade of biochemical reactions that switch on carotenoid production. The team's crystal structures of dark-state and light-state Vivid revealed a protein domain with an unusual N-terminal cap region and a loop insertion that can accommodate the biochemical cofactor flavin.

They demonstrated that a cystein-flavin adduct forms under blue light and that this in turn drives flavin protonation to trigger a conformational change at the unusual N-terminal. They also found that by swapping another, remote, sulfur-containing cysteine group for serine in Vivid, the team was able to switch off this process. "The Cys to Ser mutant that we made to shut down the signalling, was not the active site cysteine that forms the flavin adduct,"explains Crane, "but rather a cysteine, remote from the flavin that is involved in the conformational switching at the surface of the protein." He adds that this is an important distinction as it is the first time that a protein conformational change induced by photochemistry has been definitively linked to signal propagation.

"We were interested in trying to understand behaviour at the molecular level," said Crane. "This a great example of chemical biology, in that we can perturb the chemistry of a single molecule in a particular way and actually change the behaviour of a complex organism."

Whether or not these studies with fungal cycles will have any bearing on those Viagra-popping hamsters is a different matter. But, a clearer understanding of circadian rhythms at the cellular level will ultimately assist in research into the problems to which a dysfunctional cycle can give rise.
"There is much interest in how proteins are able to capture photons and convert these signals to longer time-scale, larger-amplitude changes that can influence other components of the cell," adds Crane, "Our work provides a detailed mechanism for how the Vivid protein converts photochemistry to structural changes that influence how it interacts with its targets." Such research could also have implications for understanding how the light sensors in our eyes communicate with their molecular partners.
"The proteins of our own clocks contain modules similar to Vivid," adds Crane, "Although the mechanisms by which they function are not likely to be identical to those of Vivid, they may very well be related." Crane further explains that analogous residue substitutions in these proteins may also perturb their functions, and hence the properties of the clock. "This is something that we are looking into," he told SpectroscopyNOW.

Related links:

• Science, 2007, 316, 1054-1057
• Crane's Page
• Proc Natl Acad Sci, 2007, in press

Article by David Bradley