The circadian clock is an endogenous timing system that in addition to managing sleep/wake cycles, prepares an organism for periodic environmental fluctuations and coordinates physiological and behavioral processes.
The first clocks emerged billions of years ago, when emerging life was under intense pressure to protect their genome from damaging UV radiation. One of the early mechanisms was a UV-sensitive DNA repair enzyme, which has a striking resemblance and rich evolutionary connection with a key protein in eukaryotic clocks, cryptochrome.The Earth’s rotation has instilled its constant cycle of light and dark into virtually all organisms and at all levels life, from molecularly to behaviorally.
This cycle is maintained by a 24 hour molecular timing system that relies on a transcription-translation negative feedback mechanism of four proteins. Two transcription factors, CLOCK and BMAL1 drive the positive arm of the loop by promoting the expression of clock-controlled genes, along with the components of the negative arm, Periods (PER) and Cryptochromes (CRY). The PER and CRY proteins block the activity of CLOCK-BMAL1 and inhibiting the transcription of their own and all other clock-controlled genes. The periodic accumulation, localization, and degradation of the PER and CRY proteins are necessary to manifest a circadian rhythm.
Recent work in the lab has crystallographically shown the highly unusual binding mode of CRY’s ubiquitin ligase, FBXL3, which in addition to an extensive interface, embeds a completely conserved Trp at the extreme C-terminus into a large hydrophobic pocket of CRY. The same pocket that we recently identified to bind the first direct clock-modulating small molecule, KL001, which directly competes with FBXL3 to promote CRY stability.
Previous data suggested that PER can also protect CRY from FBXL3-dependent ubiquitination as well as serve as the nodal point for circadian rhythmicity. We have also recently determined the molecular assembly of the negative arm of the loop, CRY and PER.
- Nangle SN, Rosensweig C, Koike N, Tei H, Takahashi JS*, Green CB*, Zheng N* 2014 Molecular assembly of the period-cryptochrome circadian transcriptional repressor complex. eLife. 3:e03674. (*Co-correspondence)
- Nangle SN, Xing W*, Zheng N* 2013. Crystal structure of mammalian cryptochrome in complex with a small molecule competitor of its ubiquitin ligase. Cell Research. (*Co-correspondence)
- Xing W, Busino L, Hinds TR, Marionni ST, Saifee NH, Bush MF, Pagano M, & Zheng N 2013. SCFFBXL3 ubiquitin ligase targets cryptochromes at their cofactor pocket. Nature. 496:64:68