The Importance of Disorder in the Highly Ordered Circadian Clock

The circadian clock is a tightly-regulated molecular circuit that allows an organism to anticipate the time of day in order to better regulate its physiology and behavior. It is believed that the clock enhances fitness by ensuring that many organismal functions are timed to coincide with the proper phase of the day. As clock regulation is extensive, disruptions of the clock can have a negative impact on human health, for example increased risks for ischemic, metabolic, oncological and mental health diseases. The mechanism that underlies circadian timing is a highly-conserved molecular transcription-translation negative feedback loop. This molecular clock consists of two sets of protein complexes, the positive arm and the negative arm. The positive arm drives the expression of the negative arm, which then regulates its own transcription by inhibiting the activity of the positive arm. The negative arm is then targeted for degradation, allowing for the reactivation of the positive arm, restarting the cycle. Our work has demonstrated that intrinsic disorder is essential for the timing of this circadian circuit. We have established in Neurospora crassa, a fungi and clock model organism, that proteins in the positive arm as well as in the negative arm are Intrinsically Disordered Proteins (IDPs) and that their IDP nature is important to clock function. Moreover, this IDP feature of clock proteins is conserved in higher eukaryotes. Our lab is using biochemical and biophysical methods to identify the underlying purpose for this high level of conservation of protein disorder in the circadian clock. We hypothesize that the conservation of protein disorder in clock proteins allows for conformational flexibility and that this flexibility is key in regulating the timing and interactions of clock proteins over the circadian day.