Introduction The circadian circuit, a roughly 24 h molecular feedback loop, or clock, is conserved from bacteria to animals and allows for enhanced organismal survival by facilitating the anticipation of the day/night cycle. With circadian regulation reportedly impacting as high as 80% of protein coding genes in higher eukaryotes, the protein-based circadian clock broadly regulates physiology and behavior. Due to the extensive interconnection between the clock and other cellular systems, chronic disruption of these molecular rhythms leads to a decrease in organismal fitness as well as an increase of disease rates in humans. Importantly, recent research has demonstrated that proteins comprising the circadian clock network display a significant amount of intrinsic disorder. Main body In this work, we focus on the extent of intrinsic disorder in the circadian clock and its potential mechanistic role in circadian timing. We highlight the conservation of disorder by quantifying the extent of computationally-predicted protein disorder in the core clock of the key eukaryotic circadian model organisms Drosophila melanogaster, Neurospora crassa, and Mus musculus. We further examine previously published work, as well as feature novel experimental evidence, demonstrating that the core negative arm circadian period drivers FREQUENCY (Neurospora crassa) and PERIOD-2 (PER2) (Mus musculus), possess biochemical characteristics of intrinsically disordered proteins. Finally, we discuss the potential contributions of the inherent biophysical principals of intrinsically disordered proteins that may explain the vital mechanistic roles they play in the clock to drive their broad evolutionary conservation in circadian timekeeping. Conclusion The pervasive conservation of disorder amongst the clock in the crown eukaryotes suggests that disorder is essential for optimal circadian timing from fungi to animals, providing vital homeostatic cellular maintenance and coordinating organismal physiology across phylogenetic kingdoms. Graphical abstract

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The circadian clock times cellular processes to the day/night cycle via a Transcription-Translation negative Feedback Loop (TTFL). However, a mechanistic understanding of the negative arm in both the timing of the TTFL and its control of output is lacking. We posited that the formation of negative-arm protein complexes was fundamental to clock regulation stemming from the negative arm. Using a modified peptide microarray approach termed Linear motif discovery using rational design (LOCATE), we characterized the interaction of the disordered negative-arm clock protein FREQUENCY to its partner protein FREQUENCY-Interacting RNA helicase. LOCATE identified a specific Short Linear Motif (SLiM) and interaction hotspot as well as positively charged islands that mediate electrostatic interactions, suggesting a model where negative arm proteins form a fuzzy complex essential for clock timing and robustness. Further analysis revealed that the positively charged islands were an evolutionarily conserved feature in higher eukaryotes and contributed to proper clock function.

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SUMMARYThe circadian clock employs a transcriptional/translational negative feedback loop (TTFL) to anticipate environmental changes due to the Earth’s diurnal cycle, with regulation of organismal physiology believed to stem from temporal transcriptional activation by the positive arm. However, up to 80% of oscillating proteins do not have rhythmic mRNA, establishing circadian post-transcriptional regulation through unknown mechanisms. Given the pervasive conservation of the intrinsically disordered nature of negative-arm clock proteins, we hypothesized that post-transcriptional regulation may stem from conformational shifts in negative-arm proteins that time vacillations in the constituents of negative-arm macromolecular complexes to time cellular physiology. Our investigation of the negative arm clock protein in Neurospora crassa, FREQUENCY (FRQ), demonstrated temporal conformational fluidity correlated with daily changes in physiologically diverse macromolecular complex components. A parallel investigation of the macromolecular complexes centered around Drosophila melanogaster PERIOD (dPER) and human PERIOD (hPER2) found a similar number and physiological diversity of interacting partners in higher eukaryotes. Short linear motifs (SLiMs) associated with the interactors localized to disordered and phosphorylated regions on the PERs and FRQ, with disordered interactors oscillating in the macromolecular complexes over circadian time. This oscillation correlated with oscillations in post-transcriptionally regulated proteins, suggesting the negative arm may tune cellular physiology and proteostasis post-transcriptionally via vacillations in the circadian negative-arm macromolecular protein complexes.

Organismal physiology is widely regulated by the circadian clock, a molecular circuit composed of a Transcription-Translation Feedback Loop 1,2. Protein components of the molecular clock are enriched in intrinsically disordered regions, inherently flexible regions that interact with other proteins via short linear binding motifs (SLiMs) 3–5. SLiM-driven interactions contribute to circadian timing and the circadian regulation of the cell. However, the mechanism that allows the formation of dynamic clock complexes remains unclear as structural analysis of these protein-protein interactions has been limited due to inherent protein disorder. Here, we apply a synthetic peptide microarray approach to demonstrate that the core clock forms a fuzzy complex to support circadian robustness 6,7. We found positively charged islands on the clock protein FREQUENCY (FRQ) drove a multi-valent interaction between FRQ and its partner FRQ-interacting RNA Helicase (FRH) that enabled clock robustness rather than the previously-reported feedback 8. We found these positively charged islands were a conserved molecular feature throughout clocks in fungi, insects, and mammals, and may enable the formation of fuzzy complexes. This study constitutes the first mechanistic reason for the uniquely-broad conservation of intrinsic disorder in circadian negative-arm proteins and will aid in the development of the molecular model of clock protein interactions. Furthermore, we anticipate the application of synthetic peptide microarrays to study disordered clock proteins and will be useful in characterizing sites of interaction for clock-specific drug discovery 9.

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