The Case for Kinases: A Phosphorylation Driven Model for Circadian Temperature Compensation

Circadian rhythms, ∼24-hour biological cycles, enable organisms to anticipate rhythmic environmental cycles so they can assign proper day and night functions that align with those cycles. Circadian rhythms are defined by their ability to be reset by external cues, their capacity to continue to oscillate in the absence of those cues, and their capacity to maintain the rate of the clock across a range of ambient temperatures, a property known as temperature compensation. In the Neurospora clock, the White Collar Complex (WCC) drives expression of FRQ which nucleates a complex including FRH and CK1a that phosphorylates and thereby represses WCC activity. Work to date has suggested that kinases may be involved in temperature compensation and that in Neurospora the primary target of these is FRQ. Here we investigate the genetic relationship between two clock kinases, Casein Kinase I ( ck-1a ) and Casein Kinase II ( cka ), in their regulation of temperature compensation using novel alleles, ck-1a ^D135G and Δ cka . We find that that the clock relies on Casein Kinase I more at cold temperature, but this changes as temperature increases, and the clock relies more on Casein Kinase II at warm temperatures. Using quantitative proteomics on FRQ across temperatures, we find that the FRQ phosphorylation landscape is dependent on temperature and is altered in temperature compensation mutants. This leads to the development of a phosphorylation driven model for temperature compensation, where key temperature compensation specific domains on FRQ are phosphorylated to regulate period length in response to temperature, including by Casein Kinase I and Casein Kinase II.