The Case for Kinases: a Phosphorylation Driven Model for Circadian Temperature Compensation in Neurospora crassa

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 clock speed 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 (encoded by ck-1a) and Casein Kinase II (encoded by cka), in their regulation of temperature compensation using novel alleles, ck-1aD135G and Δcka. A qualitative model supported by biochemical analyses shows that Casein kinase I activity on FRQ decreases with increasing temperature and this decrease is countered by increasing activity of Casein Kinase II. Quantitative proteomics on FRQ across temperatures shows that the FRQ phosphorylation landscape is dependent on temperature and is altered in temperature compensation mutants. In this phosphorylation-driven model for temperature compensation, key temperature compensation-specific domains on FRQ are phosphorylated by Casein Kinase I and Casein Kinase II to regulate period length in response to temperature.