Circadian protein regulation in the green lineage II. The clock gene circuit controls a phospho-dawn in Arabidopsis thaliana

The 24-hour rhythmicity in the levels of many eukaryotic mRNAs contrasts with the long half-lives of most detected proteins. Such stable molecular species are not expected to reflect the RNA rhythms, emphasizing the need to test the circadian regulation of protein abundance and modification. Here we present circadian proteomic and comparable phosphoproteomic time-series from Arabidopsis thaliana plants, estimating that 0.4% of quantified proteins and a much larger proportion of quantified phospho-sites were rhythmic under constant light conditions. Approximately half of the quantified phospho-sites were most phosphorylated at subjective dawn, when SnRK1 protein kinase was a candidate regulator. A CCA1-over-expressing line that disables the plant clock gene circuit lacked most or all circadian protein phosphorylation, indicating that the phospho-dawn is regulated by the canonical clock mechanism. The few phospho-sites that fluctuated despite CCA1-over-expression still tended to peak in abundance close to subjective dawn, indicating that their temporal regulation was less dependent on the clock gene circuit. To test the potential functional relevance of our datasets, we conduct phosphomimetic experiments using the bifunctional enzyme fructose-6-phosphate-2-kinase / phosphatase (F2KP), as an example. The clock gene circuit controls diverse protein targets in metabolism and physiology via phosphorylation, including where the bulk abundance of the cognate proteins is arrhythmic.