The proteasome-dependent degradation of CKB4 is regulated by the Arabidopsis biological clock

Most organisms have evolved an internal timing mechanism, the circadian clock, that is able to generate and maintain 24 h rhythmic oscillation in molecular, biochemical and metabolic activities. In Arabidopsis, the clock-dependent synchronization of physiology with the environment is essential for successful growth and development. The mechanisms of the Arabidopsis clockwork have been described as transcriptional feedback loops at the core of the oscillator. However, an increasing body of evidence points towards a key role of post-translational regulation of clock components as an essential mechanism of circadian function. Here, we identify CKB4, a CK2 regulatory subunit, as a component of the Arabidopsis circadian system. We demonstrate that the nuclear-localized CKB4 protein exists in vivo as different isoforms, resulting from phosphorylation on serine residues. Our findings show that the phosphorylated isoforms are the preferred substrate for ubiquitination and degradation by the proteasome pathway. We provide evidence of the involvement of the biological clock in the circadian regulation of CKB4 protein abundance, which itself is important for an accurate control of circadian period by the clock. Overexpression of CKB4 results in elevated CK2 overall activity and period-shortening of clock-controlled genes peaking at different phase angles. Restriction of CKB4 protein phosphorylation and/or degradation to specific phases within the circadian cycle might provide the cell with a fine-tuning mechanism to selectively regulate the CK2 phosphorylation activity on specific substrates.