Kinase and phosphatase: The cog and spring of the circadian clock

Reversible phosphorylation is an important regulatory mechanism for many biological processes in eukaryotic organisms. The phosphorylation state of a protein is controlled dynamically by both protein kinases and phosphatases. Phosphorylation of circadian clock proteins is an essential posttranscriptional mechanism in the regulation of circadian clocks, and several protein kinases and phosphatases have been shown to regulate key clock components in eukaryotic systems, including Arabidopsis, Neurospora, Drosophila, and mice. In this review, recent progress in the characterization of protein kinases and phosphatases involved in circadian rhythms is summarized. The protein kinase CK2 has been proposed as an evolutionary link between the divergent circadian systems of plants, animals, and fungi. The roles of CK2 in this process are discussed here in detail.