Posttranslational regulation of the mammalian circadian clock by cryptochrome and protein phosphatase 5

The molecular oscillator that drives circadian rhythmicity in mammals obtains its near 24-h periodicity from posttranslational regulation of clock proteins. Activity of the major clock kinase casein kinase I (CKI) epsilon is regulated by inhibitory autophosphorylation. Here we show that protein phosphatase (PP) 5 regulates the kinase activity of CKI epsilon. We demonstrate that cryptochrome regulates clock protein phosphorylation by modulating the effect of PP5 on CKI epsilon. Like CKI epsilon, PP5 is expressed both in the master circadian clock in the suprachiasmatic nuclei and in peripheral tissues independent of the clock. Expression of a dominant-negative PP5 mutant reduces PER phosphorylation by CKI epsilon in vivo, and down-regulation of PP5 significantly reduces the amplitude of circadian cycling in cultured human fibroblasts. Collectively, these findings indicate that PPS, CKI epsilon, and cryptochrome dynamically regulate the mammalian circadian clock.