Molecular mechanism of the repressive phase of the mammalian circadian clock

Abstract

The mammalian circadian clock consists of a transcription–translation feedback loop (TTFL) composed of CLOCK–BMAL1 transcriptional activators and CRY–PER transcriptional repressors. Previous work showed that CRY inhibits CLOCK–BMAL1-activated transcription by a “blocking”-type mechanism and that CRY–PER inhibits CLOCK–BMAL1 by a “displacement”-type mechanism. While the mechanism of CRY-mediated repression was explained by both in vitro and in vivo experiments, the CRY–PER-mediated repression in vivo seemed in conflict with the in vitro data demonstrating PER removes CRY from the CLOCK–BMAL1–E-box complex. Here, we show that CRY–PER participates in the displacement-type repression by recruiting CK1δ to the nucleus and mediating an increased local concentration of CK1δ at CLOCK–BMAL1-bound promoters/enhancers and thus promoting the phosphorylation of CLOCK and dissociation of CLOCK–BMAL1 along with CRY from the E-box. Our findings bring clarity to the role of PER in the dynamic nature of the repressive phase of the TTFL.