Michael Brunner

Casein kinase 1δ (CK1δ) is a ubiquitously expressed kinase involved in diverse cellular processes, including cell-cycle regulation. CK1δ activity is attenuated by (auto)phosphorylation. However, inhibitory phosphorylation is efficiently opposed by cellular phosphatases as CK1δ accumulates in its hypophosphorylated, active state. CK1δ is a target of the nuclear ubiquitin ligase APC/C-CDH1, yet the kinase is apparently stable. Thus, the physiological relevance of CK1δ (auto)phosphorylation, autoinhibition, and regulated turnover has remained unclear. Previously, we showed that CK1δ is dynamically regulated in a manner that is dependent on kinase activity, subcellular localization, and interaction with its binding partners, such as PER2 (Serrano et al., 2026). Here, we show that CK1δ activity and abundance are additionally coordinated in a cell cycle-dependent manner. During G1, assembled CK1δ kinase is stable while free active kinase is degraded. In S phase, unassembled CK1δ seems to be no longer degraded, likely to support functions in DNA damage signaling. Upon mitotic entry, the downregulation of phosphatases promotes CK1δ (auto)phosphorylation and consequent autoinhibition, thereby preserving a pool of kinase to rapidly re-establish the post-mitotic steady state.

Casein kinase 1δ (CK1δ) is a ubiquitously expressed kinase involved in diverse cellular processes, including cell cycle regulation. CK1δ activity is attenuated by (auto)phosphorylation. However, inhibitory phosphorylation is efficiently opposed by cellular phosphatases as CK1δ accumulates in its hypophosphorylated, active state. CK1δ is a target of the nuclear ubiquitin ligase APC/C-CDH1, yet the kinase is apparently stable. Thus, the physiological relevance of CK1δ (auto)phosphorylation, autoinhibition, and regulated turnover has remained unclear. Here we show that CK1δ activity and abundance are coordinated in a cell cycle-dependent manner. During G1, assembled CK1δ kinase is stable while free active kinase is degraded. In S phase, unassembled CK1δ seems to be no longer degraded, likely to support functions in DNA damage signaling. Upon mitotic entry, the downregulation of phosphatases promotes CK1δ (auto)phosphorylation and consequent autoinhibition, thereby preserving a pool of kinase to rapidly reestablish the post-mitotic steady state.

Casein kinase 1δ (CK1δ) is a ubiquitously expressed kinase involved in diverse cellular processes, including cell cycle regulation. CK1δ activity is attenuated by (auto)phosphorylation. However, inhibitory phosphorylation is efficiently opposed by cellular phosphatases as CK1δ accumulates in its hypophosphorylated, active state. CK1δ is a target of the nuclear ubiquitin ligase APC/C-CDH1, yet the kinase is apparently stable. Thus, the physiological relevance of CK1δ (auto)phosphorylation, autoinhibition, and regulated turnover has remained unclear. Here we show that CK1δ activity and abundance are coordinated in a cell cycle-dependent manner. During G1, assembled CK1δ kinase is stable while free active kinase is degraded. In S phase, unassembled CK1δ seems to be no longer degraded, likely to support functions in DNA damage signaling. Upon mitotic entry, the downregulation of phosphatases promotes CK1δ (auto)phosphorylation and consequent autoinhibition, thereby preserving a pool of kinase to rapidly reestablish the post-mitotic steady state.

The mammalian circadian clock is governed by a feedback loop in which the transcription activator CLOCK:BMAL1 induces expression of its inhibitors, PERs and CRYs, which form a complex with CK1δ, the main circadian kinase. However, the spatiotemporal dynamics of this feedback loop and the precise role of CK1δ remain incompletely understood. Using an inducible overexpression system, we show that nuclear availability of CK1δ is limited by both rapid nuclear degradation and active export of unassembled kinase, while cytoplasmic kinase is readily available for association with PERs. We demonstrate that CK1δ-mediated phosphorylation may disrupt PER2–CRY1 interaction, thereby resulting in cytoplasmic PER2 dimers containing substoichiometric amounts of CRY1. Analysis of endogenous PER2 localization in the context of an intact circadian clock reveals that PER2 accumulates in the cytoplasm late in the circadian cycle. Based on these findings, we propose that cytoplasmic accumulation of PER:CRY:CK1δ complexes contributes to the clearance of nuclear PER2, while the CK1δ-dependent release of CRY1 into the nucleus may sustain CLOCK:BMAL1 repression on DNA, supporting the transition from the early to the late repressive phase.

The mammalian circadian clock is governed by a feedback loop in which the transcription activator CLOCK:BMAL1 induces expression of its inhibitors, PERs and CRYs, which form a complex with CK1δ, the main circadian kinase. However, the spatiotemporal dynamics of this feedback loop and the precise role of CK1δ remain incompletely understood. Using an inducible overexpression system, we show that nuclear availability of CK1δ is limited by both rapid nuclear degradation and active export of unassembled kinase, while cytoplasmic kinase is readily available for association with PERs. We demonstrate that CK1δ-mediated phosphorylation may disrupt PER2-CRY1 interaction thereby resulting in cytoplasmic PER2 dimers containing substoichiometric amounts of CRY1. Analysis of endogenous PER2 localization in the context of an intact circadian clock reveals that PER2 accumulates in the cytoplasm late in the circadian cycle. Based on these findings, we propose that cytoplasmic accumulation of PER:CRY:CK1δ complexes contributes to the clearance of nuclear PER2, while the CK1δ-dependent release of CRY1 into the nucleus may sustain CLOCK:BMAL1 repression on DNA supporting the transition from the early to the late repressive phase.

Casein kinase 1δ (CK1δ) is a ubiquitously expressed kinase involved in diverse cellular processes, including cell cycle regulation. CK1δ activity is attenuated by (auto)phosphorylation. However, inhibitory phosphorylation is efficiently opposed by cellular phosphatases as CK1δ accumulates in its hypophosphorylated, active state. CK1δ is a target of the nuclear ubiquitin ligase APC/C-CDH1, yet the kinase is apparently stable. Thus, the physiological relevance of CK1δ (auto)phosphorylation, autoinhibition, and regulated turnover has remained unclear. Here we show that CK1δ activity and abundance are coordinated in a cell cycle-dependent manner. During G1, assembled CK1δ kinase is stable while free active kinase is degraded. In S phase, unassembled CK1δ seems to be no longer degraded, likely to support functions in DNA damage signaling. Upon mitotic entry, the downregulation of phosphatases promotes CK1δ (auto)phosphorylation and consequent autoinhibition, thereby preserving a pool of kinase to rapidly reestablish the post-mitotic steady state.

The circadian clock of Neurospora operates through a negative feedback loop in which FREQUENCY (FRQ), along with FRQ-interacting RNA helicase (FRH) and casein kinase 1a (CK1a), inhibits its transcriptional activator, WHITE COLLAR COMPLEX (WCC), via phosphorylation. CK1a, anchored to FRQ, hyperphosphorylates FRQ at its intrinsically disordered regions in a slow, temperature-independent manner, forming a module suited for molecular timekeeping. However, the molecular processes triggered by FRQ’s hyperphosphorylation have remained unclear. Here we show that FRH, the folded binding partner of disordered FRQ, decodes FRQ’s time-dependent phosphorylation state by triggering a two-step remodeling of the FRQ-FRH complex: initially, two FRH molecules bind a FRQ dimer, keeping it inactive by blocking its interaction with WCC. Slow phosphorylation eventually triggers the dissociation of one FRH, thereby activating the complex by exposing a WCC-binding site. Due to the slow and stochastic nature of phosphorylation, the release of the second FRH occurs with a delay, promoting nuclear export and subsequent degradation of FRQ. This ensures precise activation and inactivation of FRQ and positions FRH as a hub for decoding temporal phosphorylation information.

Circadian rhythms in mammals rely on cell-autonomous molecular clocks that operate in many tissues and organs. New research in The EMBO Journal quantifies the expression dynamics, distribution, mobility, and turnover of circadian clock proteins in the suprachiasmatic nucleus. The quantitative data obtained from a physiologically relevant tissue will inspire tailored in vitro approaches and help to put these findings into a physiological context to further develop our current model of the circadian clock.

The circadian clock of Neurospora operates through a negative feedback loop in which FRQ, along with FRH and CK1a, inhibits its transcriptional activator, WCC, via phosphorylation. CK1a, anchored to FRQ, hyperphosphorylates FRQ at its IDRs in a slow, temperature-independent manner, forming a module suited for molecular timekeeping. However, the molecular processes triggered by FRQ’s hyperphosphorylation have remained unclear. We show that FRH, the folded binding partner of disordered FRQ, decodes FRQ’s time-dependent phosphorylation state by triggering a two-step remodeling of the FRQ-FRH complex: initially, two FRH molecules bind a FRQ dimer, keeping it inactive by blocking its interaction with WCC. Gradual phosphorylation induces with a delay the dissociation of one FRH, exposing a binding site for WCC and activating the complex. Following a time delay, attributable to the slow and stochastic nature of phosphorylation, the release of the second FRH promotes nuclear export and subsequent degradation of FRQ. This stepwise remodeling ensures precise activation and inactivation of FRQ and positions FRH as a hub for decoding temporal phosphorylation information.

Metabolic compensation of the circadian clock ensures endogenous timing across a broad range of nutrient conditions, enabling organisms to adapt efficiently to recurrent environmental changes, even during nutrient scarcity. In this study, we have identified a novel clock‐controlled gene, rasgef (Rat Sarcoma Guanine Nucleotide Exchange Factor), that plays a crucial role in modulating the circadian clock under starvation conditions in the circadian model organism Neurospora crassa. The gene product, RasGEF—a nucleotide exchange factor for the small G protein RAS2P (Rat Sarcoma 2 Protein)—displays glucose‐dependent phosphorylation and localization. We show that deletion of rasgef hinders metabolic compensation of the circadian clock to glucose‐depleted conditions and disrupts the rhythmic expression of the output gene ccg2. Furthermore, we demonstrate in osteosarcoma cells that the period of the mammalian clock is also compensated across a wide range of extracellular glucose levels and adaptation of the clock to glucose‐starved conditions depends on the RasGEF homolog SOS1 (Son of Sevenless 1) and its downstream signaling component ERK (Extracellular Signal‐Regulated Kinase). Our results suggest a conserved role of RasGEF‐mediated signaling in the maintenance of circadian rhythm under glucose‐limited conditions.