# Casein kinase 1a mediates a two-step subunit remodeling mechanism to regulate the FRQ-FRH circadian clock complex

## TL;DR

The study reveals how phosphorylation controls the timing of a key circadian clock complex in Neurospora through a two-step process involving FRQ and FRH.

## Contribution

The paper identifies a two-step subunit remodeling mechanism triggered by phosphorylation that regulates the FRQ-FRH circadian complex.

## Key findings

- FRH decodes the phosphorylation state of FRQ through a two-step complex remodeling process.
- Phosphorylation initially activates FRQ by exposing a binding site for the WCC complex.
- Delayed dissociation of FRH promotes nuclear export and degradation of FRQ.

## Abstract

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.

The Neurospora circadian clock keeps time through a negative feedback loop involving a protein complex of FRQ bound to FRH. Here, the authors show that slow, stochastic phosphorylation of these proteins dissociates and activates them via two precisely timed switches.