# The mitotic functions of a fission yeast CK1 enzyme are regulated by Cdk1-dependent and auto-phosphorylation

## TL;DR

This study shows how a fission yeast enzyme, Hhp2, is regulated during cell division through phosphorylation, affecting cell cycle timing and checkpoint control.

## Contribution

The study identifies specific phosphorylation events on Hhp2 that regulate its activity and mitotic progression in fission yeast.

## Key findings

- Hhp2 undergoes transient hyperphosphorylation during mitosis at four autophosphorylation sites and three Cdk1-dependent sites.
- Phosphorylation inhibits Hhp2 activity, and a mutant lacking these sites accelerates mitosis and enhances checkpoint function.
- Hhp2 likely works alongside the Polo-like kinase Plo1 to regulate mitotic progression.

## Abstract

CK1 enzymes are conserved regulators of diverse cellular processes. In Schizosaccharomyces pombe, the CK1 orthologs of CK1δ and CK1ε, Hhp1 and Hhp2, are required for a mitotic checkpoint that delays cytokinesis when the mitotic spindle is disrupted. Here, we show that Hhp2, but not Hhp1, undergoes transient hyperphosphorylation during mitosis. Hhp2 autophosphorylates at four residues and is phosphorylated by the cyclin-dependent kinase Cdk1 at three additional sites. Functionally, these phosphorylation events inhibit Hhp2 catalytic activity, as phospho-ablating mutants exhibited enhanced in vitro kinase activity. In vivo, a mutant combining all seven sites (hhp2-7A) behaved as a gain-of-function mutant in the mitotic checkpoint and also had the unexpected phenotype of accelerating mitosis and cytokinesis in unperturbed conditions. Further genetic analyses indicated that Hhp2 likely promotes mitotic progression in parallel with the Polo-like kinase, Plo1. These findings establish that mitotic phosphorylation of Hhp2 serves as a negative regulatory mechanism that silences checkpoint activity and modulates cell cycle timing. Because mitotic phosphorylation of human CK1δ has been observed, our results suggest that Cdk1-mediated inhibition of CK1 enzymes is a conserved mechanism coupling the core cell cycle control machinery to CK1-dependent signaling pathways.