Summary Failure to accurately partition genetic material during cell division causes aneuploidy and drives tumorigenesis [1]. Cell-cycle checkpoints safeguard cells from such catastrophes by impeding cell-cycle progression when mistakes arise. FHA-RING E3 ligases, including human RNF8 [2] and CHFR [3] and fission yeast Dma1 [4], relay checkpoint signals by binding phosphorylated proteins via their FHA domains and promoting ubiquitination of downstream targets [5]. Upon mitotic checkpoint activation, S. pombe Dma1 concentrates at spindle pole bodies (SPBs) in an FHA-dependent manner and ubiquitinates Sid4, a scaffold of Polo kinase, to suspend cytokinesis [6]. However, the kinase or kinases that phosphoprime Sid4 for Dma1-mediated ubiquitination are unknown. Here, we report that the highly conserved protein kinase CK1 transmits the signal necessary to stall cytokinesis by phosphopriming Sid4 for Dma1-mediated ubiquitination. Like Dma1, CK1 accumulates at SPBs during a mitotic arrest and associates stably with SPB components, including Sid4. Our results establish CK1 as an integral component of a mitotic, ubiquitin-mediated checkpoint pathway.
