PP2ACdc55 dephosphorylates Pds1 to inhibit spindle elongation

DNA replication stress stalls replication forks leading to chromosome breakage and Intra-S checkpoint activation. In S. cerevisae, this checkpoint arrests the cell cycle by stabilizing securin (Pds1) and inhibiting the cyclin dependent kinase (CDK) through multiple pathways. Pds1 inhibits separase (Esp1) which cleaves the cohesin subunit Scc1 and also functions in spindle elongation. However, the role of Pds1-Esp1 in spindle elongation during replication stress response is unknown. Here, we show that Pds1 phosphorylation plays a positive role in spindle elongation through the Pds1-Esp1 interaction in unperturbed and replication stress conditions. PP2ACdc55 directly dephosphorylates Pds1 both in vivo and in vitro. Pds1 hyperphosphorylation in a cdc555 deletion mutant enhanced the Pds1-Esp1 interaction, which accelerated spindle elongation. This PP2ACdc55-dependent Pds1 dephosphorylation plays a role during replication stress and acts independently of the known Mec1, Swe1 or Spindle Assembly Checkpoint (SAC) checkpoint pathways. We propose a model where PP2ACdc55 dephosphorylates Pds1 to disrupt the Pds1-Esp1 interaction that inhibits spindle elongation during replication stress.