Stable maintenance of genetic information requires chromosome segregation to occur with high accuracy. Anaphase is triggered when ring-shaped cohesin is cleaved by separase, a protease regulated by association with its inhibitor securin. Dispensability of vertebrate securin strongly suggests additional means of separase regulation. Indeed, sister chromatid separation but not securin degradation is inhibited by constitutively active cyclin-dependent kinase 1 (Cdk1) and can be rescued solely by preventing phosphorylation of separase. We demonstrate that Cdk1-dependent phosphorylation of separase is not sufficient for inhibition. In a second step, Cdk1 stably binds phosphorylated separase via its regulatory cyclin B1 subunit. Complex formation results in inhibition of both protease and kinase, and we show that vertebrate separase is a direct inhibitor of Cdk1. This unanticipated function of separase is negatively regulated by securin but independent of separase's proteolytic activity.

Cyclin-dependent kinases (CDKs) play crucial roles in promoting DNA replication and preventing rereplication in eukaryotic cells [1-4]. In budding yeast, CDKs promote DNA replication by phosphorylating two proteins, Sld2 and Sld3, which generates binding sites for pairs of BRCT repeats (breast cancer gene 1 [BRCA1] C terminal repeats) in the Dpb11 protein [5, 6]. The Sld3-Dpb11-Sld2 complex generated by CDK phosphorylation is required for the assembly and activation of the Cdc45-Mcm2-7-GINS (CMG) replicative helicase. In response to DNA replication stress, the interaction between Sld3 and Dpb11 is blocked by the checkpoint kinase Rad53 [7], which prevents late origin firing [7, 8]. Here we show that the two key CDK sites in Sld3 are conserved in the human Sld3-related protein Treslin/ticrr and are essential for DNA replication. Moreover, phosphorylation of these two sites mediates interaction with the orthologous pair of BRCT repeats in the human Dpb11 ortholog, TopBP1. Finally, we show that DNA replication stress prevents the interaction between Treslin/ticrr and TopBP1 via the Chk1 checkpoint kinase. Our results indicate that Treslin/ticrr is a genuine ortholog of Sld3 and that the Sld3-Dpb11 interaction has remained a critical nexus of S phase regulation through eukaryotic evolution.

Cohesin pairs sister chromatids by forming a tripartite Scc1-Smc1-Smc3 ring around them. In mitosis, cohesin is removed from chromosome arms by the phosphorylation-dependent prophase pathway. Centromeric cohesin is protected by shugoshin 1 and protein phosphatase 2A (Sgo1-PP2A) and opened only in anaphase by separase-dependent cleavage of Scc1 (refs 4-6). Following chromosome segregation, centrioles loosen their tight orthogonal arrangement, which licenses later centrosome duplication in S phase. Although a role of separase in centriole disengagement has been reported, the molecular details of this process remain enigmatic. Here, we identify cohesin as a centriole-engagement factor. Both premature sister-chromatid separation and centriole disengagement are induced by ectopic activation of separase or depletion of Sgo1. These unscheduled events are suppressed by expression of non-cleavable Scc1 or inhibition of the prophase pathway. When endogenous Scc1 is replaced by artificially cleavable Scc1, the corresponding site-specific protease triggers centriole disengagement. Separation of centrioles can alternatively be induced by ectopic cleavage of an engineered Smc3. Thus, the chromosome and centrosome cycles exhibit extensive parallels and are coordinated with each other by dual use of the cohesin ring complex.

Treslin/TICRR (TopBP1-interacting, replication stimulating protein/TopBP1-interacting, checkpoint, and replication regulator), the human ortholog of the yeast Sld3 protein, is an essential DNA replication factor that is regulated by cyclin-dependent kinases and the DNA damage checkpoint. We identified MDM two binding protein (MTBP) as a factor that interacts with Treslin/TICRR throughout the cell cycle. We show that MTBP depletion by means of small interfering RNA inhibits DNA replication by preventing assembly of the CMG (Cdc45-MCM-GINS) holohelicase during origin firing. Although MTBP has been implicated in the function of the p53 tumor suppressor, we found MTBP is required for DNA replication irrespective of a cell's p53 status. We propose that MTBP acts with Treslin/TICRR to integrate signals from cell cycle and DNA damage response pathways to control the initiation of DNA replication in human cells.

DNA double strand breaks (DSBs) can be repaired by either recombination-based or direct ligation-based mechanisms. Pathway choice is made at the level of DNA end resection, a nucleolytic processing step, which primes DSBs for repair by recombination. Resection is thus under cell cycle control, but additionally regulated by chromatin and nucleosome remodellers. Here, we show that both layers of control converge in the regulation of resection by the evolutionarily conserved Fun30/SMARCAD1 remodeller. Budding yeast Fun30 and human SMARCAD1 are cell cycle-regulated by interaction with the DSB-localized scaffold protein Dpb11/TOPBP1, respectively. In yeast, this protein assembly additionally comprises the 9-1-1 damage sensor, is involved in localizing Fun30 to damaged chromatin, and thus is required for efficient long-range resection of DSBs. Notably, artificial targeting of Fun30 to DSBs is sufficient to bypass the cell cycle regulation of long-range resection, indicating that chromatin remodelling during resection is underlying DSB repair pathway choice.DOI: http://dx.doi.org/10.7554/eLife.21687.001