S-Phase Cyclin-Dependent Kinases Promote Sister Chromatid Cohesion in Budding Yeast

Abstract

Genome stability depends on faithful chromosome segregation, which relies on maintenance of chromatid cohesion during S phase. In eukaryotes, Pds1/securin is the only known inhibitor that can prevent loss of cohesion. However, pds1⌬ yeast cells and securin-null mice are viable. We sought to identify redundant mechanisms that promote cohesion within S phase in the absence of Pds1 and found that cells lacking the S-phase cyclins Clb5 and Clb6 have a cohesion defect under conditions of replication stress. Similar to the phenotype of pds1⌬ cells, loss of cohesion in cells lacking Clb5 and Clb6 is dependent on Esp1. However, Pds1 phosphorylation by Cdk-cyclin is not required for cohesion. Moreover, cells lacking Clb5, Clb6, and Pds1 are inviable and lose cohesion during an unperturbed S phase, indicating that Pds1 and specific B-type cyclins promote cohesion independently of one another. Consistent with this, we find that Mcd1/Scc1 is less abundant on chromosomes in cells lacking Clb5 and Clb6 during replication stress. However, clb5⌬ clb6⌬ cells do accumulate Mcd1/Scc1 at centromeres upon mitotic arrest, suggesting that the cyclin-dependent mechanism is S phase specific. These data indicate that Clb5 and Clb6 promote cohesion which is then protected by Pds1 and that both mechanisms are required during replication stress.

Three Different Pathways Prevent Chromosome Segregation in the Presence of DNA Damage or Replication Stress in Budding Yeast

“…To rule out that the observed phenotype results from defects specific to the pds1 deletion [ 23 , 57 , 58 ], a thermosensitive allele of cohesin ( scc1-73 ) was used in PDS1 + cells. The triple swe1 rad53 scc1-73 mutant is unable to block chromosome replication in the presence of DNA methylation damage ( S10A Fig ).…”

Systematic Triple-Mutant Analysis Uncovers Functional Connectivity between Pathways Involved in Chromosome Regulation

“…Clb5 has also been shown to play an important role in mitotic spindle positioning, a function that is not replaced by overexpressing Clb2 (Segal et al, 1998). Clb5 and Clb6 has also been implicated in the establishment of sister-chromatid cohesion in cells under replication stress; cells lacking Clb5, Clb6 and the securin, Pds1 are inviable (Hsu et al, 2011). …”

Clb6-Cdc28 Promotes Ribonucleotide Reductase Subcellular Redistribution during S Phase

“…Consistent with the different expression patterns, Clb5 is required for the firing of both early and late replication origins, while Clb6 is only involved in the firing of the early origins (37). Although the clb6Δ mutant shows no apparent S-phase defect, as opposed to the clb5Δ mutant, the clb5⌬ clb6Δ double mutant exhibits a more severe S-phase delay and an increased HU sensitivity relative to those of each single mutant (39,40), suggesting overlapping albeit distinct roles of the two cyclins (39). It is likely that the Clb5-Cdc28 and Clb6-Cdc28 kinase complexes differ in their specificities toward a subset of target substrates while sharing most others.…”