Analysis of the mechanisms involved in the new molecular interactions between Pds5, Elg1 and Cln2.

Sister chromatid cohesion is essential for correct chromosome segregation. The cohesin complex consists of an annular trimer comprising Smc1, Smc3 and Mcd1/Scc1 and Three additional subunits: Scc3, Pds5 and Wapl. Pds5 is essential for cohesion in yeast and mammals. Cohesion is established during S-phase, and the processivity clamp PCNA is critical for its regulation. Previous studies revealed that deletion of the PCNA unloading protein Elg1 suppresses the temperature sensitivity of a conditional pds5-1 mutant.

The Δelg1 allele does not bypass a complete deletion of PDS5. Using a ΔelgΔpds5 double mutant carrying a PDS5-URA3-marked plasmid, we performed a screen for mutants able to grow on 5-FOA. Sequencing results showed that about half of the suppressors inactivated the CLN2 gene, a G1 cyclins. In a secondary screen using a yeast genomic library we found that 21 plasmids carried MCD1, a member of the cohesin complex.

We found that strains carrying an auxin-inducible degron at PDS5 have low levels of Mcd1 protein grown in auxin. Opposingly, under similar conditions an AID-PDS5Δelg1Δcln2 has a wildtype equivalent Mcd1 protein levels. When checking the sister chromatid premature separation, the AID-PDS5ΔelgΔcln2 and AID-PDS5Δcln2 strains have significantly reduced the rate found in AID-PDS5 grown in auxin. The AID-PDS5Δelg1 decreased this rate but not enough to restore viability.

These results reinforce the role of Elg1 not only in PCNA processing but also in cohesion and cell cycle regulation. Moreover, we described a new relation between cohesion and cell cycle regulation mediated by the molecular interaction between Pds5 and Cln2.