Genetic Evidence That Polysumoylation Bypasses the Need for a SUMO-Targeted Ub Ligase

Saccharomyces cerevisiae cells lacking the Slx5-Slx8 SUMO-targeted Ub ligase display increased levels of sumoylated and polysumoylated proteins, and they are inviable in the absence of the Sgs1 DNA helicase. One explanation for this inviability is that one or more sumoylated proteins accumulate to toxic levels in sgs1 Delta slx5 Delta cells. To address this possibility, we isolated a second-site suppressor of sgs1 Delta slx5 Delta synthetic lethality and identified it as an allele of the ULP2 SUMO isopeptidase. The suppressor, ulp2-D623H, behaved like the ulp2 Delta allele in its sensitivity to heat, DNA replication stress, and DNA damage. Surprisingly, deletion of ULP2, which is known to promote the accumulation of poly-SUMO chains, suppressed sgs1 Delta slx5 Delta synthetic lethality and the slx5 Delta sporulation defect. Further, ulp2 Delta's growth sensitivities were found to be suppressed in ulp2 Delta slx5 Delta double mutants. This mutual suppression indicates that SLX5-SLX8 and ULP2 interact antagonistically. However, the suppressed strain sgs1 Delta slx5 Delta ulp2-D623H displayed even higher levels of sumoylated proteins than the corresponding double mutants. Thus, sgs1 Delta slx5 Delta synthetic lethality cannot be due simply to high levels of bulk sumoylated proteins. We speculate that the loss of ULP2 suppresses the toxicity of the sumoylated proteins that accumulate in slx5 Delta-slx8 Delta cells by permitting the extension of poly-SUMO chains on specific target proteins. This additional modification might attenuate the activity of the target proteins or channel them into alternative pathways for proteolytic degradation. In support of this latter possibility we find that the WSS1 isopeptidase is required for suppression by ulp2 Delta.