Normal levels of cellular sumoylation are largely dispensable for growth but facilitate heat tolerance in yeast

Sumoylation is an essential, conserved protein modification with hundreds of targets. Compared to the related modification ubiquitination, few enzymes are involved in SUMO conjugation and deconjugation, including a single conjugase, Ubc9, in yeast and mammals. This suggests that cells can simultaneously control the sumoylation level of numerous proteins by regulating just one enzyme of the SUMO pathway. Such modulated levels of cellular sumoylation are observed in response to a number of stress conditions, which typically cause a rapid and dramatic increase in overall sumoylation. Here, we demonstrate that ploidy, culture density, and nutrient availability also affect global sumoylation levels in yeast. To determine the effects of modulated cellular sumoylation on cell growth, we examined yeast strains that artificially display elevated or reduced sumoylation. Remarkably, reducing sumoylation levels by >70% has no effect on cell fitness, indicating that most SUMO conjugation events are not required for normal growth. Surprisingly, among a panel of stress conditions, only elevated temperatures severely impacted growth of cells harbouring constitutively low sumoylation levels. Consistent with the fact that Ubc9 and SUMO are essential, however, cells displaying less than ~5% of normal sumoylation levels show significantly impaired growth, even at normal temperatures. Finally, we demonstrate that many sumoylation events are highly transient, requiring constant de novo sumoylation to maintain steady state levels. Together, our results suggest that cells need only a low basal level of sumoylation for growth, but that normal levels are required pre-emptively to facilitate survival at elevated temperatures.