Sumoylation is largely dispensable for normal growth but facilitates heat tolerance in yeast

Abstract

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.

Dynamic sumoylation of promoter-bound general transcription factors facilitates transcription by RNA polymerase II

“…As shown in Fig 1A , the vast majority of sumoylated proteins in both organisms are chromatin-bound, supporting the notion that SUMO functions primarily in regulating processes associated with chromatin, such as gene regulation. To examine specifically where sumoylated proteins are stably bound to the yeast genome, we carried out a ChIP-seq experiment using a genetically unmodified, normally growing strain (W303a) and an antibody that is specific for yeast SUMO [ 45 ]. Stringent analysis of two independent SUMO ChIP-seq replicates identified a common set of 603 robust SUMO peaks across the yeast genome ( S1A Fig and S2 , S4 and S9 Tables).…”

“…The difficulty in detecting SUMO at non-RPGs promoters may be related to the highly labile nature of the modification [ 45 ]. In support of this, we found that partially inactivating the SUMO protease Ulp1 resulted in increased sumoylation of chromatin-associated Tfg1, suggesting that the protein is normally subject to some level of desumoylation.…”