Chromatin SUMOylation in heat stress: To protect, pause and organise?

Post‐translational modifications, e.g. SUMO modifications (SUMOylation), provide a mechanism for swiftly changing a protein's activity. Various stress conditions trigger a SUMO stress response (SSR) – a stress‐induced rapid change in the conjugation of SUMO to multiple proteins, which predominantly targets nuclear proteins. The SSR has been postulated to protect stressed cells by preserving the functionality of crucial proteins. However, it is unclear how it exerts its protective functions. Interestingly, heat stress (HS) increases SUMOylation of proteins at active promoters and enhancers. In promoters, HS‐induced SUMOylation correlates with gene transcription and stress‐induced RNA polymerase II (Pol2) pausing. Conversely, a disappearance of SUMOylation in HS occurs at chromatin anchor points that maintain chromatin‐looping structures and the spatial organisation of chromatin. In reviewing the literature, we hypothesise that the SSR regulates Pol2 pausing by modulating the interactions of pausing‐regulating proteins, whereas deSUMOylation alters the function of chromatin anchors. Proteotoxic stress alters SUMO modification of nuclear proteins. We hypothesise that decreased SUMOylation at chromatin anchors is linked to stress‐induced changes in chromatin structure, while increased SUMOylation is regulating promoter pausing of Pol2 at promoters.