Identification of heat induced, RNF4 regulated ubiquitylation sites

Exposure of cells to heat or oxidative stress causes misfolding of proteins. To avoid toxic protein aggregation, cells have evolved nuclear and cytosolic protein quality control (PQC) systems. In response to proteotoxic stress cells also limit protein synthesis by triggering transient storage of mRNAs and RNA binding proteins (RBPs) in cytosolic stress granules (SGs). We demonstrate that the SUMO-targeted ubiquitin ligase (StUbL) pathway, which is part of the nuclear proteostasis network, regulates SG dynamics. We provide evidence that inactivation of SUMO deconjugases under proteotoxic stress initiates SUMO-primed, RNF4-dependent ubiquitylation of RBPs that typically condense into SGs. Impairment of SUMO-primed ubiquitylation drastically delays SG resolution upon stress release. Importantly, the StUbL system regulates compartmentalization of an amyotrophic lateral sclerosis (ALS)-associated mutant of FUS in SGs. We propose that the StUbL system functions as surveillance pathway for aggregation-prone RBPs in the nucleus thereby linking the nuclear and cytosolic axis of proteotoxic stress response. Toidentify heat induced, RNF4 regulated ubiquitylation sites we performed an unbiased SILAC-based mass-spectrometry screen comparing heat-induced ubiquitylation in control and RNF4 depleted cells.