Concerted SUMO-targeted ubiquitin ligase activities of TOPORS and RNF4 are essential for stress management and cell proliferation

Abstract

SummaryProtein SUMOylation provides a principal driving force for cellular stress responses including DNA-protein crosslink (DPC) repair and arsenic-induced PML body degradation. In genome-scale screens, we identified the human E3 ligase TOPORS as a key effector of SUMO-dependent DPC resolution. We demonstrate that TOPORS promotes DPC repair by functioning as a SUMO-targeted ubiquitin ligase (STUbL) for DPCs, combining ubiquitin ligase activity through its RING domain with poly-SUMO chain binding via a cluster of SUMO-interacting motifs, analogous to the STUbL RNF4. Surprisingly, the STUbL activities of TOPORS and RNF4 are both required for SUMO-dependent DPC repair, PML degradation and other stress responses, making overlapping and distinct contributions to ubiquitin chain formation on SUMOylated targets to enable p97/VCP unfoldase recruitment. Combined loss of TOPORS and RNF4 is synthetic lethal even in unstressed cells, leading to defective clearance of SUMOylated proteins from chromatin accompanied by cell cycle arrest and apoptosis. Together, our findings establish TOPORS as a novel STUbL whose concerted action with RNF4 defines a general mechanistic principle in crucial cellular processes governed by direct SUMO-ubiquitin crosstalk.HighlightsThe RING E3 ligase TOPORS is required for SUMO-dependent DPC repairTOPORS is a novel SUMO-targeted ubiquitin ligase (STUbL)TOPORS promotes multiple STUbL-driven processes in conjunction with RNF4Combined TOPORS and RNF4 loss is synthetic lethal in human cells

Genome-Wide CRISPR-Cas9 Screening Identifies a Synergy between Hypomethylating Agents and SUMOylation Blockade in MDS/AML

“…Parallel investigations by others performing similar CRISPR screens identified TOPORS as a SUMO-targeted ubiquitin E3 ligase that acts in semi-redundant concert with RNF4 to mediate efficient proteasomal degradation of DNA-adducted DNMT1 62,63 . Nonetheless, our 20 data strongly indicate that ubiquitylation of SUMOylated DNMT1 adducts is not the only mechanism by which TOPORS protects HMA-exposed cells from DDR-induced apoptosis.…”

“…Parallel investigations by others performing similar CRISPR screens identified TOPORS as a SUMO-targeted ubiquitin E3 ligase that acts in semi-redundant concert with RNF4 to mediate efficient proteasomal degradation of DNA-adducted DNMT1 62,63 Our proteomics identified RNA splicing factors as key candidates for TOPORS-mediated SUMOylation or ubiquitylation in AML cells, even in the absence of HMA, and characterized widespread mis-splicing of DNA repair genes in TOPORS-edited cells, likely due in part to aberrant SUMO-or ubiquitin-modulation of interacting splicing factors. These results draw similarities to a recent study which showed that splicing modulators targeting SF3B1 triggered enhanced exon-skipping in DNA damage repair genes 64 .…”

Transcription-coupled repair of DNA–protein cross-links depends on CSA and CSB

“…DPC proteolysis can be initiated upon collision with the DNA replication machinery 18 causing SPRTN activation 13,19 and replication-coupled ubiquitylation, targeting DPCs for proteasomal degradation 13,20 . Additionally 3,[10][11][12] , global-genome (GG) DPC repair occurs through DPC SUMOylation and subsequent SUMO-dependent polyubiquitylation by RNF4 or TOPORS, which triggers proteasomal degradation [21][22][23][24][25] or cleavage by SPRTN 26 .…”