SUMO2 Regulates Histone Pre-mRNA Processing by Stabilizing Histone Locus Body Interactions and Promoting U7 snRNP Assembly

Histone mRNAs are the only non-polyadenylated mRNAs in eukaryotic cells and require specialized processing in nuclear organelles known as histone locus bodies (HLBs), where essential processing factors are concentrated, including the U7 snRNP complex. Recent studies have revealed that misregulation of histone pre-mRNA processing can lead to polyadenylation of histone mRNAs and disruption of histone protein homeostasis. Despite the links to human disease, the factors contributing to polyadenylation of histone mRNAs and the mechanisms underlying HLB assembly and U7 snRNP biogenesis remain unclear. Here, we report novel functions of the small ubiquitin-related modifier 2 (SUMO2) in regulating histone pre-mRNA processing. Using a SUMO2 knockout osteosarcoma cell line, we identified a defect in 3’ end cleavage and a global increase in histone mRNA polyadenylation. Subsequent analysis of HLBs revealed increased dynamics and reduced levels of the U7 snRNP complex. By over-expressing U7 snRNP-specific components, Lsm11 and U7 snRNA, we partially rescued U7 snRNP levels and processing defects in SUMO2 knockout cells. Through analysis of Lsm11, we identified a SUMO-interacting motif in its N-terminus required for efficient incorporation into U7 snRNP complexes. Collectively, we demonstrate that SUMO2 promotes histone pre-mRNA 3’ end processing through stabilizing HLB interactions and facilitating U7 snRNP assembly We previously generated U2OS SUMO1 knockout (S1KO) and SUMO2 knockout (S2KO) cell lines with CRISPR/Cas 9, as well as SUMO2 knockout rescue cell lines by stable re-expression of SUMO2 (S2R) (Bouchard, Wang et al. 2021).