In vivo modeling of polysumoylation uncovers targeting of Topoisomerase II to the nucleolus via optimal level of SUMO modification.

Conjugation of SUMO to target proteins is an essential eukaryotic regulatory pathway. Multiple potential SUMO substrates were identified among nuclear and chromatin proteins by proteomic approaches. However, the functional roles of SUMO-modified pools of individual proteins remain largely obscure, as only a small fraction of a given target is sumoylated and therefore is experimentally inaccessible. To overcome this technical difficulty in case of Topoisomerase II, we employed constitutive SUMO modification, enabling tracking of modified Top2p, not only biochemically but also cytologically and genetically. Topoisomerase II fused to a critical number of SUMO repeats is concentrated at the specific intranuclear domain, the nucleolus, when more than four SUMO moieties are added, indicating that fused SUMO repeats are biologically active. Further analysis has established that poly-sumoylation of Top2p is required for the stable maintenance of the nucleolar organizer, linking SUMO-mediated targeting to functional maintenance of ribosomal RNA gene cluster.