The nucleoporin and SUMO E3 ligase NUP358/RANBP2 regulates the mitotic factor NuSAP1 at the kinetochore/microtubule interface

Protein conjugation with SUMO (Small Ubiquitin-like MOdifier) peptides modulates protein interactions and localization in many cellular dynamic processes, e.g. DNA replication and repair, gene transcription, nuclear organization and others. Growing evidence also implicate protein SUMOylation in control of mitosis. Indeed, several factors acting in cycles of SUMOylation and deSUMOylation reside at kinetochores, including SUMO ligase enzymes (RANBP2 and members of PIAS family) and SUMO deconjugating enzymes (SENP proteins). RANBP2 is a large nucleoporin with SUMO E3 ligase and SUMO stabilizing activity. It has long been known that, after nuclear envelope breakdown and disassembly of nuclear pore complexes, RANBP2 localizes at microtubules and in part at kinetochores. By developing the Proximity Ligation Assay (PLA) technique, coupled with cell synchronization methods, we have characterized a highly regulated mechanism, involving the nuclear transport receptors Importin beta and CRM1, which timely tethers RANBP2 at kinetochores during mitosis.