Importin-β and CRM1 control a RANBP2 spatiotemporal switch essential for mitotic kinetochore function

Protein conjugation with SUMO is a post-translational modification that modulates protein interactions and localisation. RANBP2 is a large nucleoporin endowed with SUMO E3 ligase and SUMO-stabilising activity and is implicated in some cancer types. RANBP2 is part of a larger complex, comprising SUMO-modified RANGAP1, the GTP-hydrolysis activating factor for the GTPase RAN. During mitosis, the RANBP2/SUMO-RANGAP1 complex localises to the mitotic spindle and to kinetochores after microtubule attachment. Here we have addressed the mechanisms that regulate this localisation and how they affect kinetochore functions. Using proximity ligation assays, we find that nuclear transport receptors importin beta and CRM1 play critical roles in localising the RANBP2/SUMO-RANGAP1 complex away from, or at KTs, respectively. Using newly generated inducible cell lines, we show that overexpression of nuclear transport receptors affects the timing of RANBP2 localisation in opposite manners. Concomitantly, kinetochore functions are also affected, including the accumulation of SUMO-conjugated topoisomerase II alpha and kinetochore-fibre stability. These results delineate a novel mechanism through which nuclear transport receptors govern the functional state of kinetochores by regulating the timely deposition of RANBP2 at kinetochores.