Plays a role in RAN-dependent nucleocytoplasmic transport. Alleviates the TNPO1-dependent inhibition of RAN GTPase activity and mediates the dissociation of RAN from proteins involved in transport into the nucleus (By similarity). Induces a conformation change in the complex formed by XPO1 and RAN that triggers the release of the nuclear export signal of cargo proteins (By similarity). Promotes the disassembly of the complex formed by RAN and importin beta. Promotes dissociation of RAN from a complex with KPNA2 and CSE1L (By similarity). Required for normal mitotic spindle assembly and normal progress through mitosis via its effect on RAN. Does not increase the RAN GTPase activity by itself, but increases GTP hydrolysis mediated by RANGAP1. Inhibits RCC1-dependent exchange of RAN-bound GDP by GTP (By similarity). Interacts with RAN (via C-terminus of GTP-bound form) but not with GDP-bound RAN. Identified in a complex composed of RAN, RANGAP1 and RANBP1 (By similarity). Identified in a complex that contains TNPO1, RAN and RANBP1. Identified in a complex that contains CSE1L, KPNA2, RAN and RANBP1 (By similarity). Identified in a complex with nucleotide-free RAN and RCC1 (By similarity). Belongs to the RANBP1 family.