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 (PubMed:20485264). 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 (PubMed:17671426). Does not increase the RAN GTPase activity by itself, but increases GTP hydrolysis mediated by RANGAP1 (PubMed:7882974). Inhibits RCC1-dependent exchange of RAN-bound GDP by GTP (PubMed:7882974, PubMed:7616957). Interacts with RAN (via C-terminus of GTP-bound form) but not with GDP-bound RAN (PubMed:7882974, PubMed:7891706, PubMed:8896452). Identified in a complex composed of RAN, RANGAP1 and RANBP1 (PubMed:16428860, PubMed:11832950). 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 (PubMed:7882974, PubMed:7616957). Belongs to the RANBP1 family.