Functions as an U snRNP-specific nuclear import adapter. Involved in the trimethylguanosine (m3G)-cap-dependent nuclear import of U snRNPs. Binds specifically to the terminal m3G-cap U snRNAs. Component of an import snRNP complex composed of KPNB1, SNUPN, SMN1 and ZNF259. Component of a nuclear export receptor complex composed of KPNB1, Ran, SNUPN and XPO1. Found in a trimeric export complex with SNUPN, Ran and XPO1. Interacts (via IBB domain) with KPNB1; the interaction is direct. Interacts with DDX20, IPO7, SMN1, SNRPB and XPO1. Interacts directly with XPO1. Its interaction with XPO1 and binding to m3G-cap U snRNPs appears to be mutually exclusive. Nucleoplasmic shuttling protein. Its nuclear import involves the nucleocytoplasmic transport receptor importin beta. It is re-exported to the cytoplasm by the XPO1-dependent nuclear export receptor pathway. Belongs to the snurportin family.