Nucleoporin 50 kDa (NUP50) acts as a cofactor for the importin-alpha:importin-beta heterodimer, which in turn allows for transportation of many nuclear-targeted proteins through nuclear pore complexes. The C terminus of NUP50 binds importin-beta through RAN-GTP, the N terminus binds the C terminus of importin-alpha, while a central domain binds importin-beta. NUP50:importin-alpha:importin-beta then binds cargo and can stimulate nuclear import. The N-terminal domain of NUP50 is also able to actively displace nuclear localisation signals from importin-alpha [1].

1: Nup50/Npap60 function in nuclear protein import complex disassembly and importin recycling. Matsuura Y, Stewart M; EMBO J. 2005;24:3681-3689. PMID:16222336