Npap60: a new player in nuclear protein import.

Until very recently, the vertebrate protein Npap60/Nup50 was thought merely to be a component of the nuclear pore complex (NPC). This conclusion was based on the observations that Npap60/Nup50 localizes at the NPC by immunofluorescence and electron microscopy and also contains FG (Phe-Gly) repeats, a motif commonly found in nucleoporins but not in proteins located elsewhere. However, far from being a fixed structural component of the NPC, it now appears as though Npap60 can shuttle from one side of the NPC to the other. Most significantly, a recent paper shows that Npap60 enhances the nuclear import of a cargo possessing a basic nuclear localization sequence by associating directly with the import cargo-carrier complex and (presumably) moving through the NPC with it. Several NPC proteins have now been shown to be mobile in the NPC, and this new report might indicate that these 'mobile' nucleoporins play a more active role in the nuclear transport of cargo than was previously appreciated.