The nucleoporin CAN/Nup214 binds to both the cytoplasmic and the nucleoplasmic sides of the nuclear pore complex in overexpressing cells

CAN/Nup214, an essential component of the vertebrate nuclear pore complex (NPC), is required for proper cell cycle progression and nucleocytoplasmic transport. It is a member of the FG-repeat-containing family of nucleoporins and has been localized to the cytoplasmic face of the NPC. Indirect immunofluorescence studies with specific antibodies have shown that moderate overexpression of human CAN in HeLa cells causes an increase in CAN/Nup214 levels at the nuclear envelope. Here, we demonstrate that in such HeLa cells, CAN/Nup214 does not localize exclusively to the cytoplasmic side of the NPC. Cryosections, stained with CAN-specific antibodies and examined by electron microscopy, showed that about one-third of the gold-labeled NPCs were decorated at the cytoplasmic face and the remaining two-thirds at the nucleoplasmic face. These data indicate that both the cytoplasmic fibrils and the nuclear basket of the vertebrate NPC contain specific binding sites for either CAN/Nup214 or for its interacting proteins, Nup88 and hCRM1. Thus, it is conceivable that CAN/Nup214 functions in nucleocytoplasmic transport at both faces of the NPC.