Schematic of the nuclear pore complex. The NPC is embedded in a double membrane structure separating the nucleus and cytoplasm. It is composed of several subcomplexes, including the peripheral, membrane, channel FG, and scaffold Nups. Whereas the peripheral, channel, and membrane Nups turnover continuously, the scaffold Nups are stable. We propose this stability is necessary and that rapid exchange or loss of these components would result in a disrupted permeability barrier. Thus, maintenance or turnover of these scaffold complexes is necessarily slow, if at all.