Electron Microscopy: Nuclear Envelope »
 Electron Microscopy: Nuclear Pore Complex »


The Nuclear Pore Complex

The Nuclear Pore Complex (NPC) provides a channel for transport of substances between the nucleus and the cytoplasm. There are roughly 3000 NPCs situated in the nuclear envelope. Smaller molecules that are less than 9 nm in diameter, like ions and metabolites, may freely diffuse through the NPC between the nucleus and the cytoplasm. Larger molecules, between 9 and 28 nm in diameter, must be actively transported through the NPC in a controlled process that is selective and energy dependent. Each pore is capable of transport in both directions, apparently even at the same time.

The NPC is cylindrical and displays an 8-fold symmetry. At the center of the pore is a spoke assembly of 8 annular units (blue) anchored to the membrane by luminar subunits (red). Attached by column subunits (orange) are two rings (yellow), one facing the nucleus and the other the cytoplasm. The nucleoplasmic side of the NPC is decorated with fibrils. On the nucleoplasmic side a cage-like assembly is attached.

The nucleoplasmic side and the cytoplasmic side of the NPC are very different. This causes the differences in the selective transport in the two directions.