The pore

The nuclear pore complex (NPC) acts as a supramolecular sieve that spans both membranes of the nuclear envelope. The proteins that make up the pore are called nucleoporins. Parallel rings, exhibiting octagonal symmetry, are situated near the inner and outer membranes of the nuclear envelope (Web Figure 1.4.A). Various nucleoporin proteins form the other structures, such as the nuclear ring, cytoplasmic filaments, and the nuclear basket. Small metabolites diffuse freely through the NPC, but the flow of large proteins and RNA is selective. According to a recent model, selectivity is derived from a meshwork of unstructured nucleoporins within the aqueous pore. To penetrate this meshwork into or out of the nucleus, macromolecules must be associated with specific carrier proteins, such as karyopherins. Ribosomes presumably exit the nucleus by physically moving through the meshwork.

Web Figure 1.4.A  The nuclear pore complex

The proteins involved in import and export

Three proteins required for nuclear import and export have been identified: Ran, a small GTPase, and carrier proteins that traverse the nuclear pore, which are called karyopherins (importins, exportins, and transportins). Although this trafficking pathway is shared by both plants and animals, plant importins recognize a wider spectrum of nuclear localization signals than animal importins.

A model for the movement of proteins into and out of the nucleus is shown in Web Figure 1.4.B. The important feature of the model is that importin without cargo exits the nucleus as a complex with Ran-GTP, whereas importin with cargo enters the nucleus alone.

Web Figure 1.4.B  The mechanism of protein import into the nucleus. (After Lodish et al. 2004.)

For import from the cytoplasm across the nuclear envelope, specific karyopherins recognize their cargo (usually with a nuclear localization sequence, or NLS) on the cytoplasmic side of the membrane, where there are low levels of Ran-GTP. Binding to the nuclear pore complex and translocation of the karyopherin-cargo complex across it is mediated by the karyopherin. Once inside the nucleoplasm (the region inside the nuclear envelope), Ran-GTP binds to the karyopherin-cargo complex and stimulates release of the cargo. The karyopherin, with its associated Ran-GTP, is translocated through the nuclear pore back into the cytoplasm, where the GTP of the Ran is hydrolyzed to GDP and the Ran-GDP dissociates from the karyopherin, making the karyopherin available for another round of transport. Hence, during import, the affinity of the karyopherin for its cargo is decreased by association with Ran.

For export, a similar translocation process occurs, but the affinity of the export karyopherin for its cargo is increased by its association with Ran. Some karyopherins can operate in both directions, that is for both import and export, but they carry different cargo in the different directions. They can even carry certain small RNAs (e.g., tRNAs), not just protein, but use a different recognition pathway (not NLS’s) for binding this cargo.