Editing Protein targeting (section)

=== Nucleus ===
The nucleus of a cell is surrounded by a nuclear envelope consisting of two layers, with the inner layer providing structural support and anchorage for chromosomes and the nuclear lamina.<ref name="Alberts-2018" /> The outer layer is similar to the endoplasmic reticulum (ER) membrane. This envelope contains nuclear pores, which are complex structures made from around 30 different proteins.<ref name="Alberts-2018" /> These pores act as selective gates that control the flow of molecules into and out of the nucleus.

While small molecules can pass through these pores without issue, larger molecules, like RNA and proteins destined for the nucleus, must have specific signals to be allowed through.<ref name="Lodish-2008" /> These signals are known as nuclear localization signals, usually comprising short sequences rich in positively charged amino acids like lysine or arginine.<ref name="Alberts-2018" /> 

Proteins called nuclear import receptors recognize these signals and guide the large molecules through the nuclear pores by interacting with the disordered, mesh-like proteins that fill the pore.<ref name="Alberts-2018" /> The process is dynamic, with the receptor moving the molecule through the meshwork until it reaches the nucleus.<ref name="Lodish-2008" />

Once inside, a GTPase enzyme called Ran, which can exist in two different forms (one bound to GTP and the other to GDP), facilitates the release of the cargo inside the nucleus and recycles the receptor back to the cytosol.<ref name="Alberts-2018" /><ref name="Lodish-2008" /> The energy for this transport comes from the hydrolysis of GTP by Ran. Similarly, nuclear export receptors help move proteins and RNA out of the nucleus using a different signal and also harnessing Ran's energy conversion.<ref name="Alberts-2018" />

Overall, the nuclear pore complex works efficiently to transport macromolecules at high speed, allowing proteins to move in their folded state and ribosomal components as complete particles, which is distinct from how proteins are transported into most other organelles.<ref name="Alberts-2018" />