Nuclear transport

The entry and exit of large molecules from the cell nucleus is tightly controlled by the nuclear pore complexes (NPCs).

Protein that must be imported to the nucleus from the cytoplasm carry nuclear localization signals (NLS) that are bound by importins.

Nuclear export roughly reverses the import process; in the nucleus, the exportin binds the cargo and Ran-GTP and diffuses through the pore to the cytoplasm, where the complex dissociates.

This quality control mechanism is important due to tRNA's central role in translation, where it is involved in adding amino acids to a growing peptide chain.

Mutations that affect tRNA's structure inhibit its ability to bind to exportin-t, and consequentially, to be exported, providing the cell with another quality control step.

[7] Combining both NESs and NLSs promotes propagation of large proteins to more distant nuclei in muscle fibers.

Macromolecules , such as RNA and proteins , are actively transported across the nuclear membrane in a process called the Ran - GTP nuclear transport cycle.