16.3:
Nuclear Protein Sorting
Nuclear protein sorting occurs through nuclear pore complexes or NPCs embedded in the nuclear membrane.
Each NPC is a tight aqueous pore that allows the selective transport of folded peptides by dilating the channel diameter to accommodate large molecules as they squeeze through it.
Nuclear protein sorting occurs in two steps — recognition and transport.
During nuclear import, cytosolic receptors recognize and bind the cargo proteins via specific amino acid sequences called nuclear localization signals.
Hydrophobic amino acid sequences of phenylalanine and glycine called FG repeats line the NPC channel to function as binding sites for nuclear transport receptors.
The receptor-protein complex wriggles across the pore by repeated binding, dissociating, and rebinding to FG repeats.
On the nuclear side, a small monomeric GTPase—Ran, bound to GTP, associates with the receptor-cargo complex.
The receptor undergoes a conformational change to release the protein inside the nucleus and return to the cytosol for catalyzing the next round of protein transport.
The export of proteins and RNA from the nucleus occurs by a similar process and involves nuclear receptors recognizing nuclear export signals on the cargo.
16.3:
Nuclear Protein Sorting
Nuclear protein sorting is the selective trafficking of histones, polymerases, gene regulatory proteins into the nucleus and exporting RNAs and ribosomes to the cytosol. It is a tightly controlled process that regulates gene expression within a cell.
Proteins targeted to the nucleus carry nuclear localization signals or NLS recognized by import receptors in the cytosol. Similarly, proteins with nuclear export signals are recognized by export receptors. Import and export receptors are structurally related and freely move across the nucleus by facilitated diffusion. Upon cargo binding, the cargo-receptor complex docks onto the nuclear pore complex or NPC embedded on the nuclear membrane.
Two models describe docking and movement of the cargo-receptor complex across the NPC as follows:
- The Brownian affinity gating model suggests a random movement of the receptor-cargo complex across the NPC. The receptor-cargo complex selectively interacts with the nucleoporins lining the NPC to form transient bonds and facilitate their movement.
- The selective phase model suggests unstructured phenylalanine and glycine repeats called FG repeats on the inner channel of the NPC form gel-like matrices. Transient interactions with FG repeats help dissolve the matrix gel phase as the receptor-cargo move through the pore.
On the nuclear side, a small GTPase Ran, bound to GTP to the incoming cargo-importin complex, induces a conformational change in importin beta and cargo delivery inside the nucleus.
During nuclear export, Ran-GTP inside the nucleus associates with exportin to form the cargo-export complex and move to the cytosol. Ran-GTP hydrolysis on the cytoplasmic fibrils of NPC dissociates the cargo and receptor, releasing them into the cytosol.
Suggested Reading
- Alberts, Bruce, et al. Molecular Biology of the Cell. 6th ed. Garland Science, 2017. Pp 649-Pp652
- Cooper G.M, The cell, A molecular approach, 2nd edition, The nuclear envelope and traffic between the nuclear envelope and cytoplasm, NCBI bookshelf
- Alberts B et al., Molecular biology of cell, 4th edition, The transport of molecules between the nucleus and the cytosol, NCBI bookshelf
- Lu et al., Types of nuclear localization signals and mechanisms of protein import into the nucleus. Cell Commun Signal (2021) 19:60.
- IAN G. MACARA. Transport into and out of the Nucleus. MICROBIOLOGY AND MOLECULAR BIOLOGY REVIEWS, 1092-2172/01/$04.000 DOI: 10.1128/MMBR.65.4.570–594.2001 Dec. 2001, p. 570–594