Nuclear Protein Sorting

Nuclear pore complexes (NPCs) are tight aqueous pores embedded in the nuclear membrane that enable selective transport of proteins into and out of the nucleus. Each NPC dilates its channel diameter to accommodate large folded peptides as they squeeze through. NPCs contain hydrophobic FG repeats that bind nuclear transport receptors, allowing receptor-cargo complexes to wriggle across the pore through repeated binding and dissociation.

Nuclear localization signals (NLS) are specific amino acid sequences on cargo proteins recognized by cytosolic import receptors. These receptors bind the NLS-containing protein and form a receptor-cargo complex that docks onto the nuclear pore complex. The complex then translocates through the NPC, with the NLS serving as the molecular address that ensures proteins reach their correct nuclear destination.

FG repeats are hydrophobic amino acid sequences of phenylalanine and glycine that line the inner channel of the nuclear pore complex. These repeats function as binding sites for nuclear transport receptors and form gel-like matrices that facilitate selective transport. Transient interactions between receptor-cargo complexes and FG repeats help dissolve the matrix gel phase, allowing proteins to move through the pore.

On the nuclear side of the pore, Ran bound to GTP associates with the incoming receptor-cargo complex. This interaction induces a conformational change in the import receptor, causing it to release the cargo protein inside the nucleus. The conformational change also allows the receptor to return to the cytosol for the next round of protein transport, making Ran-GTP essential for completing the import cycle.

Nuclear import brings proteins into the nucleus through recognition of nuclear localization signals by import receptors. Nuclear export removes proteins and RNA from the nucleus through recognition of nuclear export signals by export receptors. Both processes use similar mechanisms involving the nuclear pore complex and Ran-GTP, but export involves Ran-GTP hydrolysis on cytoplasmic fibrils to dissociate cargo and receptors.

The Brownian affinity gating model proposes random movement of receptor-cargo complexes across the NPC through selective interactions with nucleoporins. The selective phase model suggests FG repeats form gel-like matrices that transient interactions help dissolve as the complex moves through. Both models explain how cargo selectively navigates the NPC while maintaining nuclear-cytoplasmic separation.

Nuclear protein sorting is a tightly controlled process that selectively transports histones, polymerases, and gene regulatory proteins into the nucleus while exporting RNAs and ribosomes to the cytosol. By regulating which proteins enter the nucleus and which RNA molecules exit, this process controls the availability of transcription factors and gene expression machinery, thereby regulating overall gene expression within the cell.