The Nuclear Pore Complex and Ran-GTP Gating

Phenylalanine-Glycine Hydrogel Sieves, Importin Shuttles, and Directional mRNA Export

Weighing 110 to 120 megadaltons in vertebrates—roughly thirty times the mass of a bacterial ribosome—the Nuclear Pore Complex (NPC) is the largest protein assembly in the eukaryotic cell. Each pore fuses the inner and outer nuclear membranes into a circular grommet. Roughly 1,000 nucleoporin subunits assemble around that ring in eightfold rotational symmetry.

A proliferating human cell punches 3,000 to 5,000 nuclear pore complexes through its nuclear envelope. Every individual pore simultaneously conducts bidirectional traffic at up to 1,000 translocation events per second. That traffic moves a combined mass flux of 70 to 100 megadaltons per second of ribosomal subunits, messenger RNAs, histones, and transcription factors.

Although the rigid structural scaffold of the nuclear pore frames a wide 40- to 60-nanometer central channel, inert folded proteins larger than 40 kilodaltons (about 5 nanometers wide) cannot diffuse across passively. Yet a 25-nanometer pre-ribosomal subunit bound to transport receptors flashes across that exact same channel in less than 20 milliseconds.

Unlike voltage-gated ion channels or the $20\text{S}$ proteasome, the nuclear pore complex never opens or closes a mechanical iris to let giant cargoes pass. Instead, its central channel is filled with 200 intrinsically disordered polypeptide tentacles. Those flexible chains are studded with thousands of Phenylalanine-Glycine (FG) repeats that form a self-healing permeability barrier.

Dirk Görlich demonstrated in 2006 that concentrated solutions of purified FG-repeat domains spontaneously phase-separate into an elastic hydrogel. Hydrophobic stacking between aromatic phenylalanine rings links the disordered chains into a three-dimensional molecular sieve. Its four-nanometer mesh blocks inert macromolecules while instantly dissolving hydrophobic transport receptors that carry nuclear cargoes.

How does the pore recognize which cytoplasmic proteins belong inside the nucleus? Ronald Laskey and Alan Smith showed in 1982 that nuclear proteins carry a built-in address tag. Two years later, Alan Smith and Bruce Roberts pinpointed the SV40 Large T-antigen Nuclear Localization Signal (NLS) to seven basic amino acids (PKKKRKV) where mutating one lysine halts entry.

Soluble shuttle proteins called karyopherins—importins and exportins—wrap around their cargo's NLS tag while displaying shallow hydrophobic grooves across their outer convex surface. Those grooves transiently bind and release the phenylalanine rings of the FG hydrogel. A $100\text{-kDa}$ importin-cargo complex melts through the barrier ten thousand times faster than an untagged protein.

Surprisingly, no motor protein or ATPase inside the nuclear pore channel pushes importin-cargo complexes from cytoplasm to nucleus. Single-molecule fluorescence tracking shows that cargoes diffuse back and forth through the FG plug by random Brownian motion in five milliseconds. Net directionality arises entirely from an irreversible chemical latch at the destination side.

The thermodynamic battery that powers directional nuclear transport is a steep spatial gradient of the small GTPase protein Ran. The guanine-nucleotide exchange factor RCC1 is tethered directly to nuclear chromatin, while the GTPase-activating protein RanGAP1 sits in the cytoplasm. Nuclear Ran therefore stays locked in the Ran-GTP state at more than 100 times its cytoplasmic concentration.

When an importin-$\beta$ shuttle diffuses into the nucleus with its cargo, nuclear Ran-GTP binds to importin-$\beta$ and allosterically twists its HEAT-repeat superhelix to spring open the cargo-binding jaws. Conversely, nuclear exportins like CRM1 can grip a leucine-rich export signal only when locked to Ran-GTP. They drop their cargo in the cytoplasm once RanGAP1 triggers GTP hydrolysis.

Bulk messenger RNA export bypasses the Ran-GTP cycle altogether and runs on an ATP-driven RNA helicase ratchet. The heterodimer NXF1-NXT1 ferries spliced mRNA ribonucleoproteins through the FG mesh to the cytoplasmic face. Anchored exclusively to the pore's cytoplasmic filaments, the DEAD-box helicase DDX19 (Dbp5 in yeast) strips NXF1 off the RNA so it cannot slide backward.

While dividing cells continually replace their nuclear pore proteins, post-mitotic human neurons must keep the same core scaffold ring (the Nup107-160 complex) in place for eighty years without turnover. Martin Hetzer discovered in 2009 that oxidative damage slowly erodes these long-lived scaffold nucleoporins in aging brain cells. Old nuclear pores eventually leak cytoplasmic proteins into the nucleus.