This is the same nuclear-pore transport model as the 3D version, viewed as a flat cross-section through the envelope so every particle, pore and trajectory is visible in one plane at once. The nuclear envelope (inner circle) is punctured by nuclear pore complexes (NPCs) whose central channel is filled with a disordered mesh of FG-nucleoporins. Two transport regimes decide whether a gene-therapy nanoparticle ever reaches the genome:
Passive diffusion: works only for d < d_pass ≈ 5 nm (~40 kDa)
Facilitated (NLS): works for d up to d_max ≈ 39 nm, needs importin-β
Physical cutoff: d > d_max never crosses, any NLS count
For cargo in the facilitated window, translocation probability per pore encounter follows an avidity model — more nuclear-localization-signal (NLS) copies bind more importin-β molecules, increasing the odds of engaging the FG-mesh:
P_bind = 1 − exp(−k · N_NLS), k ≈ 0.55
P_ran = RanGTP_nuclear / (RanGTP_nuclear + K_m) (Hill-type gate)
P_cross = P_bind · P_ran (per pore-contact attempt)
- Cargo diameter — below ~5 nm, cargo diffuses through the pore freely on contact, exactly like a small protein or oligonucleotide. Above ~39 nm the channel is geometrically too narrow regardless of ligands — this is why bare plasmid DNA or bulky LNP-CRISPR ribonucleoprotein complexes stall outside the nucleus in non-dividing cells.
- NLS copies — each copy is a binding site for importin-β; avidity (not single-site affinity) drives whether the complex commits to translocation, which is why gene-therapy vectors are often engineered with multiple NLS peptides.
- RanGTP gradient — a nuclear-high / cytoplasmic-low RanGTP gradient (maintained by RCC1 and RanGAP) is what makes transport directional and irreversible: RanGTP inside the nucleus dissociates the importin–cargo complex, releasing cargo into the nucleoplasm. Collapse this gradient and even NLS-tagged cargo stalls mid-pore.
- Active NPCs — the number of pores embedded in the envelope; more pores mean more simultaneous contact opportunities, raising the population-level delivery rate without changing any single particle's odds.
Real-world relevance: this bottleneck is exactly why non-viral gene-therapy payloads (LNP-mRNA, CRISPR ribonucleoproteins, naked plasmids) reach the nucleus so inefficiently in post-mitotic cells such as neurons, cardiomyocytes and hepatocytes, where the envelope never breaks down during mitosis to give cargo a free pass — unlike dividing cells, where nuclear envelope breakdown briefly bypasses the NPC bottleneck entirely.