Cell Biology
Nuclear Pore
Selective gateway between nucleus and cytoplasm — the largest protein complex
Nuclear pores are large protein complexes (~1000 proteins; ~120 MDa) that span the double membrane of the nucleus. They selectively control what passes between nucleus and cytoplasm. Small molecules (<40 kDa) diffuse freely; larger molecules require nuclear localization signals (NLS) recognized by importins (entry) or nuclear export signals (NES) recognized by exportins (exit). Run by Ran GTPase gradient: RanGTP in nucleus, RanGDP in cytoplasm. ~3000 nuclear pores per mammalian cell. Largest protein assembly in cell. Critical for: gene expression, signaling, mRNA export, transcription factor regulation.
- Size~1000 proteins; ~120 MDa
- Diameter~120 nm
- Number per cell~3000 in mammalian
- Free diffusionMolecules <40 kDa
- Active transportImportin/exportin + Ran GTPase
- Largest cellular complexNPC is one of largest protein assemblies
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Why nuclear pores matter
- Gene expression. Controls TF entry, mRNA export.
- Cell biology. Largest protein complex.
- Disease. Mutations in Nups cause diseases.
- Aging. NPC components long-lived; affect aging.
- Drug targets. Nuclear export inhibitors (selinexor; cancer).
- Viral infection. Viruses use/exploit NPCs.
- Pharmacology. Nuclear-localized drugs.
Common misconceptions
- Pores are simple holes. Highly organized; selective.
- Free diffusion only. Active transport for large molecules.
- One pore per nucleus. Thousands.
- Same as plasma membrane channels. Different scale, mechanism.
- NPC permanent. Disassembles in mitosis.
- Pore controls all nucleus contents. Some restricted by other mechanisms.
Frequently asked questions
How do nuclear pores work?
Large protein channel through nuclear envelope. Inner ring + outer ring + spoke ring + nucleoporins (FG-Nups) lining channel. FG repeats (phenylalanine-glycine) form selective barrier — disordered regions create hydrogel-like environment. Specific transport: cargo + adapter (importin/exportin) interacts with FG-Nups → translocation through pore. Fast: ~1000 events per pore per second.
What's a nuclear localization signal (NLS)?
Amino acid sequence directing protein to nucleus. Classical NLS: lysine/arginine rich (e.g., PKKKRKV from SV40 large T antigen). Bipartite NLS: two basic clusters separated by spacer. Recognized by importin α (binds NLS); importin α-importin β complex binds cargo; binds nuclear pore. Allows specific nuclear targeting. Many proteins have NLS.
How does Ran work?
Small GTPase (Ras-related). RanGTP and RanGDP differ in nucleotide bound. Compartmental gradient: high RanGTP in nucleus (RanGEF generates); high RanGDP in cytoplasm (RanGAP hydrolyzes). Importin-cargo binds in cytoplasm (RanGDP); enters nucleus; RanGTP dissociates complex (releasing cargo). Exportin-cargo binds in nucleus (RanGTP); exits; RanGTP hydrolyzed in cytoplasm; cargo released. Asymmetry drives transport.
What's translocated through pore?
Many things. Imports: histones, transcription factors, nuclear proteins, DNA repair enzymes, polymerases, etc. Exports: mRNA (with adapters), tRNA, ribosomal subunits, proteins for cytoplasmic destinations. Plus: small molecules (ions, ATP, GTP) diffuse freely. Most cellular nucleoplasmic proteins must be imported via this system.
How is mRNA exported?
Specific mechanism. mRNA + nuclear export factor (NXF1/TAP-p15) → bound to FG-Nups → exported. After export: NXF1 released; mRNA can be translated. Different from protein export (RanGTP-dependent). NXF1 system: independent of Ran; uses different mechanism. Critical for gene expression: regulates which mRNAs reach cytoplasm.
What's NPC structure?
8-fold symmetry around central channel. Cytoplasmic ring + nuclear ring + central spoke ring + nucleoplasmic basket (filaments extending into nucleus). Each ring composed of ~16 nucleoporin subunits. 30+ different nucleoporin proteins (Nups). FG-Nups line channel. Total: 1000+ proteins. Cryo-EM structures revealed in detail recently.
How is NPC formed?
During cell cycle. Vertebrates: at mitosis, NPC disassembles with nuclear envelope. Reassembles in telophase. Mechanism: ESCRT complexes facilitate annular fusion + assembly of Nups. New NPCs also formed in interphase to support growth. Insertion into existing nuclear envelope. Coordinated with cell cycle.