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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Cells keep DNA inside a membrane-bound nucleus, but the nucleus is not sealed off: nuclear pore complexes regulate the movement of molecules across its envelope. Small molecules and some small proteins can diffuse through; many larger proteins and RNAs need selective transport signals and receptor proteins. This controlled access lets cells determine which gene-regulating proteins can reach DNA.
How does a molecule cross the nuclear envelope?
The nuclear envelope consists of inner and outer membranes that separate the genome from the cytoplasm. Nuclear pore complexes span that boundary and provide routes for exchange. They are not conscious gatekeepers; molecular properties and transport machinery determine which cargo passes and how.
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Some small molecules, ions, and small proteins passively diffuse through pores. Alberts and colleagues give roughly 50 kDa as a broad textbook estimate for proteins that may diffuse, not a strict cutoff that applies to every protein or cell. Many larger macromolecules require selective transport. Cooper’s cell biology textbook describes nuclear pore complexes as fundamental to traffic between the nucleus and cytoplasm.
How do proteins get into the nucleus?
- A cargo carries an import signal. A nuclear localization signal (NLS) helps identify a protein for import.
- A transport receptor recognizes the cargo. The receptor may bind the NLS directly or connect to the cargo through an adaptor.
- The complex crosses through a nuclear pore complex. Receptor interactions with the pore machinery support passage across the envelope.
- Ran helps regulate the transport cycle. This small GTPase regulates interactions between transport receptors and cargo, contributing to the directionality of transport.
The signal and receptor make import selective: the pore is a route, while the transport machinery helps determine which larger cargo uses it.
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How does export work?
Export carries selected cargo from the nucleus to the cytoplasm. Export signals help mark cargo, and export receptors recognize and transport it through nuclear pores. Ran also helps regulate receptor-cargo interactions in export pathways. Import and export use related transport machinery, but the cargo direction and the signals involved differ. As Alberts and colleagues put it, Ran “determines the directionality of transport.” Their chapter on transport between the nucleus and cytosol explains the pathway in more detail.
How does controlling nuclear access affect gene expression?
Many proteins that regulate genes must reach DNA in the nucleus to act. By changing whether such a protein is available to the transport machinery, a cell can alter whether it enters the nucleus. That changes which gene regulators can access nuclear DNA and can therefore influence gene expression. Nuclear pores are part of this control, alongside regulation of the cargo itself.
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How many nuclear pores does a cell have?
Alberts and colleagues give an estimate of 3,000–4,000 nuclear pore complexes for a typical mammalian cell. This is a textbook estimate, not a universal count for every cell. The same textbook gives an estimate of about 100 histone molecules imported per pore per minute in the specific context of a cell synthesizing DNA and importing histones to package newly made DNA. That rate should not be generalized to other cargo or cellular conditions.
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Passive diffusion and selective transport compared
| Feature | Passive diffusion | Selective transport |
|---|---|---|
| Typical cargo | Small molecules, ions, and some small proteins | Many larger proteins and RNAs |
| Signal or receptor | No transport signal or receptor is required for diffusion | Signals such as NLSs or export signals help receptors recognize cargo |
| Direction | Diffusion follows molecular movement across the boundary | Import or export pathways use regulated receptor interactions; Ran helps establish directionality |
The distinction is general rather than an absolute size rule: the roughly 50 kDa figure is an approximate textbook description, not a precise boundary for every cargo. For a structural overview, see the 2015 review “The nuclear pore complex – structure and function at a glance.” A 2026 review record on nucleocytoplasmic transport is available through PubMed.
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