Nuclear localisation sequence and importins
A run of basic (positively charged) amino acids, the nuclear localisation sequence, acts as an address label that importins (carrier proteins) recognise to carry a protein into the nucleus.
Remember
The address label that sends a protein into the nucleus is a short run of basic amino acids, the nuclear localisation sequence, which importins recognise before carrying the protein through a nuclear pore.
All the proteins found in the nucleus are produced in the cytoplasm, so they need to be transported in. The way in is a nuclear pore, a protein channel through the nuclear envelope (the double membrane around the nucleus).
Proteins targeted for the nucleus contain an address label, the nuclear localisation sequence (NLS): a run of about 4 to 8 basic amino acids in the polypeptide chain. Basic here means positively charged, as in the amino acids lysine and arginine.
In the cytosol (the fluid part of the cell), carrier proteins called importins bind the NLS. The protein and the importins form a complex, which binds to a nuclear pore and is moved into the nucleus using energy. The cargo (the protein being carried) can stay folded throughout, unlike mitochondrial proteins, which cross their membranes in an unfolded state.
Once the complex is inside, the cargo is released. The importins then bind a protein called Ran and leave the nucleus along with it, so they can be reused for more cargo. Ran is recycled too, and this keeps transport running in one direction, into the nucleus.
Small molecules can diffuse through nuclear pores, but larger cargo is held back unless transport receptors such as importins carry it.
Key facts
- Made in
- the cytoplasm, which is where all the proteins found in the nucleus are produced before they are transported in.
- Address label
- the nuclear localisation sequence is a run of about 4 to 8 basic amino acids.
- Reader
- importins bind the nuclear localisation sequence in the cytosol and form a complex with the protein.
- Route
- the complex binds a nuclear pore and is moved through using energy.
- Folded entry
- a protein with a nuclear localisation sequence can enter the nucleus while still folded.
- Direction
- recycling of Ran keeps transport going one way, into the nucleus.
Exam traps
- A signal peptide, an address label at the start of the chain, sends a protein into the endoplasmic reticulum, so it is the wrong address for the nucleus.
- Diffusing freely through the nuclear pores without assistance does not target a protein to the nucleus: diffusion is unselective, and larger cargo is held back unless transport receptors such as importins carry it.
- A KDEL sequence at the C-terminal end (the end of the chain made last) is a retrieval signal that returns escaped endoplasmic reticulum proteins, so it is not a nuclear import signal.
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