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Nuclear import of proteins

In eukaryotic cells, a lot of proteins are selectively imported from the cytosol to the nucleus through nuclear pores.

NPCs (Nuclear Pore Complexes) are composed of many nucleoporins and are heavy proteins assembly. These complexes allow the passive diffusion of ions and the active transport of large molecules such as proteins in both directions. Because of the particular shape of NPCs, transport of large proteins take a lot of time. Thus, proteins over 60 kDa have trouble to pass through them that’s why they need complementary proteins : importins to go in the nucleus and exportins to go out of the nucleus. [1]

Selectivity of nuclear import

Proteins which have to go in the nucleus all possess a NLS (Nuclear Localization Signal) which is responsible for the selectivity of the active transport of proteins through NPCs.

A NLS is a basic residue-rich sequence with the following consensus sequence :

  • Two adjacent basic amino acids (Arg or Lys).
  • A spacer region of 10 residues.
  • At least three basic residues (Arg or Lys) in the five positions after the spacer region.[2]

NLS can be found at any place of the amino acid sequence and often have a loop structure at the surface of proteins.

The transport through NPC is not the same as transmembrane transport in organelles because in this case, it is an aquifer pore whereas transmembrane transport in organelles involves transmembrane proteins. Thus, fully folded nuclear proteins can pass through NPCs. Nevertheless, it seems that really large proteins undergo a compression when they pass through NPC. [1]

Here you can see a bound to the NLS-binding site of importin α.

  • A NLS. Thus, importin α belongs to the group of proteins containing both a ligand (NLS) and a cognate receptor (NLS-binding site). That’s why it could have a possibility of autologous ligand-receptor interactions. Nevertheless, it has been shown that NLS of importin α overlaps with the IBB. Thereby, binding of importin β to importin α covers the NLS of importin α preventing autologous ligand receptor interactions.

As for the IBB, this NLS domain is not visible in this structure as it is located in the 45-54 region. However, the overlap of the NLS by the IBB is proved by this information.

  • A CAS-binding site.

Importin α : Nup50 complex

Complex between mouse importin α(residues 70-529) and Nup50(or Npap60)

Drag the structure with the mouse to rotate


and


See Also

Contributors

Marmin Lucas/ Talide Loïc

ESBS 1A 2012

Reference

  1. ↑ 1.0 1.1 ALBERTS,B. JOHNSON,A. WALTER,P. LEWIS,J. RAFF,M. ROBERTS,K. (2007). Molecular Biology of the Cell (p. 704-712)
  2. ↑ http://prosite.expasy.org/PDOC00015
  • Matsuura Y, Stewart M. Nup50/Npap60 function in nuclear protein import complex disassembly and importin recycling. EMBO J. 2005 Nov 2;24(21):3681-9. Epub 2005 Oct 13. PMID:16222336
  • Lu Q, Lu Z, Liu Q, Guo L, Ren H, Fu J, Jiang Q, Clarke PR, Zhang C. Chromatin-bound NLS proteins recruit membrane vesicles and nucleoporins for nuclear envelope assembly via importin-alpha/beta. Cell Res. 2012 Nov;22(11):1562-75. doi: 10.1038/cr.2012.113. Epub 2012 Jul 31. PMID:22847741 doi:10.1038/cr.2012.113
  • Ogawa Y, Miyamoto Y, Oka M, Yoneda Y. The interaction between importin-alpha and Nup153 promotes importin-alpha/beta-mediated nuclear import. Traffic. 2012 Jul;13(7):934-46. doi: 10.1111/j.1600-0854.2012.01367.x. Epub 2012 , May 14. PMID:22510057 doi:10.1111/j.1600-0854.2012.01367.x
  • Moroianu J, Blobel G, Radu A. The binding site of karyopherin alpha for karyopherin beta overlaps with a nuclear localization sequence. Proc Natl Acad Sci U S A. 1996 Jun 25;93(13):6572-6. PMID:8692858
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