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From Proteopedia
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NUP50:IMPORTIN-ALPHA COMPLEX
Template:ABSTRACT PUBMED 16222336
About this Structure
2c1m is a 2 chain structure with sequence from Mus musculus. Full crystallographic information is available from OCA.
See Also
IMPORTIN ALPHA STRUCTURE
Importin α is a soluble adaptor protein also known as karyopherin α. Its function is to bind a protein containing a cNLS (classical Nuclear Localization Signal) and then bind an importin β in order to import this protein in the nucleus. 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 any 10 residues.
- At least three basic residues (Arg or Lys) in the five positions after the spacer region.
Importin α is composed of different domains:
- A flexible, hydrophilic 10kDa N-terminal Importin β binding domain (IBB domain). The IBB domain is an L-shapped molecule with an N-terminal extended moiety and a C-terminal helix running in mutually perpendicular directions. Because this domain is highly positively charged, it can binds to the inner surface of importin-β that contains many acidic residues. It has been shown that importin α contains a determinant which is sufficient for binding importin β. The consensus sequence of this determinant is KFRLLSKE. The serine contained in this sequence is present in all importin α which shows its importance. However, the upstream region is sufficient for binding importin β too. Nowadays, we think that this region contribute to the strength of the bond. This could explain the fact that the binding between importin α and β is stronger when α contains these two determinants.
- A 50kDa C-terminal NLS-binding site composed of 10 tandem armadillo (Arm) repeats. These arm repeat domains have an elongated superhelical structure and each of them contains 3 α-helices (H1, H2, H3). H3 helices define the inner concave surface of the protein and the NLS-binding site.
- 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.
- A CAS-binding site. CAS or cellular apoptosis susceptibility protein is an exportin which in the nucleus is bound to RanGTP.
Reference
- 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
