Sandbox Reserved 709
From Proteopedia
Contents |
Nuclear import of proteins
Cytoplasm to nucleus
Cargo release and protein recycling
Once the import complex (made of importin α, importin β and cargo protein) enters the nucleus, it must be dissociated to release the cargo protein. Then, importin α must be recycled to the cytoplasm. Cargo release is driven by Ran, a G-protein that is found in Ran-GTP form in the nucleus due to a high rate of Ran-GEF. It binds to importin β, causing an important change in shape. An other protein, Nup50 (also called Npap60), weakens the link between importin α and the NLS of the cargo. Thus the cargo protein is now free in the nucleoplasm. Importin β can pass through the NPC thanks to Ran-GTP, and once it is in the cytoplasm, it is released by the hydrolysis of Ran-GTP into Ran-GDP due to a higher rate of Ran-GAP in the cytoplasm. But importin α can’t leave the nucleus only with the help of Ran-GTP. It needs also interaction with CAS (Cellular Apoptosis Susceptibility protein), an exportin which is similar in shape to importin β. Binding between CAS and importin α is favoured by interactions with Nup50. The newly formed importin α, CAS and Ran-GTP complex is able to interact with nucleoplasmins of NPC, and once in the cytoplasm, Ran-GTP is hydrolysed into Ran-GDP. The complex splits, and importin α is now ready for a new import cycle.
Importin α 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 to bind an importin β in order to help the import of this protein in the nucleus. A cNLS 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.
Importin α is composed of different domains:
- A flexible and hydrophilic 10kDa N-terminal Importin β binding domain (IBB domain). The IBB domain is a 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 upstream 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). Together, 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.
Importin α : Nup50 complex
|
See Also
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
