Three main classes of protein folding accessory proteins
i) Molecular chaperones include heat shock proteins 70 (Hsp 70), the chaperonins and nucleoplasmins.
Function: prevent the improper folding and aggregation of proteins.
Chaperonins: are cylindrical protein complexes that contain chambers in which newly synthesised protein can fold without interaction from other macromolecules.
Eg: TRiC chaperonin: assist in 15% of polypeptide folding in mammalian cell.
Families of Molecular Chaperones
1. Small heat shock proteins (hsp25)
- protect against cellular stress
- prevent aggregation in the lens (cataract)
2. Hsp60 system (cpn60, GroEL) ATPase
- protein folding
3. Hsp70 system (DnaK, BiP) ATPase
- stabilization of extended chains
- membrane translocation
- regulation of the heat shock response
4. Hsp90 ATPase
- binding and stabilization/ regulation of steroid receptors, protein kinases
5. Hsp100 (Clp) ATPase
- thermotolerance, proteolysis, resolubilization of aggregates
6. Calnexin, calreticulin
- glycoprotein maturation in the ER
- quality control
ii) Protein prolyl cis-trans isomerase
In proline containing polypeptides, the conversion of X-proline peptide bonds between their cis and trans confirmation is catalyzed by this protein.
|Protein prolyl cis-trans isomerase|
iii) Protein disulphide isomerase
Catalyse shuffling of disulphide bonds in a protein until it attains correct pairing or folding.
|Protein disulphide isomerase (PDI)|