Chaperonins (CPN) are oligomeric proteins that mediate the folding of polypeptide chains. Group I CPN are found in bacteria, chloroplasts and mitochondria. For an introductory overview, see Chaperonins in Wikipedia.
The most characterized CPN are in the GroEL/GroES complex from Escherichia coli and CPN60/CPN10 from Thermus thermophilus. The larger subunit (GroEL, CPN60) contains 3 domains. The apical domain is the one which binds the substrate. Group II CPNs are found in eukaryotic cytosol and archaea. Thermosome is a CPN complex found in archaea. CCT is a CPN complex found in eukarya.
See also Chaperones.