Chaperonins, abbreviated Cpn, Cpn60, or HSP60, are a family of heat shock proteins that assist in the folding of newly synthesized proteins and refolding of misfolded proteins during stressful conditions such as high temperature. They are protein complexes consisting of about 16 protein subunits of ~60 kDa each. Chaperonins belong to a large class of molecules and complexes that assist protein folding, called molecular chaperones.
Newly made proteins usually must fold from a linear chain of amino acids into a three-dimensional tertiary structure. The energy to fold proteins is supplied by non-covalent interactions between the amino acid side chains of each protein, and by solvent effects. Most proteins spontaneously fold into their most stable three-dimensional conformation, which is usually also their functional conformation, but occasionally proteins mis-fold. Molecular chaperones catalyze protein refolding by accelerating partial unfolding of misfolded proteins, aided by energy supplied by the hydrolysis of adenosine triphosphate (ATP). Chaperonin proteins may also tag misfolded proteins to be degraded.
The original chaperonin is proposed to have evolved from a peroxiredoxin.
Classification
Group I
thumb|GroES/GroEL complex (side)
Group I chaperonins (Cpn60) are found in bacteria as well as organelles of endosymbiotic origin: chloroplasts and mitochondria.
The GroEL/GroES complex in E. coli is a Group I chaperonin and the best characterized large (~ 1 MDa) chaperonin complex.
- GroEL is a double-ring 14mer with a greasy hydrophobic patch at its opening and can accommodate the native folding of substrates 15-60 kDa in size.
- GroES (is a single-ring heptamer that binds to GroEL in the presence of ATP or transition state analogues of ATP hydrolysis, such as ADP-AlF<sub>3</sub>. It is like a cover that covers GroEL (box/bottle).
GroEL/GroES may not be able to undo protein aggregates, but kinetically it competes in the pathway of misfolding and aggregation, thereby preventing aggregate formation.
The Cpn60 subfamily was discovered in 1988. It was sequenced in 1992. The cpn10 and cpn60 oligomers also require Mg<sup>2+</sup>-ATP in order to interact to form a functional complex. The binding of cpn10 to cpn60 inhibits the weak ATPase activity of cpn60.
The RuBisCO subunit binding protein is a member of this family.
Some bacteria use multiple copies of this chaperonin, probably for different peptides.]]
Group II chaperonins (TCP-1), found in the eukaryotic cytosol and in archaea, are more poorly characterized.
- The complex in archaea is called the thermosome. A homo-16mer in some archaea, it is regarded as the prototypical type II chaperonin.