Chaperonins

Chaperonins are proteins that provide favourable conditions for the correct folding of other proteins, thus preventing aggregation. They prevent the misfolding of proteins, which prevents diseases such as Mad Cow Disease. Newly made proteins usually must fold from a linear chain of amino acids into a three-dimensional form. Chaperonins belong to a large class of molecules that assist protein folding, called molecular chaperones. The energy to fold proteins is supplied by adenosine triphosphate (ATP). Chaperonin proteins may also tag misfolded proteins to be degraded.

The structure of these chaperonins resemble two donuts stacked on top of one another to create a barrel.

Each ring is composed of either 7, 8 or 9 subunits depending on the organism in which the chaperonin is found.

Group I chaperonins are found in bacteria as well as organelles of endosymbiotic origin: chloroplasts and .

The GroEL/GroES complex in E. coli is a Group I chaperonin and the best characterized large (~ 1 MDa) chaperonin complex.

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.

Group II chaperonins, found in the eukaryotic cytosol and in archaea, are more poorly characterized.

TRiC (TCP-1 Ring Complex, also called CCT for chaperonin containing TCP-1), the eukaryotic chaperonin, is composed of two rings of eight different though related subunits, each thought to be represented once per eight-membered ring. TRiC was originally thought to fold only the cytoskeletal proteins actin and tubulin but is now known to fold dozens of substrates.

Mm cpn (Methanococcus maripaludis chaperonin), found in the archaea Methanococcus maripaludis, is composed of sixteen identical subunits (eight per ring). It has been shown to fold the mitochondrial protein rhodanese; however, no natural substrates have yet been identified.

Group II chaperonins are not thought to utilize a GroES-type cofactor to fold their substrates. They instead contain a "built-in" lid that closes in an ATP-dependent manner to encapsulate its substrates, a process that is required for optimal protein folding activity.

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