Chaperonins

Chaperonins are special proteins (also called molecular chaperones) that function to keep a newly synthesized protein from either improperly folding or aggregating. Improper folding may correspond to being trapped in a deep local minimum on the energy landscape. Aggregation is often a danger because the protein, released from the ribosome in an unfolded state, will have hydrophobic groups exposed. These will be tucked inside in normal folding, but when exposed they stand the chance of making hydrophobic interactions with other polypeptide strands and thereby aggregating.

The best studied of all chaperonins is the GroEL-ES complex from E. coli (Figure 6.26). It consists of two basic portions - GroEl is made of two rings each consisting of seven protein molecules. The center of each ring is an open cavity, accessible to the solvent at the ends. Either cavity can be "capped" with GroES, which is also a seven-membered ring of smaller subunits.

The cavities provide "shelters" in which new protein chains can be "incubated" until they have folded properly. The GroEL-ES complex does not stipulate the folding pattern - that is left up to the protein itself to do. Insulation from the environment prevents chances for aggregation or misfolding.

It is probable that only a fraction of the proteins made in E. coli (or any cells) is processed via chaperonins. Some would be too large to be accommodated within the cavity. Others can seemingly fold safely on their own.

See also: Dynamics of Protein Folding, Polypeptide Chain Folding, (from Chapter 27)

INTERNET LINK: The Chaperonin Home Page