chaperonins

Chaperonins are a family of molecular chaperones that assist the folding of proteins that may misfold or aggregate. They form a large, cylindrical complex that encloses substrate proteins within a central cavity, providing an isolated environment for folding.

Chaperonins are divided into two groups. Group I chaperonins are found in bacteria (GroEL) and in the mitochondria and chloroplasts of eukaryotes, where they function with a detachable GroES lid. Group II chaperonins occur in archaea and in the cytosol of eukaryotic cells; these typically lack a detachable lid and instead use built-in helical extensions to seal the folding chamber.

Structurally, chaperonins assemble as two stacked rings, each typically comprising seven subunits, forming a 14-subunit double ring. The central chamber is the folding cavity. In group I, a separate co-chaperonin (GroES) cap binds to the ends of the chamber to secure the lid; in group II, the lid is formed by the chaperonin itself.

Mechanistically, chaperonins operate in an ATP-dependent cycle. Substrates bind to the open chamber; ATP binding and co-chaperone or lid closure induce conformational changes that sequester the substrate. After ATP hydrolysis and release of the lid, the folded protein exits the chamber.

Chaperonins assist a broad range of proteins, especially those prone to misfolding or aggregation. They are essential for proteostasis in cells and interact with other chaperone systems, such as Hsp70, to coordinate folding.