chaperoniny

Chaperoniny, or chaperonins, are a family of molecular chaperones that catalyze the ATP-dependent folding of proteins within a sheltered chamber. They assist nascent polypeptides and stress-denatured proteins to reach their native conformations, contributing to cellular proteostasis.

Structure and mechanism: Chaperonins form large oligomeric complexes with a double-ring architecture that creates an enclosed central cavity. Group I chaperonins, such as GroEL in bacteria or the mitochondrial/chloroplast homologs, consist of two back-to-back rings of seven subunits each and use a detachable lid formed by GroES. Group II chaperonins, found in most eukaryotic cytosols (the TRiC or CCT complex) and some archaea, have built-in lids and do not require a GroES cofactor. Substrates bind to hydrophobic surfaces when the chamber is open; ATP binding and hydrolysis drive conformational changes that encapsulate the polypeptide inside the chamber, where it folds in an isolated environment. After folding, the chamber reopens and the folded protein is released.

Function and scope: Chaperonins provide a protected folding space for a subset of proteins and work in concert with other chaperone systems, particularly Hsp70 and Hsp90. They are essential for the maturation of many cytoskeletal proteins, enzymes, and secreted proteins, and play a key role in responses to heat shock and other stresses. Group I chaperonins are widespread in bacteria and in the organelles of eukaryotes, while Group II chaperonins are common in eukaryotic cytosol and some archaea.

Evolutionary and biological context: Proper chaperonin function is critical for proteostasis; defects can contribute to protein aggregation and cellular dysfunction in some contexts, and the chaperonin system is highly conserved across life.