InterPro domain: IPR037124

The chaperonins are 'helper' molecules required for correct folding and subsequent assembly of some proteins [ 1 ]. These are required for normal cell growth [ 2 ], and are stress-induced, acting to stabilise or protect disassembled polypeptides under heat-shock conditions. Type I chaperonins present in eubacteria, mitochondria and chloroplasts require the concerted action of 2 proteins, chaperonin 60 (cpn60) and chaperonin 10 (cpn10) [ 3 ].

The 10kDa chaperonin (cpn10 - or groES in bacteria) exists as a ring-shaped oligomer of between six to eight identical subunits, while the 60kDa chaperonin (cpn60 - or groEL in bacteria) forms a structure comprising 2 stacked rings, each ring containing 7 identical subunits [ 4 ]. These ring structures assemble by self-stimulation in the presence of Mg ²⁺ -ATP. The central cavity of the cylindrical cpn60 tetradecamer provides as isolated environment for protein folding whilst cpn-10 binds to cpn-60 and synchronizes the release of the folded protein in an Mg ²⁺ -ATP dependent manner [ 4 ]. The binding of cpn10 to cpn60 inhibits the weak ATPase activity of cpn60.

Escherichia coli GroES has also been shown to bind ATP cooperatively, and with an affinity comparable to that of GroEL [ 5 ]. Each GroEL subunit contains three structurally distinct domains: an apical, an intermediate and an equatorial domain. The apical domain contains the binding sites for both GroES and the unfolded protein substrate. The equatorial domain contains the ATP-binding site and most of the oligomeric contacts. The intermediate domain links the apical and equatorial domains and transfers allosteric information between them. The GroEL oligomer is a tetradecamer, cylindrically shaped, that is organised in two heptameric rings stacked back to back. Each GroEL ring contains a central cavity, known as the 'Anfinsen cage', that provides an isolated environment for protein folding. The identical 10kDa subunits of GroES form a dome-like heptameric oligomer in solution. ATP binding to GroES may be important in charging the seven subunits of the interacting GroEL ring with ATP, to facilitate cooperative ATP binding and hydrolysis for substrate protein release.

1. cDNA clones encoding Arabidopsis thaliana and Zea mays mitochondrial chaperonin HSP60 and gene expression during seed germination and heat shock. Plant Mol. Biol. 18, 873-85
2. Homologous plant and bacterial proteins chaperone oligomeric protein assembly. Nature 333, 330-4
3. Type I chaperonins: not all are created equal. FEBS Lett. 529, 1-5
4. Cloning, sequencing, mapping, and transcriptional analysis of the groESL operon from Bacillus subtilis. J. Bacteriol. 174, 3993-9
5. Identification of nucleotide-binding regions in the chaperonin proteins GroEL and GroES. Nature 366, 279-82