Chaperonin Cpn60 <p>The assembly of proteins has been thought to be the sole result of properties inherent in the primary sequence of polypeptides themselves. In some cases, however, structural information from other protein molecules is required for correct folding and subsequent assembly into oligomers [<cite idref="PUB00004022"/>]. These `helper' molecules are referred to as molecular chaperones, a subfamily of which are the chaperonins [<cite idref="PUB00004550"/>]. They are required for normal cell growth (as demonstrated by the fact that no temperature sensitive mutants for the chaperonin genes can be found in the temperature range 20 to 43 degrees centigrade [<cite idref="PUB00004022"/>]), and are stress-induced, acting to stabilise or protect disassembled polypeptides under heat-shock conditions [<cite idref="PUB00004550"/>]. Type I chaperonins present in eubacteria, mitochondria and chloroplasts require the concerted action of 2 proteins, chaperonin 60 (cpn60) and chaperonin 10 (cpn10). Type II chaperonins, found in eukaryotic cytosol and in Archaebacteria, comprise only a cpn60 member.</p><p>The 10 kDa chaperonin (cpn10 - or groES in bacteria) exists as a ring-shaped oligomer of between 6 to 8 identical subunits, whereas the 60 kDa chaperonin (cpn60 - or groEL in bacteria) forms a structure comprising 2 stacked rings, each ring containing 7 identical subunits [<cite idref="PUB00004022"/>]. These ring structures assemble by self-stimulation in the presence of Mg2+-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 Mg²⁺-ATP dependent manner [<cite idref="PUB00002183"/>, <cite idref="PUB00004550"/>]. The binding of cpn10 to cpn60 inhibits the weak ATPase activity of cpn60.</p><p>The 60 kDa form of chaperonin is the immunodominant antigen of patients with Legionnaire's disease [<cite idref="PUB00001725"/>], and is thought to play a role in the protection of the Legionella spp. bacteria from oxygen radicals within macrophages. This hypothesis is based on the finding that the cpn60 gene is upregulated in response to hydrogen peroxide, a source of oxygen radicals. Cpn60 has also been found to display strong antigenicity in many bacterial species [<cite idref="PUB00000632"/>], and has the potential for inducing immune protection against unrelated bacterial infections. The RuBisCO subunit binding protein (which has been implicated in the assembly of RuBisCO) and cpn60 have been found to be evolutionary homologues, the RuBisCO subunit binding protein having the C-terminal Gly-Gly-Met repeat found in all bacterial cpn60 sequences. Although the precise function of this repeat is unknown, it is thought to be important as it is also found in 70 kDa heat-shock proteins [<cite idref="PUB00001725"/>]. The crystal structure of <taxon tax_id="562">Escherichia coli</taxon> GroEL has been resolved to 2.8A [<cite idref="PUB00004190"/>].</p>