The chaperonins are 'helper' molecules required for correct folding and subsequent assembly of some proteins. These are required for normal cell growth, 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). The 10 kDa chaperonin (cpn10 - or groES in bacteria) exists as a ring-shaped oligomer of between six to eight identical subunits, while the 60 kDa chaperonin (cpn60 - or groEL in bacteria) forms a structure comprising 2 stacked rings, each ring containing 7 identical subunits. 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. The binding of cpn10 to cpn60 inhibits the weak ATPase activity of cpn60.
The chaperonins are 'helper' molecules required for correct folding and subsequent assembly of some proteins [ (PUBMED:1349837) ]. These are required for normal cell growth [ (PUBMED:2897629) ], 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) [ (PUBMED:12354603) ].
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 [ (PUBMED:2897629) ]. 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 [ (PUBMED:1350777) ]. 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 [ (PUBMED:7901771) ]. 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.
Type I chaperonins play an essential role in the folding of newlytranslated and stress-denatured proteins in eubacteria, mitochondria andchloroplasts. Since their discovery, the bacterial chaperonins haveprovided an excellent model system for investigating the mechanism bywhich chaperonins mediate protein folding. Due to the high conservation ofthe primary sequence among Type I chaperonins, it is generally acceptedthat organellar chaperonins function similar to the bacterial ones.However, recent studies indicate that the chloroplast and mitochondrialchaperonins possess unique structural and functional properties thatdistinguish them from their bacterial homologs. This review focuses on theunique properties of organellar chaperonins.
Structural adaptations in the specialized bacteriophage T4 co-chaperoninGp31 expand the size of the Anfinsen cage.
Cell. 1997; 90: 361-71
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The Gp31 protein from bacteriophage T4 functionally substitutes for thebacterial co-chaperonin GroES in assisted protein folding reactions bothin vitro and in vivo. But Gp31 is required for the folding and/or assemblyof the T4 major capsid protein Gp23, and this requirement cannot besatisfied by GroES. The 2.3 A crystal structure of Gp31 shows that itstertiary and quaternary structures are similar to those of GroES despitethe existence of only 14% sequence identity between the two proteins.However, Gp31 shows a series of structural adaptations which will increasethe size and the hydrophilicity of the "Anfinsen cage," the enclosedcavity within the GroEL/GroES complex that is the location of thechaperonin-assisted protein folding reaction.
cDNA clones encoding Arabidopsis thaliana and Zea mays mitochondrialchaperonin HSP60 and gene expression during seed germination and heatshock.
Plant Mol Biol. 1992; 18: 873-85
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Mitochondria contain a nuclear-encoded heat shock protein, HSP60, whichfunctions as a chaperonin in the post-translational assembly of multimericproteins encoded by both nuclear and mitochondrial genes. We have isolatedand sequenced full-length complementary DNAs coding for this mitochondrialchaperonin in Arabidopsis thaliana and Zea mays. Southern-blot analysisindicates the presence of a single hsp60 gene in the genome of A.thaliana. There is a high degree of homology at the predicted amino acidlevels (43 to 60%) between plant HSP60s and their homologues inprokaryotes and other eukaryotes which indicates that these proteins musthave similar evolutionarily conserved functions in all organisms.Northern- and western-blot analyses indicate that the expression of thehsp60 gene is developmentally regulated during seed germination. It isalso heat-inducible. Developmental regulation of the (beta-subunit ofF1-ATPase, an enzyme complex that is involved in the cyanide-sensitivemitochondrial electron transport system, indicates that imbibed embryosundergo rapid mitochondrial biogenesis through the early stages ofgermination. Based on the functional role of HSP60 in macromolecularassembly, these data collectively suggest that the presence of higherlevels of HSP60 is necessary during active mitochondrial biogenesis, whenthe need for this protein is greatest in assisting the rapid assembly ofthe oligomeric protein structures.
Cloning, sequencing, mapping, and transcriptional analysis of the groESLoperon from Bacillus subtilis.
J Bacteriol. 1992; 174: 3993-9
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Using a gene probe of the Escherichia coli groEL gene, a 1.8-kb HindIIIfragment of chromosomal DNA of Bacillus subtilis was cloned. Upstreamsequences were isolated as a 3-kb PstI fragment. Sequencing of 2,525 bprevealed two open reading frames in the order groES groEL. Alignment ofthe GroES and GroEL proteins with those of eight other eubacteria revealed50 to 65% and 72 to 84% sequence similarity, respectively. Primerextension studies revealed one potential transcription start sitepreceding the groESL operon (S) which was activated upon temperatureupshift. Northern (RNA) analysis led to the detection of two mRNA speciesof 2.2 and 1.5 kb. RNA dot blot experiments revealed an at least 10-foldincrease in the amount of specific mRNA from 0 to 5 min postinduction,remaining at this high level for 10 min and then decreasing. A 9-bpinverted repeat within the 5' leader region of the mRNA might be involvedin regulation of the heat shock response. By using PBS1 transduction, thegroESL operon was mapped at about 342 degrees.
Homologous plant and bacterial proteins chaperone oligomeric proteinassembly.
Nature. 1988; 333: 330-4
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An abundant chloroplast protein is implicated in the assembly of theoligomeric enzyme ribulose bisphosphate carboxylase-oxygenase, whichcatalyses photosynthetic CO2-fixation in higher plants. The product of theEscherichia coli groEL gene is essential for cell viability and isrequired for the assembly of bacteriophage capsids. Sequencing of thegroEL gene and the complementary cDNA encoding the chloroplast protein hasrevealed that these proteins are evolutionary homologues which we term'chaperonins'. Chaperonins comprise a class of molecular chaperones thatare found in chloroplasts, mitochondria and prokaryotes. Assistedpost-translational assembly of oligomeric protein structures is emergingas a general cellular phenomenon.
