GroEL

Abstract: An abundant chloroplast protein is implicated in the assembly of the oligomeric enzyme ribulose bisphosphate carboxylase-oxygenase, which catalyses photosynthetic CO2-fixation in higher plants. The product of the Escherichia coli groEL gene is essential for cell viability and is required for the assembly of bacteriophage capsids. Sequencing of the groEL gene and the complementary cDNA encoding the chloroplast protein has revealed that these proteins are evolutionary homologues which we term 'chaperonins'. Chaperonins comprise a class of molecular chaperones that are found in chloroplasts, mitochondria and prokaryotes. Assisted post-translational assembly of oligomeric protein structures is emerging as a general cellular phenomenon.

Abstract: The crystal structure of Escherichia coli GroEL shows a porous cylinder of 14 subunits made of two nearly 7-fold rotationally symmetrical rings stacked back-to-back with dyad symmetry. The subunits consist of three domains: a large equatorial domain that forms the foundation of the assembly at its waist and holds the rings together; a large loosely structured apical domain that forms the ends of the cylinder; and a small slender intermediate domain that connects the two, creating side windows. The three-dimensional structure places most of the mutationally defined functional sites on the channel walls and its outward invaginations, and at the ends of the cylinder.

Abstract: Chaperonins assist protein folding with the consumption of ATP. They exist as multi-subunit protein assemblies comprising rings of subunits stacked back to back. In Escherichia coli, asymmetric intermediates of GroEL are formed with the co-chaperonin GroES and nucleotides bound only to one of the seven-subunit rings (the cis ring) and not to the opposing ring (the trans ring). The structure of the GroEL-GroES-(ADP)7 complex reveals how large en bloc movements of the cis ring's intermediate and apical domains enable bound GroES to stabilize a folding chamber with ADP confined to the cis ring. Elevation and twist of the apical domains double the volume of the central cavity and bury hydrophobic peptide-binding residues in the interface with GroES, as well as between GroEL subunits, leaving a hydrophilic cavity lining that is conducive to protein folding. An inward tilt of the cis equatorial domain causes an outward tilt in the trans ring that opposes the binding of a second GroES. When combined with new functional results, this negative allosteric mechanism suggests a model for an ATP-driven folding cycle that requires a double toroid.

Abstract: ▪ Abstract Recent years have witnessed dramatic advances in our understanding of how newly translated proteins fold in the cell and the contribution of molecular chaperones to this process. Folding in the cell must be achieved in a highly crowded macromolecular environment, in which release of nonnative polypeptides into the cytosolic solution might lead to formation of potentially toxic aggregates. Here I review the cellular mechanisms that ensure efficient folding of newly translated proteins in vivo. De novo protein folding appears to occur in a protected environment created by a highly processive chaperone machinery that is directly coupled to translation. Genetic and biochemical analysis shows that several distinct chaperone systems, including Hsp70 and the cylindrical chaperonins, assist the folding of proteins upon translation in the cytosol of both prokaryotic and eukaryotic cells. The cellular chaperone machinery is specifically recruited to bind to ribosomes and protects nascent chains and foldi...