Cloning and nucleotide sequence analysis of immunodominant heat-shock protein of Yersinia enterocolitica
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Keywords:
GroES
Chaperonin
Homology
Yersinia enterocolitica
The chaperonin 60 (Cpn60) is present in all three kingdoms of life and is one of the most conserved proteins in living organisms. The Escherichia coli Cpn60 (GroEL) is the best studied representative of the huge Cpn60 family. It is an essential protein because in conjunction with the chaperonin 10 (Cpn10 or GroES) it forms a protein-folding machine required for correct folding of many proteins and for recycling of misfolded proteins. As many other chaperones, GroEL and GroES are also known as heat-shock proteins (HSPs), since heat stress leads to a strong induction of their expression, a measure to counteract the increase in misfolded proteins as a result of a high nonphysiological temperature. A large amount of literature is available which is dedicated to the elucidation of how protein folding is assisted by this molecular chaperone. However, apart from this primary task, additional so-called 'moonlighting' functions of GroEL proteins unrelated to their folding activity have emerged in the past years. In fact, it becomes apparent that GroEL proteins have diverse functions in particular in mutualistic and pathogenic microorganism-host interactions. In this brief review, we describe some of these recent findings focusing on the importance of GroEL for microorganism-insect interactions.
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The Escherichia coli chaperonin GroEL and its regulator GroES are thought to mediate adenosine triphosphate-dependent protein folding as an asymmetrical complex, with substrate protein bound within the GroEL cylinder. In contrast, a symmetrical complex formed between one GroEL and two GroES oligomers, with substrate protein binding to the outer surface of GroEL, was recently proposed to be the functional chaperonin unit. Electron microscopic and biochemical analyses have now shown that unphysiologically high magnesium concentrations and increased pH are required to assemble symmetrical complexes, the formation of which precludes the association of unfolded polypeptide. Thus, the functional significance of GroEL:(GroES)2 particles remains to be demonstrated.
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The chaperonin GroEL is an essential molecular chaperone that mediates protein folding together with its cofactor GroES in Escherichia coli. It is widely accepted that a bullet-shaped 1 : 1 GroEL-GroES complex is formed throughout the cycle, whereas a football-shaped 1 : 2 GroEL-GroES complex is not formed. However, the accepted notion has been challenged by the recent findings that indicate the existence of the football-shaped complex. Here, we present the concept that GroEL can use both the bullet cycle and the football cycle and the choice of cycle is dependent on the concentration of denatured proteins.
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Chaperonin-assisted protein folding proceeds through cycles of ATP binding and hydrolysis by the large chaperonin GroEL, which undergoes major allosteric rearrangements. Interaction between the two back-to-back seven-membered rings of GroEL plays an important role in regulating binding and release of folding substrates and of the small chaperonin GroES. Using cryo-electron microscopy, we have obtained three-dimensional reconstructions to 30 Å resolution for GroEL and GroEL–GroES complexes in the presence of ADP, ATP, and the nonhydrolyzable ATP analog, AMP-PNP. Nucleotide binding to the equatorial domains of GroEL causes large rotations of the apical domains, containing the GroES and substrate protein–binding sites. We propose a mechanism for allosteric switching and describe conformational changes that may be involved in critical steps of folding for substrates encapsulated by GroES.
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The Escherichia coli chaperonin GroEL and its cochaperone GroES facilitate protein folding in an ATP-dependent reaction cycle (for review, see 1,2). GroEL is a cylindrical complex formed by two 7-mer-toroids, which are stacked back to back (3, 4). Both toroids describe a central cavity in which denatured proteins are bound in a molten globule state. The bound substrate is sequestered inside the cavity after GroES, a dome-shaped seven-membered ring (5), closes the toroid in a cis-complex. Binding of GroES causes the release of substrate into the central cavity, which switches from a hydrophobic to a hydrophilic environment, allowing the enclosed substrate to acquire its native structure (6–8). This reaction is accompanied by ATP binding and hydrolysis in the substrate-bound ring. Subsequent ATP binding in the opposite ring causes the GroEL/GroES/substrate complex to dissociate and the native substrate is set free into solution (9). Proteins that do not reach their native state in one round of GroES binding and ATP hydrolysis rebind to GroEL on GroES release and undergo another folding cycle. Folding in a sequestered environment, effectively preventing aggregation throughout the folding pathway, facilitates high yields of chaperonin-assisted renaturation.
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