Chaperonin GroEL reassembly: an effect of protein ligands and solvent composition

The New Functional Hybrid Chaperone Protein ADGroEL-SacSm

The creation of new proteins by combining natural domains is a commonly used technique in protein engineering. In this work, we have tested the possibilities and limitations of using circular homo-oligomeric Sm-like proteins as a basis for attaching other domains. Attachment to such a stable base should bring target domains together and keep them in the correct mutual orientation. We chose a circular homoheptameric Sm-like protein from Sulfolobus acidocaldarius as a stable backbone and the apical domain of the GroEL chaperone protein as the domain of study. This domain by itself, separated from the rest of the GroEL molecule, does not form an oligomeric ring. In our design, the hyperstable SacSm held the seven ADGroELs together and forced them to oligomerize. The designed hybrid protein was obtained and studied with various physical and chemical methods. Stepwise assembly and self-organization of this protein have been shown. First, the SacSm base was assembled, and then ADGroEL was folded on it. Functional testing showed that the obtained fusion protein was able to bind the same non-native proteins as the full-length GroEL chaperone. It also reduced the aggregation of a number of proteins when they were heated, which confirms its chaperone activity. Thus, the engineering path we chose made it possible to create an efficient thermostable chaperone. The result obtained shows the productivity of the way we chose for the creation and stabilization of oligomeric proteins.

Influence of Chaperones on Amyloid Formation of Аβ Peptide

Background:

An extensive study of the folding and stability of proteins and their complexes has revealed a number of problems and questions that need to be answered. One of them is the effect of chaperones on the process of fibrillation of various proteins and peptides.

Methods:

We studied the effect of molecular chaperones, such as GroEL and α-crystallin, on the fibrillogenesis of the Aβ(1-42) peptide using electron microscopy and surface plasmon resonance.

Results:

Recombinant GroEL and Aβ(1-42) were isolated and purified. It was shown that the assembly of GroEL occurs without the addition of magnesium and potassium ions, as is commonly believed. According to the electron microscopy results, GroEL insignificantly affects the fibrillogenesis of the Aβ(1-42) peptide, while α-crystallin prevents the elongation of the Aβ(1-42) peptide fibrils. We have demonstrated that GroEL interacts nonspecifically with Aβ(1-42), while α-crystallin does not interact with Aβ(1-42) at all using surface plasmon resonance.

Conclusion:

The data obtained will help us understand the process of amyloid formation and the effect of various components on it.

Back to GroEL-Assisted Protein Folding: GroES Binding-Induced Displacement of Denatured Proteins from GroEL to Bulk Solution

The main events in chaperone-assisted protein folding are the binding and ligand-induced release of substrate proteins. Here, we studied the location of denatured proteins previously bound to the GroEL chaperonin resulting from the action of the GroES co-chaperonin in the presence of Mg-ATP. Fluorescein-labeled denatured proteins (α-lactalbumin, lysozyme, serum albumin, and pepsin in the presence of thiol reagents at neutral pH, as well as an early refolding intermediate of malate dehydrogenase) were used to reveal the effect of GroES on their interaction with GroEL. Native electrophoresis has demonstrated that these proteins tend to be released from the GroEL-GroES complex. With the use of biotin- and fluorescein-labeled denatured proteins and streptavidin fused with luciferase aequorin (the so-called streptavidin trap), the presence of denatured proteins in bulk solution after GroES and Mg-ATP addition has been confirmed. The time of GroES-induced dissociation of a denatured protein from the GroEL surface was estimated using the stopped-flow technique and found to be much shorter than the proposed time of the GroEL ATPase cycle.

Improvement of structural stability and functional efficiency of chaperonin GroEL mediated by mixed salt

GroEL is the most commonly used chaperonin protein for both in-vitro refolding of aggregating proteins as well as in-vivo solubilization of over-expressed aggregation-prone proteins of therapeutic and biotechnological applications. But sometimes the stress conditions like heat and a load of over-expressed/unfolded/misfolded proteins lead to a decrease in structural stability and functional efficiency of GroEL, which results in less recovery of substrate protein through the chaperone-mediated refolding process. So, to amend it, we have been able to optimize physicochemical conditions utilizing a cumulation of (NH₄)₂SO₄/MgCl₂ in the buffer. Interestingly, we found a consequential enhancement in the aggregation prevention efficiency, refolding of the denatured substrate and ATPase activity of GroEL protein. The reason for the increased refolding and aggregation prevention efficiency might be the exposure of hydrophobic sites and enhanced ATP hydrolysis rate in presence of buffer containing (NH₄)₂SO₄/MgCl₂. The present study withal shows that GroEL under optimized conditions exhibits consequential amelioration in thermal aggregation at high temperature. Hence the optimized buffer conditions are utilizable for the folding of substrate proteins under a broad temperature range.

Folding and unfolding pathway of chaperonin GroEL monomer and elucidation of thermodynamic parameters

The conformation and thermodynamic stability of monomeric GroEL were studied by CD and fluorescence spectroscopy. GroEL denaturation with urea and dilution in buffer leads to formation of a folded GroEL monomer. The monomeric nature of this protein was verified by size-exclusion chromatography and native PAGE. It has a well-defined secondary and tertiary structure, folding activity (prevention of aggregation) for substrate protein and is resistant to proteolysis. Being a properly folded and reversibly refoldable, monomeric GroEL is amenable for the study of thermodynamic stability by unfolding transition methods. We present the equilibrium unfolding of monomeric GroEL as studied by urea and heat mediated unfolding processes. The urea mediated unfolding shows two transitions and a single transition in the heat mediated unfolding process. In the case of thermal unfolding, some residual structure unfolds at a higher temperature (70-75°C). The process of folding/unfolding is reversible in both cases. Analysis of folding/unfolding data provides a measure of ΔG_NU^H₂^O, T_m, ΔH^van and ΔS^van of monomeric GroEL. The thermodynamic stability parameter ΔG_NU^H₂^O is similar with both CD and intrinsic fluorescence i.e. 7.10±1.0kcal/mol. The calculated T_m, ΔH^van and ΔS^van from the thermal unfolding transition is 46±0.5°C, 43.3±0.1kcal/mol and 143.9±0.1cal/mol/k respectively.

Separation of E. coli chaperonin groEL from β-galactosidase without denaturation

Chaperonins are a class of ubiquitous proteins that assist and accelerate protein folding in the cell. The Escherichia coli groEL is the best known and forms a complex with its co-chaperonin groES in the presence of ATP and assists in the folding of nascent and misfolded substrate proteins. The purification of recombinant groEL results in a nearly homogeneous sample that consistently co-purifies with the major contaminant E. coli β-galactosidase. Removal of β-galactosidase using column chromatography alone is exceedingly difficult. This is due to the fact that the overall size, surface charge, isoelectric point and hydrophobicity of groEL and β-galactosidase are very similar. Therefore purification of groEL chaperonin to homogeneity requires denaturation of the complex into monomers with urea for separating the groEL from contaminating β-galactosidase followed by reassembly of the chaperonin complex. Here, we present a simple procedure for separating β-galactosidase along with many other impurities from groEL preparations under non-denaturing conditions. The groEL is first salted out with 50% ammonium sulfate. This step also precipitates β-galactosidase but this is then salted out by the addition of magnesium chloride which leaves groEL in solution. All remaining contaminants are removed by column chromatography.