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Therefore, the ATP-independent chaperones can be regarded as efficient 'holding' components. Cooperation of different chaperone machineries creates a synergistic network of folding helpers in the cell, which allows to maintain protein homeostasis under conditions nonpermissive for spontaneous folding.
It is composed of two stacked rings of GroEL proteins, colored blue and green here, and a cap on one side composed of GroES, colored red and yellow at the bottom. As seen in the top view, seven GroEL proteins form a ring with a protein-sized cavity inside. Unfolded proteins enter this cavity and fold up inside. HSP chaperonin DnaK top and prefoldin bottom. Smaller chaperones protect proteins just after they leave the ribosome.
At this stage, they may have very little folded structure, so stretches of the chain with lots of exposed carbon atoms are particularly susceptible to aggregation. HSP shown at the top finds these stretches and binds to them, shielding them from neighbors. Then, powered by ATP, the chaperone releases the chain when it is ready to fold. HSP is composed of two domains: one that binds ATP and controls the process, shown on the left side of the molecule from PDB entry 1dkg , and one that binds to carbon-rich peptides, shown here on the right side using coordinates from PDB entry 1dkz.
A little peptide, colored pink, is bound in the deep protein-binding cleft. The odd jelly-fish shaped prefoldin, shown at the bottom from PDB entry 1fxk , performs a similar job, engulfing protein chains when they are in the process of folding. Chaperonins have two forms: the binding form and the enclosed state. In the binding form, ATP is bound, allowing unfolded proteins to enter into the stacked rings. ATP hydrolysis then activates the enclosed or folding-active state.
During this brief period, which approximately lasts for 15 seconds, the proteins are prevented from leaving the chamber and are folded into the correct conformation. Once the enclosed state ends, the properly folded proteins are released into the cytoplasm.
Molecular chaperones are essential to protein folding and can prevent protein aggregation by binding to non-specific proteins. This assay is suitable for the simple and rapid estimation of protein concentration.
This assay is based on a single Coomassie dye based reagent. The binding of protein to the dye results in a change of color from brown to blue.
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