9.8
Molecular Chaperones and Protein Folding
The native conformation of a protein is formed by interactions between the side chains of its constituent amino acids. When the amino acids cannot for…
As they are synthesized in the cell, most proteins do not fold spontaneously into their native conformation but require a special class of proteins called chaperones to help fold them.
Of the several types of molecular chaperones found in prokaryotes and eukaryotes, the two major families are the heat shock proteins - hsp70 and hsp60.
In a correctly folded protein, hydrophobic patches are buried in the interior.
In misfolded proteins, hydrophobic patches are exposed. Such patches on different protein molecules can bind to each other, leading to irreversible protein aggregation.
Chaperones recognize these exposed hydrophobic patches and prevent protein aggregation, facilitating the folding of the proteins.
The hsp70 machinery often acts before the protein leaves the ribosome, with each ATP-bound monomer recognizing a small stretch of hydrophobic amino acids on a protein’s surface.
A set of smaller hsp40 proteins interacts with this complex and triggers ATP hydrolysis. As a result, parts of hsp70 come together like jaws, trapping the unfolded protein inside.
Next, ATP binds the complex again inducing the dissociation of hsp70 and releasing the bound polypeptide, allowing it a chance to re-fold. If folding does not occur rapidly enough, the polypeptide may bind again, and the process is repeated until the protein is folded into its native conformation.
Alternatively, a fully synthesized and partially folded polypeptide may be delivered to a chaperonin.
Chaperonins are large barrel-shaped protein complexes that provide an isolated chamber for protein folding, with one half of the symmetric barrel operating on a client protein at a time.
In E. coli, the chaperonin system is called GroEL/GroES, while its eukaryotic analog is called Hsp60.
A misfolded protein is captured by hydrophobic interactions with the exposed surface of the opening. This initial binding often helps to unfold a misfolded protein.
Once the protein is inside, ATP binding seals the chamber with a cap.
The interior of the chamber is lined with hydrophilic surfaces, where the protein can fold in isolation.
ATP hydrolysis weakens the binding of the cap and binding of additional ATP molecules ejects the cap.
The substrate protein, whether folded or not, is released from the chamber.
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Q1: Why do proteins need molecular chaperones to fold correctly?
Most proteins cannot fold spontaneously into their native conformation as they are synthesized. Chaperones recognize exposed hydrophobic patches on unfolded proteins that would otherwise bind to other protein molecules, causing irreversible aggregation. By stabilizing unfolded or partially folded polypeptides, chaperones prevent misfolding and aggregation, allowing proteins to reach their correct three-dimensional structure.
Q2: What are the two major families of molecular chaperones?
The two major families of molecular chaperones are heat shock proteins hsp70 and hsp60. Hsp70 often acts while the protein is still being synthesized on the ribosome, recognizing hydrophobic amino acid stretches and trapping unfolded proteins. Hsp60, also called chaperonins, are large barrel-shaped complexes that provide an isolated chamber where proteins can fold in a protected environment away from other cellular proteins.
Q3: How does the hsp70 chaperone system work during protein synthesis?
The hsp70 machinery recognizes small stretches of hydrophobic amino acids on a protein's surface while it remains attached to the ribosome. Hsp40 proteins trigger ATP hydrolysis, causing hsp70 to clamp around the unfolded protein like jaws. When ATP rebinds, hsp70 releases the polypeptide, allowing it to attempt folding. If folding fails, the cycle repeats until the protein achieves its native conformation.
Q4: What happens inside a chaperonin barrel during protein folding?
A misfolded protein enters the chaperonin barrel through hydrophobic interactions at the opening, which often helps unfold the protein further. ATP binding seals the chamber with a cap, creating an isolated environment lined with hydrophilic surfaces. The protein folds in this protected space away from other cellular proteins. ATP hydrolysis and additional ATP binding eventually eject the cap and release the substrate protein.
Q5: How do chaperones assist with multi-subunit protein assembly?
Chaperones play a critical role in assembling complex proteins comprising multiple polypeptide chains. Each subunit must be correctly folded individually, then assembled in a specific manner. Chaperones stabilize unassociated components while other parts of the protein undergo assembly, ensuring proper folding and correct spatial arrangement of all subunits in the final protein complex.
Q6: Can chaperones help proteins cross cellular membranes?
Yes, chaperones stabilize partially unfolded polypeptides during transport across subcellular membranes, such as during mitochondrial protein import from the cytosol. Partially unfolded conformations are easier to transport across membranes than fully folded proteins. Chaperones maintain these unfolded states during transit, then facilitate proper refolding once the protein reaches its destination organelle.
Q7: Does a protein's amino acid sequence determine its final folded structure?
Yes, the native conformation of a protein is determined solely by its amino acid sequence through interactions between side chains of constituent amino acids. Chaperones do not provide additional folding information; they catalyze the folding process without becoming part of the final protein. The amino acid sequence contains all the information needed for correct folding, and chaperones simply stabilize intermediates to prevent misfolding or aggregation.