Chaperons

 Key Points to Remember

·   Molecular chaperones help proteins fold correctly and prevent misfolding or aggregation.

·        They are found in all cellular compartments and function under normal and stress conditions.

·        Major types include Hsp60, Hsp70, Hsp90, and small heat shock proteins (Hsps).

·        Macromolecular crowding increases the need for chaperones to maintain protein stability.

·        Hsp70, discovered in Drosophila by Ferruccio Ritossa, plays a key role in protein folding and repair.

Keywords

Molecular chaperones, Hsp70, Heat shock proteins, Protein folding, Macromolecular crowding.

Molecular Chaperones

Definition

Molecular chaperones are specialized proteins that assist other proteins in folding correctly, preventing misfolding and aggregation inside the cell. They do not form part of the final structure of the protein but ensure that proteins achieve their correct three-dimensional conformation essential for biological function.

Historical Background

The concept of molecular chaperones emerged in the late 1970s when scientists observed that some proteins required assistance during folding.
In 1987, the term “molecular chaperone” was introduced by R. John Ellis, who described proteins that prevent improper interactions during folding.
The discovery of heat shock proteins (Hsps) in bacteria exposed to high temperatures further confirmed that these chaperones protect cells under stress.

Location

Molecular chaperones are found in all living cells, from bacteria to humans, and are located in several cellular compartments:

• Cytoplasm
• Endoplasmic Reticulum (ER)
• Mitochondria
• Nucleus
• Chloroplasts (in plants)

Their widespread presence reflects their fundamental role in maintaining protein homeostasis (proteostasis).

Functions of Molecular Chaperones

1.     Assist Protein Folding – Help newly synthesized polypeptides fold into functional structures.

2.     Prevent Aggregation – Stop partially folded or denatured proteins from clumping together.

3.     Refold Damaged Proteins – Help proteins recover their original structure after stress conditions.

4.     Assist Protein Transport – Guide proteins across cellular membranes.

5.     Regulate Protein Degradation – Direct irreversibly damaged proteins to degradation pathways.

Types of Molecular Chaperones

Molecular chaperones are classified into several major families based on their structure and function:

Chaperone Family	Example	Function
Hsp60 Family (Chaperonins)	GroEL/GroES	Assist folding of large proteins inside a protective chamber
Hsp70 Family	DnaK (in bacteria), Hsp70 (in eukaryotes)	Binds to nascent polypeptides to prevent aggregation
Hsp90 Family	Hsp90	Stabilizes signaling and regulatory proteins
Small Hsps (Hsp27, Hsp20)	α-crystallin	Prevents aggregation under heat stress
Hsp100 Family	ClpA, ClpB	Protein disaggregation and unfolding for degradation

Properties of Molecular Chaperones

• Highly conserved across species.
• Function both under normal and stress conditions.
• Require ATP hydrolysis for activity (e.g., Hsp70, Hsp90).
• Can interact with multiple client proteins.
• Exhibit dynamic binding and release cycles.

Macromolecular Crowding

The cellular environment is not empty; it is filled with thousands of macromolecules—proteins, nucleic acids, lipids, and carbohydrates—occupying up to 40% of the cell volume.
This dense condition is known as macromolecular crowding.

In crowded conditions:

• The effective concentration of proteins increases.
• The rate of folding, binding, and assembly of biomolecules changes.
• The risk of protein misfolding and aggregation also rises.

Importance of Molecular Crowding

1.     Mimics Real Cellular Environment – Laboratory conditions are dilute, but crowding better represents real intracellular conditions.

2.     Influences Protein Folding and Stability – Crowding promotes compact and stable protein conformations.

3.     Enhances Chaperone Efficiency – Chaperones become more essential as crowding increases, preventing unwanted interactions.

4.     Regulates Reaction Rates – Affects diffusion and binding kinetics within the cytoplasm.

Thus, understanding macromolecular crowding helps explain how chaperones maintain proteostasis inside living cells.

Example of a Molecular Chaperone: Heat Shock Protein (Hsp)

The Heat Shock Proteins (Hsps) are the most well-studied class of molecular chaperones.
They are expressed in response to heat, oxidative stress, toxins, and infections.
The major families include Hsp60, Hsp70, Hsp90, Hsp100, and small Hsps.
Among them, Hsp70 is one of the most important and evolutionarily conserved.

Discovery of Hsp70

The Hsp70 family was first discovered in Drosophila melanogaster (fruit fly) by Ferruccio Ritossa in the early 1960s.
He observed that when flies were exposed to heat shock, certain chromosomal regions showed “puffing”—indicating the transcription of specific genes now known as heat shock genes.
Later studies identified these proteins as Hsp70, which help the cell survive under heat and stress conditions.

Structure of Hsp70

Hsp70 has a two-domain structure:

1.     N-terminal ATPase Domain (~44 kDa):

o    Binds and hydrolyzes ATP.

o    Controls the binding and release of client proteins.

2.     C-terminal Substrate-Binding Domain (~25 kDa):

o    Recognizes and binds short hydrophobic sequences of unfolded proteins.

o    Contains a “lid” that closes over the bound substrate.

A flexible linker region connects the two domains, enabling coordinated activity.

Functions of Hsp70

• Prevents aggregation of newly synthesized or stress-denatured proteins.
• Assists in folding of nascent polypeptides on ribosomes.
• Helps in protein translocation across mitochondrial and ER membranes.
• Refolds misfolded proteins and supports cellular recovery from stress.
• Works with co-chaperones like Hsp40 (DnaJ) and nucleotide exchange factors (NEFs) to regulate its activity cycle.

Conclusion

Molecular chaperones, particularly Hsp70, are essential guardians of protein quality inside cells.
They ensure that proteins fold correctly, remain soluble, and function efficiently even under stressful conditions such as heat or oxidative damage.
Understanding their mechanisms not only enhances our knowledge of cellular stress responses but also aids in developing treatments for protein misfolding diseases like Alzheimer’s and Parkinson’s.