Chaperones & Chaperonin

 By: Dr. A. K. Verma, Assistant Professor, Cotton University, Guwahati, Assam

What are Chaperones 

Chaperones are a type of molecular chaperones responsible for the folding and assembly of proteins into their native structures. Moreover, they are responsible for the remodeling of proteins with wrong conformations. Most chaperones are heat shock proteins (HSPs). They are also monomers with a molecular weight of 70-100 kDa. Furthermore, the three families of chaperones are the Hsp70 family, the Hsp90 family, and the Hsp33 family. 

The Hsp70 Family 

The Hsp70 family consists of the protein, Hsp70, which has the molecular weight of approximately 70 kDa. Also, it shows ATPase activity. Significantly, in the cytosol, DnaK is the type of Hsp70 in bacteria while Hsp72, which is stress-inducible, and Hsp73, which is constitutive, are the types of Hsp70 in higher eukaryotes. On the other hand, Hsp70 interacts with Hsp40 (DnaJ in bacteria) and GrpE. Here, Hsp40 stimulates the hydrolysis of ATP while GrpE serves as a factor in nucleotide exchange.  

The Hsp90 Family 

The Hsp90 family is less representative than the Hsp70 family. Moreover, cells contain a large amount of Hsp90, which is stress-dependent. HtpG is the protein of the Hsp90 family in bacteria. 

The Hsp33 Family 

The Hsp33 family contains active cysteines and Zn. The synthesis is induced by heat shock and activated by oxidative shock.  Furthermore, chaperones can be either foldases or holdases. Here, foldases assist the protein folding in an ATP-dependent manner. Examples of foldases include GroEL/GroES, DnaK, DnaJ, and GrpE. In contrast, holdases are responsible for preventing the aggregation of folding intermediates by binding to them. 

The Hsp60 Family 

In bacteria, the Hsp60 family consists of the protein GroEL, which has two rings of seven subunits, each 60 kDa. Moreover, it has an ATPase activity. Also, the cofactor of GroEL is GroES, which promotes the folding of polypeptides. On the other hand, in higher eukaryotes, Hsp60 and its cofactor Hsp10 are the proteins of the Hsp60 family. These proteins also occur in mitochondria. However, the proteins of the Hsp60 family called Cpn60 and Cpn20 occur in chloroplasts of higher eukaryotes. 

Similarities Between Chaperones and Chaperonins 

·         Chaperones and chaperonins are two groups of proteins, aiding in protein folding and unfolding. 

·         Furthermore, they assist the assembly and disassembly of proteins. 

·         Therefore, their main function is to maintain protein homeostasis. 

·         These proteins are highly conserved in evolution. 

·         Moreover, they show an ATPase activity. 

·         Most of them are heat shock proteins (HSPs). 

 

Difference Between Chaperones and Chaperonins 

Definition 

Chaperones refer to the proteins which assist the covalent folding or unfolding and assembly and disassembly of other macromolecular structures while chaperonins refer to the proteins which provide favorable conditions for the correct folding of denatured proteins, preventing aggregation. Thus, this is the fundamental difference between chaperones and chaperonins.

Size 

While chaperones are monomers with a molecular weight of 70-100 kDa, chaperonins are oligomers with a molecular weight of 800 kDa. 

Shape 

Most of the chaperones are heat shock proteins (HSPs) while chaperonins have a shape of two donuts stacked on top of one another to create a barrel. 

Function 

Furthermore, another difference between chaperones and chaperonins is that the chaperones are responsible for the folding, unfolding, assembly, and disassembly of proteins, while the chaperonins are responsible for the correct folding of denatured proteins, which prevent aggregation.  

Examples 

The chaperones include DnaK, DnaJ, GrpE, HtpG, and Hsp33 while chaperonins include GroEL/GroES and TRiC. 

 

The main difference between chaperones and chaperonins is that chaperones are proteins that assist the covalent folding or unfolding and the assembly or disassembly of other macromolecular structures, whereas chaperonins are a class of molecular chaperones which provide favorable conditions for the correct folding of denatured proteins, thus preventing aggregation.  Furthermore, chaperones are monomers with a molecular weight of 70-100 kDa while chaperonins are oligomers with a molecular weight of 800 kDa.  Chaperones and chaperonins are two groups of molecular chaperone proteins primarily responsible for the folding of proteins. Generally, most of them are heat shock proteins (HSPs).