Chaperones are proteins that assist the non-covalent folding or unfolding and the assembly or disassembly of other macromolecular structures. The first protein to be called a chaperone assists the assembly of Nucleosome from folded histones and DNA. These chaperones are concerned with the assembly of folded subunits into oligomeric structures.
Chaperones do not convey steric information required for proteins to fold. The major function of chaperones is to prevent both newly synthesized polypeptide chains and assembled subunits from aggregating into nonfunctional structures.Many chaperones are heat shock proteins, that is, proteins expressed in response to elevated temperatures or other cellular stresses.Protein folding is severely affected by heat and, therefore, some chaperones act to repair the potential damage caused by misfolding. Other chaperones are involved in folding newly made proteins as they are extruded from the ribosome.
Some chaperones are involved in transport across membranes of mitochondria and endoplasmic reticulum in eukaryotes. Bacterial translocation-specific chaperone maintains newly synthesized precursor polypeptide chains in a translocation-competent state and guides them to the translocon. In the endoplasmic reticulum (ER) there are general, lectin- and non-classical molecular chaperones helping to fold proteins.
Some of the Chaperons are
1. General chaperones: BiP, GRP94, GRP170.
2. Lectin chaperones: Calnexin and Calreticulin
3. Non-classical molecular chaperones: HSP47 and ERp29
4. Folding chaperones: Protein disulfide isomerase (PDI)
Peptidyl prolyl cis-trans-isomerase (PPI)
The Chaperon family
There are many different families of chaperones. Each family of Chaperon acts to help protein folding in a different way. In bacteria like E. coli, many of these proteins are highly expressed under conditions of high stress, when placed in high temperatures. The term “heat shock protein” has been used to name these chaperones since they express when subject to high temperature. The prefix “Hsp” designates that the protein is a heat shock protein. For example, the Hsp60(GroEL/GroES complex) in E. coli is the best characterized large chaperone complex. This also acts in mitochondrial matrix as molecular chaperone.
Hsp70 is perhaps the best characterized small (~ 70 kDa) chaperone. Eg. The DnaK in E. coli. Increased expression of Hsp70 proteins in the cell results in a decreased tendency towards apoptosis.
Hsp90 may be the least understood chaperone. Example is HtpG in E. coli .Its molecular weight is about 90 kDa. It is necessary for viability in eukaryotes .Heat shock protein 90 (Hsp90) is a molecular chaperone essential for activating many signaling proteins in the eukaryotic cell.
Hsp100 proteins have been studied in vivo and in vitro for their ability to target and unfold tagged and misfolded proteins. Example is Clp family in E. coli
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