Chaperone

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For the person who accompanies another during social situations, see chaperon.

In biology, chaperones are proteins whose function is to assist other proteins in achieving proper folding. Many chaperones are heat shock proteins, that is, proteins expressed in response to elevated temperatures or other cellular stresses. The reason for this behaviour is that 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. Although most newly synthesized proteins can fold in absence of chaperones, a minority strictly requires them. Chaperones were co-discovered by Art Horwich, now a professor at Yale University and the Howard Hughes Medical Institute, and Ulrich Hartl.

More information on the various types and mechanisms of a subset of chaperones which encapsulate their folding substrates can be found in the article for chaperonins. Chaperonins are characterized by a stacked double-ring structure and are found in prokaryotes, in the cytosol of eukaryotes, and in mitochondria.

Other types of chaperones are involved in transport across membranes, for example in the mitochondria and endoplasmic reticulum. New functions for chaperones continue to be discovered, such as assistance in protein degradation and in responding to diseases linked to protein aggregation (see prion).

[edit] Nomenclature and examples of bacterial and archeal chaperones

There are many different families of chaperones; each family acts to aid protein folding in a different way. In bacteria like E. coli, many of these proteins are highly expressed under conditions of high stress, for example, when placed in high temperatures. For this reason, the term "heat shock protein" has historically been used to name these chaperones. The prefix "Hsp" designates that the protein is a heat shock protein.

Hsp60 (GroEL/GroES complex in E. coli) is the best characterized large (~ 1 MDa) chaperone complex. GroEL is a double-ring 14mer with a greasy hydrophobic patch at its opening; it is so large it can accommodate native folding of 54-kDa GFP in its lumen. GroES is a single-ring heptamer that binds to GroEL in the presence of ATP or ADP. GroEL/GroES may not be able to undo previous aggregation, but it does compete in the pathway of misfolding and aggregation. See Fenton and Horwich (2003), and the articles on GroEL/GroES for more information. Also acts in mitochondrial matrix as molecular chaperone.

Hsp70 (DnaK in E. coli) is perhaps the best characterized small (~ 70 kDa) chaperone. The Hsp70 proteins are aided by Hsp40 proteins (DnaJ in E. coli), which increase the ATP consumption rate and activity of the Hsp70s. It has been noted that increased expression of Hsp70 proteins in the cell results in a decreased tendency towards apoptosis. Although a precise mechanistic understanding has yet to be determined, it is known that Hsp70's have a high-affinity bound state to unfolded proteins when bound to ADP, and a low-affinity state when bound to ATP. It is thought that many Hsp70s crowd around an unfolded substrate, stabilizing it and preventing aggregation until the unfolded molecule folds properly, at which time the Hsp70s lose affinity for the molecule and diffuse away. For more information, see (Mayer and Bukau, 2005). Hsp70 also acts as a mitochondrial molecular chaporone in eukaryotes.

Hsp90 (HtpG in E. coli) may be the least well-understood chaperone. Its molecular weight is about 90 kDa, and it is necessary for viability in eukaryotes (possibly for prokaryotes as well). Each Hsp90 has an ATP-binding domain, a middle domain, and a dimerization domain. They are thought to clamp onto their substrate protein upon binding ATP, and may require co-chaperones like Hsp70. See also (Terasawa et al, 2005).

Hsp100 (Clp family in E. coli) proteins have been studied in vivo and in vitro for their ability to target and unfold tagged and misfolded proteins. Proteins in the Hsp100/Clp family form large hexameric structures with unfoldase activity in the presence of ATP. These proteins are thought to function as chaperones by processively threading client proteins through a small 20 Å (2 nm) pore, thereby giving each client protein a second chance to fold. Some of these Hsp100 chaperones, like ClpA and ClpX, associate with the double-ringed tetradecameric serine protease ClpP; instead of catalyzing the refolding of client proteins, these complexes are responsible for the targeted destruction of tagged and misfolded proteins.

[edit] References

"Hsp70 chaperones: cellular functions and molecular mechanism." Mayer, MP and Bukau, B Cell Mol Life Sci 62: 670-684, 2005. Entrez PubMed 15770419
Terasawa, et al, J Biochemistry (Tokyo), 137(4): 443-447, 2005.
"Chaperonin-mediated protein folding: fate of substrate polypeptide." Fenton, WA and Horwich, AL Q Rev Biophys 36(2): 229-256, 2003. Entrez PubMed 14686103