Inmolecular biology,molecular chaperones areproteins that assist the conformational folding or unfolding of large proteins or macromolecular protein complexes. There are a number of classes of molecular chaperones, all of which function to assist large proteins in properprotein folding during or after synthesis, and after partial denaturation. Chaperones are also involved in the translocation of proteins forproteolysis.
The first molecular chaperones discovered were a type of assembly chaperones which assist in the assembly ofnucleosomes from foldedhistones andDNA.[1][2] One major function of molecular chaperones is to prevent the aggregation of misfolded proteins, thus many chaperone proteins are classified asheat shock proteins, as the tendency for protein aggregation is increased by heat stress.
The majority of molecular chaperones do not convey anysteric information for protein folding, and instead assist in protein folding by binding to and stabilizing folding intermediates until the polypeptide chain is fullytranslated. The specific mode of function of chaperones differs based on their target proteins and location. Various approaches have been applied to study the structure,dynamics and functioning of chaperones. Bulk biochemical measurements have informed us on the protein folding efficiency, and prevention of aggregation when chaperones are present during protein folding. Recent advances in single-molecule analysis[3] have brought insights into structural heterogeneity of chaperones, folding intermediates and affinity of chaperones for unstructured and structured protein chains.
Many chaperones areheat shock proteins, that is, proteinsexpressed in response to elevated temperatures or other cellular stresses.[4] Heat shock protein chaperones are classified based on their observed molecular weights into Hsp60,Hsp70, Hsp90, Hsp104, and small Hsps.[5] The Hsp60 family of protein chaperones are termedchaperonins, and are characterized by a stacked double-ring structure and are found in prokaryotes, in the cytosol of eukaryotes, and in mitochondria.
Some chaperone systems work asfoldases: they support the folding of proteins in an ATP-dependent manner (for example, theGroEL/GroES or theDnaK/DnaJ/GrpE system). Although most newly synthesized proteins can fold in absence of chaperones, a minority strictly requires them for the same. Other chaperones work asholdases: they bind folding intermediates to prevent their aggregation, for exampleDnaJ orHsp33.[6]Chaperones can also work as disaggregases, which interact with aberrant protein assemblies and revert them to monomers.[7] Some chaperones can assist inprotein degradation, leading proteins toprotease systems, such as theubiquitin-proteasome system ineukaryotes.[8] Chaperone proteins participate in the folding of over half of all mammalian proteins.[citation needed]
Macromolecular crowding may be important in chaperone function. The crowded environment of thecytosol can accelerate the folding process, since a compact folded protein will occupy less volume than an unfolded protein chain.[9] However, crowding can reduce the yield of correctly folded protein by increasingprotein aggregation.[10][11] Crowding may also increase the effectiveness of the chaperone proteins such asGroEL,[12] which could counteract this reduction in folding efficiency.[13] Some highly specific 'steric chaperones' convey unique structural information onto proteins, which cannot be folded spontaneously. Such proteins violateAnfinsen's dogma,[14] requiringprotein dynamics to fold correctly.
Other types of chaperones are involved in transport acrossmembranes, for example membranes of themitochondria andendoplasmic reticulum (ER) ineukaryotes. Abacterial translocation-specific chaperoneSecB maintains newly synthesizedprecursorpolypeptide chains in atranslocation-competent (generally unfolded) state and guides them to thetranslocon.[15]
New functions for chaperones continue to be discovered, such asbacterial adhesin activity, induction of aggregation towards non-amyloid aggregates,[16] suppression of toxic protein oligomers via their clustering,[17][18] and in responding to diseases linked to protein aggregation[19] and cancer maintenance.[20]
In human cell lines, chaperone proteins were found to make up ~10% of the gross proteome mass,[21] and are ubiquitously and highly expressed across human tissues.
Chaperones are found extensively in the endoplasmic reticulum (ER), sinceprotein synthesis often occurs in this area.
In the endoplasmic reticulum (ER) there are general, lectin- and non-classical molecular chaperones that moderate protein folding.
There are many different families of chaperones; each family acts to aid protein folding in a different way. In bacteria likeE. coli, many of these proteins are highly expressed under conditions of high stress, for example, when the bacterium is placed in high temperatures, thus heat shock protein chaperones are the most extensive.
A variety of nomenclatures are in use for chaperones. As heat shock proteins, the names are classically formed by "Hsp" followed by the approximate molecular mass inkilodaltons; such names are commonly used for eukaryotes such as yeast. The bacterial names have more varied forms, and refer directly to their apparent function at discovery. For example, "GroEL" originally stands for "phage growth defect, overcome by mutation in phage gene E, large subunit".[25]
Hsp10/60 (GroEL/GroES complex inE. coli) is the best characterized large (~ 1 MDa) chaperone complex.GroEL (Hsp60) is a double-ring 14mer with ahydrophobic patch at its opening; it is so large it can accommodate native folding of 54-kDaGFP in its lumen.GroES (Hsp10) 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.[26] Also acts in themitochondrial matrix as a molecular chaperone.
Hsp70 (DnaK inE. coli) is perhaps the best characterized small (~ 70 kDa) chaperone. TheHsp70 proteins are aided by Hsp40 proteins (DnaJ inE. coli), which increase the ATP consumption rate and activity of the Hsp70s. The two proteins are named "Dna" in bacteria because they were initially identified as being required forE. coli DNA replication.[27]
It has been noted that increased expression of Hsp70 proteins in the cell results in a decreased tendency towardapoptosis. Although a precise mechanistic understanding has yet to be determined, it is known that Hsp70s have a high-affinity bound state to unfolded proteins when bound toADP, and a low-affinity state when bound toATP.
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.[28] Hsp70 also acts as a mitochondrial and chloroplastic molecular chaperone in eukaryotes.
Hsp90 (HtpG inE. coli[a]) may be the least understood chaperone. Its molecular weight is about 90 kDa, and it is necessary for viability in eukaryotes (possibly for prokaryotes as well). Heat shock protein 90 (Hsp90) is a molecular chaperone essential for activating many signaling proteins in the eukaryotic cell.
Each Hsp90 has an ATP-binding domain, a middledomain, and adimerization domain. Originally thought to clamp onto their substrate protein (also known as a client protein) upon binding ATP, the recently published structures by Vaughanet al. and Aliet al. indicate that client proteins may bind externally to both the N-terminal and middle domains of Hsp90.[29][30]
Hsp90 may also requireco-chaperones-likeimmunophilins,Sti1, p50 (Cdc37), andAha1, and also cooperates with the Hsp70 chaperone system.[31][32]
Hsp100 (Clp family inE. coli) proteins have been studiedin vivo andin vitro for their ability to target and unfold tagged and misfolded proteins.
Proteins in the Hsp100/Clp family form largehexameric 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 (2nm) pore, thereby giving each client protein a second chance to fold.
Some of these Hsp100 chaperones, like ClpA and ClpX, associate with the double-ringedtetradecamericserine protease ClpP; instead of catalyzing the refolding of client proteins, these complexes are responsible for the targeted destruction of tagged and misfolded proteins.
Hsp104, the Hsp100 ofSaccharomyces cerevisiae, is essential for the propagation of manyyeast prions. Deletion of the HSP104 gene results in cells that are unable to propagate certainprions.
Thegenes ofbacteriophage (phage) T4 that encodeproteins with a role in determining phage T4 structure were identified using conditional lethalmutants.[33] Most of these proteins proved to be either major or minor structural components of the completed phage particle. However among the gene products (gps) necessary for phage assembly, Snustad[34] identified a group of gps that actcatalytically rather than being incorporated themselves into the phage structure. These gps were gp26, gp31, gp38, gp51, gp28, and gp4 [gene 4 is synonymous with genes 50 and 65, and thus the gp can be designated gp4(50)(65)]. The first four of these six gene products have since been recognized as being chaperone proteins. Additionally, gp40, gp57A, gp63 and gpwac have also now been identified as chaperones.
Phage T4morphogenesis is divided into three independent pathways: the head, the tail and the long tail fiber pathways as detailed by Yap and Rossman.[35] With regard to head morphogenesis, chaperone gp31 interacts with the bacterial host chaperoneGroEL to promote proper folding of the major headcapsid protein gp23.[36][35] Chaperone gp40 participates in the assembly of gp20, thus aiding in the formation of the connector complex that initiates head procapsid assembly.[36][35] Gp4(50)(65), although not specifically listed as a chaperone, acts catalytically as a nuclease that appears to be essential for morphogenesis by cleaving packaged DNA to enable the joining of heads to tails.[37]
During overall tail assembly, chaperone proteins gp26 and gp51 are necessary for baseplate hub assembly.[38] Gp57A is required for correct folding of gp12, a structural component of the baseplate short tail fibers.[38]
Synthesis of the long tail fibers depends on the chaperone protein gp57A that is needed for thetrimerization of gp34 and gp37, the major structural proteins of the tail fibers.[36][35] The chaperone protein gp38 is also required for the proper folding of gp37.[38][39] Chaperone proteins gp63 and gpwac are employed in attachment of the long tail fibers to the tail baseplate.[38]
The investigation of chaperones has a long history.[40] The term "molecular chaperone" appeared first in the literature in 1978, and was invented byRon Laskey to describe the ability of a nuclear protein callednucleoplasmin to prevent the aggregation of folded histone proteins with DNA during the assembly of nucleosomes.[41] The term was later extended byR. John Ellis in 1987 to describe proteins that mediated the post-translational assembly of protein complexes.[42] In 1988, it was realised that similar proteins mediated this process in both prokaryotes and eukaryotes.[43] The details of this process were determined in 1989, when the ATP-dependent protein folding was demonstratedin vitro.[44]
There are many disorders associated with mutations in genes encoding chaperones (i.e.multisystem proteinopathy) that can affect muscle, bone and/or the central nervous system.[45]
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