HOME
TheInfoList



Chaperone proteins participate in the folding of over half of all mammalian proteins. In molecular biology, molecular chaperones are
protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing m ...
s that assist the conformational folding or unfolding and the assembly or disassembly of other macromolecular structures. Chaperones are present when the macromolecules perform their normal biological functions and have correctly completed the processes of folding and/or assembly. The chaperones are concerned primarily with
protein folding Protein folding is the physical process by which a protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of func ...

protein folding
. The first protein to be called a chaperone assists the assembly of
nucleosome A nucleosome is the basic structural unit of DNA packaging in eukaryotes. The structure of a nucleosome consists of a segment of DNA wound around eight histone proteins and resembles thread wrapped around a spool. The nucleosome is the fundamen ...

nucleosome
s from folded
histone Image:Nucleosome structure.png, 300px, Schematic representation of the assembly of the core histones into the nucleosome. In biology, histones are highly Base_(chemistry), basic proteins abundant in lysine and arginine residues that are found in euk ...
s and DNA and such assembly chaperones, especially in the nucleus, are concerned with the assembly of folded subunits into oligomeric structures. One major function of chaperones is to prevent both newly synthesised polypeptide chains and assembled subunits from aggregating into nonfunctional structures. It is for this reason that many chaperones, but by no means all, are
heat shock proteinHeat shock proteins (HSP) are a family of proteins that are produced by cells in response to exposure to stressful conditions. They were first described in relation to heat shock, but are now known to also be expressed during other stresses inclu ...
s because the tendency to aggregate increases as proteins are denatured by stress. In this case, chaperones do not convey any additional
steric Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions and molecules. Steric effects complement electronic effects, which dictate the shape and reactivity of molecules. Steric repulsive forces ...
information required for proteins to fold. However, some highly specific 'steric chaperones' do convey unique structural (steric) information onto proteins, which cannot be folded spontaneously. Such proteins violate Anfinsen's dogma, requiring
protein dynamicsProtein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catal ...
to fold correctly. The hsp70 (70-kDa heat shock protein ) family of chaperones bind short sequence of hydrophobic amino acid that emerge while a new polypeptide is being synthesised shielding them from solvent. The hsp60 family is called chaperonines differ in its sequence and structure from hsp70 and it's homologs hsp60 act later in the folding process often together with an hsp70 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 have brought insights into structural heterogeneity of chaperones, folding intermediates and affinity of chaperones for unstructured and structured protein chains.


Location and functions

Some chaperone systems work as foldases: they support the folding of proteins in an ATP-dependent manner (for example, the
GroEL
GroEL
/ GroES or the DnaK/ 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 as holdases: they bind folding intermediates to prevent their aggregation, for example DnaJ or Hsp33. Chaperones can also work as disaggregases, i.e. they can interact with aberrant protein assemblies and revent them to monomers. Some chaperones can assist in protein degradation, leading proteins to protease systems, such as the ubiquitin-proteasome system in eukaryotes. Many chaperones are
heat shock proteinHeat shock proteins (HSP) are a family of proteins that are produced by cells in response to exposure to stressful conditions. They were first described in relation to heat shock, but are now known to also be expressed during other stresses inclu ...
s, that is, proteins gene expression, expressed in response to elevated temperatures or other cellular stresses. The reason for this behaviour is that
protein folding Protein folding is the physical process by which a protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of func ...

protein folding
is severely affected by heat and, therefore, some chaperones act to prevent or correct damage caused by misfolding. Macromolecular crowding may be important in chaperone function. The crowded environment of the cytosol can accelerate the folding process, since a compact folded protein will occupy less volume than an unfolded protein chain. However, crowding can reduce the yield of correctly folded protein by increasing protein aggregation. Crowding may also increase the effectiveness of the chaperone proteins such as
GroEL
GroEL
, which could counteract this reduction in folding efficiency. More information on the various types and mechanisms of a subset of chaperones that encapsulate their folding substrates (e.g. GroES) 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 biological membrane, membranes, for example membranes of the mitochondria and endoplasmic reticulum (ER) in eukaryotes. A bacterial translocation—specific chaperone maintains newly synthesized Protein precursor, precursor polypeptide chains in a Protein targeting, translocation-competent (Denaturation (biochemistry)#Background, generally unfolded) state and guides them to the translocon. New functions for chaperones continue to be discovered, such as bacterial adhesin activity, induction of aggregation towards non-amyloid aggregates, suppression of toxic protein oligomers via their clustering, and in responding to diseases linked to protein aggregation (e.g. see prion) and cancer maintenance.


Human chaperone proteins

Chaperones are found in, for example, the endoplasmic reticulum (ER), since protein synthesis often occurs in this area.


Endoplasmic reticulum

In the endoplasmic reticulum (ER) there are general, lectin- and non-classical molecular chaperones helping to fold proteins. *General chaperones: Binding immunoglobulin protein, GRP78/BiP, GRP94, GRP170. *Lectin chaperones: calnexin and calreticulin *Non-classical molecular chaperones: HSP47 and ERp29 *Folding chaperones: **Protein disulfide isomerase (PDI), **''Peptidyl prolyl cis-trans isomerase'' (PPI), Prolyl isomerase **ERp57


Nomenclature and examples of bacterial and archaeal chaperones

There are many different families of chaperones; each family acts to aid protein folding in a different way. In bacteria like ''Escherichia coli, E. coli'', many of these proteins are highly expressed under conditions of high stress, for example, when the bacterium is placed in high temperatures. For this reason, the term "
heat shock proteinHeat shock proteins (HSP) are a family of proteins that are produced by cells in response to exposure to stressful conditions. They were first described in relation to heat shock, but are now known to also be expressed during other stresses inclu ...
" has historically been used to name these chaperones. The prefix "Hsp" designates that the protein is a heat shock protein.


Hsp60

HSP60, Hsp60 (GroEL/GroES complex in ''E. coli'') is the best characterized large (~ 1 MDa) chaperone complex.
GroEL
GroEL
is a double-ring 14mer with a hydrophobic patch at its opening; it is so large it can accommodate native folding of 54-kDa Green fluorescent protein, 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. Also acts in Matrix (biology), mitochondrial matrix as molecular chaperone.


Hsp70

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 toward apoptosis. 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 to Adenosine diphosphate, ADP, and a low-affinity state when bound to Adenosine triphosphate, 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. Hsp70 also acts as a mitochondrial and chloroplastic molecular chaperone in eukaryotes.


Hsp90

Hsp90 (HtpG in ''E. coli'') 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 middle Protein domain, domain, and a protein dimer, dimerization domain. Originally thought to clamp onto their substrate protein (also known as a client protein) upon binding ATP, the recently published structures by Vaughan ''et al.'' and Ali ''et al.'' indicate that client proteins may bind externally to both the N-terminal and middle domains of Hsp90. Hsp90 may also require co-chaperones-like immunophilins, STIP1, Sti1, p50 (Cdc37), and Aha1, and also cooperates with the Hsp70 chaperone system.


Hsp100

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 Quaternary structure#Nomenclature of quaternary structures, 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 Ă… (2 nanometer, 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 Quaternary structure#Nomenclature of quaternary structures, 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. Hsp104, the Hsp100 of Saccharomyces cerevisiae, is essential for the propagation of many yeast prions. Deletion of the HSP104 gene results in cells that are unable to propagate certain prions.


Bacteriophage

The genes of Escherichia virus T4, bacteriophage (phage) T4 that encode
protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing m ...
s with a role in determining phage T4 structure were identified using conditional lethal mutants. 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 identified a group of gps that act Catalysis, catalytically 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 T4 morphogenesis is divided into three independent pathways: the head, the tail and the long tail fiber pathways as detailed by Yap and Rossman.Yap ML, Rossmann MG. Structure and function of bacteriophage T4. Future Microbiol. 2014;9(12):1319-1327. doi:10.2217/fmb.14.91 With regard to head morphogenesis, chaperone gp31 interacts with the bacterial host chaperone
GroEL
GroEL
to promote proper folding of the major head capsid protein gp23.Marusich EI, Kurochkina LP, Mesyanzhinov VV. Chaperones in bacteriophage T4 assembly. Biochemistry (Mosc). 1998;63(4):399-406 Chaperone gp40 participates in the assembly of gp20, thus aiding in the formation of the connector complex that initiates head procapsid assembly. 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. During overall tail assembly, chaperone proteins gp26 and gp51 are necessary for baseplate hub assembly.Leiman PG, Arisaka F, van Raaij MJ, et al. Morphogenesis of the T4 tail and tail fibers. Virol J. 2010;7:355. Published 2010 Dec 3. doi:10.1186/1743-422X-7-355 Gp57A is required for correct folding of gp12, a structural component of the baseplate short tail fibers. Synthesis of the long tail fibers depends on the chaperone protein gp57A that is needed for the protein trimer, trimerization of gp34 and gp37, the major structural proteins of the tail fibers. The chaperone protein gp38 is also required for the proper folding of gp37.Leiman PG, Arisaka F, van Raaij MJ, et al. Morphogenesis of the T4 tail and tail fibers. Virol J. 2010;7:355. Published 2010 Dec 3. doi:10.1186/1743-422X-7-355 Chaperone proteins gp63 and gpwac are employed in attachment of the long tail fibers to the tail baseplate.


History

The investigation of chaperones has a long history. The term "molecular chaperone" appeared first in the literature in 1978, and was invented by Ron Laskey to describe the ability of a nuclear protein called nucleoplasmin to prevent the aggregation of folded histone proteins with DNA during the assembly of nucleosomes. The term was later extended by R. John Ellis in 1987 to describe proteins that mediated the post-translational assembly of protein complexes. In 1988, it was realised that similar proteins mediated this process in both prokaryotes and eukaryotes. The details of this process were determined in 1989, when the ATP-dependent protein folding was demonstrated ''in vitro''.


Clinical significance

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.


See also

* Molecular machine#Biological, Biological machines * Chaperome * Chaperonin * Chemical chaperones * Heat shock protein * HSF1, Heat shock factor 1 * Molecular chaperone therapy * Pharmacoperone * Proteasome * Protein dynamics


References

{{Chaperones Molecular chaperones, Protein biosynthesis