Molecular chaperones are proteins and protein complexes that bind to misfolded or unfolded polypeptide chains and affect the subsequent folding processes of these chains. All proteins are created at the ribosome as straight chains of amino acids, but must be folded into a precise, three-dimensional shape (conformation) in order to perform their specific functions. The misfolded or unfolded polypeptide chains to which chaperones bind are said to be "non-native," meaning that they are not folded into their functional conformation. Chaperones are found in all types of cells and cellular compartments, and have a wide range of binding specificities and functional roles.
Discovery of Chaperones
Chaperones were originally identified in the mid-1980s from studies of protein folding and assembly in plant chloroplasts. A new protein was identified that was required for correct folding of a large enzyme complex in chloroplasts, yet the mysterious protein was not associated with the final assembled complex. It was quickly determined that this "chaperone" protein directing correct assembly was identical to one of the many proteins expressed at high levels when cells are grown at high temperatures (hence the common alternative name, "heat-shock protein," or Hsp).
It was later discovered that chaperones recognize the non-native, partially misfolded states of proteins that accumulate during high temperature stress. Most chaperones are also abundantly expressed under normal cell growth conditions, where they recognize non-native conformations occurring during both protein synthesis (prior to correct polypeptide chain folding), and later misfolding events.
Recognizing and Correcting Mistakes
Careful study, both in vivo and in the test tube, has demonstrated that molecular chaperones bind to their non-native substrate proteins by recognizing exposed non-polar surfaces ("non-polar" means that they are not attracted to water). In correctly folded proteins, these surfaces are usually buried away from the watery environment surrounding the protein. Chaperones promote correct folding of their substrate proteins by unfolding incorrect polypeptide chain conformations, and, in some cases, by providing a sequestered environment in which correct protein folding can occur. The activity of chaperones often requires the binding and hydrolysis of adenosine triphosphate (ATP).
Although only 20 to 30 percent of polypeptide chains require the assistance of a chaperone for correct folding under normal growth conditions, molecular chaperones are absolutely required for cell viability. Discussed below are a few of the most common classes of molecular chaperones and their effects on protein folding in the cell.
Two Common Chaperone Systems: Hsp70 and Hsp60
Hsp70 chaperones (so called because their size is approximately 70,000 daltons, or atomic mass units) are a very large family of proteins whose amino acid sequences are very similar, indicating how important their structure is to their function. A single cell or cellular compartment may contain multiple Hsp70 chaperones, each with a specific function. In addition, the Hsp70 chaperones often work in concert with one or more smaller co-chaperone proteins, which serve to modulate the activity of the chaperone.
Some of the well-studied Hsp70 chaperones include DnaK from the bacterium Escherichia coli, the Ssa and Ssb proteins from yeast, and BiP (for "binding protein") from the mammalian endoplasmic reticulum . Hsp70 chaperones are often located where unfolded polypeptide chains typically appear. For example, Ssb chaperones associate with ribosomes, so that they are close to newly synthesized, unstructured polypeptide chains. It is thought that the binding of Hsp70 chaperones to these unfolded chains prevents inappropriate partial folding until the entire polypeptide chain is available for correct folding.
Hsp60 chaperones (also called "chaperonins") are barrel-shaped structures composed of fourteen to sixteen subunits of proteins that are approximately 60,000 daltons in size. Each subunit has a patch of non-polar amino acid groups lining the inner surface of the barrel; this patch recognizes the exposed non-polar amino acids of misfolded proteins. The binding and hydrolysis of ATP triggers conformational changes within the barrel, which (1) unfolds the misfolded conformation and releases the unfolded chain into the center of the barrel, (2) closes the top of the barrel with the binding of a co-chaperone "cap," and thereby (3) provides a protected environment in which correct folding can occur. Upon dissociation of the co-chaperone, the fully or partially folded protein is released into the general cellular environment.
The most extensively studied Hsp60 chaperones include GroEL from E. coli and TRiC/CCT from eukaryotic cells. GroEL appears to function as a general chaperone and interacts with 10 to 15 percent of all E. coli polypeptide chains, with a definite bias toward proteins that are small enough to fit within its central cavity. TRiC/CCT recognizes a much smaller set of proteins, and appears to play an additional role in the assembly of multiprotein complexes.
Other Chaperone Systems
While Hsp70 and Hsp60 chaperones are the most extensively studied chaperone systems, there are many other chaperones with distinct cellular functions. These functions include modifying polypeptides after formation by altering the bonds within and between chains. It appears that some chaperones, in addition to attempting to rescue partially misfolded proteins, also alert the protein degradation system of the cell to the presence of substrate proteins that are too misfolded for rescue. It is expected, with the explosion of information provided by genome sequencing efforts, that many additional chaperones will be identified in the near future.
Chaperones and Human Disease
It is clear that molecular chaperones assist with the folding of newly synthesized proteins and correct protein misfolding. Recent studies now suggest that defects in molecular chaperone/substrate interactions may also play a substantial role in human disease. For example, mutations linked to Alzheimer's disease have been shown to disrupt the expression of chaperones in the endoplasmic reticulum. In addition, several genes linked to eye degeneration diseases have recently been identified as putative molecular chaperones.
see also Cell, Eukaryotic; Post-translational Control; Proteins; Ribosome; Translation.
Patricia L. Clark
Frydman, Judith. "Folding of Newly Translated Proteins In Vivo: The Role of Molecular Chaperones." Annual Review of Biochemistry 70 (2001): 603-647.
Wickner, Sue, Michael R. Maurizi, and Susan Gottesman. "Posttranslational Quality Control: Folding, Refolding, and Degrading Proteins." Science 286 (1999): 1888-1893.
"Chaperone." Nurse Minerva. <http://www.nurseminerva.co.uk/chaperon.htm>.
"Innovations." Environmental Health Perspectives. National Institutes of Health. <http://ehpnet1.niehs.nih.gov/docs/1994/102-6-7/innovations.html>.
"Molecular Chaperones." Federation of American Societies for Experimental Biology. <http://www.faseb.org/opar/protfold/molechap.html>.
The last two decades of the twentieth century saw the discovery of the heat-shock or cell-stress response, changes in the expression of certain proteins, and the unraveling of the function of proteins that mediate this essential cell-survival strategy. The proteins made in response to the stresses are called heat-shock proteins, stress proteins, or molecular chaperones. A large number of chaperones have been identified in bacteria (including archaebacteria), yeast , and eukaryotic cells. Fifteen different groups of proteins are now classified as chaperones. Their expression is often increased by cellular stress. Indeed, many were identified as heat-shock proteins, produced when cells were subjected to elevated temperatures. Chaperones likely function to stabilize proteins under less than ideal conditions.
The term chaperone was coined only in 1978, but the existence of chaperones is ancient, as evidenced by the conservation of the peptide sequences in the chaperones from prokaryotic and eukaryotic organisms, including humans.
Chaperones function 1) to stabilize folded proteins, 2) unfold them for translocation across membranes or for degradation, or 3) to assist in the proper folding of the proteins during assembly. These functions are vital. Accumulation of unfolded proteins due to improper functioning of chaperones can be lethal for cells. Prions serve as an example. Prions are an infectious agent composed solely of protein. They are present in both healthy and diseased cells. The difference is that in diseased cells the folding of the protein is different. Accumulation of the misfolded proteins in brain tissue kills nerve cells. The result for the affected individual can be dementia and death, as in the conditions of kuru, Creutzfeld-Jakob disease and "mad cow" disease (bovine spongiform encephalopthy).
Chaperones share several common features. They interact with unfolded or partially folded protein subunits, nascent chains emerging from the ribosome, or extended chains being translocated across subcellular membranes. They do not, however, form part of the final folded protein molecule. Chaperones often facilitate the coupling of cellular energy sources (adenosine triphosphate; ATP) to the folding process. Finally, chaperones are essential for viability.
Chaperones differ in that some are non-specific, interacting with a wide variety of polypeptide chains, while others are restricted to specific targets. Another difference concerns their shape; some are donut-like, with the central zone as the direct interaction region, while others are block-like, tunnel-like, or consist of paired subunits.
The reason for chaperone's importance lies with the environment within cells. Cells have a watery environment, yet many of the amino acids in a protein are hydrophobic (water hating). These are hidden in the interior of a correctly folded protein, exposing the hydrophilic (water loving) amino acids to the watery interior solution of the cell. If folded in such a correct manner, tensions are minimized and the three-dimensional structure of the protein is stable. Chaperons function to aid the folding process, ensuring protein stability and proper function.
Protein folding occurs by trial and error. If the protein folds the wrong way, it is captured by a chaperone, and another attempt at folding can occur. Even correctly folded proteins are subject to external stress that can disrupt structure. The chaperones, which are produced in greater amounts when a cell is exposed to higher temperatures, function to stabilize the unraveling proteins until the environmental crisis passes.
Non-biological molecules can also participate as chaperones. In this category, protein folding can be increased by the addition of agents such as glycerol, guanidium chloride, urea, and sodium chloride. Folding is likely due to an electrostatic interaction between exposed charged groups on the unfolded protein and the anions.
Increasing attention is being paid to the potential roles of chaperones in human diseases, including infection and idiopathic conditions such as arthritis and atherosclerosis. One subgroup of chaperones, the chaperonins, has received the most attention in this regard, because, in addition to facilitating protein folding, they also act as cell-to-cell signaling molecules.
See also Proteins and enzymes
chap·er·one / ˈshapəˌrōn/ (also chap·er·on ) • n. a person who accompanies and looks after another person or group of people, in particular: ∎ dated an older woman responsible for the decorous behavior of a young unmarried girl at social occasions. ∎ a person who takes charge of a child or group of children in public. • v. [tr.] accompany and look after or supervise. DERIVATIVES: chap·er·on·age / -ˌrōnij; ˌshapəˈrōnij/ n.