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.
Bibliography
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.
Internet Resources
"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.
—Patricia L. Clark
