1. Introduction
Proteins are the workhorses of the cell, carrying out a variety of essential functions that are crucial for cellular survival and homeostasis. To conduct these functions, proteins must adopt their native three-dimensional structure, which is determined by their amino acid sequence. Proteins are frequently susceptible to misfolding, aggregation, and degradation due to the congested and complex cellular environment.
To ensure proper protein folding and prevent aggregation, cells have evolved a complex system of chaperone proteins that aid in the folding and maintenance of other proteins’ structures. Chaperone proteins perform a crucial role in maintaining proteostasis – the equilibrium between protein synthesis, folding, and degradation – in all organisms, from bacteria to humans.
This review examines the intriguing world of chaperone proteins, including their classification, mechanisms of action, regulation, functions in health and disease, and potential as therapeutic targets.
2. Categorization of Chaperone Proteins
Chaperone proteins can be categorized according to their molecular weight, sequence similarity, and function. Notable chaperone protein families include Hsp70, Hsp60, Hsp90, small heat shock proteins (sHsps), and others.
2.1 Members of the Hsp70 Family
The Hsp70 family consists of a group of highly conserved molecular chaperones involved in diverse cellular processes, such as protein folding, assembly, translocation, and degradation.Hsp70 proteins consist of an N-terminal nucleotide-binding domain (NBD) and a C-terminal substrate-binding domain (SBD) and have a molecular weight of approximately 70 kDa.
Hsp70 proteins function by binding to exposed hydrophobic regions of unfolded or nascent polypeptides, thereby preventing their aggregation and promoting their proper folding. Cochaperones, such as J-domain proteins, modulate the activity of Hsp70 proteins by stimulating their ATPase activity and promoting substrate binding and release.
2.2 Hsp60 Family
The Hsp60 family, also known as chaperonins, consists of ring-shaped chaperone proteins that aid in the ATP-dependent folding of proteins. The most well-known members of this family are the GroEL-GroES complex in bacteria and the Hsp60-Hsp10 complex in eukaryotic mitochondria.
Chaperonins consist of two rings, each of which contains multiple subunits, forming a barrel-like structure. In the central cavity of the chaperonin, unfolded proteins are protected from aggregation and allowed to fold in a controlled environment. Hydrolysis of ATP regulates the binding and release of substrates by driving conformational changes in the chaperonin complex.
2.3 Hsp90 Family
Hsp90 is a highly conserved molecular chaperone that is essential for the folding, stabilization, and activation of a vast array of client proteins, the majority of which are involved in signal transduction, cell cycle regulation, and development. Hsp90 proteins, like Hsp70 proteins, contain an N-terminal nucleotide-binding domain and a C-terminal substrate-binding domain, with an intermediate domain that facilitates conformational changes and client protein interactions.
Hsp90 is a dimer that endures ATP-driven conformational changes to aid in the folding and activation of client proteins. Numerous cochaperones, such as p23, Aha1, and Hop, modulate the activity of Hsp90 by regulating its ATPase activity, substrate binding, and dimerization.
2.4 Small heat shock proteins (sHsps)
Small heat shock proteins (sHsps) are a diverse family of chaperone proteins distinguished by their low molecular weight (12-43 kDa) and -crystallin domain. sHsps serve as molecular chaperones chiefly by binding to partially unfolded proteins and preventing their aggregation.
Unlike other chaperone families, sHsps do not actively facilitate protein folding; instead, they function as a “holdase” to maintain the solubility of misfolded proteins until they can be refolded by other chaperone systems, such as Hsp70 and Hsp90, or degraded by the proteasome.
2.5 Additional Chaperone Proteins
There are numerous other chaperone proteins with specialized functions in protein folding, assembly, and degradation, in addition to the main chaperone families described above. The protein disulfide isomerase (PDI) family catalyzes the formation and rearrangement of disulfide bonds in the endoplasmic reticulum, and the Hsp100 family participates in protein disaggregation and refolding alongside Hsp70.
3. Mechanisms of Effect
Chaperone proteins employ numerous mechanisms to aid in protein folding and preserve proteostasis, including:
3.1 Folding and Unfolding of Proteins
Chaperone proteins, such as Hsp70 and Hsp90, bind to nascent polypeptides as they exit the ribosome or to unfolded proteins under conditions of stress. Chaperones prevent nonspecific aggregation and facilitate the folding process by interacting with exposed hydrophobic regions.Additionally, chaperones can facilitate the unfolding of misfolded proteins, allowing for their refolding into the correct conformation.
3.2 Protein Aggregation Prevention
Chaperone proteins, such as sHsps and certain members of the Hsp70 and Hsp90 families, can bind to partially unfolded proteins and sequester them in a soluble state to prevent their aggregation. This “holdase” activity permits chaperones to prevent protein aggregation during times of duress or when other folding machinery is overloaded.
3.3 Disaggregation and Refolding of Proteins
Some chaperone proteins, such as the Hsp100 family in conjunction with the Hsp70 system, are able to actively disaggregate and refold aggregated proteins. These chaperones utilize the energy derived from ATP hydrolysis to mechanically disrupt protein aggregates and aid in their refolding into their functional conformations.
3.4 Protein Degradation
If misfolded or aggregated proteins cannot be refolded, chaperone proteins can direct them to degradation pathways, such as the ubiquitin-proteasome system or the autophagy-lysosome pathway. Chaperones, such as Hsp70 and Hsp90, can interact with protein degradation machinery components to target misfolded proteins for degradation, thereby preserving proteostasis.
4. Controlling Chaperone Proteins
Under various cellular conditions, the expression and activity of chaperone proteins are tightly regulated at multiple levels to ensure appropriate protein folding and proteostasis.
4.1 Gene Expression Regulation
In response to cellular stress, such as thermal shock, oxidative stress, or protein misfolding, the expression of chaperone proteins is often regulated at the transcriptional level. Heat shock factor 1 (HSF1) and other stress-responsive transcription factors bind to heat shock elements (HSEs) in the promoter regions of chaperone genes and activate their transcription. This chaperone expression induction aids in maintaining proteostasis during times of duress.
4.2 Regulation Post-translation
Post-translational modifications such as phosphorylation, acetylation, and ubiquitination can also be used to regulate the activity of chaperone proteins. These modifications can alter the conformation, stability, substrate specificity, and interactions with cochaperones of chaperone proteins, fine-tuning their function in response to various cellular conditions.
For instance, Hsp90 can be phosphorylated at multiple sites by various kinases, which can modulate its ATPase activity, client protein binding, and cochaperone interactions either positively or negatively. Similar to acetylation, ubiquitination can target Hsp70 for degradation or alter its function in protein quality control pathways.
In addition to post-translational modifications, interactions between cochaperones modulate the substrate-binding specificity, ATPase activity, and conformational transitions of chaperone proteins. J-domain proteins, for example, stimulate the ATPase activity of Hsp70 and promote substrate binding and release, whereas tetratricopeptide repeat (TPR) domain-containing cochaperones, such as Hop, facilitate the transfer of substrates between Hsp70 and Hsp90 systems.
5. The Role of Chaperone Proteins in Health and Illness
Given their essential function in protein folding and proteostasis, it should come as no surprise that chaperone proteins are implicated in a vast array of biological processes and human diseases. It has been shown that chaperone proteins perform crucial functions in the following areas:
5.1 Protein Misfolding Diseases
Protein misfolding diseases, also known as conformational diseases, are a group of conditions characterized by the toxic accumulation of misfolded proteins and their aggregates in cells and tissues. Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis (ALS), and Huntington’s disease are some examples. It has been demonstrated that chaperone proteins modulate the folding, aggregation, clearance, and toxicity of disease-associated proteins in various protein misfolding diseases, highlighting their potential as therapeutic targets.
5.2 Malignancy
Frequently, cancer cells have elevated levels of chaperone proteins, which enable them to withstand the proteotoxic stress caused by rapid proliferation, metabolic reprogramming, and the unfolded protein response. It has been demonstrated that chaperone proteins, such as Hsp90, stabilize and activate numerous oncogenic client proteins, contributing to cancer cell survival, growth, and therapy resistance. Targeting chaperone proteins alone or in conjunction with other therapies has emerged as a promising cancer treatment strategy.
5.3 Infectious Conditions
Chaperone proteins are essential for the replication and pathogenesis of diverse viruses, bacteria, and parasites. They facilitate the folding and assembly of viral proteins, the translocation and secretion of bacterial virulence factors, and the modulation of the host immune system. As a novel method for treating infectious diseases, targeting pathogen chaperone proteins or modifying their function in host cells has been proposed.
5.4 Aging
Aging is associated with a decline in proteostasis, which results in the accumulation of damaged and misfolded proteins that can contribute to the development of age-related diseases such as neurodegeneration, cardiovascular disease, and diabetes. It has been demonstrated that chaperone proteins play a crucial role in maintaining proteostasis during aging and modulating the lifespan of numerous model organisms, highlighting their potential as anti-aging intervention targets.
6. Targeting Therapeutically Chaperone Proteins
Given their importance in protein folding and proteostasis, chaperone proteins have been intensively studied as potential therapeutic targets for a variety of diseases. Several strategies to modulate the expression, activity, or client protein interactions of chaperone proteins have been developed, including:
Several small molecules that can inhibit the ATPase activity or disrupt the client protein interactions of chaperone proteins, such as Hsp90 and Hsp70, have been identified. In preclinical and clinical trials for a variety of diseases, including cancer and neurodegeneration, these inhibitors have demonstrated promising efficacy.
Induction of chaperone expression: Methods to increase the expression of chaperone proteins, such as the activation of HSF1 or the application of gene therapy, have been investigated as a means to improve cellular proteostasis and treat protein misfolding diseases.
Targeting cochaperones, post-translational modifications, or protein-protein interactions can be used to modulate the function and specificity of chaperone proteins, opening up additional therapeutic avenues.
7. Conclusion
Chaperone proteins are essential proteome custodians, as they ensure proper protein folding and prevent the accumulation of misfolded proteins and aggregates. They perform essential functions in a variety of cellular processes and are implicated in a number of human diseases, including protein misfolding disorders, cancer, infectious diseases, and aging. Chaperone proteins have emerged as promising therapeutic targets due to their essential functions, and numerous strategies have been developed to modulate their expression, activity, and client protein interactions.
Even though significant progress has been made in comprehending the molecular mechanisms of chaperone proteins and their roles in health and disease, numerous obstacles and questions still exist. The substrate specificity and client protein recognition mechanisms of chaperone proteins, for instance, are not yet fully understood, and the development of selective and potent small molecule modulators remains a challenge. Additionally, the potential off-target effects and toxicity of chaperone-targeting therapies must be thoroughly evaluated.
Future research will continue to elucidate the intricate functions and regulation of chaperone proteins and their networks, casting light on their roles in proteostasis and disease pathogenesis. These discoveries will not only advance our comprehension of the fundamental principles of protein folding and quality control, but they will also pave the way for the development of novel therapeutic strategies targeting chaperone proteins and their networks in a variety of human diseases.
