The heat shock response (HSR) is a cell stress response that increases the number of molecular chaperones to combat the negative effects on proteins caused by stressors such as increased temperatures, oxidative stress, and heavy metals. [1] In a normal cell, proteostasis (protein homeostasis) must be maintained because proteins are the main functional units of the cell. [2] Many proteins take on a defined configuration in a process known as protein folding in order to perform their biological functions. If these structures are altered, critical processes could be affected, leading to cell damage or death. [3] The heat shock response can be employed under stress to induce the expression of heat shock proteins (HSP), many of which are molecular chaperones, that help prevent or reverse protein misfolding and provide an environment for proper folding. [4]
Protein folding is already challenging due to the crowded intracellular space where aberrant interactions can arise; it becomes more difficult when environmental stressors can denature proteins and cause even more non-native folding to occur. [5] If the work by molecular chaperones is not enough to prevent incorrect folding, the protein may be degraded by the proteasome or autophagy to remove any potentially toxic aggregates. [6] Misfolded proteins, if left unchecked, can lead to aggregation that prevents the protein from moving into its proper conformation and eventually leads to plaque formation, which may be seen in various diseases. [7] Heat shock proteins induced by the HSR can help prevent protein aggregation that is associated with common neurodegenerative diseases such as Alzheimer's, Huntington's, or Parkinson's disease. [8]
With the introduction of environmental stressors, the cell must be able to maintain proteostasis. Acute or chronic subjection to these harmful conditions elicits a cytoprotective response to promote stability to the proteome. [9] HSPs (e.g. HSP70, HSP90, HSP60, etc.) are present under normal conditions but under heat stress, they are upregulated by the transcription factor heat shock factor 1 (HSF1). [10] [11] There are four different transcription factors found in vertebrates (HSF 1–4) where the main regulator of HSPs is HSF1, while σ32 is the heat shock transcription factor in E. coli. [12] [13] When not bound to DNA, HSF1 is in a monomeric state where it is inactive and negatively regulated by chaperones. [14] When a stress occurs, these chaperones are released due to the presence of denatured proteins and various conformational changes to HSF1 cause it to undergo nuclear localization where it becomes active through trimerization. [15] [14] Newly trimerized HSF1 will bind to heat shock elements (HSE) located in promoter regions of different HSPs to activate transcription of HSP mRNA. The mRNA will eventually be transcribed and comprise the upregulated HSPs that can alleviate the stress at hand and restore proteostasis. [12] HSF1 will also regulate expression of HSPs through epigenetic modifications. The HSR will eventually attenuate as HSF1 returns to its monomeric form, negatively regulated through association with HSP70 and HSP90 along with additional post-translational modifications. [16] The HSR is not only involved with increasing transcription levels of HSPs; other facets include stress-induced mRNA stability preventing errors in mRNA and enhanced control during translation to thwart misfolding. [17]
Molecular chaperones are typically referred to as proteins that associate with and help other proteins reach a native conformation while not being present in the end state. [18] Chaperones bind to their substrate (i.e. a misfolded protein) in an ATP-dependent manner to perform a specific function. [19] Exposed hydrophobic residues are a major problem with regards to protein aggregation because they can interact with one another and form hydrophobic interactions. [20] It is the job of chaperones to prevent this aggregation by binding to the residues or providing proteins a "safe" environment to fold properly. [21] Heat shock proteins are also believed to play a role in the presentation of pieces of proteins (or peptides) on the cell surface to help the immune system recognize diseased cells. [22] The major HSPs involved in the HSR include HSP70, HSP90, and HSP60. [5] Chaperones include the HSP70s and HSP90s while HSP60s are considered to be chaperonins. [17]
The HSP70 chaperone family is the main HSP system within cells, playing a key role in translation, post-translation, prevention of aggregates and refolding of aggregated proteins. [23] When a nascent protein is being translated, HSP70 is able to associate with the hydrophobic regions of the protein to prevent faulty interactions until translation is complete. [24] Post-translational protein folding occurs in a cycle where the protein becomes bound/released from the chaperone allowing burying hydrophobic groups and aiding in overcoming the energy needed to fold in a timely fashion. [25] HSP70 plays a part in de-aggregating proteins using the aforementioned mechanism; the chaperone will bind to exposed hydrophobic residues and either partially or fully disassemble the protein, allowing HSP70 to assist in the proper refolding. [26] When proteins are beyond the point of refolding, HSP70s can help direct these potentially toxic aggregates to be degraded by the proteasome or through autophagy. [27] HSP90s are parallel to HSP70s with respect to the refolding or proteins and use in protein clearance. [4] One difference between the two HSPs is HSP90s ability to keep proteins in an unfolded yet stable configuration until a signal causes the protein to translocate and complete its folding. [24]
Sometimes, HSP70 is unable to effectively aid a protein in reaching its final 3-D structure; The main reason being the thermodynamic barriers for folding are too high for the chaperone to meet. [23] Because the intracellular space is very crowded, sometimes proteins need an isolated space to prevent aberrant interactions between other proteins, which is provided by chaperonins or HSP60s . [7] HSP60s are barrel shaped and suited to bind to the hydrophobic residues of proteins. [28] Once a cap binds to the chaperonin, the protein is free within the barrel to undergo hydrophobic collapse and reach a stable conformation. [29] Once the cap is removed, the protein can either be correctly folded and move on to perform its function or return to a HSP if it is still not folded accurately. [30] These chaperones function to remove aggregation and significantly speed up protein folding. [20]
Discovery of the heat shock response is attributed to Italian geneticist Ferruccio Ritossa, who observed changes called chromosomal "puffs" in response to heat exposure while working with the polytene chromosomes of Drosophila . [31] [32] By his own account, the discovery was the serendipitous result of unintentional elevated temperature in a laboratory incubator. [33] Ritossa's observations, reported in 1962, [34] were later described as "the first known environmental stress acting directly on gene activity" [31] but were not initially widely cited. [31] [35] The significance of these observations became clearer in the 1970s, as a distinct class of heat shock proteins were discovered in the laboratory of Herschel K. Mitchell, [36] and as heat shock responses were reported in other organisms and came to be recognized as universal. [31] [35] [37]
In molecular biology, molecular chaperones are proteins 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 proper protein folding during or after synthesis, and after partial denaturation. Chaperones are also involved in the translocation of proteins for proteolysis.
Heat shock proteins (HSP) are a family of proteins 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 including exposure to cold, UV light and during wound healing or tissue remodeling. Many members of this group perform chaperone functions by stabilizing new proteins to ensure correct folding or by helping to refold proteins that were damaged by the cell stress. This increase in expression is transcriptionally regulated. The dramatic upregulation of the heat shock proteins is a key part of the heat shock response and is induced primarily by heat shock factor (HSF). HSPs are found in virtually all living organisms, from bacteria to humans.
The 70 kilodalton heat shock proteins are a family of conserved ubiquitously expressed heat shock proteins. Proteins with similar structure exist in virtually all living organisms. Intracellularly localized Hsp70s are an important part of the cell's machinery for protein folding, performing chaperoning functions, and helping to protect cells from the adverse effects of physiological stresses. Additionally, membrane-bound Hsp70s have been identified as a potential target for cancer therapies and their extracellularly localized counterparts have been identified as having both membrane-bound and membrane-free structures.
Hsp90 is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. It also stabilizes a number of proteins required for tumor growth, which is why Hsp90 inhibitors are investigated as anti-cancer drugs.
GroEL is a protein which belongs to the chaperonin family of molecular chaperones, and is found in many bacteria. It is required for the proper folding of many proteins. To function properly, GroEL requires the lid-like cochaperonin protein complex GroES. In eukaryotes the organellar proteins Hsp60 and Hsp10 are structurally and functionally nearly identical to GroEL and GroES, respectively, due to their endosymbiotic origin.
Hop, occasionally written HOP, is an abbreviation for Hsp70-Hsp90 Organizing Protein. It functions as a co-chaperone which reversibly links together the protein chaperones Hsp70 and Hsp90.
Heat shock 70 kDa protein 8 also known as heat shock cognate 71 kDa protein or Hsc70 or Hsp73 is a heat shock protein that in humans is encoded by the HSPA8 gene on chromosome 11. As a member of the heat shock protein 70 family and a chaperone protein, it facilitates the proper folding of newly translated and misfolded proteins, as well as stabilize or degrade mutant proteins. Its functions contribute to biological processes including signal transduction, apoptosis, autophagy, protein homeostasis, and cell growth and differentiation. It has been associated with an extensive number of cancers, neurodegenerative diseases, cell senescence, and aging.
Co-chaperones are proteins that assist chaperones in protein folding and other functions. Co-chaperones are the non-client binding molecules that assist in protein folding mediated by Hsp70 and Hsp90. They are particularly essential in stimulation of the ATPase activity of these chaperone proteins. There are a great number of different co-chaperones however based on their domain structure most of them fall into two groups: J-domain proteins and tetratricopeptide repeats (TPR).
Heat shock 70 kDa protein 1, also termed Hsp72, is a protein that in humans is encoded by the HSPA1A gene. As a member of the heat shock protein 70 family and a chaperone protein, it facilitates the proper folding of newly translated and misfolded proteins, as well as stabilize or degrade mutant proteins. In addition, Hsp72 also facilitates DNA repair. Its functions contribute to biological processes including signal transduction, apoptosis, protein homeostasis, and cell growth and differentiation. It has been associated with an extensive number of cancers, neurodegenerative diseases, cell senescence and aging, and inflammatory diseases such as Diabetes mellitus type 2 and rheumatoid arthritis.
Heat shock protein HSP 90-alpha is a protein that in humans is encoded by the HSP90AA1 gene.
Human gene HSPA1B is an intron-less gene which encodes for the heat shock protein HSP70-2, a member of the Hsp70 family of proteins. The gene is located in the major histocompatibility complex, on the short arm of chromosome 6, in a cluster with two paralogous genes, HSPA1A and HSPA1L. HSPA1A and HSPA1B produce nearly identical proteins because the few differences in their DNA sequences are almost exclusively synonymous substitutions or in the three prime untranslated region, heat shock 70kDa protein 1A, from HSPA1A, and heat shock 70kDa protein 1B, from HSPA1B. A third, more modified paralog to these genes exists in the same region, HSPA1L, which shares a 90% homology with the other two.
Heat shock factor 1 (HSF1) is a protein that in humans is encoded by the HSF1 gene. HSF1 is highly conserved in eukaryotes and is the primary mediator of transcriptional responses to proteotoxic stress with important roles in non-stress regulation such as development and metabolism.
Heat shock protein HSP 90-beta also called HSP90beta is a protein that in humans is encoded by the HSP90AB1 gene.
Heat shock 70 kDa protein 4 is a protein that in humans is encoded by the HSPA4 gene.
DnaJ homolog subfamily B member 1 is a protein that in humans is encoded by the DNAJB1 gene.
In molecular biology, protein aggregation is a phenomenon in which intrinsically-disordered or mis-folded proteins aggregate either intra- or extracellularly. Protein aggregates have been implicated in a wide variety of diseases known as amyloidoses, including ALS, Alzheimer's, Parkinson's and prion disease.
Richard I. Morimoto is a Japanese American molecular biologist. He is the Bill and Gayle Cook Professor of Biology and Director of the Rice Institute for Biomedical Research at Northwestern University.
Proteostasis is the dynamic regulation of a balanced, functional proteome. The proteostasis network includes competing and integrated biological pathways within cells that control the biogenesis, folding, trafficking, and degradation of proteins present within and outside the cell. Loss of proteostasis is central to understanding the cause of diseases associated with excessive protein misfolding and degradation leading to loss-of-function phenotypes, as well as aggregation-associated degenerative disorders. Therapeutic restoration of proteostasis may treat or resolve these pathologies.
The mitochondrial unfolded protein response (UPRmt) is a cellular stress response related to the mitochondria. The UPRmt results from unfolded or misfolded proteins in mitochondria beyond the capacity of chaperone proteins to handle them. The UPRmt can occur either in the mitochondrial matrix or in the mitochondrial inner membrane. In the UPRmt, the mitochondrion will either upregulate chaperone proteins or invoke proteases to degrade proteins that fail to fold properly. UPRmt causes the sirtuin SIRT3 to activate antioxidant enzymes and mitophagy.
Chaperones, also called molecular chaperones, are proteins that assist other proteins in assuming their three-dimensional fold, which is necessary for protein function. However, the fold of a protein is sensitive to environmental conditions, such as temperature and pH, and thus chaperones are needed to keep proteins in their functional fold across various environmental conditions. Chaperones are an integral part of a cell's protein quality control network by assisting in protein folding and are ubiquitous across diverse biological taxa. Since protein folding, and therefore protein function, is susceptible to environmental conditions, chaperones could represent an important cellular aspect of biodiversity and environmental tolerance by organisms living in hazardous conditions. Chaperones also affect the evolution of proteins in general, as many proteins fundamentally require chaperones to fold or are naturally prone to misfolding, and therefore mitigates protein aggregation.