Heat shock proteins and other components of cellular machinery for protein synthesis are up-regulated in vascular endothelial cell growth factor-activated human endothelial cells

Proteome analysis of human umbilical endothelial cells was performed to identify proteins that are modified during vascular endothelial cell growth factor (VEGF)-induced transition from the quiescent into the proliferating-migrative phenotype. Subtractive analysis of two-dimensional gel patterns of human endothelial cells, before and after stimulation with VEGF(165), revealed differences in 85 protein spots. All proteins were identified by peptide sequencing and peptide mass fingerprinting using an electrospray spectrometer. The proteins identified were members of specific families including Ca(2+)-binding proteins, fatty-acid binding proteins, structural proteins, and chaperones. Remarkably, there was a massive activation of cellular machinery for both protein synthesis and protein degradation. Thus, among up-regulated proteins there were members of all groups of heat shock proteins (HSPs; HSP 27, HSP 60, HSP 70p5, HSP 70p8, HSP 90, and HSP 96) and some other proteins showing either chaperone activity or which participate in assembly of multimolecular structures (TCP-1, desmoplakins, junction plakoglobin, GRP 94, thioredoxin related protein, and peptidylprolyl isomerase). The increased expression of HSPs was confirmed at the mRNA level at different stages of treatment with VEGF. Similarly, components of the proteolytic machinery for the degradation of misfolded proteins (ER-60, cathepsin D, proteasome subunits, and protease inhibitor 6) were also up-regulated. On the other hand, changes in the expression of structural proteins (T-plastin, vimentin, alpha tubulin, actin, and myosin) could account, at least in part, for the different morphologies displayed by migrating endothelial cells. In summary, our data show that VEGF levels similar to those during physiological stresses induce a number of genes and multiple endogenous pathways seem to be engaged in restoring cellular homeostasis. To ensure cell survival, the molecular chaperones (the heat shock family of stress proteins) are highly up-regulated providing protein-folding machinery to repair or degrade misfolded proteins.     

Over‐expression of heat shock factor 1 phenocopies the effect of chronic inhibition of TOR by rapamycin and is sufficient to ameliorate Alzheimer's‐like deficits in mice modeling the disease

Rapamycin, an inhibitor of target‐of‐rapamycin, extends lifespan in mice, possibly by delaying aging. We recently showed that rapamycin halts the progression of Alzheimer's (AD)‐like deficits, reduces amyloid‐beta (Aβ) and induces autophagy in the human amyloid precursor protein (PDAPP) mouse model. To delineate the mechanisms by which chronic rapamycin delays AD we determined proteomic signatures in brains of control‐ and rapamycin‐treated PDAPP mice. Proteins with reported chaperone‐like activity were overrepresented among proteins up‐regulated in rapamycin‐fed PDAPP mice and the master regulator of the heat‐shock response, heat‐shock factor 1, was activated. This was accompanied by the up‐regulation of classical chaperones/heat shock proteins (HSPs) in brains of rapamycin‐fed PDAPP mice. The abundance of most HSP mRNAs except for alpha B‐crystallin, however, was unchanged, and the cap‐dependent translation inhibitor 4E‐BP was active, suggesting that increased expression of HSPs and proteins with chaperone activity may result from preferential translation of pre‐existing mRNAs as a consequence of inhibition of cap‐dependent translation. The effects of rapamycin on the reduction of Aβ, up‐regulation of chaperones, and amelioration of AD‐like cognitive deficits were recapitulated by transgenic over‐expression of heat‐shock factor 1 in PDAPP mice. These results suggest that, in addition to inducing autophagy, rapamycin preserves proteostasis by increasing chaperones. We propose that the failure of proteostasis associated with aging may be a key event enabling AD, and that chronic inhibition of target‐of‐rapamycin may delay AD by maintaining proteostasis in brain. Read the Editorial Highlight for this article on doi: 10.1111/jnc.12098 .     

Proteomic profiling of heat shock protein 70 family members as biomarkers for hepatitis C virus-related hepatocellular carcinoma

To identify proteins linked to the pathogenesis of hepatocellular carcinoma (HCC) associated with hepatitis C virus (HCV), we profiled protein expression levels in samples of HCC. To identify essential proteins, ten samples of HCV-related HCC were analyzed by two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization-time of flight mass spectrometry. These experiments revealed increased levels of nine proteins in cancerous tissues compared to levels in corresponding noncancerous liver tissues. We focused on four members of the heat shock protein 70 family: 78 kDa glucose-regulated protein (GRP78), heat shock cognate 71 kDa protein (HSC70), 75 kDa glucose-regulated protein (GRP75), and heat shock 70 kDa protein 1 (HSP70.1). These results were confirmed by immunoblot analysis. In an additional 11 samples, the same expression patterns of these four proteins were observed. In total, 21 samples showed statistically significant up-regulation of GRP78, GRP75 and HSP70.1 in cancerous tissues. HSC70 showed a tendency toward overexpression. There has been no report describing overexpression of these four proteins simultaneously in HBV-related HCC as well as nonviral HCC. Our results suggest that these four proteins play important roles in the pathogenesis of HCV-related HCC and could be molecular targets for diagnosis and treatment of this disease.     

Thermosensitization of tumor cells with inhibitors of chaperone activity and expression

Effects of inhibitors of the heat shock protein 90 (HSP90) chaperone activity and inhibitors of the heat shock protein (HSP) expression on sensitivity of HeLa tumor cells to hyperthermia were studied. It was found that nanomolar concentrations of inhibitors of the HSP90 activity (17AAG or radicicol) slowed down the chaperone-dependent reactivation of a thermolabile reporter (luciferase) in heat-stressed HeLa cells and slightly enhanced their death following the incubation for 60 min at 43°C. The inhibitors of HSP90 activity stimulated de novo induction of additional chaperones (HSP70 and HSP27) that significantly increased intracellular HSP levels. Treatment of the cells with 17AAG or radicicol along with an inhibitor of the HSP induction (e.g. quercetin or triptolide, or NZ28) completely prevented the increase in the intracellular chaperone levels resulting from the inhibition of HSP90 activity and subsequent heating. Combination of all three treatments (inhibition of the HSP90 activity + inhibition of the HSP induction + heating at 43°C for 60 min) resulted in more potent inhibition of the reporter reactivation and a sharp (2–3-fold) increase in cell death. Such enhancement of the cytotoxicity may be attributed to the “chaperone deficiency” when prior to heat stress both the functional activity of constitutive HSP90 and the expression of additional (inducible) chaperones are blocked in the cells.     

辅分子伴侣调控热休克蛋白HSP90功能的研究

热休克蛋白90(HSP90)是一类ATPase依赖性蛋白,作为分子伴侣,可在辅分子伴侣协助下,通过自身构象改变,参与众多细胞的生物学事件,从而协助新合成蛋白的正确折叠、成功装配、功能稳定及异常蛋白的降解过程。HSP90功能的发挥依赖于辅分子伴侣及氨基末端结合的核苷酸。辅分子伴侣是一类可与分子伴侣(如,HSP90)结合并调节其功能的蛋白,通过参与ATPase循环从而调节HSP90分子伴侣的功能。近年来,辅分子伴侣的研究得到越来越多的关注,本文就辅分子伴侣调控HSP90功能的作用进行综述。     

Inhibitors of the heat shock response: biology and pharmacology

A number of human diseases can be linked to aberrations in protein folding which cause an imbalance in protein homeostasis. Molecular chaperones, including heat shock proteins, act to assist protein folding, stability and activity in the cell. Attention has begun to focus on modulating the expression and/or activity of this group of proteins for the treatment of a wide variety of human diseases. This review will describe the progress made to date in developing pharmacological modulators of the heat shock response, including both agents which affect the entire heat shock response and those that specifically target the HSP70 and HSP90 chaperone families.     

Meta-analysis of heat- and chemically upregulated chaperone genes in plant and human cells

Molecular chaperones are central to cellular protein homeostasis. In mammals, protein misfolding diseases and aging cause inflammation and progressive tissue loss, in correlation with the accumulation of toxic protein aggregates and the defective expression of chaperone genes. Bacteria and non-diseased, non-aged eukaryotic cells effectively respond to heat shock by inducing the accumulation of heat-shock proteins (HSPs), many of which molecular chaperones involved in protein homeostasis, in reducing stress damages and promoting cellular recovery and thermotolerance. We performed a meta-analysis of published microarray data and compared expression profiles of HSP genes from mammalian and plant cells in response to heat or isothermal treatments with drugs. The differences and overlaps between HSP and chaperone genes were analyzed, and expression patterns were clustered and organized in a network. HSPs and chaperones only partly overlapped. Heat-shock induced a subset of chaperones primarily targeted to the cytoplasm and organelles but not to the endoplasmic reticulum, which organized into a network with a central core of Hsp90s, Hsp70s, and sHSPs. Heat was best mimicked by isothermal treatments with Hsp90 inhibitors, whereas less toxic drugs, some of which non-steroidal anti-inflammatory drugs, weakly expressed different subsets of Hsp chaperones. This type of analysis may uncover new HSP-inducing drugs to improve protein homeostasis in misfolding and aging diseases.     

Effects of chronic heat stress on the expressions of heat shock proteins 60, 70, 90, A2, and HSC70 in the rabbit testis

Few studies have focused on the expression of heat shock proteins (HSPs) after chronic heat stress. The objective of this study was to investigate the effect of chronic high temperature–humidity index treatment on the expressions of HSP60, HSP70, HSP90, HSPA2 and HSC70, in the Rex rabbit testis and the expressions of these proteins after recovery from the chronic heat shock. Thirty mature male rabbits of the same age were randomly divided into three groups: control, heat stress, and recovery. The western blot results showed that the expressional levels of HSP60, HSP90, and HSC70 increased significantly and HSPA2 was elevated slightly after a 9-week heat treatment. HSP70 was absent in the control testis and had a high level of expression after heat stress. All of these proteins partially reverted back to normal levels after a 9-week recovery. The immunohistochemical results indicated that the expression patterns of HSP60, HSP90, HSPA2, and HSC70 did not change.     

Evidence of prognostic relevant expression profiles of heat-shock proteins and glucose-regulated proteins in oesophageal adenocarcinomas

A high percentage of oesophageal adenocarcinomas show an aggressive clinical behaviour with a significant resistance to chemotherapy. Heat-shock proteins (HSPs) and glucose-regulated proteins (GRPs) are molecular chaperones that play an important role in tumour biology. Recently, novel therapeutic approaches targeting HSP90/GRP94 have been introduced for treating cancer. We performed a comprehensive investigation of HSP and GRP expression including HSP27, phosphorylated (p)-HSP27^(Ser15), p-HSP27^(Ser78), p-HSP27^(Ser82), HSP60, HSP70, HSP90, GRP78 and GRP94 in 92 primary resected oesophageal adenocarcinomas by using reverse phase protein arrays (RPPA), immunohistochemistry (IHC) and real-time quantitative RT-PCR (qPCR). Results were correlated with pathologic features and survival. HSP/GRP protein and mRNA expression was detected in all tumours at various levels. Unsupervised hierarchical clustering showed two distinct groups of tumours with specific protein expression patterns: The hallmark of the first group was a high expression of p-HSP27^{(Ser15, Ser78, Ser82)} and low expression of GRP78, GRP94 and HSP60. The second group showed the inverse pattern with low p-HSP27 and high GRP78, GRP94 and HSP60 expression. The clinical outcome for patients from the first group was significantly improved compared to patients from the second group, both in univariate analysis (p = 0.015) and multivariate analysis (p = 0.029). Interestingly, these two groups could not be distinguished by immunohistochemistry or qPCR analysis. In summary, two distinct and prognostic relevant HSP/GRP protein expression patterns in adenocarcinomas of the oesophagus were detected by RPPA. Our approach may be helpful for identifying candidates for specific HSP/GRP-targeted therapies.     

The Constitutive Heat Shock Protein-70 Is Required for Optimal Expression of Myelin Basic Protein During Differentiation of Oligodendrocytes

To further elucidate the role of the constitutive heat shock protein-70 (HSC70) as a chaperone for the synthesis of myelin basic protein (MBP), HSC70 content was decreased in oligodendrocyte precursor cells prior to MBP expression either by transfection with an antisense oligonucleotide specific for HSC70, or by exposure to low levels of quercetin, a bioflavonoid known to decrease synthesis of HSC70. As these cells underwent differentiation in vitro, antisense treatment decreased HSC70 levels to 66% of controls. At the same time, a sharp induction resulted in the stress-inducible heat shock protein-70 (HSP70). Levels of two other stress proteins increased as well, namely, the 25-kDa heat shock protein (HSP25) and the 78-kDa glucose regulated protein (GRP78). MBP synthesis proceeded over a normal time course, but at only 50% of control values. As HSC70 content returned to normal, MBP synthesis was also restored to normal levels. Quercetin reduced the expression of HSC70 to an even greater extent than transfection, and prevented the induction of HSP70. In contrast to antisense-treated cells, MBP synthesis was essentially blocked in quercetin-treated cells even though levels of HSP25 and GRP78 increased. Taken together, these observations (a) indicate that HSP70 partially compensates for decreased chaperoning of nascent MBP by HSC70 (HSC70 and HSP70 are closely related and perform similar functions); (b) preclude the involvement of HSP25 and GRP78 in MBP synthesis; and (c) emphasize the requirement of HSC70 for optimal synthesis of MBP.     

Plasma membrane rafts and chaperones in cytokine/STAT signaling

We and others have recently obtained data suggesting that cytokine-STAT signaling in many different cell-types is a chaperoned pathway initiated at the level of specialized plasma membrane microdomains called "rafts" (the "raft-STAT signaling hypothesis"). These findings are of broad significance in that all cytokines and growth factors initiate signaling in target cells by interacting with respective cell-surface receptors. The new data suggest that raft microdomains represent the units of function at the cell-surface through which ligand-stimulated STAT signaling is initiated. Moreover, recent evidence shows the involvement of chaperone proteins in regulating the STAT signaling pathway. These chaperones include the human homolog of the tumorous imaginal disc 1 protein (hTid1) which associates with Janus kinase 2 (JAK2) at the level of the plasma membrane, heat shock protein 90 (HSP90) which associates with STAT3 and STAT1 proteins in caveolin-1-containing raft and cytoplasmic complexes, and glucose regulated protein 58 (GRP58/ER-60/ERp57), a thiol dependent protein-disulfide isomerase, found in association with STAT3 "statosome" complexes in the cytosol and in the raft fraction. We suggest a function of the HSP90 chaperone system in preserving IL-6/STAT3 signaling in liver cells in the context of fever. The identification and function of protein partners associated with specific STAT species in rafts and in cytosolic complexes, and in the efficient departure of cytokine-activated STATs from the cytosolic face of rafts towards the cell nucleus are now areas of active investigation.     

Blocking microsomal triglyceride transfer protein interferes with apoB secretion without causing retention or stress in the ER

Microsomal triglyceride transfer protein (MTP) is an intraluminal protein in the endoplasmic reticulum (ER) that is essential for the assembly of apolipoprotein B (apoB)-containing lipoproteins. In this study, we examine how the livers of mice respond to two distinct methods of blocking MTP function: Cre-mediated disruption of the gene for MTP and chemical inhibition of MTP activity. Blocking MTP significantly reduced plasma levels of triglycerides, cholesterol, and apoB-containing lipoproteins in both wild-type C57BL/6 and LDL receptor-deficient mice. While treating LDL receptor-deficient mice with an MTP inhibitor for 7 days lowered plasma lipids to control levels, liver triglyceride levels were increased by only 4-fold. Plasma levels of apoB-100 and apoB-48 fell by >90% and 65%, respectively, but neither apoB isoform accumulated in hepatic microsomes. Surprisingly, loss of MTP expression was associated with a nearly complete absence of apoB-100 in hepatic microsomes. Levels of microsomal luminal chaperone proteins [e.g., protein disulfide isomerase, glucose-regulated protein 78 (GRP78), and GRP94] and cytosolic heat shock proteins (HSPs) (e.g., HSP60, HSC, HSP70, and HSP90) were unaffected by MTP inhibition. These findings show that the liver responds rapidly to inhibition of MTP by degrading apoB and preventing its accumulation in the ER. The rapid degradation of secretion-incompetent apoB in the ER may block the induction of proteins associated with unfolded protein and heat shock responses.     

Molecular chaperones: proposal of a systematic computer-oriented nomenclature and construction of a centralized database

Molecular chaperones are a wide group of unrelated protein families whose role is to assist others proteins. Comparably, under environmental stress, stress proteins behave as biocatalysts of protein stabilization. Stress proteins include a large class of proteins that were originally termed heat shock proteins (HSPs) due to their initial discovery in tissues exposed to elevated temperatures. Many, but not all, stress proteins and HSPs are molecular chaperones. Moreover, not all HSPs are derivable from stress. HSPs are structurally diversified by the contribution of various domains having specific roles. HSPs have been grouped, mainly on the basis of their molecular masses, into specific families that include small HSPs (sHSPs)/alpha-crystallins, HSP10s, HSP40s, HSP60s, HSP70s, HSP90s, HSP100s and HSP110s. The names of these major families are historical artefacts with limited information content. Using the current databases, names and proteic domains of many molecular chaperones in different species were analyzed. Although traditional names of HSPs are trivial, it is unrealistic to suggest replacing them, because they are preferred and widely used. Here we suggest that these traditional names be chaperoned, in silico, by a systematic nomenclature. Thus, for example, with the same intent of use of [trioxygen: O3] for ozone, we propose here C7HSP70[Ehsa]ER-P11021 for GRP78 (78 kDa endoplasmic Human molecular chaperone in HSP70 superfamily with P11021 as its accession number in the database of the National Center for Biotechnology Information (NCBI)). The proposed systematic computer-oriented naming and classification method is designed for HSPs and also their partners based on the number of amino acids, domain structure, phylogenetic domain, localization in the cell and accession number as stated in the NCBI. Arabidopsis thaliana was analyzed as a model, because it contains a large number of various HSPs localized in several organelles. Overall, this naming system helps in building, optimizing and managing a novel online database entirely devoted to HSPs. The purported taxonomy, coupled with the newly constructed database, can contribute to studies involving large amounts of stored data on HSPs.     

The small heat shock protein 20 RSI2 interacts with and is required for stability and function of tomato resistance protein I‐2

Race‐specific disease resistance in plants depends on the presence of resistance (R) genes. Most R genes encode NB‐ARC‐LRR proteins that carry a C‐terminal leucine‐rich repeat (LRR). Of the few proteins found to interact with the LRR domain, most have proposed (co)chaperone activity. Here, we report the identification of RSI2 (Required for Stability of I‐2) as a protein that interacts with the LRR domain of the tomato R protein I‐2. RSI2 belongs to the family of small heat shock proteins (sHSPs or HSP20s). HSP20s are ATP‐independent chaperones that form oligomeric complexes with client proteins to prevent unfolding and subsequent aggregation. Silencing of RSI2‐related HSP20s in Nicotiana benthamiana compromised the hypersensitive response that is normally induced by auto‐active variants of I‐2 and Mi‐1, a second tomato R protein. As many HSP20s have chaperone properties, the involvement of RSI2 and other R protein (co)chaperones in I‐2 and Mi‐1 protein stability was examined. RSI2 silencing compromised the accumulation of full‐length I‐2 in planta, but did not affect Mi‐1 levels. Silencing of heat shock protein 90 (HSP90) and SGT1 led to an almost complete loss of full‐length I‐2 accumulation and a reduction in Mi‐1 protein levels. In contrast to SGT1 and HSP90, RSI2 silencing led to accumulation of I‐2 breakdown products. This difference suggests that RSI2 and HSP90/SGT1 chaperone the I‐2 protein using different molecular mechanisms. We conclude that I‐2 protein function requires RSI2, either through direct interaction with, and stabilization of I‐2 protein or by affecting signalling components involved in initiation of the hypersensitive response.     

Chaperones and foldases in endoplasmic reticulum stress signaling in plants

Molecular chaperones and foldases are a diverse group of proteins that in vivo bind to misfolded or unfolded proteins (non-native or unstable proteins) and play important role in their proper folding. Stress conditions compel altered and heightened chaperone and foldase expression activity in the endoplasmic reticulum (ER), which highlights the role of these proteins, due to which several of the proteins under these classes were identified as heat shock proteins. Different chaperones and foldases are active in different cellular compartment performing specific tasks. The review will discuss the role of ER chaperones and foldases under stress conditions, to maintain proper protein folding dynamics in the plant cells and recent advances in the field. The ER chaperones and foldases, which are described in article, are binding protein (BiP), glucose regulated protein (GRP94), protein-disulfide isomerase (PDI), peptidyl-prolyl isomerases (PPI) or immunophilins, calnexin and calreticulin.Key words: Abiotic stress, chaperones, endoplasmic reticulum, foldases, immunophilins, protein folding, signal transduction     

Alpha-lipoic acid does not alter stress protein response to acute exercise in diabetic brain

Heat shock proteins (HSPs) are molecular chaperones which may act protective in cerebrovascular insults and peripheral diabetic neuropathy. We hypothesized that alpha-lipoic acid (LA), a natural thiol antioxidant, may enhance brain HSP response in diabetes. Rats with or without streptozotocin-induced diabetes were treated with LA or saline for 8 weeks. Half of the rats were subjected to exhaustive exercise to investigate HSP induction, and the brain tissue was analyzed. Diabetes increased constitutive HSC70 mRNA, and decreased HSP90 and glucose-regulated protein 75 (GRP75) mRNA without affecting protein levels. Exercise increased HSP90 protein and mRNA, and also GRP75 and heme oxygenase-1 (HO-1) mRNA only in non-diabetic animals. LA had no significant effect on brain HSPs, although LA increased HSC70 and HO-1 mRNA in diabetic animals and decreased HSC70 mRNA in non-diabetic animals. Eukaryotic translation elongation factor-2, essential for protein synthesis, was decreased by diabetes and suggesting a mechanism for the impaired HSP response related to translocation of the nascent chain during protein synthesis. LA supplementation does not offset the adverse effects of diabetes on brain HSP mRNA expression. Diabetes may impair HSP translation through elongation factors related to nascent chain translocation and subsequent responses to acute stress.     

GRP94: An HSP90-like protein specialized for protein folding and quality control in the endoplasmic reticulum

Glucose-regulated protein 94 is the HSP90-like protein in the lumen of the endoplasmic reticulum and therefore it chaperones secreted and membrane proteins. It has essential functions in development and physiology of multicellular organisms, at least in part because of this unique clientele. GRP94 shares many biochemical features with other HSP90 proteins, in particular its domain structure and ATPase activity, but also displays distinct activities, such as calcium binding, necessitated by the conditions in the endoplasmic reticulum. GRP94's mode of action varies from the general HSP90 theme in the conformational changes induced by nucleotide binding, and in its interactions with co-chaperones, which are very different from known cytosolic co-chaperones. GRP94 is more selective than many of the ER chaperones and the basis for this selectivity remains obscure. Recent development of molecular tools and functional assays has expanded the spectrum of clients that rely on GRP94 activity, but it is still not clear how the chaperone binds them, or what aspect of folding it impacts. These mechanistic questions and the regulation of GRP94 activity by other proteins and by post-translational modification differences pose new questions and present future research avenues. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).     

Chaperone signalling complexes in Alzheimer's disease

Molecular chaperones and heat shock proteins (Hsp) have emerged as critical regulators of proteins associated with neurodegenerative disease pathologies. The very nature of the chaperone system, which is to maintain protein quality control, means that most nascent proteins come in contact with chaperone proteins. Thus, amyloid precursor protein (APP), members of the gamma-secretase complex (presenilin 1 [PS1] collectively), the microtubule-associated protein tau (MAPT) as well as a number of neuroinflammatory components are all in contact with chaperones from the moment of their production. Chaperones are often grouped together as one machine presenting abnormal or mutant proteins to the proteasome for degradation, but this is not at all the case. In fact, the chaperone family consists of more than 100 proteins in mammalian cells, and the primary role for most of these proteins is to protect clients following synthesis and during stress; only as a last resort do they facilitate protein degradation. To the best of our current knowledge, the chaperone system in eukaryotic cells revolves around the ATPase activities of Hsp70 and Hsp90, the two primary chaperone scaffolds. Other chaperones and co-chaperones manipulate the ATPase activities of Hsp70 and Hsp90, facilitating either folding of the client or its degradation. In the case of Alzheimer's disease (AD), a number of studies have recently emerged describing the impact that these chaperones have on the proteotoxic effects of tau and amyloid-β accumulation. Here, we present the current understandings of chaperone biology and examine the literature investigating these proteins in the context of AD.     

Identification of two molecular chaperons (HSX70, HSC70) in mature human erythrocytes

Two-dimensional gel electrophoresis of cytosolic proteins from mature human erythrocytes combined with immunoblotting revealed the presence of a group of heat shock proteins (HSPs) that included two molecular chaperons of the HSP70 family (HSX70, inducible; HSC70, constitutively expressed) and HSP90. As expected for cells devoid of organelles, erythrocytes do not contain stress proteins that are localized either in the mitochondria (HSP60, glucose-regulated protein (GRP 75) or in the endoplasmic reticulum (GRP78 or Ig heavy chain-binding protein, endoplasmin). Since red cells are unable to replace proteins whose structure has been damaged by environmental changes the results are taken to imply a role for chaperons in monitoring, protecting, and maintaining the structure and stability of erythrocyte proteins.