Regulation of 70-kDa heat-shock-protein ATPase activity and substrate binding by human DnaJ-like proteins, HSJ1a and HSJ1b

CHIP: A Co-chaperone for Degradation by the Proteasome and Lysosome

Protein homeostasis relies on a balance between protein folding and protein degradation. Molecular chaperones like Hsp70 and Hsp90 fulfill well-defined roles in protein folding and conformational stability via ATP-dependent reaction cycles. These folding cycles are controlled by associations with a cohort of non-client protein co-chaperones, such as Hop, p23, and Aha1. Pro-folding co-chaperones facilitate the transit of the client protein through the chaperone-mediated folding process. However, chaperones are also involved in proteasomal and lysosomal degradation of client proteins. Like folding complexes, the ability of chaperones to mediate protein degradation is regulated by co-chaperones, such as the C-terminal Hsp70-binding protein (CHIP/STUB1). CHIP binds to Hsp70 and Hsp90 chaperones through its tetratricopeptide repeat (TPR) domain and functions as an E3 ubiquitin ligase using a modified RING finger domain (U-box). This unique combination of domains effectively allows CHIP to network chaperone complexes to the ubiquitin-proteasome and autophagosome-lysosome systems. This chapter reviews the current understanding of CHIP as a co-chaperone that switches Hsp70/Hsp90 chaperone complexes from protein folding to protein degradation.

Chaperone-assisted E3 ligase CHIP: A double agent in cancer

The carboxy-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin ligase and co-chaperone belonging to Ubox family that plays a crucial role in the maintenance of cellular homeostasis by switching the equilibrium of the folding-refolding mechanism towards the proteasomal or lysosomal degradation pathway. It links molecular chaperones viz. HSC70, HSP70 and HSP90 with ubiquitin proteasome system (UPS), acting as a quality control system. CHIP contains charged domain in between N-terminal tetratricopeptide repeat (TPR) and C-terminal Ubox domain. TPR domain interacts with the aberrant client proteins via chaperones while Ubox domain facilitates the ubiquitin transfer to the client proteins for ubiquitination. Thus, CHIP is a classic molecule that executes ubiquitination for degradation of client proteins. Further, CHIP has been found to be indulged in cellular differentiation, proliferation, metastasis and tumorigenesis. Additionally, CHIP can play its dual role as a tumor suppressor as well as an oncogene in numerous malignancies, thus acting as a double agent. Here, in this review, we have reported almost all substrates of CHIP established till date and classified them according to the hallmarks of cancer. In addition, we discussed about its architectural alignment, tissue specific expression, sub-cellular localization, folding-refolding mechanisms of client proteins, E4 ligase activity, normal physiological roles, as well as involvement in various diseases and tumor biology. Further, we aim to discuss its importance in HSP90 inhibitors mediated cancer therapy. Thus, this report concludes that CHIP may be a promising and worthy drug target towards pharmaceutical industry for drug development.

Exported plasmodial J domain protein, PFE0055c, and PfHsp70-x form a specific co-chaperone-chaperone partnership

Plasmodium falciparum is a unicellular protozoan parasite and causative agent of a severe form of malaria in humans, accounting for very high worldwide fatality rates. At the molecular level, survival of the parasite within the human host is mediated by P. falciparum heat shock proteins (PfHsps) that provide protection during febrile episodes. The ATP-dependent chaperone activity of Hsp70 relies on the co-chaperone J domain protein (JDP), with which it forms a chaperone-co-chaperone complex. The exported P. falciparum JDP (PfJDP), PFA0660w, has been shown to stimulate the ATPase activity of the exported chaperone, PfHsp70-x. Furthermore, PFA0660w has been shown to associate with another exported PfJDP, PFE0055c, and PfHsp70-x in J-dots, highly mobile structures found in the infected erythrocyte cytosol. Therefore, the present study aims to conduct a structural and functional characterization of the full-length exported PfJDP, PFE0055c. Recombinant PFE0055c was successfully expressed and purified and found to stimulate the basal ATPase activity of PfHsp70-x to a greater extent than PFA0660w but, like PFA0660w, did not significantly stimulate the basal ATPase activity of human Hsp70. Small-molecule inhibition assays were conducted to determine the effect of known inhibitors of JDPs (chalcone, C86) and Hsp70 (benzothiazole rhodacyanines, JG231 and JG98) on the basal and PFE0055c-stimulated ATPase activity of PfHsp70-x. In this study, JG231 and JG98 were found to inhibit both the basal and PFE0055c-stimulated ATPase activity of PfHsp70-x. C86 only inhibited the PFE0055c-stimulated ATPase activity of PfHsp70-x, consistent with PFE0055c binding to PfHsp70-x through its J domain. This research has provided further insight into the molecular basis of the interaction between these exported plasmodial chaperones, which could inform future antimalarial drug discovery studies.

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

Skeletal muscle and the nervous system depend on efficient protein quality control, and they express chaperones and cochaperones at high levels to maintain protein homeostasis. Mutations in many of these proteins cause neuromuscular diseases, myopathies, and hereditary motor and sensorimotor neuropathies. In this review, we cover mutations in DNAJB6, DNAJB2, αB-crystallin (CRYAB, HSPB5), HSPB1, HSPB3, HSPB8, and BAG3, and discuss the molecular mechanisms by which they cause neuromuscular disease. In addition, previously unpublished results are presented, showing downstream effects of BAG3 p.P209L on DNAJB6 turnover and localization.

Screening for Small Molecule Modulators of Trypanosoma brucei Hsp70 Chaperone Activity Based upon Alcyonarian Coral-Derived Natural Products

The Trypanosoma brucei Hsp70/J-protein machinery plays an essential role in survival, differentiation, and pathogenesis of the protozoan parasite, and is an emerging target against African Trypanosomiasis. This study evaluated a set of small molecules, inspired by the malonganenones and nuttingins, as modulators of the chaperone activity of the cytosolic heat inducible T. brucei Hsp70 and constitutive TbHsp70.4 proteins. The compounds were assessed for cytotoxicity on both the bloodstream form of T. b. brucei parasites and a mammalian cell line. The compounds were then investigated for their modulatory effect on the aggregation suppression and ATPase activities of the TbHsp70 proteins. A structure-activity relationship for the malonganenone-class of alkaloids is proposed based upon these results.

Fine Tuning: Effects of Post-Translational Modification on Hsp70 Chaperones

The discovery of heat shock proteins shaped our view of protein folding in the cell. Since their initial discovery, chaperone proteins were identified in all domains of life, demonstrating their vital and conserved functional roles in protein homeostasis. Chaperone proteins maintain proper protein folding in the cell by utilizing a variety of distinct, characteristic mechanisms to prevent aberrant intermolecular interactions, prevent protein aggregation, and lower entropic costs to allow for protein refolding. Continued study has found that chaperones may exhibit alternative functions, including maintaining protein folding during endoplasmic reticulum (ER) import and chaperone-mediated degradation, among others. Alternative chaperone functions are frequently controlled by post-translational modification, in which a given chaperone can switch between functions through covalent modification. This review will focus on the Hsp70 class chaperones and their Hsp40 co-chaperones, specifically highlighting the importance of post-translational control of chaperones. These modifications may serve as a target for therapeutic intervention in the treatment of diseases of protein misfolding and aggregation.

DNAJC13 p.Asn855Ser, implicated in familial parkinsonism, alters membrane dynamics of sorting nexin 1

DNAJC13 (RME-8) is a core co-chaperone that facilitates membrane recycling and cargo sorting of endocytosed proteins. DNAJ/Hsp40 (heat shock protein 40) proteins are highly conserved throughout evolution and mediate the folding of nascent proteins, and the unfolding, refolding or degradation of misfolded proteins while assisting in associated-membrane translocation. DNAJC13 is one of five DNAJ 'C' class chaperone variants implicated in monogenic parkinsonism. Here we examine the effect of the DNAJC13 disease-linked mutation (p.Asn855Ser) on its interacting partners, focusing on sorting nexin 1 (SNX1) membrane dynamics in primary cortical neurons derived from a novel Dnajc13 p.Asn855Ser knock-in (DKI) mouse model. Dnajc13 p.Asn855Ser mutant and wild type protein expression were equivalent in mature heterozygous cultures (DIV21). While SNX1-positive puncta density, area, and WASH-retromer assembly were comparable between cultures derived from DKI and wild type littermates, the formation of SNX1-enriched tubules in DKI neuronal cultures was significantly increased. Thus, Dnajc13 p.Asn855Ser disrupts SNX1 membrane-tubulation and trafficking, analogous to results from RME-8 depletion studies. The data suggest the mutation confers a dominant-negative gain-of-function in RME-8. Implications for the pathogenesis of Parkinson's disease are discussed.

Cholesterol bound Plasmodium falciparum co-chaperone 'PFA0660w' complexes with major virulence factor 'PfEMP1' via chaperone 'PfHsp70-x'

Lethality of Plasmodium falciparum caused malaria results from ‘cytoadherence’, which is mainly effected by exported Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family. Several exported P. falciparum proteins (exportome) including chaperones alongside cholesterol rich microdomains are crucial for PfEMP1 translocation to infected erythrocyte surface. An exported Hsp40 (heat shock protein 40) ‘PFA0660w’ functions as a co-chaperone of ‘PfHsp70-x’, and these co-localize to specialized intracellular mobile structures termed J-dots. Our studies attempt to understand the function of PFA0660w-PfHsp70-x chaperone pair using recombinant proteins. Biochemical assays reveal that N and C-terminal domains of PFA0660w and PfHsp70-x respectively are critical for their activity. We show the novel direct interaction of PfHsp70-x with the cytoplasmic tail of PfEMP1, and binding of PFA0660w with cholesterol. PFA0660w operates both as a chaperone and lipid binding molecule via its separate substrate and cholesterol binding sites. PfHsp70-x interacts with cholesterol bound PFA0660w and PfEMP1 simultaneously in vitro to form a complex. Collectively, our results and the past literature support the hypothesis that PFA0660w-PfHsp70-x chaperone pair assists PfEMP1 transport across the host erythrocyte through cholesterol containing ‘J-dots’. These findings further the understanding of PfEMP1 export in malaria parasites, though their in vivo validation remains to be performed.

Protein kinase CK2 modulates HSJ1 function through phosphorylation of the UIM2 domain

HSJ1 (DNAJB2), a member of the DNAJ family of molecular chaperones, is a key player in neuronal proteostasis maintenance. It binds ubiquitylated proteins through its Ubiquitin Interacting Motifs (UIMs) and facilitates their delivery to the proteasome for degradation. Mutations in the DNAJB2 gene lead to inherited neuropathies such as Charcot-Marie-Tooth type-2, distal hereditary motor neuropathies, spinal muscular atrophy with parkinsonism and the later stages can resemble amyotrophic lateral sclerosis. HSJ1 overexpression can reduce aggregation of neurodegeneration-associated proteins in vitro and in vivo; however, the regulation of HSJ1 function is little understood. Here we show that CK2, a ubiquitous and constitutively active protein kinase, phosphorylates HSJ1 within its second UIM, at the dominant site Ser250 and the hierarchical site Ser247. A phospho-HSJ1 specific antibody confirmed phosphorylation of endogenous HSJ1a and HSJ1b. A tandem approach of phospho-site mutation and treatment with CK2 specific inhibitors demonstrated that phosphorylation at these sites is accompanied by a reduced ability of HSJ1 to bind ubiquitylated clients and to exert its chaperone activity. Our results disclose a novel interplay between ubiquitin- and phosphorylation-dependent signalling, and represent the first report of a regulatory mechanism for UIM-dependent function. They also suggest that CK2 inhibitors could release the full neuroprotective potential of HSJ1, and deserve future interest as therapeutic strategies for neurodegenerative disease.

The development of modified human Hsp70 (HSPA1A) and its production in the milk of transgenic mice

The production of major human heat shock protein Hsp70 (HSPA1A) in a eukaryotic expression system is needed for testing and possible medical applications. In this study, transgenic mice were produced containing wild-type human Hsp70 allele in the vector providing expression in the milk. The results indicated that human Hsp70 was readily expressed in the transgenic animals but did not apparently preserve its intact structure and, hence, it was not possible to purify the protein using conventional isolation techniques. It was suggested that the protein underwent glycosylation in the process of expression, and this quite common modification for proteins expressed in the milk complicated its isolation. To check this possibility, we mutated all presumptive sites of glycosylation and tested the properties of the resulting modified Hsp70 expressed in E. coli. The investigation demonstrated that the modified protein exhibited all beneficial properties of the wild-type Hsp70 and was even superior to the latter for a few parameters. Based on these results, a transgenic mouse strain was obtained which expressed the modified Hsp70 in milk and which was easy to isolate using ATP columns. Therefore, the developed construct can be explored in various bioreactors for reliable manufacture of high quality, uniform, and reproducible human Hsp70 for possible medical applications including neurodegenerative diseases and cancer.

The heat shock response plays an important role in TDP-43 clearance: evidence for dysfunction in amyotrophic lateral sclerosis

Detergent-resistant, ubiquitinated and hyperphosphorylated Tar DNA binding protein 43 (TDP-43, encoded by TARDBP) neuronal cytoplasmic inclusions are the pathological hallmark in ∼95% of amyotrophic lateral sclerosis and ∼60% of frontotemporal lobar degeneration cases. We sought to explore the role for the heat shock response in the clearance of insoluble TDP-43 in a cellular model of disease and to validate our findings in transgenic mice and human amyotrophic lateral sclerosis tissues. The heat shock response is a stress-responsive protective mechanism regulated by the transcription factor heat shock factor 1 (HSF1), which increases the expression of chaperones that refold damaged misfolded proteins or facilitate their degradation. Here we show that manipulation of the heat shock response by expression of dominant active HSF1 results in a dramatic reduction of insoluble and hyperphosphorylated TDP-43 that enhances cell survival, whereas expression of dominant negative HSF1 leads to enhanced TDP-43 aggregation and hyperphosphorylation. To determine which chaperones were mediating TDP-43 clearance we over-expressed a range of heat shock proteins (HSPs) and identified DNAJB2a (encoded by DNAJB2, and also known as HSJ1a) as a potent anti-aggregation chaperone for TDP-43. DNAJB2a has a J domain, allowing it to interact with HSP70, and ubiquitin interacting motifs, which enable it to engage the degradation of its client proteins. Using functionally deleted DNAJB2a constructs we demonstrated that TDP-43 clearance was J domain-dependent and was not affected by ubiquitin interacting motif deletion or proteasome inhibition. This indicates that TDP-43 is maintained in a soluble state by DNAJB2a, leaving the total levels of TDP-43 unchanged. Additionally, we have demonstrated that the levels of HSF1 and heat shock proteins are significantly reduced in affected neuronal tissues from a TDP-43 transgenic mouse model of amyotrophic lateral sclerosis and patients with sporadic amyotrophic lateral sclerosis. This implies that the HSF1-mediated DNAJB2a/HSP70 heat shock response pathway is compromised in amyotrophic lateral sclerosis. Defective refolding of TDP-43 is predicted to aggravate the TDP-43 proteinopathy. The finding that the pathological accumulation of insoluble TDP-43 can be reduced by the activation of HSF1/HSP pathways presents an exciting opportunity for the development of novel therapeutics.

The Malarial Exported PFA0660w Is an Hsp40 Co-Chaperone of PfHsp70-x

Plasmodium falciparum, the human pathogen responsible for the most dangerous malaria infection, survives and develops in mature erythrocytes through the export of proteins needed for remodelling of the host cell. Molecular chaperones of the heat shock protein (Hsp) family are prominent members of the exportome, including a number of Hsp40s and a Hsp70. PFA0660w, a type II Hsp40, has been shown to be exported and possibly form a complex with PfHsp70-x in the infected erythrocyte cytosol. However, the chaperone properties of PFA0660w and its interaction with human and parasite Hsp70s are yet to be investigated. Recombinant PFA0660w was found to exist as a monomer in solution, and was able to significantly stimulate the ATPase activity of PfHsp70-x but not that of a second plasmodial Hsp70 (PfHsp70-1) or a human Hsp70 (HSPA1A), indicating a potential specific functional partnership with PfHsp70-x. Protein binding studies in the presence and absence of ATP suggested that the interaction of PFA0660w with PfHsp70-x most likely represented a co-chaperone/chaperone interaction. Also, PFA0660w alone produced a concentration-dependent suppression of rhodanese aggregation, demonstrating its chaperone properties. Overall, we have provided the first biochemical evidence for the possible role of PFA0660w as a chaperone and as co-chaperone of PfHsp70-x. We propose that these chaperones boost the chaperone power of the infected erythrocyte, enabling successful protein trafficking and folding, and thereby making a fundamental contribution to the pathology of malaria.

70-kDa heat shock protein coated magnetic nanocarriers as a nanovaccine for induction of anti-tumor immune response in experimental glioma

Nanovaccines based on superparamagnetic iron oxide nanoparticles (SPIONs) provide a novel approach to induce the humoral and cell-based immune system to fight cancer. Herein, we increased the immunostimulatory capacity of SPIONs by coating them with recombinant heat shock protein 70 (Hsp70) which is known to chaperone antigenic peptides. After binding, Hsp70-SPIONs deliver immunogenic peptides from tumor lysates to dendritiс cells (DCs) and thus stimulate a tumor-specific, CD8+ cytotoxic T cell response. We could show that binding activity of Hsp70-SPIONs to the substrate-binding domain (SBD) is highly dependent on the ATPase activity of its nucleotide-binding domain NBD), as shown by (31)P NMR spectroscopy. Immunization of C6 glioma-bearing rats with DCs pulsed with Hsp70-SPIONs and tumor lysates resulted in a delayed tumor progression (as measured by MRI) and an increased overall survival. In parallel an increased IFNγ secretion were detected in the serum of these animals and immunohistological analysis of subsequent cryosections of the glioma revealed an enhanced infiltration of memory CD45RO+ and cytotoxic CD8+ T cells. Taken together the study demonstrates that magnetic nanocarriers such as SPIONs coated with Hsp70 can be applied as a platform for boosting anti-cancer immune responses.

Isoform-selective Genetic Inhibition of Constitutive Cytosolic Hsp70 Activity Promotes Client Tau Degradation Using an Altered Co-chaperone Complement

The constitutively expressed heat shock protein 70 kDa (Hsc70) is a major chaperone protein responsible for maintaining proteostasis, yet how its structure translates into functional decisions regarding client fate is still unclear. We previously showed that Hsc70 preserved aberrant Tau, but it remained unknown if selective inhibition of the activity of this Hsp70 isoform could facilitate Tau clearance. Using single point mutations in the nucleotide binding domain, we assessed the effect of several mutations on the functions of human Hsc70. Biochemical characterization revealed that one mutation abolished both Hsc70 ATPase and refolding activities. This variant resembled the ADP-bound conformer at all times yet remained able to interact with cofactors, nucleotides, and substrates appropriately, resembling a dominant negative Hsc70 (DN-Hsc70). We then assessed the effects of this DN-Hsc70 on its client Tau. DN-Hsc70 potently facilitated Tau clearance via the proteasome in cells and brain tissue, in contrast to wild type Hsc70 that stabilized Tau. Thus, DN-Hsc70 mimics the action of small molecule pan Hsp70 inhibitors with regard to Tau metabolism. This shift in Hsc70 function by a single point mutation was the result of a change in the chaperome associated with Hsc70 such that DN-Hsc70 associated more with Hsp90 and DnaJ proteins, whereas wild type Hsc70 was more associated with other Hsp70 isoforms. Thus, isoform-selective targeting of Hsc70 could be a viable therapeutic strategy for tauopathies and possibly lead to new insights in chaperone complex biology.

CHIP: a co-chaperone for degradation by the proteasome

Protein homeostasis relies on a balance between protein folding and protein degradation. Molecular chaperones like Hsp70 and Hsp90 fulfil well-defined roles in protein folding and conformational stability via ATP dependent reaction cycles. These folding cycles are controlled by associations with a cohort of non-client protein co-chaperones, such as Hop, p23 and Aha1. Pro-folding co-chaperones facilitate the transit of the client protein through the chaperone mediated folding process. However, chaperones are also involved in ubiquitin-mediated proteasomal degradation of client proteins. Similar to folding complexes, the ability of chaperones to mediate protein degradation is regulated by co-chaperones, such as the C terminal Hsp70 binding protein (CHIP). CHIP binds to Hsp70 and Hsp90 chaperones through its tetratricopeptide repeat (TPR) domain and functions as an E3 ubiquitin ligase using a modified RING finger domain (U-box). This unique combination of domains effectively allows CHIP to network chaperone complexes to the ubiquitin-proteasome system. This chapter reviews the current understanding of CHIP as a co-chaperone that switches Hsp70/Hsp90 chaperone complexes from protein folding to protein degradation.

Molecular characterization of DnaJ 5 homologs in silkworm Bombyx mori and its expression during egg diapause

A comparison of the cDNA sequences (1 056 bp) of Bombyx mori DnaJ 5 homolog with B. mori genome revealed that unlike in other Hsps, it has an intron of 234 bp. The DnaJ 5 homolog contains 351 amino acids, of which 70 contain the conserved DnaJ domain at the N-terminal end. This homolog of B. mori has all desirable functional domains similar to other insects, and the 13 different DnaJ homologs identified in B. mori genome were distributed on different chromosomes. The expressed sequence tag database analysis of Hsp40 gene expression revealed higher expression in wing disc followed by diapause-induced eggs. Microarray analysis revealed higher expression of DnaJ 5 homolog at 18th h after oviposition in diapause-induced eggs. Further validation of DnaJ 5 expression through qPCR in diapause-induced and nondiapause eggs at different time intervals revealed higher expression in diapause eggs at 18 and 24 h after oviposition, which coincided with the expression of Hsp70 as the Hsp 40 is its co-chaperone. This study thus provides an outline of the genome organization of Hsp40 gene, and its role in egg diapause induction in B. mori.

The E3 ligase CHIP: insights into its structure and regulation

The carboxy-terminus of Hsc70 interacting protein (CHIP) is a cochaperone E3 ligase containing three tandem repeats of tetratricopeptide (TPR) motifs and a C-terminal U-box domain separated by a charged coiled-coil region. CHIP is known to function as a central quality control E3 ligase and regulates several proteins involved in a myriad of physiological and pathological processes. Recent studies have highlighted varied regulatory mechanisms operating on the activity of CHIP which is crucial for cellular homeostasis. In this review article, we give a concise account of our current knowledge on the biochemistry and regulation of CHIP.

ERdj3 regulates BiP occupancy in living cells

Co-chaperones regulate chaperone activities and are likely to impact a protein-folding environment as much as the chaperone itself. As co-chaperones are expressed substoichiometrically, the ability of co-chaperones to encounter a chaperone is crucial for chaperone activity. ERdj3, an abundant soluble endoplasmic reticulum (ER) co-chaperone of the Hsp70 BiP, stimulates the ATPase activity of BiP to increase BiP's affinity for client (or substrate) proteins. We investigated ERdj3 availability, how ERdj3 levels impact BiP availability, and the significance of J proteins for regulating BiP binding of clients in living cells. FRAP analysis revealed that overexpressed ERdj3-sfGFP dramatically decreases BiP-GFP mobility in a client-dependent manner. By contrast, ERdj3-GFP mobility remains low regardless of client protein levels. Native gels and co-immunoprecipitations established that ERdj3 associates with a large complex including Sec61α. Translocon binding probably ensures rapid encounters between emerging nascent peptides and stimulates BiP activity in the crucial early stages of secretory protein folding. Importantly, mutant BiP exhibited significantly increased mobility when it could not interact with any ERdjs. Thus, ERdjs appear to play the dual roles of increasing BiP affinity for clients and regulating delivery of clients to BiP. Our data suggest that BiP engagement of clients is enhanced in ER subdomains enriched in ERdj proteins.

Design principles of protein biosynthesis-coupled quality control

The protein biosynthetic machinery, composed of ribosomes, chaperones, and localization factors, is increasingly found to interact directly with factors dedicated to protein degradation. The coupling of these two opposing processes facilitates quality control of nascent polypeptides at each stage of their maturation. Sequential checkpoints maximize the overall fidelity of protein maturation, minimize the exposure of defective products to the bulk cellular environment, and protect organisms from protein misfolding diseases.

The C-terminal helices of heat shock protein 70 are essential for J-domain binding and ATPase activation

The J-domain co-chaperones work together with the heat shock protein 70 (HSP70) chaperone to regulate many cellular events, but the mechanism underlying the J-domain-mediated HSP70 function remains elusive. We studied the interaction between human-inducible HSP70 and Homo sapiens J-domain protein (HSJ1a), a J domain and UIM motif-containing co-chaperone. The J domain of HSJ1a shares a conserved structure with other J domains from both eukaryotic and prokaryotic species, and it mediates the interaction with and the ATPase cycle of HSP70. Our in vitro study corroborates that the N terminus of HSP70 including the ATPase domain and the substrate-binding β-subdomain is not sufficient to bind with the J domain of HSJ1a. The C-terminal helical α-subdomain of HSP70, which was considered to function as a lid of the substrate-binding domain, is crucial for binding with the J domain of HSJ1a and stimulating the ATPase activity of HSP70. These fluctuating helices are likely to contribute to a proper conformation of HSP70 for J-domain binding other than directly bind with the J domain. Our findings provide an alternative mechanism of allosteric activation for functional regulation of HSP70 by its J-domain co-chaperones.

Functional analysis of the exported type IV HSP40 protein PfGECO in Plasmodium falciparum gametocytes

During Plasmodium falciparum infection, host red blood cell (RBC) remodeling is required for the parasite's survival. Such modifications are mediated by the export of parasite proteins into the RBC that alter the architecture of the RBC membrane and enable cytoadherence. It is probable that some exported proteins also play a protective role against the host defense response. This may be of particular importance for the gametocyte stage of the life cycle that is responsible for malaria transmission, since the gametocyte remains in contact with blood as it proceeds through five morphological stages (I to V) during its 12-day maturation. Using microarray analysis, we identified several genes with encoded secretory or export sequences that were differentially expressed during early gametocytogenesis. One of these, PfGECO, encodes a predicted type IV heat shock protein 40 (HSP40) that we show is expressed in gametocyte stages I to IV and is exported to the RBC cytoplasm. HSPs are traditionally induced under stressful conditions to maintain homeostasis, but PfGECO expression was not increased upon heat shock, suggesting an alternate function. Targeted disruption of PfGECO indicated that the gene is not essential for gametocytogenesis in vitro, and quantitative reverse transcriptase PCR (RT-PCR) showed that there was no compensatory expression of the other type IV HSP40 genes. Although P. falciparum HSP40 members are implicated in the trafficking of proteins to the RBC surface, removal of PfGECO did not affect the targeting of other exported gametocyte proteins. This work has expanded the repertoire of known gametocyte-exported proteins to include a type IV HSP40, PfGECO.

Human DNAJ in cancer and stem cells

The heat shock protein 40kDa (HSP40/DNAJ) co-chaperones constitute the largest and most diverse sub-group of the heat shock protein (HSP) family. DNAJ are widely accepted as regulators of HSP70 function, but also have roles as co-chaperones for the HSP90 chaperone machine, and a growing number of biological functions that may be independent of either of these chaperones. The DNAJ proteins are differentially expressed in human tissues and demonstrate the capacity to function to both promote and suppress cancer development by acting as chaperones for tumour suppressors or oncoproteins. We review the current literature on the function and expression of DNAJ in cancer, stem cells and cancer stem cells. Combining data from gene expression, proteomics and studies in other systems, we propose that DNAJ will be key regulators of cancer, stem cell and possibly cancer stem cell function. The diversity of DNAJ and their assorted roles in a range of biological functions means that selected DNAJ, provided there is limited redundancy and that a specific link to malignancy can be established, may yet provide an attractive target for specific and selective drug design for the development of anti-cancer treatments.

Plasmodium falciparum encodes a single cytosolic type I Hsp40 that functionally interacts with Hsp70 and is upregulated by heat shock

Heat shock protein 70 (Hsp70) and heat shock protein 40 (Hsp40) function as molecular chaperones during the folding and trafficking of proteins within most cell types. However, the Hsp70-Hsp40 chaperone partnerships within the malaria parasite, Plasmodium falciparum, have not been elucidated. Only one of the 43 P. falciparum Hsp40s is predicted to be a cytosolic, canonical Hsp40 (termed PfHsp40) capable of interacting with the major cytosolic P. falciparum-encoded Hsp70, PfHsp70. Consistent with this hypothesis, we found that PfHsp40 is upregulated under heat shock conditions in a similar pattern to PfHsp70. In addition, PfHsp70 and PfHsp40 reside mainly in the parasite cytosol, as assessed using indirect immunofluorescence microscopy. Recombinant PfHsp40 stimulated the ATP hydrolytic rates of both PfHsp70 and human Hsp70 similar to other canonical Hsp40s of yeast (Ydj1) and human (Hdj2) origin. In contrast, the Hsp40-stimulated plasmodial and human Hsp70 ATPase activities were differentially inhibited in the presence of pyrimidinone-based small molecule modulators. To further probe the chaperone properties of PfHsp40, protein aggregation suppression assays were conducted. PfHsp40 alone suppressed protein aggregation, and cooperated with PfHsp70 to suppress aggregation. Together, these data represent the first cellular and biochemical evidence for a PfHsp70-PfHsp40 partnership in the malaria parasite, and furthermore that the plasmodial and human Hsp70-Hsp40 chaperones possess unique attributes that are differentially modulated by small molecules.

Co-chaperone HSJ1a dually regulates the proteasomal degradation of ataxin-3

Homo sapiens J domain protein (HSJ1) is a J-domain containing co-chaperone that is known to stimulate ATPase activity of HSP70 chaperone, while it also harbors two ubiquitin (Ub)-interacting motifs (UIMs) that may bind with ubiquitinated substrates and potentially function in protein degradation. We studied the effects of HSJ1a on the protein levels of both normal and the disease–related polyQ-expanded forms of ataxin-3 (Atx3) in cells. The results demonstrate that the N-terminal J-domain and the C-terminal UIM domain of HSJ1a exert opposite functions in regulating the protein level of cellular overexpressed Atx3. This dual regulation is dependent on the binding of the J-domain with HSP70, and the UIM domain with polyUb chains. The J-domain down-regulates the protein level of Atx3 through HSP70 mediated proteasomal degradation, while the UIM domain may alleviate this process via maintaining the ubiquitinated Atx3. We propose that co-chaperone HSJ1a orchestrates the balance of substrates in stressed cells in a Yin-Yang manner.

Screening for small molecule modulators of Hsp70 chaperone activity using protein aggregation suppression assays: inhibition of the plasmodial chaperone PfHsp70-1

Plasmodium falciparum heat shock protein 70 (PfHsp70-1) is thought to play an essential role in parasite survival and virulence in the human host, making it a potential antimalarial drug target. A malate dehydrogenase based aggregation suppression assay was adapted for the screening of small molecule modulators of Hsp70. A number of small molecules of natural (marine prenylated alkaloids and terrestrial plant naphthoquinones) and related synthetic origin were screened for their effects on the protein aggregation suppression activity of purified recombinant PfHsp70-1. Five compounds (malonganenone A-C, lapachol and bromo-β-lapachona) were found to inhibit the chaperone activity of PfHsp70-1 in a concentration dependent manner, with lapachol preferentially inhibiting PfHsp70-1 compared to another control Hsp70. Using growth inhibition assays on P. falciparum infected erythrocytes, all of the compounds, except for malonganenone B, were found to inhibit parasite growth with IC50 values in the low micromolar range. Overall, this study has identified two novel classes of small molecule inhibitors of PfHsp70-1, one representing a new class of antiplasmodial compounds (malonganenones). In addition to demonstrating the validity of PfHsp70-1 as a possible drug target, the compounds reported in this study will be potentially useful as molecular probes for fundamental studies on Hsp70 chaperone function.

RDJ2 (DNAJA2) chaperones neural G protein signaling pathways

A number of structurally divergent proteins with J domains, called J proteins, interact with and activate the ATPase of Hsp70s, thereby harnessing the ATPase activity for conformational work on target proteins. The precise role of most mammalian J proteins remains undefined. In this paper, we demonstrate that transient expression of the J protein, Rdj2, in HEK 293 cells increased cellular cyclic adenosine monophosphate (cAMP) levels in the presence of the beta-adrenergic agonist isoproterenol. In CNS-derived catecholaminergic neuronal cell line (CAD) neuroblastoma cells, expression of Rdj2 increased isoproterenol-stimulated phosphorylation of cAMP response element binding protein (CREB). Moreover, we have characterized the binding properties of Rdj2 and observed a direct interaction between Rdj2 and receptor-coupled trimeric GTP-binding proteins (G proteins). We further show that the composition of the Rdj2-chaperone complex and the cysteine string protein (CSPalpha)-chaperone complex, another J protein, is distinct. Our data demonstrate that Rdj2 modulates G protein signaling and further suggest that chaperoning G proteins is an emerging theme of the J protein network.

Hsp40 Molecules That Target to the Ubiquitin-proteasome System Decrease Inclusion Formation in Models of Polyglutamine Disease

We studied the ability of heat shock, DnaJ-like-1 (HSJ1) proteins (which contain DnaJ and ubiquitin-interacting motifs) to reduce polyglutamine-mediated inclusion formation. The experiments demonstrated that expression of heat shock protein 70 (hsp70), hsp40, HSJ1a, and HSJ1b significantly reduced protein inclusion formation in a model of spinal and bulbar muscular atrophy (SBMA). HSJ1a also mediated a significant decrease in the number of inclusions formed in a primary neuronal model of protein aggregation. Studies to elucidate the mechanisms underlying these reductions showed that hsp70 and hsp40 increased chaperone-mediated refolding. In contrast, expression of HSJ1 proteins did not promote chaperone activity but caused an increase in ubiquitylation. Furthermore, HSJ1a was associated with a ubiquitylated luciferase complex, and in the presence of HSJ1a but not an HSJ1a UIM mutant (HSJ1a-deltaUIM) there was a reduction in luciferase protein levels. Together these results show that HSJ1 proteins mediated an increase in target protein degradation via the ubiquitin-proteasome system (UPS). We also found that the expression of HSJ1a significantly decreased the number of neurons containing inclusions in an in vivo model of polyglutamine disease. These findings indicate that targeted modification of the UPS to facilitate degradation of misfolded proteins may represent a highly effective therapeutic avenue for the treatment of polyglutamine disease.

Cytosolic and ER J-domains of mammalian and parasitic origin can functionally interact with DnaK

Both prokaryotic and eukaryotic cells contain multiple heat shock protein 40 (Hsp40) and heat shock protein 70 (Hsp70) proteins, which cooperate as molecular chaperones to ensure fidelity at all stages of protein biogenesis. The Hsp40 signature domain, the J-domain, is required for binding of an Hsp40 to a partner Hsp70, and may also play a role in the specificity of the association. Through the creation of chimeric Hsp40 proteins by the replacement of the J-domain of a prokaryotic Hsp40 (DnaJ), we have tested the functional equivalence of J-domains from a number of divergent Hsp40s of mammalian and parasitic origin (malarial Pfj1 and Pfj4, trypanosomal Tcj3, human ERj3, ERj5, and Hsj1, and murine ERj1). An in vivo functional assay was used to test the functionality of the chimeric proteins on the basis of their ability to reverse the thermosensitivity of a dnaJ cbpA mutant Escherichia coli strain (OD259). The Hsp40 chimeras containing J-domains originating from soluble (cytosolic or endoplasmic reticulum (ER)-lumenal) Hsp40s were able to reverse the thermosensitivity of E. coli OD259. In all cases, modified derivatives of these chimeric proteins containing an His to Gln substitution in the HPD motif of the J-domain were unable to reverse the thermosensitivity of E. coli OD259. This suggested that these J-domains exerted their in vivo functionality through a specific interaction with E. coli Hsp70, DnaK. Interestingly, a Hsp40 chimera containing the J-domain of ERj1, an integral membrane-bound ER Hsp40, was unable to reverse the thermosensitivity of E. coli OD259, suggesting that this J-domain was unable to functionally interact with DnaK. Substitutions of conserved amino acid residues and motifs were made in all four helices (I–IV) and the loop regions of the J-domains, and the modified chimeric Hsp40s were tested for functionality using the in vivo assay. Substitution of a highly conserved basic residue in helix II of the J-domain was found to disrupt in vivo functionality for all the J-domains tested. We propose that helix II and the HPD motif of the J-domain represent the fundamental elements of a binding surface required for the interaction of Hsp40s with Hsp70s, and that this surface has been conserved in mammalian, parasitic and bacterial systems.

A Drosophila ortholog of the human MRJ modulates polyglutamine toxicity and aggregation

In the Drosophila eye, proteins with an expanded polyglutamine (polyQ) tract form nuclear and cytoplasmic inclusions and produce cytotoxicity, demonstrated as loss of eye pigmentation and structural integrity. An EP P-element that suppressed the loss of eye pigmentation was inserted 9.7 kb upstream of dmrj, a gene that encodes an ortholog of a brain-enriched cochaperone, the human MRJ (mammalian relative of DnaJ). Despite the large distance between them, quantitative polymerase chain reaction indicated that the EP could overexpress dmrj. In the retina and other neurons, transgenic dMRJ suppressed polyQ toxicity and colocalized with its inclusions. In the photoreceptors, expression of another suppressor with a J domain, dHDJ1, but not dMRJ, prior to expression of expanded polyQs dramatically promoted cytoplasmic aggregation. However, both proteins increased the level of detergent-soluble, monomeric polyQ-expanded proteins. These findings exemplify the functional similarities and differences between J domain proteins in suppressing polyQ toxicity.

Cystamine and cysteamine increase brain levels of BDNF in Huntington disease via HSJ1b and transglutaminase

There is no treatment for the neurodegenerative disorder Huntington disease (HD). Cystamine is a candidate drug; however, the mechanisms by which it operates remain unclear. We show here that cystamine increases levels of the heat shock DnaJ-containing protein 1b (HSJ1b) that are low in HD patients. HSJ1b inhibits polyQ-huntingtin-induced death of striatal neurons and neuronal dysfunction in Caenorhabditis elegans. This neuroprotective effect involves stimulation of the secretory pathway through formation of clathrin-coated vesicles containing brain-derived neurotrophic factor (BDNF). Cystamine increases BDNF secretion from the Golgi region that is blocked by reducing HSJ1b levels or by overexpressing transglutaminase. We demonstrate that cysteamine, the FDA-approved reduced form of cystamine, is neuroprotective in HD mice by increasing BDNF levels in brain. Finally, cysteamine increases serum levels of BDNF in mouse and primate models of HD. Therefore, cysteamine is a potential treatment for HD, and serum BDNF levels can be used as a biomarker for drug efficacy.

Approaches to the isolation and characterization of molecular chaperones

Molecular chaperones are integral components of the cellular machinery involved in ensuring correct protein folding and the continued maintenance of protein structure. An understanding of these ubiquitous molecules is key to finding cures to protein misfolding diseases such as Alzheimer's and Creutzfeldt-Jacob diseases. In addition, further understanding of chaperones will enhance our comprehension of the way the body copes with the environmental stresses that humans encounter daily. Our laboratory and our collaborators specialize in the production and characterization of chaperones from a wide variety of sources in order to gain a fuller understanding of how chaperones function in the cell. In this review, we primarily use the Hsp70/Hsp40 chaperone pair as an example to discuss recent advances in technology and reductions in cost that lend themselves to chaperone purification from both native and recombinant sources. Common assays to assess purified chaperone activity are also discussed.

Neuronal DnaJ proteins HSJ1a and HSJ1b: a role in linking the Hsp70 chaperone machine to the ubiquitin–proteasome system?

The heat-shock protein 70 chaperone machine is functionally connected to the ubiquitin–proteasome system by the co-chaperone CHIP. In this article, we discuss evidence that the neuronal DnaJ proteins HSJ1a and HSJ1b may represent a further link between the cellular protein folding and degradation machineries. We have demonstrated that HSJ1 proteins contain putative ubiquitin interaction motifs and can modulate the cellular processing of rhodopsin, a protein that is targeted for degradation by the proteasome when it is misfolded.

Heat shock proteins in the regulation of apoptosis: new strategies in tumor therapy: a comprehensive review

Heat shock proteins (Hsp) form the most ancient defense system in all living organisms on earth. These proteins act as molecular chaperones by helping in the refolding of misfolded proteins and assisting in their elimination if they become irreversibly damaged. Hsp interact with a number of cellular systems and form efficient cytoprotective mechanisms. However, in some cases, wherein it is better if the cell dies, there is no reason for any further defense. Programmed cell death is a widely conserved general phenomenon helping in many processes involving the reconstruction of multicellular organisms, as well as in the elimination of old or damaged cells. Here, we review some novel elements of the apoptotic process, such as its interrelationship with cellular senescence and necrosis, as well as bacterial apoptosis. We also give a survey of the most important elements of the apoptotic machinery and show the various modes of how Hsp interact with the apoptotic events in detail. We review caspase-independent apoptotic pathways and anoikis as well. Finally, we show the emerging variety of pharmacological interventions inhibiting or, just conversely, inducing Hsp and review the emergence of Hsp as novel therapeutic targets in anticancer protocols.

NMR study of nucleotide-induced changes in the nucleotide binding domain of Thermus thermophilus Hsp70 chaperone DnaK: implications for the allosteric mechanism

We present an NMR investigation of the nucleotide-dependent conformational properties of a 44-kDa nucleotide binding domain (NBD) of an Hsp70 protein. Conformational changes driven by ATP binding and hydrolysis in the N-terminal NBD are believed to allosterically regulate substrate affinity in the C-terminal substrate binding domain. Several crystal structures of Hsc70 NBDs in different nucleotide states have, however, not shown significant structural differences. We have previously reported the NMR assignments of the backbone resonances of the NBD of the bacterial Hsp70 homologue Thermus thermophilus DnaK in the ADP-bound state. In this study we show, by assigning the NBD with the ATP/transition state analogue, ADP.AlFx, bound, that it closely mimics the ATP-bound state. Chemical shift difference mapping of the two nucleotide states identified differences in a cluster of residues at the interface between subdomains 1A and 1B. Further analysis of the spectra revealed that the ATP state exhibited a single conformation, whereas the ADP state was in slow conformational exchange between a form similar to the ATP state and another state unique to the ADP-bound form. A model is proposed of the allosteric mechanism based on the nucleotide state altering the balance of a dynamic equilibrium between the open and closed states. The observed chemical shift perturbations were concentrated in an area close to a previously described J-domain binding channel, confirming the importance of that region in the allosteric mechanism.

Overexpression of the cochaperone CHIP enhances Hsp70-dependent folding activity in mammalian cells

CHIP is a cochaperone of Hsp70 that inhibits Hsp70-dependent refolding in vitro. However, the effect of altered expression of CHIP on the fate of unfolded proteins in mammalian cells has not been determined. Surprisingly, we found that overexpression of CHIP in fibroblasts increased the refolding of proteins after thermal denaturation. This effect was insensitive to geldanamycin, an Hsp90 inhibitor, and required the tetratricopeptide repeat motifs but not the U-box domain of CHIP. Inhibition of Hsp70 chaperone activity abolished the effects of CHIP on protein folding, indicating that the CHIP-mediated events were Hsp70 dependent. Hsp40 competitively inhibited the CHIP-dependent refolding, which is consistent with in vitro data indicating that these cofactors act on Hsp70 in the ATP-bound state and have opposing effects on Hsp70 ATPase activity. Consistent with these observations, CHIP overexpression did not alter protein folding in the setting of ATP depletion, when Hsp70 is in the ADP-bound state. Concomitant with its effects on refolding heat-denatured substrates, CHIP increased the fraction of nascent chains coimmunoprecipitating with Hsc70, but only when sufficient ATP was present to allow Hsp70 to cycle rapidly. Our data suggest that, consistent with in vitro studies, CHIP attenuates the Hsp70 cycle in living cells. The impact of this effect on the fate of unfolded proteins in cells, however, is different from what might be expected from the in vitro data. Rather than resulting in inhibited refolding, CHIP increases the folding capacity of Hsp70 in eukaryotic cells.

The chaperone environment at the cytoplasmic face of the endoplasmic reticulum can modulate rhodopsin processing and inclusion formation

The human DnaJ (Hsp40) proteins HSJ1a and HSJ1b are type II DnaJ proteins with different C termini generated by alternate splicing. Both protein isoforms can regulate the ATPase activity and substrate binding of Hsp70. In this study, we have confirmed the neuronal expression of HSJ1a and HSJ1b proteins and localized their expression in human neural retina using isoform-specific antisera. HSJ1a and HSJ1b were enriched in photoreceptors, particularly the inner segments, but had different intracellular localization due to the prenylation of HSJ1b by a geranylgeranyl moiety. Because of their enrichment at the site of rhodopsin production, we investigated the effect of HSJ1 isoforms on the cellular processing of wild-type and mutant rhodopsin apoprotein in SK-N-SH cells. The expression of HSJ1b, but not HSJ1a, inhibited the normal cellular processing of wild-type rhodopsin-GFP, which co-localized with HSJ1b at the ER. HSJ1b expression also increased the incidence of inclusion formation by the wild-type protein. Both isoforms were recruited to mutant P23H rhodopsin inclusions, but only HSJ1b enhanced inclusion formation. Investigation of a prenylation-null mutant showed that the modulation of rhodopsin processing and inclusion formation was dependent on the correct subcellular targeting of HSJ1b to the cytosolic face of the ER. An Hsp70 interaction-null mutant of HSJ1b had the same effect as HSJ1b, suggesting that these phenomena were independent of Hsp70 and, furthermore, overexpression of Hsp70 with HSJ1b did not modulate the HSJ1b effect on inclusion formation, showing that Hsp70 was not limiting. The data provide evidence that cytoplasmic chaperones when targeted to the ER can influence the folding and processing of a GPCR and show that DnaJ protein function can be further specialized by alternative splicing and post-translational modification.

Differential inhibition of Hsc70 activities by two Hsc70-binding peptides

The ability of two high-affinity Hsc70-binding peptides [FYQLALT (peptide-Phi) and NIVRKKK (peptide-K)] to differentially inhibit Hsc70-dependent processes in rabbit reticulocyte lysate (RRL) was examined. Both peptide-Phi and peptide-K inhibited chaperone-dependent renaturation of luciferase in RRL. Peptide-Phi, but not peptide-K, blocked Hsp90/Hsc70-dependent transformation of the heme-regulated eIF2 alpha kinase (HRI) into an active, heme-regulatable kinase. In contrast, peptide-K, but not peptide-Phi, inhibited Hsc70-mediated suppression of the activation of mature-transformed HRI. Furthermore, HDJ2 (Human DnaJ homologue 2), but not HDJ1, potentiated the ability of Hsc70 to suppress the activation of HRI in RRL. Mechanistically, peptide-K inhibited, while peptide-Phi enhanced, HDJ2-induced stimulation of Hsc70 ATPase activity in vitro. The data presented support the hypotheses that peptide-Phi acts to inhibit Hsc70 function by binding to the hydrophobic peptide-binding cleft of Hsc70, while peptide-K acts through binding to a site that modulates the interaction of Hsc70 with DnaJ homologues. Overall, the data indicate that peptide-Phi and peptide-K have differential effects on Hsc70 functions under quasi-physiological conditions in RRL, and suggest that therapeutically valuable peptide mimetics can be designed to inhibit specific functions of Hsc70.

Identification and characterization of a human mitochondrial homologue of the bacterial co-chaperone GrpE

We have identified a novel human cDNA with a predicted protein sequence that has 28% amino acid identity with the E. coli Hsp70 co-chaperone GrpE and designated it HMGE. Even with this low level of amino acid identity the human sequence could be efficiently modelled on the X-ray structure of the E. coli protein, suggesting that there may be significant functional conservation. Indeed, HMGE expressed in E. coli as a GST fusion protein co-purified with the E. coli Hsp70 protein DnaK in the absence of ATP. DnaK could be released from the GST-HMGE with a Mg-ATP wash. Subcellular fractionation and immunocytochemistry studies using antisera raized against HMGE show that it is a mitochondrial protein. In contrast to studies of rat GrpE, however, HMGE also appears to bind the constitutive cytosolic Hsp70, Hsc70, in addition to mitochondrial Hsp70, Mt-Hsp70. We have previously shown that Hsc70 nucleotide-exchange is rate limiting in the presence of the DnaJ-protein, HSJ1b. However, HMGE was found to inhibit the HSJ1b-enhanced Hsc70 ATPase activity and may mediate this inhibition by binding the DnaJ-protein, HSJ1b. This is the first description of a direct interaction between a DnaJ protein and GrpE-like protein. These studies suggest that the structure of GrpE has been conserved throughout evolution and that the conserved structure can interact with several forms of Hsp70, but that HMGE cannot form part of the reaction cycle for cytosolic Hsc70.

Molecular chaperone function of mammalian Hsp70 and Hsp40--a review

Virtually all organisms respond to up-shifts in temperature (heat shock) by synthesizing a set of proteins called heat shock proteins (HSPs). The HSPs are induced not only by heat shock but also by various other environmental stresses. Induction of HSPs is regulated by the trans-acting heat shock factors (HSFs) and cis-acting heat shock element (HSE) present at the promoter region of each heat shock gene. Usually, HSPs are also expressed constitutively at normal growth temperatures and have basic and indispensable functions in the life cycle of proteins as molecular chaperones, as well as playing a role in protecting cells from the deleterious stresses. Molecular chaperones are able to inhibit the aggregation of partially denatured proteins and refold them using the energy of ATP. Recently, there are expectations for the use of molecular chaperones for the protection against and therapeutic treatment of inherited diseases caused by protein misfolding. In this review, the focus will be on the mammalian Hsp40, a homologue of bacterial DnaJ heat shock protein, and the beneficial functions of molecular chaperones.

Mammalian HSP40/DNAJ homologs: cloning of novel cDNAs and a proposal for their classification and nomenclature

We have cloned 10 novel full-length cDNAs of mouse and human HSP40/DNAJ homologs using expressed sequence tag (EST) clones found in the DDBJ/GenBank/EMBL DNA database. In this report, we tentatively designated them mHsp40, mDj3, mDj4, mDj5, mDj6, mDj7, mDj8, hDj9, mDj10, and mDj11. Based on the identity of the deduced amino acid sequences, mHsp40, mDj3, and mDj11 are orthologs of human Hsp40, rat Rdj2, and human Tpr2, respectively. We determined that mDj4 is identical with the recently isolated mouse Mrj (mammalian relative of DnaJ). PSORT analysis (a program that predicts the subcellular localization site of a given protein from its amino acid sequences) revealed that hDj9 has an N-terminal signal peptide; hence, its localization might be extracellular, suggesting that there may be a partner Hsp70 protein that acts together with the hDj9 outside of the cell. The same analysis indicated that mDj7 and mDj10 may have transmembrane domains. In order to simplify the complicated and confusing nomenclature of recently identified mammalian HSP40/DNAJ homologs, we propose here some new rules for their nomenclature. This proposed nomenclature includes the name of species with 2 lowercase letters such as hs (Homo sapiens), mm (Mus musculus) and rn (Rattus norvegicus); Dj standing for DnaJ; the name of types with A, B, and C, which were previously classified as type I, II, and III according to the domain structure of the homologs; and finally Arabic numerals according to the chronological order of registration of the sequence data into the database.

Dual role for Hsc70 in the biogenesis and regulation of the heme-regulated kinase of the alpha subunit of eukaryotic translation initiation factor 2

The heme-regulated kinase of the alpha subunit of eukaryotic initiation factor 2 (HRI) is activated in rabbit reticulocyte lysate (RRL) in response to a number of environmental conditions, including heme deficiency, heat shock, and oxidative stress. Activation of HRI causes an arrest of initiation of protein synthesis. Recently, we have demonstrated that the heat shock cognate protein Hsc70 negatively modulates the activation of HRI in RRL in response to these environmental conditions. Hsc70 is also known to be a critical component of the Hsp90 chaperone machinery in RRL, which plays an obligatory role for HRI to acquire and maintain a conformation that is competent to activate. Using de novo-synthesized HRI in synchronized pulse-chase translations, we have examined the role of Hsc70 in the regulation of HRI biogenesis and activation. Like Hsp90, Hsc70 interacted with nascent HRI and HRI that was matured to a state which was competent to undergo stimulus-induced activation (mature-competent HRI). Interaction of HRI with Hsc70 was required for the transformation of HRI, as the Hsc70 antagonist clofibric acid inhibited the folding of HRI into a mature-competent conformation. Unlike Hsp90, Hsc70 also interacted with transformed HRI. Clofibric acid disrupted the interaction of Hsc70 with transformed HRI that had been matured and transformed in the absence of the drug. Disruption of Hsc70 interaction with transformed HRI in heme-deficient RRL resulted in its hyperactivation. Furthermore, activation of HRI in response to heat shock or denatured proteins also resulted in a similar blockage of Hsc70 interaction with transformed HRI. These results indicate that Hsc70 is required for the folding and transformation of HRI into an active kinase but is subsequently required to negatively attenuate the activation of transformed HRI.

Mechanism of regulation of hsp70 chaperones by DnaJ cochaperones

Hsp70 chaperones assist a large variety of protein folding processes within the entire lifespan of proteins. Central to these activities is the regulation of Hsp70 by DnaJ cochaperones. DnaJ stimulates Hsp70 to hydrolyze ATP, a key step that closes its substrate-binding cavity and thus allows stable binding of substrate. We show that DnaJ stimulates ATP hydrolysis by Escherichia coli Hsp70, DnaK, very efficiently to >1000-fold, but only if present at high (micromolar) concentration. In contrast, the chaperone activity of DnaK in luciferase refolding was maximal at several hundredfold lower concentration of DnaJ. However, DnaJ was capable of maximally stimulating the DnaK ATPase even at this low concentration, provided that protein substrate was present, indicating synergistic action of DnaJ and substrate. Peptide substrates were poorly effective in this synergistic action. DnaJ action required binding of protein substrates to the central hydrophobic pocket of the substrate-binding cavity of DnaK, as evidenced by the reduced ability of DnaJ to stimulate ATP hydrolysis by a DnaK mutant with defects in substrate binding. At high concentrations, DnaJ itself served as substrate for DnaK in a process considered to be unphysiological. Mutant analysis furthermore revealed that DnaJ-mediated stimulation of ATP hydrolysis requires communication between the ATPase and substrate-binding domains of DnaK. This mechanism thus allows DnaJ to tightly couple ATP hydrolysis by DnaK with substrate binding and to avoid jamming of the DnaK chaperone with peptides. It probably is conserved among Hsp70 family members and is proposed to account for their functional diversity.

DnaJ dramatically stimulates ATP hydrolysis by DnaK: insight into targeting of Hsp70 proteins to polypeptide substrates

Most, if not all, of the cellular functions of Hsp70 proteins require the assistance of a DnaJ homologue, which accelerates the weak intrinsic ATPase activity of Hsp70 and serves as a specificity factor by binding and targeting specific polypeptide substrates for Hsp70 action. We have used pre-steady-state kinetics to investigate the interaction of the Escherichia coli DnaJ and DnaK proteins, and the effects of DnaJ on the ATPase reaction of DnaK. DnaJ accelerates hydrolysis of ATP by DnaK to such an extent that ATP binding by DnaK becomes rate-limiting for hydrolysis. At high concentrations of DnaK under single-turnover conditions, the rate-limiting step is a first-order process, apparently a change of DnaK conformation, that accompanies ATP binding and proceeds at 12-15 min-1 at 25 degrees C and 1-1.5 min-1 at 5 degrees C. By prebinding ATP to DnaK and subsequently adding DnaJ, the effects of this slow step may be bypassed, and the maximal rate-enhancement of DnaJ on the hydrolysis step is approximately 15 000-fold at 5 degrees C. The interaction of DnaJ with DnaK.ATP is likely a rapid equilibrium relative to ATP hydrolysis, and is relatively weak, with a KD of approximately 20 microM at 5 degrees C, and weaker still at 25 degrees C. In the presence of saturating DnaJ, the maximal rate of ATP hydrolysis by DnaK is similar to previously reported rates for peptide release from DnaK.ATP. This suggests that when DnaK encounters a DnaJ-bound polypeptide or protein complex, a significant fraction of such events result in ATP hydrolysis by DnaK and concomitant capture of the polypeptide substrate in a tight complex with DnaK.ADP. Furthermore, a broadly applicable kinetic mechanism for DnaJ-mediated specificity of Hsp70 action arises from these observations, in which the specificity arises largely from the acceleration of the hydrolysis step itself, rather than by DnaJ-dependent modulation of the affinity of Hsp70 for substrate polypeptides.

Oligomerization of the 17-kDa peptide-binding domain of the molecular chaperone HSC70

Crystallographic and biochemical studies have indicated that the peptide-binding site of the molecular chaperone HSC70 is located in a small subdomain comprising a beta-sheet motif followed by a helical region, and there is some evidence of the involvement of this site in oligomerization of the protein. To determine the structure of this subdomain in solution and examine its involvement in oligomerization of HSC70, a 17-kDa protein (residues 385-540 of HSC70) consisting mainly of the peptide-binding site was constructed and analyzed for oligomerization properties. This small domain was found to bind peptides and to form oligomers in solution, probably tetramers, which dissociated into monomers on peptide binding in a manner comparable with that observed for the whole protein. Furthermore, in the 60-kDa fragment of HSC70, which is composed of the 17-kDa domain and the 44-kDa ATPase domain, not only were the oligomerization properties conserved, but dissociation of multimeric species into monomers on ATP binding also occurred and peptide stimulation of ATPase activity was restored. These results indicate that the isolated 17-kDa peptide-binding domain is necessary and sufficient for oligomerization of the whole protein, suggesting that the peptide-binding site may be involved in the oligomerization process.

Isolation and characterization of a DnaJ-like protein in rats: the C-terminal 10-kDa domain of hsc70 is not essential for stimulating the ATP-hydrolytic activity of hsc70 by a DnaJ-like protein

A DnaJ-like protein, RDJ1, was isolated from a rat brain cDNA library. The protein is predicted to have 397 amino acid residues and shares 99% identity to that of HDJ2, a human DnaJ-like protein. RDJ1 was also shown to rescue the temperature-sensitive lethality of a strain containing a mutated cytosolic DnaJ in yeast, ydj1-151. Fragments containing the J-domain of RDJ1 either with or without the G/F motif were expressed in Escherichia coli. The purified proteins stimulated the ATPase activity of hsc70 and of the 60-kDa N-terminal fragment of hsc70. These results imply that RDJ1 can interact with the N-terminal 60-kDa fragment of hsc70 to activate ATP hydrolysis by hsc70.

The molecular chaperone function of the secretory vesicle cysteine string proteins

The "J" domains of eukaryotic DnaJ-like proteins specify interaction with various Hsp70s. The conserved tripeptide, HPD, present in all J domains has been shown to be important for the interaction between yeast and bacterial DnaJ/Hsp70 protein pairs. We have characterized mutations in the HPD motif of the synaptic vesicle protein cysteine-string protein (Csp). Mutation of the histidine (H43Q) or aspartic acid (D45A) residues of this motif reduced the ability of Csp to stimulate the ATPase activity of mammalian Hsc70. The H43Q and D45A mutant proteins were not able to stimulate the ATPase activity of Hsc70 to any significant extent. The mutant proteins were characterized by competition assays, tryptic digestion analysis, and direct binding analysis from which it was seen that these proteins were defective in binding to Hsc70. Thus, the HPD motif of Csp is required for binding to Hsc70. We also analyzed the interaction between Csp and a model substrate protein, denatured firefly luciferase. Both Csp1 and the C-terminally truncated isoform Csp2 were able to prevent aggregation of heat-denatured luciferase, and they also cooperated with Hsc70 to prevent aggregation. In addition, complexes of Csp1 or Csp2 with Hsc70 and luciferase were isolated, confirming that these proteins interact and that Csps can bind directly to denatured proteins. Csp1 and Csp2 isoforms must differ in some aspect other than interaction with Hsc70 and substrate protein. These results show that both Csp1 and Csp2 can bind a partially unfolded protein and act as chaperones. This suggests that Csps may have a general chaperone function in regulated exocytosis.