Inactivation of the PKR protein kinase and stimulation of mRNA translation by the cellular co-chaperone P58(IPK) does not require J domain function

The Essential Functions of Molecular Chaperones and Folding Enzymes in Maintaining Endoplasmic Reticulum Homeostasis

It has been estimated that up to one-third of the proteins encoded by the human genome enter the endoplasmic reticulum (ER) as extended polypeptide chains where they undergo covalent modifications, fold into their native structures, and assemble into oligomeric protein complexes. The fidelity of these processes is critical to support organellar, cellular, and organismal health, and is perhaps best underscored by the growing number of disease-causing mutations that reduce the fidelity of protein biogenesis in the ER. To meet demands encountered by the diverse protein clientele that mature in the ER, this organelle is populated with a cadre of molecular chaperones that prevent protein aggregation, facilitate protein disulfide isomerization, and lower the activation energy barrier of cis-trans prolyl isomerization. Components of the lectin (glycan-binding) chaperone system also reside within the ER and play numerous roles during protein biogenesis. In addition, the ER houses multiple homologs of select chaperones that can recognize and act upon diverse peptide signatures. Moreover, redundancy helps ensure that folding-compromised substrates are unable to overwhelm essential ER-resident chaperones and enzymes. In contrast, the ER in higher eukaryotic cells possesses a single member of the Hsp70, Hsp90, and Hsp110 chaperone families, even though several homologs of these molecules reside in the cytoplasm. In this review, we discuss specific functions of the many factors that maintain ER quality control, highlight some of their interactions, and describe the vulnerabilities that arise from the absence of multiple members of some chaperone families.

Dual topology of co-chaperones at the membrane of the endoplasmic reticulum

Dual topologies of proteins at the ER membrane are known for a variety of proteins allowing the same protein to exert different functions according to the topology adopted. A dual topology of the co-chaperone ERdj4, which resides in the endoplasmic reticulum (ER), was proposed recently, a thesis that we found to align all published data and existing controversies into one whole picture. The aim of this review is to reassess all primary data available in the literature on ER-resident Hsp40 co-chaperones with respect to their topology. After careful and critical analyses of all experimental data published so far, we identified, next to ERdj4, two other co-chaperones, ERdj3 and ERdj6, that also display features of a dual topology at the ER membrane. We assume that during cellular stress subpools of some ER-resident J protein can alter their topology so that these proteins can exert different functions in order to adapt to cellular stress.

The unfolded protein response plays dual roles in rice stripe virus infection through fine-tuning the movement protein accumulation

The movement of plant viruses is a complex process that requires support by the virus-encoded movement protein and multiple host factors. The unfolded protein response (UPR) plays important roles in plant virus infection, while how UPR regulates viral infection remains to be elucidated. Here, we show that rice stripe virus (RSV) elicits the UPR in Nicotiana benthamiana. The RSV-induced UPR activates the host autophagy pathway by which the RSV-encoded movement protein, NSvc4, is targeted for autophagic degradation. As a counteract, we revealed that NSvc4 hijacks UPR-activated type-I J-domain proteins, NbMIP1s, to protect itself from autophagic degradation. Unexpectedly, we found NbMIP1 stabilizes NSvc4 in a non-canonical HSP70-independent manner. Silencing NbMIP1 family genes in N. benthamiana, delays RSV infection, while over-expressing NbMIP1.4b promotes viral cell-to-cell movement. Moreover, OsDjA5, the homologue of NbMIP1 family in rice, behaves in a similar manner toward facilitating RSV infection. This study exemplifies an arms race between RSV and the host plant, and reveals the dual roles of the UPR in RSV infection though fine-tuning the accumulation of viral movement protein.

The search for a PKR code-differential regulation of protein kinase R activity by diverse RNA and protein regulators

The interferon-inducible protein kinase R (PKR) is a key component of host innate immunity that restricts viral replication and propagation. As one of the four eIF2α kinases that sense diverse stresses and direct the integrated stress response (ISR) crucial for cell survival and proliferation, PKR's versatile roles extend well beyond antiviral defense. Targeted by numerous host and viral regulators made of RNA and proteins, PKR is subject to multiple layers of endogenous control and external manipulation, driving its rapid evolution. These versatile regulators include not only the canonical double-stranded RNA (dsRNA) that activates the kinase activity of PKR, but also highly structured viral, host, and artificial RNAs that exert a full spectrum of effects. In this review, we discuss our deepening understanding of the allosteric mechanism that connects the regulatory and effector domains of PKR, with an emphasis on diverse structured RNA regulators in comparison to their protein counterparts. Through this analysis, we conclude that much of the mechanistic details that underlie this RNA-regulated kinase await structural and functional elucidation, upon which we can then describe a "PKR code," a set of structural and chemical features of RNA that are both descriptive and predictive for their effects on PKR.

Reconstitution of a Mycobacterium tuberculosis proteostasis network highlights essential cofactor interactions with chaperone DnaK

During host infection, Mycobacterium tuberculosis (Mtb) encounters several types of stress that impair protein integrity, including reactive oxygen and nitrogen species and chemotherapy. The resulting protein aggregates can be resolved or degraded by molecular machinery conserved from bacteria to eukaryotes. Eukaryotic Hsp104/Hsp70 and their bacterial homologs ClpB/DnaK are ATP-powered chaperones that restore toxic protein aggregates to a native folded state. DnaK is essential in Mycobacterium smegmatis, and ClpB is involved in asymmetrically distributing damaged proteins during cell division as a mechanism of survival in Mtb, commending both proteins as potential drug targets. However, their molecular partners in protein reactivation have not been characterized in mycobacteria. Here, we reconstituted the activities of the Mtb ClpB/DnaK bichaperone system with the cofactors DnaJ1, DnaJ2, and GrpE and the small heat shock protein Hsp20. We found that DnaJ1 and DnaJ2 activate the ATPase activity of DnaK differently. A point mutation in the highly conserved HPD motif of the DnaJ proteins abrogates their ability to activate DnaK, although the DnaJ2 mutant still binds to DnaK. The purified Mtb ClpB/DnaK system reactivated a heat-denatured model substrate, but the DnaJ HPD mutants inhibited the reaction. Finally, either DnaJ1 or DnaJ2 is required for mycobacterial viability, as is the DnaK-activating activity of a DnaJ protein. These studies lay the groundwork for strategies to target essential chaperone-protein interactions in Mtb, the leading cause of death from a bacterial infection.

J domain independent functions of J proteins

Heat shock proteins of 40 kDa (Hsp40s), also called J proteins, are obligate partners of Hsp70s. Via their highly conserved and functionally critical J domain, J proteins interact and modulate the activity of their Hsp70 partners. Mutations in the critical residues in the J domain often result in the null phenotype for the J protein in question. However, as more J proteins have been characterized, it is becoming increasingly clear that a significant number of J proteins do not "completely" rely on their J domains to carry out their cellular functions, as previously thought. In some cases, regions outside the highly conserved J domain have become more important making the J domain dispensable for some, if not for all functions of a J protein. This has profound effects on the evolution of such J proteins. Here we present selected examples of J proteins that perform J domain independent functions and discuss this in the context of evolution of J proteins with dispensable J domains and J-like proteins in eukaryotes.

p58IPK is an inhibitor of the eIF2α kinase GCN2 and its localization and expression underpin protein synthesis and ER processing capacity

One of the key cellular responses to stress is the attenuation of mRNA translation and protein synthesis via the phosphorylation of eIF2α (eukaryotic translation initiation factor 2α). This is mediated by four eIF2α kinases and it has been suggested that each kinase is specific to the cellular stress imposed. In the present study, we show that both PERK (PKR-like endoplasmic reticulum kinase/eIF2α kinase 3) and GCN2 (general control non-derepressible 2/eIF2α kinase 4) are required for the stress responses associated with conditions encountered by cells overexpressing secreted recombinant protein. Importantly, whereas GCN2 is the kinase that is activated following cold-shock/hypothermic culturing of mammalian cells, PERK and GCN2 have overlapping functions since knockdown of one of these at the mRNA level is compensated for by the cell by up-regulating levels of the other. The protein p58IPK {also known as DnaJ3C [DnaJ heat-shock protein (hsp) 40 homologue, subfamily C, member 3]} is known to inhibit the eIF2α kinases PKR (dsRNA-dependent protein kinase/eIF2α kinase 2) and PERK and hence prevent or delay eIF2α phosphorylation and consequent inhibition of translation. However, we show that p58IPK is a general inhibitor of the eIF2α kinases in that it also interacts with GCN2. Thus forced overexpression of cytoplasmic p58 delays eIF2α phosphorylation, suppresses GCN2 phosphorylation and prolongs protein synthesis under endoplasmic reticulum (ER), hypothermic and prolonged culture stress conditions. Taken together, our data suggest that there is considerable cross talk between the eIF2α kinases to ensure that protein synthesis is tightly regulated. Their activation is controlled by p58 and the expression levels and localization of this protein are crucial in the capacity the cells to respond to cellular stress via control of protein synthesis rates and subsequent folding in the ER.

Deletion of P58(IPK), the Cellular Inhibitor of the Protein Kinases PKR and PERK, Causes Bone Changes and Joint Degeneration in Mice

OBJECTIVE

Protein kinase-like endoplasmic reticulum kinase (PERK) and protein kinase R (PKR) are implicated in endoplasmic reticulum stress-induced arthritis and pro-inflammatory cytokine-mediated cartilage degradation in vitro, respectively. We determined whether knockout of the cellular inhibitor of PERK and PKR, P58(IPK) causes joint degeneration in vivo and whether these molecules are activated in human osteoarthritis (OA).

MATERIALS AND METHODS

Sections of knee joints from P58(IPK)-null and wild-type mice aged 12-13 and 23-25 months were stained with toluidine blue and scored for degeneration using the osteoarthritis research society international (OARSI) system. Bone changes were assessed by radiology and high-resolution micro-computed tomography of hind limbs. Sections from the medial tibial plateaus of two human knees, removed in total knee replacement surgery for OA, were immunolabelled for phosphorylated PERK and PKR and P58(IPK).

RESULTS

Knockout mice exhibited narrower tibiae (p = 0.0031) and smaller epiphyses in tibiae (p = 0.0004) and femora (p = 0.0214). Older knockout mice had reduced total volume inside the femoral periosteal envelope (p = 0.023), reduced tibial (p = 0.03), and femoral (p = 0.0012) bone volumes (BV) and reduced femoral BV fraction (p = 0.025). Compared with wild-types, younger P58(IPK)-null mice had increased OARSI scores in medial femoral condyles (p = 0.035). Thirty four percent of null mice displayed severe joint degeneration with complete articular cartilage loss from the medial compartment and heterotopic chondro-osseous tissue in the medial joint capsule. Phosphorylated PERK and PKR were localized throughout human osteoarthritic tibial plateaus but, in particular, in areas exhibiting the most degeneration. There was limited expression of P58(IPK).

CONCLUSION

This study is the first to reveal a critical role for P58(IPK) in maintaining joint integrity in vivo, implicating the PKR and PERK stress signaling pathways in bony changes underlying the pathogenesis of joint degeneration.

Interaction of Nck1 and PERK phosphorylated at Y⁵⁶¹ negatively modulates PERK activity and PERK regulation of pancreatic β-cell proinsulin content

PERK is phosphorylated at Y⁵⁶¹ in the juxtamembrane domain, and the adaptor protein Nck1, by directly interacting with phospho-Y⁵⁶¹ PERK, negatively regulates PERK activity. Strong evidence is given supporting the biological relevance of Nck1 regulation of PERK function in modulating pancreatic β-cell proinsulin content.

PERK, the PKR-like endoplasmic reticulum (ER) kinase, is an ER transmembrane serine/threonine protein kinase activated during ER stress. In this study, we provide evidence that the Src-homology domain–containing adaptor Nck1 negatively regulates PERK. We show that Nck directly binds to phosphorylated Y⁵⁶¹ in the PERK juxtamembrane domain through its SH2 domain. We demonstrate that mutation of Y⁵⁶¹ to a nonphosphorylatable residue (Y561F) promotes PERK activity, suggesting that PERK phosphorylation at Y⁵⁶¹ (pY⁵⁶¹PERK) negatively regulates PERK. In agreement, we show that pY⁵⁶¹PERK delays PERK activation and signaling during ER stress. Compatible with a role for PERK in pancreatic β-cells, we provide strong evidence that Nck1 contributes to PERK regulation of pancreatic β-cell proteostasis. In fact, we demonstrated that down-regulation of Nck1 in mouse insulinoma MIN6 cells results in faster dephosphorylation of pY⁵⁶¹PERK, which correlates with enhanced PERK activation, increased insulin biosynthesis, and PERK-dependent increase in proinsulin content. Furthermore, we report that pancreatic islets in whole-body Nck1-knockout mice contain more insulin than control littermates. Together our data strongly suggest that Nck1 negatively regulates PERK by interacting with PERK and protecting PERK from being dephosphorylated at its inhibitory site pY⁵⁶¹ and in this way affects pancreatic β-cell proinsulin biogenesis.

The eIF2α kinases: their structures and functions

Cell signaling in response to an array of diverse stress stimuli converges on the phosphorylation of the α-subunit of eukaryotic initiation factor 2 (eIF2). Phosphorylation of eIF2α on serine 51 results in a severe decline in de novo protein synthesis and is an important strategy in the cell's armory against stressful insults including viral infection, the accumulation of misfolded proteins, and starvation. The phosphorylation of eIF2α is carried out by a family of four kinases, PERK (PKR-like ER kinase), PKR (protein kinase double-stranded RNA-dependent), GCN2 (general control non-derepressible-2), and HRI (heme-regulated inhibitor). Each primarily responds to a distinct type of stress or stresses. Thus, while significant sequence similarity exists between the eIF2α kinases in their kinase domains, underlying their common role in phosphorylating eIF2α, additional unique features determine the regulation of these four proteins, that is, what signals activate them. This review will describe the structure of each eIF2α kinase and discuss how this is linked to their activation and function. In parallel to the general translational attenuation elicited by eIF2α kinase activation the translation of stress-induced mRNAs, most notably activating transcription factor 4 (ATF4) is enhanced and these set in motion cascades of gene expression constituting the integrated stress response (ISR), which seek to remediate stress and restore homeostasis. Depending on the cellular context and concurrent signaling pathways active, however, translational attenuation can also facilitate apoptosis. Accordingly, the role of the kinases in determining cell fate will also be discussed.

Characterization of porcine P58IPK gene and its up-regulation after H1N1 or H3N2 influenza virus infection

BACKGROUND

The 58-kDa inhibitor of the interferon-induced double-stranded RNA-activated protein kinase (P58IPK) is a cellular protein that is activated during influenza virus infection. Although the function of human P58IPK has been studied for a long time, porcine P58IPK (pP58IPK) has little been studied except for its cloning.

OBJECTIVE

In this study, we aimed to investigate the characteristics of the pP58IPK gene, determine its subcellular localization, and find its expression change during H1N1 or H3N2 infection.

STUDY DESIGN

First, the sequence and structure of pP58IPK were analyzed. Second, pP58IPK gene was cloned into pEGFP-N1 and pEGFP-C1 vectors, respectively, which were transfected into cells to determine its subcellular localization. Third, Lung tissues of piglets from H1N1 infected, H3N2 infected and control groups were analyzed using histopathology, real-time PCR, and immunohistochemistry.

RESULTS

The sequence and structure of pP58IPK was highly similar to the counterpart of human. pP58IPK protein distributed only in the cytoplasm. Lung tissues of piglets infected by H1N1 or H3N2 appeared obvious pathological changes, and the expression of pP58IPK in both mRNA and protein level was up-regulated by approximate 1.5-fold in piglets infected by H1N1 or H3N2 comparing with control piglets.

CONCLUSIONS

We analyzed the characteristics of the pP58IPK gene, constructed a phylogenetic tree, determined its subcellular localization, and investigated its expression changes during H1N1 or H3N2 infection. The fundamental data accumulated in this study provides a potential medical model for investigating the function of P58IPK during influenza A viruses infection.

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.

The crystal structure of the human co-chaperone P58(IPK)

P58^IPK is one of the endoplasmic reticulum- (ER-) localised DnaJ (ERdj) proteins which interact with the chaperone BiP, the mammalian ER ortholog of Hsp70, and are thought to contribute to the specificity and regulation of its diverse functions. P58^IPK, expression of which is upregulated in response to ER stress, has been suggested to act as a co-chaperone, binding un- or misfolded proteins and delivering them to BiP. In order to give further insights into the functions of P58^IPK, and the regulation of BiP by ERdj proteins, we have determined the crystal structure of human P58^IPK to 3.0 Å resolution using a combination of molecular replacement and single wavelength anomalous diffraction. The structure shows the human P58^IPK monomer to have a very elongated overall shape. In addition to the conserved J domain, P58^IPK contains nine N-terminal tetratricopeptide repeat motifs, divided into three subdomains of three motifs each. The J domain is attached to the C-terminal end via a flexible linker, and the structure shows the conserved Hsp70-binding histidine-proline-aspartate (HPD) motif to be situated on the very edge of the elongated protein, 100 Å from the putative binding site for unfolded protein substrates. The residues that comprise the surface surrounding the HPD motif are highly conserved in P58^IPK from other organisms but more varied between the human ERdj proteins, supporting the view that their regulation of different BiP functions is facilitated by differences in BiP-binding.

Structural insight into the protective role of P58(IPK) during unfolded protein response

P58(IPK) has been identified as an ER molecular chaperone to maintain protein-folding homeostasis. P58(IPK) expression can be significantly upregulated during unfolded protein responses (UPR), and it may play important roles in suppressing the ER protein aggregations. To investigate the mechanism how P58(IPK) functions to promote protein folding within ER, we have determined the crystal structure of P58(IPK) TPR domain at 2.5Å resolution. P58(IPK) contains nine TPR motifs and a C-terminal J domain within its primary sequence. The crystal structure of P58(IPK) revealed three subdomains (I, II, and III) with similar folds and each domain contains three TPR motifs. Our data also showed that P58(IPK) acts as a molecular chaperone by interacting with the unfolded proteins such as luciferase, rhodanese, and insulin. The P58(IPK) structure reveals a conserved hydrophobic patch located in subdomain I that may be involved in binding the misfolded polypeptides. We have proposed a working model for P58(IPK) to act together with Bip to prevent protein aggregations and promote protein foldings within ER.

Cwc23, an essential J protein critical for pre-mRNA splicing with a dispensable J domain

J proteins are structurally diverse, obligatory cochaperones of Hsp70s, each with a highly conserved J domain that plays a critical role in the stimulation of Hsp70's ATPase activity. The essential protein, Cwc23, is one of 13 J proteins found in the cytosol and/or nucleus of Saccharomyces cerevisiae. We report that a partial loss-of-function CWC23 mutant has severe, global defects in pre-mRNA splicing. This mutation leads to accumulation of the excised, lariat form of the intron, as well as unspliced pre-mRNA, suggesting a role for Cwc23 in spliceosome disassembly. Such a role is further supported by the observation that this mutation results in reduced interaction between Cwc23 and Ntr1 (SPP382), a known component of the disassembly pathway. However, Cwc23 is a very atypical J protein. Its J domain, although functional, is dispensable for both cell viability and pre-mRNA splicing. Nevertheless, strong genetic interactions were uncovered between point mutations encoding alterations in Cwc23's J domain and either Ntr1 or Prp43, a DExD/H-box helicase essential for spliceosome disassembly. These genetic interactions suggest that Hsp70-based chaperone machinery does play a role in the disassembly process. Cwc23 provides a unique example of a J protein; its partnership with Hsp70 plays an auxiliary, rather than a central, role in its essential cellular function.

Dissection of Swa2p/auxilin domain requirements for cochaperoning Hsp70 clathrin-uncoating activity in vivo

The auxilin family of J-domain proteins load Hsp70 onto clathrin-coated vesicles (CCVs) to drive uncoating. In vitro, auxilin function requires its ability to bind clathrin and stimulate Hsp70 ATPase activity via its J-domain. To test these requirements in vivo, we performed a mutational analysis of Swa2p, the yeast auxilin ortholog. Swa2p is a modular protein with three N-terminal clathrin-binding (CB) motifs, a ubiquitin association (UBA) domain, a tetratricopeptide repeat (TPR) domain, and a C-terminal J-domain. In vitro, clathrin binding is mediated by multiple weak interactions, but a Swa2p truncation lacking two CB motifs and the UBA domain retains nearly full function in vivo. Deletion of all CB motifs strongly abrogates clathrin disassembly but does not eliminate Swa2p function in vivo. Surprisingly, mutation of the invariant HPD motif within the J-domain to AAA only partially affects Swa2p function. Similarly, a TPR point mutation (G388R) causes a modest phenotype. However, Swa2p function is abolished when these TPR and J mutations are combined. The TPR and J-domains are not functionally redundant because deletion of either domain renders Swa2p nonfunctional. These data suggest that the TPR and J-domains collaborate in a bipartite interaction with Hsp70 to regulate its activity in clathrin disassembly.

Hijacking of the host-cell response and translational control during influenza virus infection

Influenza virus is a major public health problem with annual deaths in the US of 36,000 with pandemic outbreaks, such as in 1918, resulting in deaths exceeding 20 million worldwide. Recently, there is much concern over the introduction of highly pathogenic avian influenza H5N1 viruses into the human population. Influenza virus has evolved complex translational control strategies that utilize cap-dependent translation initiation mechanisms and involve the recruitment of both viral and host-cell proteins to preferentially synthesize viral proteins and prevent activation of antiviral responses. Influenza virus is a member of the Orthomyxoviridae family of negative-stranded, segmented RNA viruses and represents a particularly attractive model system as viral replication strategies are closely intertwined with normal cellular processes including the host defense and stress pathways. In this chapter, we review the parallels between translational control in influenza virus infected cells and in stressed cells with a focus on selective translation of viral mRNAs and the antagonism of the dsRNA and host antiviral responses. Moreover, we will discuss how the use of genomic technologies such as DNA microarrays and high through-put proteomics can be used to gain new insights into the control of protein synthesis during viral infection and provide a near comprehensive view of virus-host interactions.

Not all J domains are created equal: implications for the specificity of Hsp40-Hsp70 interactions

Heat shock protein 40s (Hsp40s) and heat shock protein 70s (Hsp70s) form chaperone partnerships that are key components of cellular chaperone networks involved in facilitating the correct folding of a broad range of client proteins. While the Hsp40 family of proteins is highly diverse with multiple forms occurring in any particular cell or compartment, all its members are characterized by a J domain that directs their interaction with a partner Hsp70. Specific Hsp40-Hsp70 chaperone partnerships have been identified that are dedicated to the correct folding of distinct subsets of client proteins. The elucidation of the mechanism by which these specific Hsp40-Hsp70 partnerships are formed will greatly enhance our understanding of the way in which chaperone pathways are integrated into finely regulated protein folding networks. From in silico analyses, domain swapping and rational protein engineering experiments, evidence has accumulated that indicates that J domains contain key specificity determinants. This review will critically discuss the current understanding of the structural features of J domains that determine the specificity of interaction between Hsp40 proteins and their partner Hsp70s. We also propose a model in which the J domain is able to integrate specificity and chaperone activity.

Activation of pleiotropic drug resistance by the J-protein and Hsp70-related proteins, Zuo1 and Ssz1

Ssz1 (Pdr13) and Zuo1, members of the Hsp70 and J-protein molecular chaperone families, respectively, form a heterodimer and function on the ribosome with the Hsp70, Ssb, presumably assisting folding of newly synthesized polypeptides. As it has also been reported that Ssz1 induces pleiotropic drug resistance (PDR) when overexpressed, a possible role for Zuo1 in PDR was investigated. The C-terminal domain of Zuo1, which is dispensable for Zuo1's chaperone function on the ribosome, is both necessary and sufficient for PDR induction by Zuo1. A single domain of Ssz1, the N-terminal ATPase domain, is sufficient for PDR induction as well, indicating that Ssz1 does not function as a chaperone in PDR. No role for Ssb was found in PDR; overexpression did not affect PDR, nor was its presence required for Ssz1's or Zuo1's effect on PDR. As our results also indicate that Ssz1 and Zuo1 must be free of ribosomes to induce PDR, we propose that Ssz1's and Zuo1's function in PDR is distinct from their role as ribosome-associated co-chaperones and may be regulatory in nature.

Recruitment of Hsp70 chaperones: a crucial part of viral survival strategies

Virus proliferation depends on the successful recruitment of host cellular components for their own replication, protein synthesis, and virion assembly. In the course of virus particle production a large number of proteins are synthesized in a relatively short time, whereby protein folding can become a limiting step. Most viruses therefore need cellular chaperones during their life cycle. In addition to their own protein folding problems viruses need to interfere with cellular processes such as signal transduction, cell cycle regulation and induction of apoptosis in order to create a favorable environment for their proliferation and to avoid premature cell death. Chaperones are involved in the control of these cellular processes and some viruses reprogram their host cell by interacting with them. Hsp70 chaperones, as central components of the cellular chaperone network, are frequently recruited by viruses. This review focuses on the function of Hsp70 chaperones at the different stages of the viral life cycle emphasizing mechanistic aspects.

Simian virus 40 T antigens and J domains: analysis of Hsp40 cochaperone functions in Escherichia coli

The N-terminal exon of DNA tumor virus T antigens represents a J domain that can direct interaction with the host-encoded Hsp70 chaperones. We have taken advantage of rapid Hsp40 cochaperone assays with Escherichia coli to assess simian virus 40 (SV40)-encoded J-domain loss of function. We found a strong correlation between loss of cochaperone function in E. coli and defective SV40 growth, suggesting that the major role of the J domain in DNA tumor viruses is to provide cochaperone function. We also report the expression of native SV40 virus T antigens in E. coli. Our results show that small t antigen, but not large T antigen (LT) or LT truncation TN125 or TN136, can functionally replace under limited growth conditions DnaJ (Hsp40) function in vivo. In addition, purified small t antigen can efficiently stimulate E. coli DnaK's (Hsp70) ATPase in vitro, thus behaving like a bona fide cochaperone. Furthermore, small t amino acids 83 to 174, which are adjacent to the viral J domain, can replace the E. coli DnaJ J-domain glycine-phenylalanine-rich domain, immediately adjacent to the J-domain sequences, even in the absence of significant amino acid similarity to their DnaJ counterpart. Taken together, our studies demonstrate that functionally related Hsp40 proteins from mammalian viral systems can be rapidly studied in bacteria and exploited to probe the universally conserved Hsp70 chaperone machine mechanism.

Control of PERK eIF2alpha kinase activity by the endoplasmic reticulum stress-induced molecular chaperone P58IPK

P58(IPK) is an Hsp40 family member known to inhibit the interferon (IFN)-induced, double-stranded RNA-activated, eukaryotic initiation factor 2alpha (eIF2alpha) protein kinase R (PKR) by binding to its kinase domain. We find that the stress of unfolded proteins in the endoplasmic reticulum (ER) activates P58(IPK) gene transcription through an ER stress-response element in its promoter region. P58(IPK) interacts with and inhibits the PKR-like ER-localized eIF2alpha kinase PERK, which is normally activated during the ER-stress response to protect cells from ER stress by attenuating protein synthesis and reducing ER client protein load. Levels of phosphorylated eIF2alpha were lower in ER-stressed P58(IPK)-overexpressing cells and were enhanced in P58(IPK) mutant cells. In the ER-stress response, PKR-like ER kinase (PERK)-mediated translational repression is transient and is followed by translational recovery and enhanced expression of genes that increase the capacity of the ER to process client proteins. The absence of P58(IPK) resulted in increased expression levels of two ER stress-inducible genes, BiP and Chop, consistent with the enhanced eIF2alpha phosphorylation in the P58(IPK) deletion cells. Our studies suggest that P58(IPK) induction during the ER-stress response represses PERK activity and plays a functional role in the expression of downstream markers of PERK activity in the later phase of the ER-stress response.