Human Hsp70 and Hsp40 chaperone proteins facilitate human papillomavirus-11 E1 protein binding to the origin and stimulate cell-free DNA replication

Mechanisms of Viral DNA Replication of Human Papillomavirus: E2 Protein-Dependent Recruitment of E1 DNA Helicase to the Origin of DNA Replication

Human papillomaviruses (HPVs) are small double-stranded DNA viruses that infect epithelial cells and cause cervical, anogenital, and oropharyngeal cancers. HPV genome replication relies on the viral E1 and E2 proteins to initiate DNA replication. The first step is the assembly of the E1-E2 complex at the origin of replication. We have examined the role of full-length HPV E1 helicase and its interaction with E2 in pre-initiation complex formation. Electrophoretic mobility shift assays (EMSAs) with purified E1 and E2 proteins revealed that the HPV genome does not have a specific E1 binding site, or such a sequence is not required for pre-initiation complex formation. E1 alone did not show any binding to the origin DNA sequences, while E2 facilitated E1 recruitment to the origin, forming the E1-E2-DNA ternary complex. Formation of such a complex required at least two E2 binding sites. These findings led us to propose a novel mechanism in which E2 dimers serve as the primary recruiters of E1 to form the pre-initiation complex. This study provides new insights into the mechanistic role of E2 in the recruitment of E1 at the origin of HPV DNA replication, enhancing our understanding of HPV biology and potentially informing future therapeutic strategies.

DNAJA4 Promotes the Replication of the Chinese Giant Salamander Iridovirus

The DNAJ family, a class of chaperone proteins involved in protein folding, assembly, and transport, plays an essential role in viral infections. However, the role of DNAJA4 (DnaJ Heat Shock Protein Family (Hsp40) Member A4) in the ranavirus infection has not been reported. This study demonstrates the function of the epithelial papilloma of carp (EPC) DNAJA4 in Chinese giant salamander (Andrias davidianus) iridovirus (CGSIV) replication. DNAJA4 consists of 1479 base pairs and encodes a 492 amino acid polypeptide. Sequence analysis has shown that EPC DNAJA4 contains a conserved J domain and shares 84% homology with Danio rerio DNAJA4 and 68% homology with Homo sapiens DNAJA4. EPC DNAJA4 was localized in the cytoplasm, and its expression was significantly upregulated after CGSIV infection. Overexpression of EPC DNAJA4 promotes CGSIV replication and CGSIV DNA replication. siRNA knockdown of DNAJA4 expression attenuates CGSIV replication and viral DNA replication. Overexpression and interference experiments have proved that EPC DNAJA4 is a pro-viral factor. Co-IP, GST-pulldown, and immunofluorescence confirmed the interaction between EPC DNAJA4 and CGSIV proliferating cell nuclear antigen (PCNA). Our results demonstrate for the first time that EPC DNAJA4 is involved in viral infection by promoting viral DNA replication and interacting with proteins associated with viral replication.

Transcriptome Sequencing of the Spleen Reveals Antiviral Response Genes in Chickens Infected with CAstV

Astrovirus infections pose a significant problem in the poultry industry, leading to multiple adverse effects such as a decreased egg production, breeding disorders, poor weight gain, and even increased mortality. The commonly observed chicken astrovirus (CAstV) was recently reported to be responsible for the "white chicks syndrome" associated with an increased embryo/chick mortality. CAstV-mediated pathogenesis in chickens occurs due to complex interactions between the infectious pathogen and the immune system. Many aspects of CAstV-chicken interactions remain unclear, and there is no information available regarding possible changes in gene expression in the chicken spleen in response to CAstV infection. We aim to investigate changes in gene expression triggered by CAstV infection. Ten 21-day-old SPF White Leghorn chickens were divided into two groups of five birds each. One group was inoculated with CAstV, and the other used as the negative control. At 4 days post infection, spleen samples were collected and immediately frozen at -70 °C for RNA isolation. We analyzed the isolated RNA, using RNA-seq to generate transcriptional profiles of the chickens' spleens and identify differentially expressed genes (DEGs). The RNA-seq findings were verified by quantitative reverse-transcription PCR (qRT-PCR). A total of 31,959 genes was identified in response to CAstV infection. Eventually, 45 DEGs (p-value < 0.05; log2 fold change > 1) were recognized in the spleen after CAstV infection (26 upregulated DEGs and 19 downregulated DEGs). qRT-PCR performed on four genes (IFIT5, OASL, RASD1, and DDX60) confirmed the RNA-seq results. The most differentially expressed genes encode putative IFN-induced CAstV restriction factors. Most DEGs were associated with the RIG-I-like signaling pathway or more generally with an innate antiviral response (upregulated: BLEC3, CMPK2, IFIT5, OASL, DDX60, and IFI6; downregulated: SPIK5, SELENOP, HSPA2, TMEM158, RASD1, and YWHAB). The study provides a global analysis of host transcriptional changes that occur during CAstV infection in vivo and proves that, in the spleen, CAstV infection in chickens predominantly affects the cell cycle and immune signaling.

Heat Shock Factor 1 Prevents Age-Related Hearing Loss by Decreasing Endoplasmic Reticulum Stress

Endoplasmic reticulum (ER) stress is a common stress factor during the aging process. Heat shock factor 1 (HSF1) plays a critical role in ER stress; however, its exact function in age-related hearing loss (ARHL) has not been fully elucidated. The purpose of the present study was to identify the role of HSF1 in ARHL. In this study, we demonstrated that the loss of inner and outer hair cells and their supporting cells was predominant in the high-frequency region (basal turn, 32 kHz) in ARHL cochleae. In the aging cochlea, levels of the ER stress marker proteins p-eIF2α and CHOP increased as HSF1 protein levels decreased. The levels of various heat shock proteins (HSPs) also decreased, including HSP70 and HSP40, which were markedly downregulated, and the expression levels of Bax and cleaved caspase-3 apoptosis-related proteins were increased. However, HSF1 overexpression showed significant hearing protection effects in the high-frequency region (basal turn, 32 kHz) by decreasing CHOP and cleaved caspase-3 and increasing the HSP40 and HSP70 proteins. These findings were confirmed by HSF1 functional studies using an auditory cell model. Therefore, we propose that HSF1 can function as a mediator to prevent ARHL by decreasing ER stress-dependent apoptosis in the aging cochlea.

Diversity in heat shock protein families: functional implications in virus infection with a comprehensive insight of their role in the HIV-1 life cycle

Heat shock proteins (HSPs) are a group of cellular proteins that are induced during stress conditions such as heat stress, cold shock, UV irradiation and even pathogenic insult. They are classified into families based on molecular size like HSP27, 40, 70 and 90 etc, and many of them act as cellular chaperones that regulate protein folding and determine the fate of mis-folded or unfolded proteins. Studies have also shown multiple other functions of these proteins such as in cell signalling, transcription and immune response. Deregulation of these proteins leads to devastating consequences, such as cancer, Alzheimer's disease and other life threatening diseases suggesting their potential importance in life processes. HSPs exist in multiple isoforms, and their biochemical and functional characterization still remains a subject of active investigation. In case of viral infections, several HSP isoforms have been documented to play important roles with few showing pro-viral activity whereas others seem to have an anti-viral role. Earlier studies have demonstrated that HSP40 plays a pro-viral role whereas HSP70 inhibits HIV-1 replication; however, clear isoform-specific functional roles remain to be established. A detailed functional characterization of all the HSP isoforms will uncover their role in cellular homeostasis and also may highlight some of them as potential targets for therapeutic strategies against various viral infections. In this review, we have tried to comprehend the details about cellular HSPs and their isoforms, their role in cellular physiology and their isoform-specific functions in case of virus infection with a specific focus on HIV-1 biology.

The Hsp70-Chaperone Machines in Bacteria

The ATP-dependent Hsp70s are evolutionary conserved molecular chaperones that constitute central hubs of the cellular protein quality surveillance network. None of the other main chaperone families (Tig, GroELS, HtpG, IbpA/B, ClpB) have been assigned with a comparable range of functions. Through a multitude of functions Hsp70s are involved in many cellular control circuits for maintaining protein homeostasis and have been recognized as key factors for cell survival. Three mechanistic properties of Hsp70s are the basis for their high versatility. First, Hsp70s bind to short degenerate sequence motifs within their client proteins. Second, Hsp70 chaperones switch in a nucleotide-controlled manner between a state of low affinity for client proteins and a state of high affinity for clients. Third, Hsp70s are targeted to their clients by a large number of cochaperones of the J-domain protein (JDP) family and the lifetime of the Hsp70-client complex is regulated by nucleotide exchange factors (NEF). In this review I will discuss advances in the understanding of the molecular mechanism of the Hsp70 chaperone machinery focusing mostly on the bacterial Hsp70 DnaK and will compare the two other prokaryotic Hsp70s HscA and HscC with DnaK.

Human Papillomavirus infection requires the CCT Chaperonin Complex

Human papillomavirus (HPV) infection is a multi-step process that implies complex interactions of the viral particles with cellular proteins. The HPV capsid includes the two structural proteins L1 and L2, that play crucial roles on infectious viral entry. L2 is particularly relevant for the intracellular trafficking of the viral DNA towards the nucleus. Here, using proteomic studies we identified CCT proteins as novel interaction partners of HPV-16 L2. The CCT multimeric complex is an essential chaperonin which interacts with a large number of protein targets. We analysed the binding of different components of the CCT complex to L2. We confirmed the interaction of this structural viral protein with the CCT subunit 3 (CCT3) and we found that this interaction requires the N-terminal region of L2. Defects in HPV-16 pseudoviral particle (PsVs) infection were revealed by siRNA-mediated knockdown of some CCT subunits. While a substantial drop in the viral infection was associated with the ablation of CCT component 2, even more pronounced effects on infectivity were observed upon depletion of CCT component 3. Using confocal immunofluorescence assays, CCT3 co-localised with HPV PsVs at early times after infection, with L2 being required for this to occur. Further analysis showed the colocalization of several other subunits of CCT with the PsVs. Moreover, we observed a defect in capsid uncoating and a change in PsVs intracellular normal processing when ablating CCT3. Taken together, these studies demonstrate the importance of CCT chaperonin during HPV infectious entry.ImportanceSeveral of the mechanisms that function during the infection of target cells by HPV particles have been previously described. However, many aspects of this process remain unknown. In particular, the role of cellular proteins functioning as molecular chaperones during HPV infections has been only partially investigated. To the best of our knowledge, we describe here for the first time, a requirement of the CCT chaperonin for HPV infection. The role of this cellular complex seems to be determined by the binding of its component 3 to the viral structural protein L2. However, CCT's effect on HPV infection most probably comprises the whole chaperonin complex. Altogether, these studies define an important role for the CCT chaperonin in the processing and intracellular trafficking of HPV particles and in subsequent viral infectious entry.

Are Hsp90 inhibitors good candidates against Covid-19?

Drug reposition, or repurposing, has become a promising strategy in therapeutics due to its advantages in several aspects of drug therapy. General drug development is expensive and can take more than 10 years to go through the designing, development, and necessary approval steps. However, established drugs have already overcome these steps and thus a potential candidate may be already available decreasing the risks and costs involved. Viruses invade cells, usually provoking biochemical changes, leading to tissue damage, alteration of normal physiological condition in organisms and can even result in death. Inside the cell, the virus finds the machinery necessary for its multiplication, as for instance the protein quality control system, which involves chaperones and Hsps (heat shock proteins) that, in addition to physiological functions, help in the stabilization of viral proteins. Recently, many inhibitors of Hsp90 have been developed as therapeutic strategies against diseases such as the Hsp90 inhibitors used in anticancer therapy. Several shreds of evidence indicate that these inhibitors can also be used as therapeutic strategies against viruses. Therefore, since a drug treatment for COVID-19 is urgently needed, this review aims to discuss the potential use of Hsp90 inhibitors in the treatment of this globally threatening disease.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

HSC70 is required for infectious bursal disease virus (IBDV) infection in DF-1 cells

Background

Infectious bursal disease (IBD) is a highly contagious infectious disease that causes severe immunosuppression and damage to the bursa of Fabricius in chickens. Several proteins involved in IBD virus (IBDV) infection, such as surface immunoglobulin M, integrin, annexin A2 and chicken heat shock protein 90, have been identified. However, the main protein that plays key roles in virus infection has not yet been confirmed.

Methods

DF-1 cell line was transfected with the pcDNA-VP2 plasmid and analyzed by immunofluorescence assay. The proteins reacted with VP2 of IBDV in DF-1 cells were pulldown with the monoclonal antibody and identified by mass spectrometry. Heat shock cognate protein 70 (HSC70), one of these proteins, was selected to be investigated in the function in IBDV infection by specific antibody and its inhibitor.

Results

The DF-1 cell line was transfected with the pcDNA-VP2 plasmid, and expression of IBDV VP2 in DF-1 cells was confirmed by immunofluorescence assays. Heat shock cognate protein 70 (HSC70) was one of the proteins identified by coimmunoprecipitation using a monoclonal antibody (2H11) against VP2 and mass spectrometry analysis. IBDV infection in DF-1 cells was strongly inhibited by both an anti-HSC70 antibody and a HSC70 inhibitor (VER155008).

Conclusion

These results suggest that HSC70 may be an essential factor for IBDV infection.

Cold Exposure-Induced Up-Regulation of Hsp70 Positively Regulates PEDV mRNA Synthesis and Protein Expression In Vitro

Porcine epidemic diarrhea (PED) is a highly contagious, intestinal infectious disease caused by porcine epidemic diarrhea virus (PEDV). PEDV as an emerging and re-emerging epizootic virus of swine causes substantial economic losses to the pig industry in China and other countries. In China, the occurrence of PED shows significant seasonal variations, usually outbreak during the winter season. The epidemic characteristics of PED may be highly correlated with the changes of ambient temperature. However, molecular mechanism on the seasonal occurrence of PED still remains unclear. It has been widely observed that low ambient temperature up-regulates the expression of host heat shock protein 70 (Hsp70). Here, we showed that nucleotide and protein levels of Hsp70 were up-regulated in the intestinal of cold exposed pig and cold exposed Vero E6 cells. We found that overexpression of Hsp70 could increase PEDV mRNA synthesis and protein expression in Vero E6 and IPEC-J2 cells, while the siRNAs mediated knockdown of Hsp70 and VER155008 mediated inhibition of Hsp70 resulted in inhibition of viral mRNA synthesis and protein expression in Vero E6 cells. These data suggested that Hsp70 positively regulated PEDV mRNA synthesis and protein expression, which being helpful for understanding the seasonality of PED epidemics and development of novel antiviral therapies in the future.

Proteostasis in Viral Infection: Unfolding the Complex Virus-Chaperone Interplay

Viruses are obligate intracellular parasites that rely on their hosts for protein synthesis, genome replication, and viral particle production. As such, they have evolved mechanisms to divert host resources, including molecular chaperones, facilitate folding and assembly of viral proteins, stabilize complex structures under constant mutational pressure, and modulate signaling pathways to dampen antiviral responses and prevent premature host death. Biogenesis of viral proteins often presents unique challenges to the proteostasis network, as it requires the rapid and orchestrated production of high levels of a limited number of multifunctional, multidomain, and aggregation-prone proteins. To overcome such challenges, viruses interact with the folding machinery not only as clients but also as regulators of chaperone expression, function, and subcellular localization. In this review, we summarize the main types of interactions between viral proteins and chaperones during infection, examine evolutionary aspects of this relationship, and discuss the potential of using chaperone inhibitors as broad-spectrum antivirals.

The Nuclear and DNA-Associated Molecular Chaperone Network

Maintenance of a healthy and functional proteome in all cellular compartments is critical to cell and organismal homeostasis. Yet, our understanding of the proteostasis process within the nucleus is limited. Here, we discuss the identified roles of the major molecular chaperones Hsp90, Hsp70, and Hsp60 with client proteins working in diverse DNA-associated pathways. The unique challenges facing proteins in the nucleus are considered as well as the conserved features of the molecular chaperone system in facilitating DNA-linked processes. As nuclear protein inclusions are a common feature of protein-aggregation diseases (e.g., neurodegeneration), a better understanding of nuclear proteostasis is warranted.

Heat shock proteins in infection

Heat shock proteins (HSPs) are constitutively expressed under physiological conditions in most organisms but their expression can significantly enhance in response to four types of stimuli including physical (e.g., radiation or heat shock), chemical and microbial (e.g., pathogenic bacteria, viruses, parasites and fungi) stimuli, and also dietary. These proteins were identified for their role in protecting cells from high temperature and other forms of stress. HSPs control physiological activities or virulence through interaction with various regulators of cellular signaling pathways. Several roles were determined for HSPs in the immune system including intracellular roles (e.g., antigen presentation and expression of innate receptors) as well as extracellular roles (e.g., tumor immunosurveillance and autoimmunity). It was observed that exogenously administered HSPs induced various immune responses in immunotherapy of cancer, infectious diseases, and autoimmunity. Moreover, virus interaction with HSPs as molecular chaperones showed important roles in regulating viral infections including cell entry and nuclear import, viral replication and gene expression, folding/assembly of viral protein, apoptosis regulation, and host immunity. Viruses could regulate host HSPs at different levels such as transcription, translation, post-translational modification and cellular localization. In this review, we attempt to overview the roles of HSPs in a variety of infectious diseases.

Elevated HPV16 E1 Expression Is Associated with Cervical Cancer Progression

OBJECTIVES

The primary replication protein, HPV E1, has been shown to play a role in mitigating host defence and disrupting normal cell cycle processes, leading to the development of cancer. This study investigated the expression profile of HPV16 E1 in various stages of cervical cancer development and the factors that control E1 expression.

METHODS

One hundred and twenty-four HPV16-positive cervical samples ranging from normal to CIN 1, CIN 2/3, and SCC lesions were studied. E1 mRNA expression was determined by ddPCR. Methylation of promoters p97 and p670 was quantified by pyrosequencing, while PCR, qPCR, and sequencing were used to determine the physical state and variations of the HPV16 E1 genome.

RESULTS

Increased E1 mRNA expression related to disease progression (normal 0.18, CIN 1 0.41, CIN 2/3 0.65, and SCC 0.79) was demonstrated with a significant positive correlation (r = 0.661, p = 0.019). No association between physical state and E1 expression was found. Methylation of p97 and p670 promoters showed significant elevation in SCC compared to normal samples. Only 4.2% showed genomic variations of HPV16 E1 63-bp duplication.

CONCLUSION

E1 may play a role in cancer development. The detection of E1 mRNA and promoter methylation may be useful as cancer prognostic markers.

Hsp70 Isoforms Are Essential for the Formation of Kaposi's Sarcoma-Associated Herpesvirus Replication and Transcription Compartments

Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic herpesvirus associated with various AIDS-related malignancies. Like other herpesviruses, multiple processes required for KSHV lytic replication, including viral transcription, viral DNA synthesis and capsid assembly occur in virus-induced intranuclear structures, termed replication and transcription compartments (RTCs). Here we utilised a novel methodology, combining subcellular fractionation and quantitative proteomics, to identify cellular proteins which are recruited to KSHV-induced RTCs and thus play a key role in KSHV lytic replication. We show that several isoforms of the HSP70 chaperone family, Hsc70 and iHsp70, are redistributed from the cytoplasm into the nucleus coinciding with the initial formation of KSHV-induced RTCs. We demonstrate that nuclear chaperone foci are dynamic, initially forming adjacent to newly formed KSHV RTCs, however during later time points the chaperones move within KSHV RTCs and completely co-localise with actively replicating viral DNA. The functional significance of Hsp70 isoforms recruitment into KSHV RTCs was also examined using the specific Hsp70 isoform small molecule inhibitor, VER-155008. Intriguingly, results highlight an essential role of Hsp70 isoforms in the KSHV replication cycle independent of protein stability and maturation. Notably, inhibition of Hsp70 isoforms precluded KSHV RTC formation and RNA polymerase II (RNAPII) relocalisation to the viral genome leading to the abolishment of global KSHV transcription and subsequent viral protein synthesis and DNA replication. These new findings have revealed novel mechanisms that regulate KSHV lytic replication and highlight the potential of HSP70 inhibitors as novel antiviral agents.

A systems biology analysis of the changes in gene expression via silencing of HPV-18 E1 expression in HeLa cells

Previous studies have reported the detection of a truncated E1 mRNA generated from HPV-18 in HeLa cells. Although it is unclear whether a truncated E1 protein could function as a replicative helicase for viral replication, it would still retain binding sites for potential interactions with different host cell proteins. Furthermore, in this study, we found evidence in support of expression of full-length HPV-18 E1 mRNA in HeLa cells. To determine whether interactions between E1 and cellular proteins play an important role in cellular processes other than viral replication, genome-wide expression profiles of HPV-18 positive HeLa cells were compared before and after the siRNA knockdown of E1 expression. Differential expression and gene set enrichment analysis uncovered four functionally related sets of genes implicated in host defence mechanisms against viral infection. These included the toll-like receptor, interferon and apoptosis pathways, along with the antiviral interferon-stimulated gene set. In addition, we found that the transcriptional coactivator E1A-binding protein p300 (EP300) was downregulated, which is interesting given that EP300 is thought to be required for the transcription of HPV-18 genes in HeLa cells. The observed changes in gene expression produced via the silencing of HPV-18 E1 expression in HeLa cells indicate that in addition to its well-known role in viral replication, the E1 protein may also play an important role in mitigating the host's ability to defend against viral infection.

Inhibition of HSP70 reduces porcine reproductive and respiratory syndrome virus replication in vitro

Background

Successful viral infection requires the involvement of host cellular factors in their life cycle. Heat shock protein 70 (HSP70) can be recruited by numerous viruses to promote the folding, maturation, or assembly of viral proteins. We have previously shown that HSP70 is significantly elevated in porcine reproductive and respiratory syndrome virus (PRRSV)-infected lungs, suggesting HSP70 may play a potential role during PRRSV infection. In this study, we tried to investigate the role of HSP70 during PRRSV infection.

Results

In this study, we observed that PRRSV infection induced the expression of HSP70. The down-regulation of HSP70 using quercetin, a HSPs synthesis inhibitor, or small interfering RNAs (siRNA) reduced the viral protein level and viral production. Notably, these inhibitory effects on PRRSV infection could be attenuated by heat shock treatment. In addition, HSP70 was found to colocalize with the viral double-stranded RNA (dsRNA) and knockdown of HSP70 decreased the dsRNA levels, suggesting HSP70 is involved in the formation of viral replication and transcription complex (RTC) and thus affects the viral replication.

Conclusions

Our study revealed that HSP70 is an essential host factor required for the replication of PRRSV. The inhibition of HSP70 significantly reduced PRRSV replication, which may be applied as an effective antiviral strategy.

Human polyoma JC virus minor capsid proteins, VP2 and VP3, enhance large T antigen binding to the origin of viral DNA replication: evidence for their involvement in regulation of the viral DNA replication

JC virus (JCV) lytically infects the oligodendrocytes in the central nervous system in a subset of immunocompromized patients and causes the demyelinating disease, progressive multifocal leukoencephalopathy. JCV replicates and assembles into infectious virions in the nucleus. However, understanding the molecular mechanisms of its virion biogenesis remains elusive. In this report, we have attempted to shed more light on this process by investigating molecular interactions between large T antigen (LT-Ag), Hsp70 and minor capsid proteins, VP2/VP3. We demonstrated that Hsp70 interacts with VP2/VP3 and LT-Ag; and accumulates heavily in the nucleus of the infected cells. We also showed that VP2/VP3 associates with LT-Ag through their DNA binding domains resulting in enhancement in LT-Ag DNA binding to Ori and induction in viral DNA replication. Altogether, our results suggest that VP2/VP3 and Hsp70 actively participate in JCV DNA replication and may play critical roles in coupling of viral DNA replication to virion encapsidation.

The E1 proteins

E1, an ATP-dependent DNA helicase, is the only enzyme encoded by papillomaviruses (PVs). It is essential for replication and amplification of the viral episome in the nucleus of infected cells. To do so, E1 assembles into a double-hexamer at the viral origin, unwinds DNA at the origin and ahead of the replication fork and interacts with cellular DNA replication factors. Biochemical and structural studies have revealed the assembly pathway of E1 at the origin and how the enzyme unwinds DNA using a spiral escalator mechanism. E1 is tightly regulated in vivo, in particular by post-translational modifications that restrict its accumulation in the nucleus. Here we review how different functional domains of E1 orchestrate viral DNA replication, with an emphasis on their interactions with substrate DNA, host DNA replication factors and modifying enzymes. These studies have made E1 one of the best characterized helicases and provided unique insights on how PVs usurp different host-cell machineries to replicate and amplify their genome in a tightly controlled manner.

Human papillomavirus infections: warts or cancer?

Human papillomaviruses (HPVs) are prevalent pathogens of mucosal and cutaneous epithelia. Productive infections of squamous epithelia lead to benign hyperproliferative warts, condylomata, or papillomas. Persistent infections of the anogenital mucosa by high-risk HPV genotypes 16 and 18 and closely related types can infrequently progress to high-grade intraepithelial neoplasias, carcinomas-in-situ, and invasive cancers in women and men. HPV-16 is also associated with a fraction of head and neck cancers. We discuss the interactions of the mucosotropic HPVs with the host regulatory proteins and pathways that lead to benign coexistence and enable HPV DNA amplification or, alternatively, to cancers that no longer support viral production.

Targeting human papillomavirus genome replication for antiviral drug discovery

Human papillomavirus (HPV) infections are a major human health problem; they are the cause of recurrent benign warts and of several cancers of the anogenital tract and head and neck region. Although there are two prophylactic HPV vaccines that could, if used universally, prevent as many as two-thirds of HPV-induced cancers, as well as several cytotoxic and immunomodulatory agents for localized treatment of infections, there are currently no HPV antiviral drugs in our arsenal of therapeutic agents. This review examines the status of past and ongoing research into the development of HPV antivirals, focused primarily upon approaches targeting the replication of the viral genome. The only HPV enzyme, E1, is a DNA helicase that interfaces with the cellular DNA replication machinery to replicate the HPV genome. To date, searches for small molecule inhibitors of E1 for use as antivirals have met with limited success. The lack of other viral enzymes has meant that the search for antivirals has shifted to a large degree to the modulation of protein-protein interactions. There has been some success in identifying small molecule inhibitors targeting interactions between HPV proteins but with activity against a small subset of viral types only. As noted in this review, it is thought that targeting E1 interactions with cellular replication proteins may provide inhibitors with broader activity against multiple HPV types. Herein, we outline the steps in HPV DNA replication and discuss those that appear to provide the most advantageous targets for the development of anti-HPV therapeutics.

The role of mutational robustness in RNA virus evolution

RNA viruses face dynamic environments and are masters at adaptation. During their short 'lifespans', they must surmount multiple physical, anatomical and immunological challenges. Central to their adaptative capacity is the enormous genetic diversity that characterizes RNA virus populations. Although genetic diversity increases the rate of adaptive evolution, low replication fidelity can present a risk because excess mutations can lead to population extinction. In this Review, we discuss the strategies used by RNA viruses to deal with the increased mutational load and consider how this mutational robustness might influence viral evolution and pathogenesis.

Recruitment of Brd4 to the human papillomavirus type 16 DNA replication complex is essential for replication of viral DNA

Replication of the human papillomavirus (HPV) DNA genome relies on viral factors E1 and E2 and the cellular replication machinery. Bromodomain-containing protein 4 (Brd4) interacts with viral E2 protein to mediate papillomavirus (PV) genome maintenance and viral transcription. However, the functional role of Brd4 in the HPV life cycle remains to be clearly defined. In this study, we provide the first look into the E2-Brd4 interaction in the presence of other important viral factors, such as the HPV16 E1 protein and the viral genome. We show that Brd4 is recruited to actively replicating HPV16 origin foci together with HPV16 E1, E2, and a number of the cellular replication factors: replication protein A70 (RPA70), replication factor C1 (RFC1), and DNA polymerase δ. Mutagenesis disrupting the E2-Brd4 interaction abolishes the formation of the HPV16 replication complex and impairs HPV16 DNA replication in cells. Brd4 was further demonstrated to be necessary for HPV16 viral DNA replication using a cell-free replication system in which depletion of Brd4 by small interfering RNA (siRNA) silencing leads to impaired HPV16 viral DNA replication and recombinant Brd4 protein is able to rescue viral DNA replication. In addition, releasing endogenous Brd4 from cellular chromatin by using the bromodomain inhibitor JQ1(+) enhances HPV16 DNA replication, demonstrating that the role of Brd4 in HPV DNA replication could be uncoupled from its function in chromatin-associated transcriptional regulation and cell cycle control. Our study reveals a new role for Brd4 in HPV genome replication, providing novel insights into understanding the life cycle of this oncogenic DNA virus.

Virus-heat shock protein interaction and a novel axis for innate antiviral immunity

Virus infections induce heat shock proteins that in turn enhance virus gene expression, a phenomenon that is particularly well characterized for the major inducible 70 kDa heat shock protein (hsp70). However, hsp70 is also readily induced by fever, a phylogenetically conserved response to microbial infections, and when released from cells, hsp70 can stimulate innate immune responses through toll like receptors 2 and 4 (TLR2 and 4). This review examines how the virus-hsp70 relationship can lead to host protective innate antiviral immunity, and the importance of hsp70 dependent stimulation of virus gene expression in this host response. Beginning with the well-characterized measles virus-hsp70 relationship and the mouse model of neuronal infection in brain, we examine data indicating that the innate immune response is not driven by intracellular sensors of pathogen associated molecular patterns, but rather by extracellular ligands signaling through TLR2 and 4. Specifically, we address the relationship between virus gene expression, extracellular release of hsp70 (as a damage associated molecular pattern), and hsp70-mediated induction of antigen presentation and type 1 interferons in uninfected macrophages as a novel axis of antiviral immunity. New data are discussed that examines the more broad relevance of this protective mechanism using vesicular stomatitis virus, and a review of the literature is presented that supports the probable relevance to both RNA and DNA viruses and for infections both within and outside of the central nervous system.

Measles virus neurovirulence and host immunity

Measles virus is highly neuroinvasive, yet host immune responses are highly effective at limiting neurovirulence in humans. We know that neurons are an important target of infection and that both IFN-γ and -β expression are observed in the measles virus-infected human brain. Rodent models can be used to understand how this response is orchestrated. Constitutive expression of the major inducible 70-kDa heat-shock protein is a feature of primate tissues that is lacking in mice. This article examines the importance of addressing this difference when modeling outcomes of brain infection in mice, particularly in terms of understanding how infected neurons may activate uninfected brain macrophages to produce IFN-β and support T-cell production of IFN-γ, a mediator of noncytolytic viral clearance. New and historical data suggest that the virus heat-shock protein 70 relationship is key to a protective host immune response and has potential broad relevance.

Analyses of porcine public SNPs in coding-gene regions by re-sequencing and phenotypic association studies

The Porcine SNP database has a huge number of SNPs, but these SNPs are mostly found by computer data-mining procedures and have not been well characterized. We re-sequenced 1,439 porcine public SNPs from four commercial pig breeds and one Korean domestic breed (Korean Native pig, KNP) by using two DNA pools from eight unrelated animals in each breed. These SNPs were from 419 protein-coding genes covering the 18 autosomes, and the re-sequencing in breeds confirmed 690 public SNPs (47.9%) and 226 novel mutations (173 SNPs and 53 insertions/deletions). Thus, totally, 916 variations were found from our study. Of the 916 variations, 148 SNPs (16.2%) were found across all the five breeds, and 199 SNPs (21.7%) were breed specific polymorphisms. According to the SNP locations in the gene sequences, these 916 variations were categorized into 802 non-coding SNPs (785 in intron, 17 in 3'-UTR) and 114 coding SNPs (86 synonymous SNPs, 28 non-synonymous SNPs). The nucleotide substitution analyses for these SNPs revealed that 70.2% were from transitions, 20.0% from transversions, and the remaining 5.79% were deletions or insertions. Subsequently, we genotyped 261 SNPs from 180 genes in an experimental KNP × Landrace F2 cross by the Sequenom MassARRAY system. A total of 33 traits including growth, carcass composition and meat quality were analyzed for the phenotypic association tests using the 132 SNPs in 108 genes with minor allele frequency (MAF)>0.2. The association results showed that five marker-trait combinations were significant at the 5% experiment-wise level (ADCK4 for rear leg, MYH3 for rear leg, Hunter B, Loin weight and Shearforce) and four at the 10% experiment-wise level (DHX38 for average daily gain at live weight, LGALS9 for crude lipid, NGEF for front leg and LIFR for pH at 24 h). In addition, 49 SNPs in 44 genes showing significant association with the traits were detected at the 1% comparison-wise level. A large number of genes that function as enzymes, transcription factors or signalling molecules were considered as genetic markers for pig growth (RNF103, TSPAN31, DHX38, ABCF1, ABCC10, SCD5, KIAA0999 and FKBP10), muscling (HSPA5, PTPRM, NUP88, ADCK4, PLOD1, DLX1 and GRM8), fatness (PTGIS, IDH3B, RYR2 and NOL4) and meat quality traits (DUSP4, LIFR, NGEF, EWSR1, ACTN2, PLXND1, DLX3, LGALS9, ENO3, EPRS, TRIM29, EHMT2, RBM42, SESN2 and RAB4B). The SNPs or genes reported here may be beneficial to future marker assisted selection breeding in pigs.

Viral interaction with molecular chaperones: role in regulating viral infection

As essential effectors in protein quality control, molecular chaperones serve as the primary checkpoint to assist proper protein folding and prevent misfolded proteins from denaturation and aggregation. In addition, chaperones can function to direct terminally misfolded proteins to the proteolytic system for degradation. Viruses rely on host cell machineries for productive infection. Like for many other processes, various viruses have been shown to evolve mechanisms to utilize or subvert the host protein quality control machinery to support the completion of their life cycle. Furthermore, recent studies suggest that some viruses encode for their own chaperone-like proteins to enhance their infectivity. This review summarizes the current understanding of the interplay between molecular chaperones and viral proteins, highlights the chaperone activities of a number of viral proteins, and discusses potential antiviral therapeutic strategies targeting the virus-chaperone interactions.

High affinity binding between Hsp70 and the C-terminal domain of the measles virus nucleoprotein requires an Hsp40 co-chaperone

The major inducible 70 kDa heat shock protein (hsp70) binds the measles virus (MeV) nucleocapsid with high affinity in an ATP-dependent manner, stimulating viral transcription and genome replication, and profoundly influencing virulence in mouse models of brain infection. Binding is mediated by two hydrophobic motifs (Box-2 and Box-3) located within the C-terminal domain (N(TAIL)) of the nucleocapsid protein, with N(TAIL) being an intrinsically disordered domain. The current work showed that high affinity hsp70 binding to N(TAIL) requires an hsp40 co-chaperone that interacts primarily with the hsp70 nucleotide binding domain (NBD) and displays no significant affinity for N(TAIL). Hsp40 directly enhanced hsp70 ATPase activity in an N(TAIL)-dependent manner, and formation of hsp40-hsp70-N(TAIL) intracellular complexes required the presence of N(TAIL) Box-2 and 3. Results are consistent with the functional interplay between hsp70 nucleotide and substrate binding domains (SBD), where ATP hydrolysis is rate limiting to high affinity binding to client proteins and is enhanced by hsp40. As such, hsp40 is an essential variable in understanding the outcome of MeV-hsp70 interactions.

Concerted loading of Mcm2-7 double hexamers around DNA during DNA replication origin licensing

The licensing of eukaryotic DNA replication origins, which ensures once-per-cell-cycle replication, involves the loading of six related minichromosome maintenance proteins (Mcm2-7) into prereplicative complexes (pre-RCs). Mcm2-7 forms the core of the replicative DNA helicase, which is inactive in the pre-RC. The loading of Mcm2-7 onto DNA requires the origin recognition complex (ORC), Cdc6, and Cdt1, and depends on ATP. We have reconstituted Mcm2-7 loading with purified budding yeast proteins. Using biochemical approaches and electron microscopy, we show that single heptamers of Cdt1*Mcm2-7 are loaded cooperatively and result in association of stable, head-to-head Mcm2-7 double hexamers connected via their N-terminal rings. DNA runs through a central channel in the double hexamer, and, once loaded, Mcm2-7 can slide passively along double-stranded DNA. Our work has significant implications for understanding how eukaryotic DNA replication origins are chosen and licensed, how replisomes assemble during initiation, and how unwinding occurs during DNA replication.

Inducible heat shock protein 70 enhances HPV31 viral genome replication and virion production during the differentiation-dependent life cycle in human keratinocytes

Increasing data indicate heat shock proteins (HSPs) including inducible HSP70 (HSP70i) are involved in the replicative cycles of various viruses including adenoviruses (Ads), polyomaviruses (PyVs), and some RNA viruses. Cell-free system studies implicate HSP70i in human papillomavirus type 11 (HPV11) genome replication with E1 and E2 proteins, and there is evidence that HSP70 is involved in capsid assembly and disassembly for PyVs and HPVs. HSP70 expression is increased in HPV16 E6/E7 gene transduced human primary keratinocytes, and frequently detected in early stage uterine cervical cancer at levels in conjunction with lesion severity. In this study we carry out analyses in the natural host epithelial tissues to assess the role of inducible HSP70 (HSP70i) in the HPV infectious life cycle. For these studies we used the organotypic (raft) culture system to recapitulate the full viral life cycle of the high-risk HPV31. Upon heat shock of HPV31-infected organotypic tissues, we find high and sustained expression of HSP70i coincident with enhanced HPV genome replication and virion production. Whereas there is no clear effect on L1 expression levels, we find HSP70i and L1 interact and HSP70i colocalizes with and enhances the nuclear localization of L1 in differentiated cells. Ad-mediated gene transfer was used to study the effects of HSP70i in naturally HPV-infected differentiating tissues and showed results similar to those in heat shocked rafts. These results indicate that increased HSP70i augments late activities in the viral life cycle. We conclude that HSP70i contributes directly to HPV replicative viral activities and the production of infectious virions.

Role of Heat Shock Proteins in Viral Infection

One of the most intriguing and less known aspects of the interaction between viruses and their host is the impact of the viral infection on the heat shock response (HSR). While both a positive and a negative role of different heat shock proteins (HSP) in the control of virus replication has been hypothesized, HSP function during the virus replication cycle is still not well understood. This chapter describes different aspects of the interactions between viruses and heat shock proteins during infection of mammalian cells: the first part focuses on the modulation of the heat shock response by human viral pathogens; the second describes the interactions of HSP and other chaperones with viral components, and their function during different steps of the virus replication cycle; the last part summarizes our knowledge on the effect of hyperthermia and HSR modulators on virus replication.

The co-chaperone BAG3 regulates Herpes Simplex Virus replication

Viruses as obligate intracellular parasites use host cell proteins to ensure efficient replication and spread. Cellular proteins are required for several stages of a virus life cycle. Here, we identify BAG3, a co-chaperone, as a regulator of herpes virus immediate early gene expression. We report that a herpes simplex virus lacking the gene encoding a potent transcriptional activator, ICP0, is compromised for replication in cells silenced for BAG3 in a multiplicity of infection-dependent manner. We also show a requirement for BAG3 to augment virus gene expression and demonstrate that the co-chaperone acts independently of promyelocytic leukemia to increase herpes simplex virus replication.

Chaperones in cell cycle regulation and mitogenic signal transduction: a review

Chaperones/heat shock proteins (HSPs) of the HSP90 and HSP70 families show elevated levels in proliferating mammalian cells and a cell cycle-dependent expression. They transiently associate with key molecules of the cell cycle control system such as Cdk4, Wee-1, pRb, p53, p27/Kip1 and are involved in the nuclear localization of regulatory proteins. They also associate with viral oncoproteins such as SV40 super T, large T and small t antigen, polyoma large and middle S antigen and EpsteinBarr virus nuclear antigen. This association is based on a J-domain in the viral proteins and may assist their targeting to the pRb/E2F complex. Small HSPs and their state of phosphorylation and oligomerization also seem to be involved in proliferation and differentiation. Chaperones/HSPs thus play important roles within cell cycle processes. Their exact functioning, however, is still a matter of discussion. HSP90 in particular, but also HSP70 and other chaperones associate with proteins of the mitogen-activated signal cascade, particularly with the Src kinase, with tyrosine receptor kinases, with Raf and the MAP-kinase activating kinase (MEK). This apparently serves the folding and translocation of these proteins, but possibly also the formation of large immobilized complexes of signal transducing molecules (scaffolding function).

The human TPR protein TTC4 is a putative Hsp90 co-chaperone which interacts with CDC6 and shows alterations in transformed cells

Background

The human TTC4 protein is a TPR (tetratricopeptide repeat) motif-containing protein. The gene was originally identified as being localized in a genomic region linked to breast cancer and subsequent studies on melanoma cell lines revealed point mutations in the TTC4 protein that may be associated with the progression of malignant melanoma.

Methodology/Principle Findings

Here we show that TTC4 is a nucleoplasmic protein which interacts with HSP90 and HSP70, and also with the replication protein CDC6. It has significant structural and functional similarities with a previously characterised Drosophila protein Dpit47. We show that TTC4 protein levels are raised in malignant melanoma cell lines compared to melanocytes. We also see increased TTC4 expression in a variety of tumour lines derived from other tissues. In addition we show that TTC4 proteins bearing some of the mutations previously identified from patient samples lose their interaction with the CDC6 protein.

Conclusions/Significance

Based on these results and our previous work with the Drosophila Dpit47 protein we suggest that TTC4 is an HSP90 co-chaperone protein which forms a link between HSP90 chaperone activity and DNA replication. We further suggest that the loss of the interaction with CDC6 or with additional client proteins could provide one route through which TTC4 could influence malignant development of cells.

Human papillomavirus E1 helicase interacts with the WD repeat protein p80 to promote maintenance of the viral genome in keratinocytes

Due to the limited coding capacity of their small genomes, human papillomaviruses (HPV) rely extensively on host factors for the completion of their life cycles. Accordingly, most HPV proteins, including the replicative helicase E1, engage in multiple protein interactions. The fact that conserved regions of E1 have not yet been ascribed a function prompted us to use tandem affinity protein purification (TAP) coupled to mass spectrometry to identify novel targets of this helicase. This method led to the discovery of a novel interaction between the N-terminal 40 amino acids of HPV type 11 (HPV11) E1 and the cellular WD repeat protein p80 (WDR48). We found that interaction with p80 is conserved among E1 proteins from anogenital HPV but not among cutaneous or animal types. Colocalization studies showed that E1 can redistribute p80 from the cytoplasm to the nucleus in a manner that is dependent on the E1 nuclear localization signal. Three amino acid substitutions in E1 proteins from HPV11 and -31 were identified that abrogate binding to p80 and its relocalization to the nucleus. In HPV31 E1, these substitutions reduced but did not completely abolish transient viral DNA replication. HPV31 genomes encoding two of the mutant E1 proteins were not maintained as episomes in immortalized primary keratinocytes, whereas one encoding the third mutant protein was maintained at a very low copy number. These findings suggest that the interaction of E1 with p80 is required for efficient maintenance of the viral episome in undifferentiated keratinocytes.

Remodeling of the human papillomavirus type 11 replication origin into discrete nucleoprotein particles and looped structures by the E2 protein

The human papillomavirus (HPV) DNA replication origin (ori) shares a common theme with many DNA control elements in having multiple binding sites for one or more proteins spaced over several hundreds of base pairs. The HPV type 11 ori spans 103 bp and contains three palindromic E2 binding sites (E2BS-2, E2BS-3, and E2BS-4) for the dimeric E2 ori binding protein. These sites are separated by 64 and 3 bp. E2BS-1 is located 288 bp upstream of E2BS-2 and is not required for efficient transient or cell-free replication. In this study, electron microscopy was used to visualize complexes of HPV-11 DNA ori bound by purified E2 protein. DNA containing only E2BS-2 showed a single E2 dimer bound. DNA containing E2BS-3 and E2BS-4 showed two side-by-side E2 dimers, while DNA containing E2BS-2, E2BS-3, and E2BS-4 exhibited a large disk/ring-shaped protein particle bound, indicating that the DNA had been remodeled into a discrete complex, likely containing an E2 hexamer. With all four binding sites present, up to 27% of the DNA molecules were arranged into loops by E2, the majority of which spanned E2BS-1 and one of the other three sites. Studies on the dependence of looping on salt, ATP, and DTT using full-length E2 and an E2 protein containing only the carboxyl-terminal DNA binding and protein dimerization domain suggest that looping is dependent on the N-terminal domain and factors that may affect the manner in which E2 scans DNA for binding sites. The role of these structures in the modeling and regulation of the HPV-11 ori is discussed.

Activation profiles of HSPA5 during the glomerular mesangial cell stress response to chemical injury

Environmental injury has been associated with endoplasmic reticulum (ER) stress, a response characterized by activation of the unfolded protein response, proteasomal degradation of proteins, and induction of HSPA5, also known as GRP78 or BiP. Although HSPA5 has been implicated in the stress response to environmental injury in several cell types, its role in the glomerular ER stress response is unknown. In this study, we evaluated HSPA5 activation profiles in rat glomerular mesangial cells (rGMCs) challenged with heavy metals (HgCl2 or Pb2+ acetate) or polycyclic aromatic hydrocarbons (PAHs, ie, benzo(a)pyrene [BaP]). Challenge of rGMCs with 1 or 10 microM HgCl2 or Pb2+ acetate increased HSPA5 mRNA and protein levels. The induction response was sensitive to transcriptional and translational inhibition by actinomycin D (AD) and cyclohexamide, respectively. HSPA5 mRNA was induced by 3 microM BaP in an AD-sensitive manner, but this response was unaffected by the presence of heavy metals. A promoter construct containing sequences that mediate thapsigargin (TH) inducibility of the HSPA5 promoter was refractory to both heavy metals and BaP. The HSPA5 induction response in rGMCs is conserved because it was reproduced with fidelity in immunolocalization experiments of HSPA5 protein in M15 and HEK293 cells in embryonic lines of murine and human origin, respectively. Collectively, these findings identify HSPA5 in the stress response of rGMCs and implicate regulatory mechanisms that are distinct from those involved in TH inducibility.

BAG3, a host cochaperone, facilitates varicella-zoster virus replication

Varicella-zoster virus (VZV) establishes a lifelong latent infection in the dorsal root ganglia of the host. During latency, a subset of virus-encoded regulatory proteins is detected; however, they are excluded from the nucleus. ORF29p, a single-stranded DNA binding protein, is one of these latency-associated proteins. We searched for cell proteins that interact with ORF29p and identified BAG3. BAG3, Hsp70/Hsc70, and Hsp90 colocalize with ORF29p in nuclear transcription/replication factories during lytic replication of VZV. Pharmacological intercession of Hsp90 activity with ansamycin antibiotics or depletion of BAG3 by small interfering RNA results in inhibition of virus replication. Replication in BAG3-depleted cell lines is restored by complementation with exogenous BAG3. Alteration of host chaperone activity provides a novel means of regulating virus replication.

Mitogen-activated protein kinases activate the nuclear localization sequence of human papillomavirus type 11 E1 DNA helicase to promote efficient nuclear import

Human and animal papillomavirus DNA replicates as multicopy nuclear plasmids. Replication requires two viral proteins, the origin-recognition protein E2 and the replicative DNA helicase E1. Using genetic, biochemical, and immunofluorescence assays, we demonstrated that efficient nuclear import of the human papillomavirus (HPV) type 11 E1 protein depends on a codominant bipartite nuclear localization sequence (NLS) and on phosphorylation of the serine residues S89 and S93 by the mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase, and c-Jun N-terminal protein kinase. The NLS and the MAPK substrates are located within a 50-amino-acid-long peptide near the amino terminus, previously designated the localization regulatory region (LRR). The downstream NLS overlaps the cyclin-binding motif RRL, which is necessary for phosphorylation by the cyclin-dependent kinases to inactivate a dominant nuclear export sequence, also in the LRR. Alanine mutations of the MAPK substrates significantly impaired nuclear import, whereas phospho-mimetic mutations partially restored nuclear import. We further identified two MAPK docking motifs near the C terminus of E1 that are conserved among E1 proteins of many HPVs and bovine papillomavirus type 1. Mutations of these MAPK docking motifs or addition of specific MAPK inhibitors significantly reduced nuclear import. Interestingly, a fraction of the NLS-minus E1 protein was cotransported with the E2 protein into the nucleus and supported transient viral DNA replication. In contrast, E1 proteins mutated in the MAPK docking motifs were completely inactive in transient replication, an indication that additional properties were adversely affected by those changes.

Hsp70 negatively controls rotavirus protein bioavailability in caco-2 cells infected by the rotavirus RF strain

Previous studies demonstrated that the induction of the heat shock protein Hsp70 in response to viral infection is highly specific and differs from one cell to another and for a given virus type. However, no clear consensus exists so far to explain the likely reasons for Hsp70 induction within host cells during viral infection. We show here that upon rotavirus infection of intestinal cells, Hsp70 is indeed rapidly, specifically, and transiently induced. Using small interfering RNA-Hsp70-transfected Caco-2 cells, we observed that Hsp70 silencing was associated with an increased virus protein level and enhanced progeny virus production. Upon Hsp70 silencing, we observed that the ubiquitination of the main rotavirus structural proteins was strongly reduced. In addition, the use of proteasome inhibitors in infected Caco-2 cells was shown to induce an accumulation of structural viral proteins. Together, these results are consistent with a role of Hsp70 in the control of the bioavailability of viral proteins within cells for virus morphogenesis.

hsp72, a host determinant of measles virus neurovirulence

Transient hyperthermia such as that experienced during febrile episodes increases expression of the major inducible 70-kDa heat shock protein (hsp72). Despite the relevance of febrile episodes to viral pathogenesis and the multiple in vitro roles of heat shock proteins in viral replication and gene expression, the in vivo significance of virus-heat shock protein interactions is unknown. The present work determined the in vivo relationship between hsp72 levels and neurovirulence of an hsp72-responsive virus using the mouse model of measles virus (MV) encephalitis. Transgenic C57BL/6 mice were created to constitutively overexpress hsp72 in neurons, and these mice were inoculated intracranially with Edmonston MV (Ed MV) at 42 h of age. The mean viral RNA burden in brain was approximately 2 orders of magnitude higher in transgenic animals than in nontransgenic animals 2 to 4 weeks postinfection, and this increased burden was associated with a fivefold increase in mortality. Mice were also challenged with an Ed MV variant exhibiting an attenuated in vitro response to hsp72-dependent stimulation of viral transcription (Ed N-522D). This virus exhibited an attenuated neuropathogenicity in transgenic mice, where mortality and viral RNA burdens were not significantly different from nontransgenic mice infected with either Ed N-522D or parent Ed MV. Collectively, these results indicate that hsp72 levels can serve as a host determinant of viral neurovirulence in C57BL/6 mice, reflecting the direct influence of hsp72 on viral gene expression.

Dynamic localization of the human papillomavirus type 11 origin binding protein E2 through mitosis while in association with the spindle apparatus

Papillomaviral DNA replicates as extrachromosomal plasmids in squamous epithelium. Viral DNA must segregate equitably into daughter cells to persist in dividing basal/parabasal cells. We have previously reported that the viral origin binding protein E2 of human papillomavirus types 11 (HPV-11), 16, and 18 colocalized with the mitotic spindles. In this study, we show the localization of the HPV-11 E2 protein to be dynamic. It colocalized with the mitotic spindles during prophase and metaphase. At anaphase, it began to migrate to the central spindle microtubules, where it remained through telophase and cytokinesis. It was additionally observed in the midbody at cytokinesis. A peptide spanning residues 285 to 308 in the carboxyl-terminal domain of HPV-11 E2 (E2C) is necessary and sufficient to confer localization on the mitotic spindles. This region is conserved in HPV-11, -16, and -18 and bovine papillomavirus type 4 (BPV-4) E2 and is also required for the respective E2C to colocalize with the mitotic spindles. The E2 protein of bovine papillomavirus type 1 is tethered to the mitotic chromosomes via the cellular protein Brd4. However, the HPV-11 E2 protein did not associate with Brd4 during mitosis. Lastly, a chimeric BPV-1 E2C containing the spindle localization domain from HPV-11 E2C gained the ability to localize to the mitotic spindles, whereas the reciprocal chimera lost the ability. We conclude that this region of HPV E2C is critical for localization with the mitotic apparatus, enabling the HPV DNA to sustain persistent infections.

Cellular topoisomerase I modulates origin binding by bovine papillomavirus type 1 E1

In addition to viral proteins E1 and E2, bovine papillomavirus type 1 (BPV1) depends heavily on host replication machinery for genome duplication. It was previously shown that E1 binds to and recruits cellular replication proteins to the BPV1 origin of replication, including DNA polymerase alpha-primase, replication protein A (RPA), and more recently, human topoisomerase I (Topo I). Here, we show that Topo I specifically stimulates the origin binding of E1 severalfold but has no effect on nonorigin DNA binding. This is highly specific, as binding to nonorigin DNA is not stimulated, and other cellular proteins that bind E1, such as RPA and polymerase alpha-primase, show no such effect. The stimulation of E1's origin binding by Topo I is not synergistic with the stimulation by E2. Although the enhanced origin binding of E1 by Topo I requires ATP and Mg2+ for optimal efficiency, ATP hydrolysis is not required. Using an enzyme-linked immunosorbent assay, we showed that the interaction between E1 and Topo I is decreased in the presence of DNA. Our results suggest that Topo I participates in the initiation of papillomavirus DNA replication by enhancing E1 binding to the BPV1 origin.

Mechanisms of genetic robustness in RNA viruses

Two key features of RNA viruses are their compacted genomes and their high mutation rate. Accordingly, deleterious mutations are common and have an enormous impact on viral fitness. In their multicellular hosts, robustness can be achieved by genomic redundancy, including gene duplication, diploidy, alternative metabolic pathways and biochemical buffering mechanisms. However, here we review evidence suggesting that during RNA virus evolution, alternative robustness mechanisms may have been selected. After briefly describing how genetic robustness can be quantified, we discuss mechanisms of intrinsic robustness arising as consequences of RNA-genome architecture, replication peculiarities and quasi-species population dynamics. These intrinsic robustness mechanisms operate efficiently at the population level, despite the mutational sensitivity shown by individual genomes. Finally, we discuss the possibility that viruses might exploit cellular buffering mechanisms for their own benefit, producing a sort of extrinsic robustness.

Human papillomaviruses: basic mechanisms of pathogenesis and oncogenicity

Human papillomaviruses (HPVs) are small double-stranded DNA viruses that infect the cutaneous and mucosal epithelium. Infection by specific HPV types has been linked to the development of cervical carcinoma. HPV infects epithelial cells that undergo terminal differentiation and so encode multiple mechanisms to override the normal regulation of differentiation to produce progeny virions. Two viral proteins, E6 and E7, alter cell cycle control and are the main arbitrators of HPV-induced oncogenesis. Recent data suggest that E6 and E7 also play a major role in the inhibition of the host cell innate immune response to HPV. The E1 and E2 proteins, in combination with various cellular factors, mediate viral replication. In addition, E2 has been implicated in both viral and cellular transcriptional control. Despite decades of research, the function of other viral proteins still remains unclear. While prophylactic vaccines to block genital HPV infection will soon be available, the widespread nature of HPV infection requires greater understanding of both the HPV life cycle as well as the mechanisms underlying HPV-induced carcinogenesis.

Activation of budding yeast replication origins and suppression of lethal DNA damage effects on origin function by ectopic expression of the co-chaperone protein Mge1

Initiation of DNA replication in eukaryotes requires the origin recognition complex (ORC) and other proteins that interact with DNA at origins of replication. In budding yeast, the temperature-sensitive orc2-1 mutation alters these interactions in parallel with defects in initiation of DNA replication and in checkpoints that depend on DNA replication forks. Here we show that DNA-damaging drugs modify protein-DNA interactions at budding yeast replication origins in association with lethal effects that are enhanced by the orc2-1 mutation or suppressed by a different mutation in ORC. A dosage suppressor screen identified the budding yeast co-chaperone protein Mge1p as a high copy suppressor of the orc2-1-specific lethal effects of adozelesin, a DNA-alkylating drug. Ectopic expression of Mge1p also suppressed the temperature sensitivity and initiation defect conferred by the orc2-1 mutation. In wild type cells, ectopic expression of Mge1p also suppressed the lethal effects of adozelesin in parallel with the suppression of adozelesin-induced alterations in protein-DNA interactions at origins, stimulation of initiation of DNA replication, and binding of the precursor form of Mge1p to nuclear chromatin. Mge1p is the budding yeast homologue of the Escherichia coli co-chaperone protein GrpE, which stimulates initiation at bacterial origins of replication by promoting interactions of initiator proteins with origin sequences. Our results reveal a novel, proliferation-dependent cytotoxic mechanism for DNA-damaging drugs that involves alterations in the function of initiation proteins and their interactions with DNA.

Cyclin/CDK regulates the nucleocytoplasmic localization of the human papillomavirus E1 DNA helicase

Cyclin-dependent kinases (CDKs) play key roles in eukaryotic DNA replication and cell cycle progression. Phosphorylation of components of the preinitiation complex activates replication and prevents reinitiation. One mechanism is mediated by nuclear export of critical proteins. Human papillomavirus (HPV) DNA replication requires cellular machinery in addition to the viral replicative DNA helicase E1 and origin recognition protein E2. E1 phosphorylation by cyclin/CDK is critical for efficient viral DNA replication. We now show that E1 is phosphorylated by CDKs in vivo and that phosphorylation regulates its nucleocytoplasmic localization. We identified a conserved regulatory region for localization which contains a dominant leucine-rich nuclear export sequence (NES), the previously defined cyclin binding motif, three serine residues that are CDK substrates, and a putative bipartite nuclear localization sequence. We show that E1 is exported from the nucleus by a CRM1-dependent mechanism unless the NES is inactivated by CDK phosphorylation. Replication activities of E1 phosphorylation site mutations are reduced and correlate inversely with their increased cytoplasmic localization. Nuclear localization and replication activities of most of these mutations are enhanced or restored by mutations in the NES. Collectively, our data demonstrate that CDK phosphorylation controls E1 nuclear localization to support viral DNA amplification. Thus, HPV adopts and adapts the cellular regulatory mechanism to complete its reproductive program.

Mechanisms for regulation of Hsp70 function by Hsp40

The Hsp70 family members play an essential role in cellular protein metabolism by acting as polypeptide-binding and release factors that interact with nonnative regions of proteins at different stages of their life cycles. Hsp40 cochaperone proteins regulate complex formation between Hsp70 and client proteins. Herein, literature is reviewed that describes the mechanisms by which Hsp40 proteins interact with Hsp70 to specify its cellular functions.