Intranuclear delivery of an antiviral peptide mediated by the B subunit of Escherichia coli heat-labile enterotoxin

Specific inhibition of the interaction between pseudorabies virus DNA polymerase subunits UL30 and UL42 by a synthetic peptide

Pseudorabies virus (PRV) is a ubiquitous and economically important swine alphaherpesvirus that causes devastating swine diseases worldwide. PRV-encoded DNA-dependent DNA polymerase, comprised of the catalytic subunit UL30 and the accessory subunit UL42, is essential for viral replication. PRV UL30 and UL42 act as a heterodimer with UL30 harboring inherent DNA polymerase activity and UL42 conferring processivity on the DNA polymerase holoenzyme. The formation of PRV UL30/UL42 heterodimer holoenzyme through protein-protein interactions is indispensable for viral replication. In work described here, we defined the key domains that mediate PRV UL30/UL42 interaction, and found that the 41 carboxy-terminal amino acids region of PRV UL30 is critical for its interaction with UL42. Intriguingly, a synthetic peptide corresponding to these 41 carboxy-terminal amino acid residues efficiently disrupted PRV UL30/UL42 interaction through competitively binding to UL42. These findings suggest that the peptides from the PRV DNA polymerase UL30/UL42 subunit interface may represent potential targets for designing a novel intervention strategy against PRV infection. This work further strengthens the concept that the herpesvirus DNA polymerase catalytic subunits utilize their extreme carboxy-terminal domains as a conserved mechanism to associate with their cognate accessory subunits, providing us the opportunity of designing novel antiviral agents against herpesvirus infection through disruption of the herpesvirus DNA polymerase subunit interactions.

Herpesvirus DNA polymerase: Structures, functions, and mechanisms

Herpesviruses comprise a family of DNA viruses that cause a variety of human and veterinary diseases. During productive infection, mammalian, avian, and reptilian herpesviruses replicate their genomes using a set of conserved viral proteins that include a two subunit DNA polymerase. This enzyme is both a model system for family B DNA polymerases and a target for inhibition by antiviral drugs. This chapter reviews the structure, function, and mechanisms of the polymerase of herpes simplex viruses 1 and 2 (HSV), with only occasional mention of polymerases of other herpesviruses such as human cytomegalovirus (HCMV). Antiviral polymerase inhibitors have had the most success against HSV and HCMV. Detailed structural information regarding HSV DNA polymerase is available, as is much functional information regarding the activities of the catalytic subunit (Pol), which include a DNA polymerization activity that can utilize both DNA and RNA primers, a 3'-5' exonuclease activity, and other activities in DNA synthesis and repair and in pathogenesis, including some remaining to be biochemically defined. Similarly, much is known regarding the accessory subunit, which both resembles and differs from sliding clamp processivity factors such as PCNA, and the interactions of this subunit with Pol and DNA. Both subunits contribute to replication fidelity (or lack thereof). The availability of both pharmacologic and genetic tools not only enabled the initial identification of Pol and the pol gene, but has also helped dissect their functions. Nevertheless, important questions remain for this long-studied enzyme, which is still an attractive target for new drug discovery.

Pseudorabies Virus DNA Polymerase Processivity Factor UL42 Inhibits Type I IFN Response by Preventing ISGF3-ISRE Interaction

Alphaherpesviruses are large dsDNA viruses with an ability to establish persistent infection in hosts, which rely partly on their ability to evade host innate immune responses, notably the type I IFN response. However, the relevant molecular mechanisms are not well understood. In this study, we report the UL42 proteins of alphaherpesvirus pseudorabies virus (PRV) and HSV type 1 (HSV1) as a potent antagonist of the IFN-I-induced JAK-STAT signaling pathway. We found that ectopic expression of UL42 in porcine macrophage CRL and human HeLa cells significantly suppresses IFN-α-mediated activation of the IFN-stimulated response element (ISRE), leading to a decreased transcription and expression of IFN-stimulated genes (ISGs). Mechanistically, UL42 directly interacts with ISRE and interferes with ISG factor 3 (ISGF3) from binding to ISRE for efficient gene transcription, and four conserved DNA-binding sites of UL42 are required for this interaction. The substitution of these DNA-binding sites with alanines results in reduced ISRE-binding ability of UL42 and impairs for PRV to evade the IFN response. Knockdown of UL42 in PRV remarkably attenuates the antagonism of virus to IFN in porcine kidney PK15 cells. Our results indicate that the UL42 protein of alphaherpesviruses possesses the ability to suppress IFN-I signaling by preventing the association of ISGF3 and ISRE, thereby contributing to immune evasion. This finding reveals UL42 as a potential antiviral target.

Promotion of CTL epitope presentation by a nanoparticle with environment-responsive stability and phagolysosomal escape capacity

Vaccines that induce cytotoxic T lymphocyte (CTL)-mediated immune responses constitute an important class of medical tools to fend off diseases like infections and malignancy. Epitope peptides, as a format of CTL vaccines, are being tested preclinically and clinically. To elicit CTL responses, epitope vaccines go through an epitope presentation pathway in dendritic cells (DCs) that has multiple bottleneck steps and hence is inefficient. Here, we report the development of a strategy to overcome one of these barriers, phagolysosomal escape in DCs. First, we furnished a previously established carrier-an immune-tolerant elastin-like polypeptide nanoparticle (iTEP NP)-with the peptides that are derived from the DNA polymerase of herpes simplex virus 1 (Pol peptides). Pol peptides were reported to facilitate phagolysosomal escape. In this study, while we found that Pol peptides promoted the CTL epitope presentation; we also discovered Pol peptides disrupted the formation of the iTEP NP. Thus, we engineered a series of new iTEPs and identified several iTEPs that could accommodate Pol peptides and maintain their NP structure at the same time. We next optimized one of these NPs so that its stability is responsive to its redox environment. This environment-responsive NP further strengthened the CTL epitope presentation and CTL responses. Lastly, we revealed how this NP and Pol peptides utilized biological cues of phagolysosomes to realize phagolysosomal escape and epitope release. In summary, we developed iTEP NP carriers with a new phagolysosomal escape function. These carriers, with their priorly incorporated functions, resolve three bottleneck issues in the CTL epitope presentation pathway: vaccine uptake, phagolysosomal escape, and epitope release.

Classification of human Herpesviridae proteins using Domain-architecture Aware Inference of Orthologs (DAIO)

We developed a computational approach called Domain-architecture Aware Inference of Orthologs (DAIO) for the analysis of protein orthology by combining phylogenetic and protein domain-architecture information. Using DAIO, we performed a systematic study of the proteomes of all human Herpesviridae species to define Strict Ortholog Groups (SOGs). In addition to assessing the taxonomic distribution for each protein based on sequence similarity, we performed a protein domain-architecture analysis for every protein family and computationally inferred gene duplication events. While many herpesvirus proteins have evolved without any detectable gene duplications or domain rearrangements, numerous herpesvirus protein families do exhibit complex evolutionary histories. Some proteins acquired additional domains (e.g., DNA polymerase), whereas others show a combination of domain acquisition and gene duplication (e.g., betaherpesvirus US22 family), with possible functional implications. This novel classification system of SOGs for human Herpesviridae proteins is available through the Virus Pathogen Resource (ViPR, www.viprbrc.org).

Expression of B subunit of E. coli heat-labile enterotoxin in the progenies of transgenic tobacco bred by crossing nuclear- and chloroplast-transgenic lines

The B subunit of Escherichia coli heat-labile toxin (LTB) is a model antigen that induces a strong immune response upon oral administration and enhances immune responses to conjugated and co-administered antigens. We previously examined high expression levels of LTB in plants by chloroplast and synthetic LTB gene expression and found substantially higher expression levels of LTB, compared to nuclear LTB expression in wild-type plants. The 2.5% LTB protein of total soluble protein that was observed by chloroplast transformation was approximately 250-fold greater expression than that of LTB via nuclear genome integration. In addition, the amount of LTB protein found in transgenic tobacco leaves using a synthetic LTB gene was 2.2% of the total soluble plant protein, which was approximately 200-fold higher than that in plants with native LTB gene expression. The purpose of our experiment was to increase LTB levels in plants by crossing chloroplast-transformed and synthetic LTB transgenic lines produced previously to express higher LTB levels. LTB protein levels in the F1 transgenic tobacco plants was significantly higher (3.3%), compared to the 2.2% of chloroplast-transformed line or 2.8% of synthetic LTB gene line. Our results suggest that LTB expression was successfully enhanced in the F1 hybrid generation of transgenic tobacco plants.

Engineering an AB5 Protein Carrier

The promise of biologic therapeutics is hindered by the challenge to deliver their activity to biochemically relevant sites within diseased cells. The favourable application of the natural protein carriers of the AB₅ toxin family to this challenge has been restricted owing to still unresolved requirements for assembling non-native cargo into carrier complexes. Here, we clarify the properties of fusion peptides which allow co-assembly of a selected fluorescent protein cargo with the non-toxic B subunit of a heat-labile enterotoxin. We establish the influence of sequence length, sequence identity and secondary structure of these linking domains on the assembly and disassembly of the complexes. Through our engineering framework we identify several non-native, reduced length fusion sequences that robustly assemble with the native carriers, maintain their ability to deliver protein cargo to cells, and demonstrate substantially refined in vitro properties. Constructs based upon these sequences should prove directly applicable to a variety of protein delivery challenges, and the described design framework should find immediate application to other members of the AB₅ protein carrier family.

MODERN ETHIOTROPIC CHEMOTHERAPY OF HERPESVIRUS INFECTIONS: ADVANCES, NEW TRENDS AND PERSPECTIVES. ALPHAHERPESVIRINAE (part I)

Modern therapy of infections caused by alpha-herpesviruses is based on drugs belonging to the class of modified nucleosides (acyclovir) and their metabolic progenitors - valine ester of acyclovir and famciclovir (prodrug of penciclovir). The biological activity of these compounds is determined by the similarity of their structure to natural nucleosides: modified nucleosides compete with natural nucleosides for binding to DNA-polymerase and, due to their structural features, inhibit its activity. However, the emergence of variants of viruses resistant to the antiviral drugs available in the arsenal of modern medicine necessitates the search for new compounds able of effectively inhibiting the reproduction of viruses. These compounds should be harmless to the macroorganisms, convenient to use, and overcoming the drug resistance barrier in viruses. The search for literature in international databases (PubMed, MedLine, RINC, etc.) in order to obtain information on promising developments that open new possibilities for treating herpesvirus infection and subsequent analysis of the collected data made it possible to determine not only the main trends in the search for new antiviral agents, but also to provide information on the compounds most promising for the development of anti-herpesvirus drugs.

The B subunit of Escherichia coli enterotoxin helps control the in vivo growth of solid tumors expressing the Epstein-Barr virus latent membrane protein 2A

Latent membrane protein 2A (LMP2A) is expressed on almost all Epstein–Barr virus (EBV)-associated tumors and is a potential target for immunotherapeutic intervention and vaccination. However, LMP2A is not efficiently processed and presented on major histocompatibility antigens class I molecules to generate potent cytotoxic T-lymphocytes (CTL) responses capable of killing these tumors. The B subunit of Escherichia coli enterotoxin (EtxB), causes rapid internalization and processing of membrane-bound LMP2A on EBV-infected B cells, and facilitates loading of processed-LMP2A peptides onto MHC class I. This re-directed trafficking/delivery of LMP2A to the MHC class I machinery enhances recognition and killing by LMP2A-specific CTL in vitro. To test the potential of EtxB to enhance immune targeting of LMP2A expressed in solid tumors, we generated a murine tumor model (Renca-LMP2A), in which LMP2A is expressed as a transgenic neoantigen on a renal carcinoma (Renca) cell line and forms solid tumors when injected subcutaneously into BALB/c mice. The data show that in BALB/c mice which have only low levels of peripheral K^d-LMP2A-specific CD8⁺ T cells, merely a transient inhibition of tumor growth is achieved compared with naïve mice; suggesting that there is suboptimal LMP2A-specifc CTL recognition and poorly targeted tumor killing. However, importantly, treatment of these mice with EtxB led to a significant delay in the onset of tumor growth and significantly lower tumor volumes compared with similar mice that did not receive EtxB. Moreover, this remarkable effect of EtxB was achieved despite progressive reduction in tumor expression of LMP2A and MHC class I molecules. These data clearly demonstrate the potential efficacy of EtxB as a novel therapeutic agent that could render EBV-associated tumors susceptible to immune control.

Small molecule inhibitors of influenza A and B viruses that act by disrupting subunit interactions of the viral polymerase

Influenza viruses are the cause of yearly epidemics and occasional pandemics that represent a significant challenge to public health. Current control strategies are imperfect and there is an unmet need for new antiviral therapies. Here, we report the identification of small molecule compounds able to effectively and specifically inhibit growth of influenza A and B viruses in cultured cells through targeting an assembly interface of the viral RNA-dependent RNA polymerase. Using an existing crystal structure of the primary protein-protein interface between the PB1 and PA subunits of the influenza A virus polymerase, we conducted an in silico screen to identify potential small molecule inhibitors. Selected compounds were then screened for their ability to inhibit the interaction between PB1 and PA in vitro using an ELISA-based assay and in cells, to inhibit nuclear import of a binary PB1-PA complex as well as transcription by the full viral ribonucleoprotein complex. Two compounds emerged as effective inhibitors with IC(50) values in the low micromolar range and negligible cytotoxicity. Of these, one compound also acted as a potent replication inhibitor of a variety of influenza A virus strains in Madin-Darby canine kidney (MDCK) cells, including H3N2 and H1N1 seasonal and 2009 pandemic strains. Importantly, this included an oseltamivir-resistant isolate. Furthermore, potent inhibition of influenza B viruses but not other RNA or DNA viruses was seen. Overall, these compounds provide a foundation for the development of a new generation of therapeutic agents exhibiting high specificity to influenza A and B viruses.

Early inhibitors of human cytomegalovirus: state-of-art and therapeutic perspectives

Human cytomegalovirus (HCMV) infection is associated with severe morbidity and mortality in immunocompromised individuals, mainly transplant recipients and AIDS patients, and is the most frequent cause of congenital malformations in newborn children. To date, few drugs are licensed for the treatment of HCMV infections, most of which target the viral DNA polymerase and suffer from many drawbacks, including long-term toxicity, low potency, and poor bioavailability. In addition, the emergence of drug-resistant viral strains is becoming an increasing problem for disease management. Finally, none of the current anti-HCMV drugs have been approved for the treatment of congenital infections. For all these reasons, there is still a strong need for new anti-HCMV drugs with novel mechanisms of action. The first events of the virus replication cycle, including attachment, entry, immediate-early gene expression, and immediate-early functions—in particular that of Immediate-Early 2 protein—represent attractive targets for the development of novel antiviral compounds. Such inhibitors would block not only the expression of viral immediate-early proteins, which play a key role in the pathogenesis of HCMV infection, but also the host immunomodulation and the changes to cell physiology induced by the first events of virus infection. This review describes the current knowledge on the initial phases of HCMV replication, their validation as potential novel antiviral targets, and the development of compounds that block such processes.

Bacterial toxins as immunomodulators

Bacterial toxins are the causative agent at pathology in a variety of diseases. Although not always the primary target of these toxins, many have been shown to have potent immunomodulatory effects, for example, inducing immune responses to co-administered antigens and suppressing activation of immune cells. These abilities of bacterial toxins can be harnessed and used in a therapeutic manner, such as in vaccination or the treatment of autoimmune diseases. Furthermore, the ability of toxins to gain entry to cells can be used in novel bacterial toxin based immuno-therapies in order to deliver antigens into MHC Class I processing pathways. Whether the immunomodulatory properties of these toxins arose in order to enhance bacterial survival within hosts, to aid spread within the population or is pure serendipity, it is interesting to think that these same toxins potentially hold the key to preventing or treating human disease.

Evidence that the C-terminal PB2-binding region of the influenza A virus PB1 protein is a discrete alpha-helical domain

The influenza A virus RNA-dependent RNA polymerase is a heterotrimer composed of PB1, PB2 and PA subunits and essential for viral replication. However, little detailed structural information is available for this important enzyme. We show by circular dichroism spectroscopy that polypeptides from the C-terminus of PB1 that are capable of binding efficiently to PB2 fold into stable alpha-helical structures. Structure prediction analysis of this region of PB1 indicates that it likely consists of a three-helical bundle. Deletion of any of the helices abrogated transcriptional function. Thus, PB1 contains a C-terminal alpha-helical PB2-binding domain that is essential for nucleotide polymerization activity.

Disruption of the interactions between the subunits of herpesvirus DNA polymerases as a novel antiviral strategy

Most biological processes depend on the co-ordinated formation of protein-protein interactions. Besides their importance for virus replication, several interactions between virus proteins have been proposed as attractive targets for antiviral drug discovery, as the exquisite specificity of such cognate interactions affords the possibility of interfering with them in a highly specific and effective manner. There is a considerable need for new drugs active against herpesviruses, since available agents, most of which target the polymerisation activity of the virus DNA polymerase, are limited by pharmacokinetic issues, toxicity and antiviral resistance. A potential novel target for anti-herpesvirus drugs is the interaction between the two subunits of the virus DNA polymerase. This review focuses on recent developments using peptides and small molecules to inhibit protein-protein interactions between herpesvirus DNA polymerase subunits.

Disruption of protein-protein interactions: towards new targets for chemotherapy

Protein-protein interactions play a key role in various mechanisms of cellular growth and differentiation, and in the replication of pathogen organisms in host cells. Thus, inhibition of these interactions is a promising novel approach for rational drug design against a wide number of cellular and microbial targets. In the past few years, attempts to inhibit protein-protein interactions using antibodies, peptides, and synthetic or natural small molecules have met with varying degrees of success, and these will be the focus of this review.

Identification of a small molecule that inhibits herpes simplex virus DNA Polymerase subunit interactions and viral replication

The interaction between the catalytic subunit Pol and the processivity subunit UL42 of herpes simplex virus DNA polymerase has been characterized structurally and mutationally and is a potential target for novel antiviral drugs. We developed and validated an assay for small molecules that could disrupt the interaction of UL42 and a Pol-derived peptide and used it to screen approximately 16,000 compounds. Of 37 "hits" identified, four inhibited UL42-stimulated long-chain DNA synthesis by Pol in vitro, of which two exhibited little inhibition of polymerase activity by Pol alone. One of these specifically inhibited the physical interaction of Pol and UL42 and also inhibited viral replication at concentrations below those that caused cytotoxic effects. Thus, a small molecule can inhibit this protein-protein interaction, which provides a starting point for the discovery of new antiviral drugs.

Specific residues in the connector loop of the human cytomegalovirus DNA polymerase accessory protein UL44 are crucial for interaction with the UL54 catalytic subunit

The human cytomegalovirus DNA polymerase includes an accessory protein, UL44, which has been proposed to act as a processivity factor for the catalytic subunit, UL54. How UL44 interacts with UL54 has not yet been elucidated. The crystal structure of UL44 revealed the presence of a connector loop analogous to that of the processivity subunit of herpes simplex virus DNA polymerase, UL42, which is crucial for interaction with its cognate catalytic subunit, UL30. To investigate the role of the UL44 connector loop, we replaced each of its amino acids (amino acids 129 to 140) with alanine. We then tested the effect of each substitution on the UL44-UL54 interaction by glutathione S-transferase pulldown and isothermal titration calorimetry assays, on the stimulation of UL54-mediated long-chain DNA synthesis by UL44, and on the binding of UL44 to DNA-cellulose columns. Substitutions that affected residues 133 to 136 of the connector loop measurably impaired the UL44-UL54 interaction without altering the ability of UL44 to bind DNA. One substitution, I135A, completely disrupted the binding of UL44 to UL54 and inhibited the ability of UL44 to stimulate long-chain DNA synthesis by UL54. Thus, similar to the herpes simplex virus UL30-UL42 interaction, a residue of the connector loop of the accessory subunit is crucial for UL54-UL44 interaction. However, while alteration of a polar residue of the UL42 connector loop only partially reduced binding to UL30, substitution of a hydrophobic residue of UL44 completely disrupted the UL54-UL44 interaction. This information may aid the discovery of small-molecule inhibitors of the UL44-UL54 interaction.

Trafficking of exogenous peptides into proteasome-dependent major histocompatibility complex class I pathway following enterotoxin B subunit-mediated delivery

The B-subunit component of Escherichia coli heat-labile enterotoxin (EtxB), which binds to cell surface GM1 ganglioside receptors, was recently shown to be a highly effective vehicle for delivery of conjugated peptides into the major histocompatibility complex (MHC) class I pathway. In this study we have investigated the pathway of epitope delivery. The peptides used contained the epitope either located at the C terminus or with a C-terminal extension. Pretreatment of cells with cholesterol-disrupting agents blocked transport of EtxB conjugates to the Golgi/endoplasmic reticulum, but did not affect EtxB-mediated MHC class I presentation. Under these conditions, EtxB conjugates entered EEA1-positive early endosomes where peptides were cleaved and translocated into the cytosol. Endosome acidification was required for epitope presentation. Purified 20 S immunoproteasomes were able to generate the epitope from peptides in vitro, but 26 S proteasomes were not. Only presentation from the C-terminal extended peptide was proteasome-dependent in cells, and this was found to be significantly slower than presentation from peptides with the epitope at the C terminus. These results implicate the proteasome in the generation of the correct C terminus of the epitope and are consistent with proteasome-independent N-terminal trimming. Epitope presentation was blocked in a TAP-deficient cell line, providing further evidence that conjugated peptides enter the cytosol as well as demonstrating a requirement for the peptide transporter. Our findings demonstrate the utility of EtxB-mediated peptide delivery for rapid and efficient loading of MHC class I epitopes in several different cell types. Conjugated peptides are released from early endosomes into the cytosol where they gain access to proteasomes and TAP in the "classical" pathway of class I presentation.

Residues of human cytomegalovirus DNA polymerase catalytic subunit UL54 that are necessary and sufficient for interaction with the accessory protein UL44

The human cytomegalovirus DNA polymerase contains a catalytic subunit, UL54, and an accessory protein, UL44. Recent studies suggested that UL54 might interact via its extreme C terminus with UL44 (A. Loregian, R. Rigatti, M. Murphy, E. Schievano, G. Palu', and H. S. Marsden, J. Virol. 77:8336-8344, 2003). To address this hypothesis, we quantitatively measured the binding of peptides corresponding to the extreme C terminus of UL54 to UL44 by using isothermal titration calorimetry. A peptide corresponding to the last 22 residues of UL54 was sufficient to bind specifically to UL44 in a 1:1 complex with a dissociation constant of ca. 0.7 microM. To define individual residues in this segment that are crucial for interacting with UL44, we engineered a series of mutations in the C-terminal region of UL54. The UL54 mutants were tested for their ability to interact with UL44 by glutathione S-transferase pulldown assays, for basal DNA polymerase activity, and for long-chain DNA synthesis in the presence of UL44. We observed that deletion of the C-terminal segment or substitution of alanine for Leu1227 or Phe1231 in UL54 greatly impaired both the UL54-UL44 interaction in pulldown assays and long-chain DNA synthesis without affecting basal polymerase activity, identifying these residues as important for subunit interaction. Thus, like the herpes simplex virus UL30-UL42 interaction, a few specific side chains in the C terminus of UL54 are crucial for UL54-UL44 interaction. However, the UL54 residues important for interaction with UL44 are hydrophobic and not basic. This information might aid in the rational design of new drugs for the treatment of human cytomegalovirus infection.

The Sp1 transcription factor does not directly interact with the HIV-1 Tat protein

The role of Sp1 in regulating the trans-activating activity of the human immunodeficiency virus type 1 (HIV-1) Tat protein has not yet been clearly defined. In fact, studies on the physical and functional interaction between Sp1 and Tat have yielded contradictory results. Here we investigated whether a physical interaction between Sp1 and Tat indeed occurs, exploiting both biochemical and genetic techniques that allow detection of direct protein-protein interactions. Studies performed with the yeast two-hybrid system indicate that Sp1 does not directly interact with the HIV-1 Tat protein. Control experiments demonstrated that both proteins are functionally expressed in the yeast cells. In vitro binding assays further confirmed that Sp1 does not physically bind Tat. These data suggest that in vivo Tat and Sp1 most likely take part of a multicomponent complex and thus encourage the search of the molecule(s) which mediate Tat-Sp1 interaction.

Inhibition of human cytomegalovirus DNA polymerase by C-terminal peptides from the UL54 subunit

In common with other herpesviruses, the human cytomegalovirus (HCMV) DNA polymerase contains a catalytic subunit (Pol or UL54) and an accessory protein (UL44) that is thought to increase the processivity of the enzyme. The observation that antisense inhibition of UL44 synthesis in HCMV-infected cells strongly inhibits viral DNA replication, together with the structural similarity predicted for the herpesvirus processivity subunits, highlights the importance of the accessory protein for virus growth and raises the possibility that the UL54/UL44 interaction might be a valid target for antiviral drugs. To investigate this possibility, overlapping peptides spanning residues 1161 to 1242 of UL54 were synthesized and tested for inhibition of the interaction between purified UL54 and UL44 proteins. A peptide, LPRRLHLEPAFLPYSVKAHECC, corresponding to residues 1221 to 1242 at the very C terminus of UL54, disrupted both the physical interaction between the two proteins and specifically inhibited the stimulation of UL54 by UL44. A mutant peptide lacking the two carboxy-terminal cysteines was markedly less inhibitory, suggesting a role for these residues in the UL54/UL44 interaction. Circular dichroism spectroscopy indicated that the UL54 C-terminal peptide can adopt a partially alpha-helical structure. Taken together, these results indicate that the two subunits of HCMV DNA polymerase most likely interact in a way which is analogous to that of the two subunits of herpes simplex virus DNA polymerase, even though there is no sequence homology in the binding site, and suggest that the UL54 peptide, or derivatives thereof, could form the basis for developing a new class of anti-HCMV inhibitors that act by disrupting the UL54/UL44 interaction.

Membrane traffic exploited by protein toxins

A large number of protein toxins having enzymatically active A- and B-moieties that bind to cell surface receptors must be endocytosed before the A-moiety is translocated into the cytosol where it exerts its cytotoxic action. The accumulated information about the most well-studied toxins has provided a detailed picture of how they exploit the membrane trafficking systems of cells, and studies of toxin trafficking have revealed the existence of new pathways. The complexity of different endocytic mechanisms, as well as the multiple routes between endosomes and the Golgi apparatus and retrogradely to the endoplasmic reticulum (ER), are being unravelled by investigations of how toxins gain access to their targets. With increasing information about the internalization and intracellular trafficking of these opportunistic toxins, new avenues have been opened for their application in areas of medicine such as drug delivery and therapy.

Antiherpesvirus drugs: a promising spectrum of new drugs and drug targets

In the absence of effective vaccines to control herpesvirus infections, nucleosidic antiviral drugs have been the mainstay of clinical treatment since their development in the late 1970s. However, given the drawbacks of these drugs, including the increasing emergence of drug-resistant clinical isolates, new strategies for treating herpesvirus infections are warranted. A range of promising new drugs with novel molecular targets has been developed, but will they cure latent infections?

The B subunit of Escherichia coli heat-labile enterotoxin enhances CD8+ cytotoxic-T-lymphocyte killing of Epstein-Barr virus-infected cell lines

Epstein-Barr virus (EBV) is associated with a number of important human cancers, including nasopharyngeal carcinoma, gastric carcinoma, and Hodgkin's lymphoma. These tumors express a viral nuclear antigen, EBV nuclear antigen 1 (EBNA1), which cannot be presented to T cells in a major histocompatibility complex class I context, and the viral latent membrane proteins (LMPs). Although the LMPs are expressed in these tumors, no effective immune response is made. We report here that exposure to the cholera-like enterotoxin B subunit (EtxB) in EBV-infected lymphoblastoid cell lines (LCLs) enhances their susceptibility to killing by LMP-specific CD8(+) cytotoxic T lymphocytes (CTLs) in a HLA class I-restricted manner. CTL killing of LCLs is dramatically increased through both transporter-associated protein-dependent and -independent epitopes after EtxB treatment. The use of mutant B subunits revealed that the enhanced susceptibility of LCLs to CTL killing is dependent on the B subunit's interaction with GM(1) but not its signaling properties. These important findings could underpin the development of novel approaches to treating EBV-associated malignancies and may offer a general approach to increasing the presentation of other tumor and viral antigens.

Transport of protein toxins into cells: pathways used by ricin, cholera toxin and Shiga toxin

Ricin, cholera, and Shiga toxin belong to a family of protein toxins that enter the cytosol to exert their action. Since all three toxins are routed from the cell surface through the Golgi apparatus and to the endoplasmic reticulum (ER) before translocation to the cytosol, the toxins are used to study different endocytic pathways as well as the retrograde transport to the Golgi and the ER. The toxins can also be used as vectors to carry other proteins into the cells. Studies with protein toxins reveal that there are more pathways along the plasma membrane to ER route than originally believed.

Nuclear targeting of Porphyromonas gingivalis W50 protease in epithelial cells

Porphyromonas gingivalis is an important pathogen associated with destructive periodontal disease and is able to invade the epithelial cell barrier. Its cysteine proteases are recognized as major virulence factors, and in this study, we examined the interaction of the arginine-specific protease with epithelial cells in culture. Three cell lines (KB, HeLa, and SCC4) were incubated with strain W50 culture supernatant; stained with monoclonal antibody 1A1, which recognizes an epitope on the adhesin (beta) component of the cysteine protease-adhesin (alpha/beta) heterodimer; and viewed using immunofluorescence microscopy. Within 1 h, the protease traversed the plasma membrane and was localized around the nucleus before becoming concentrated in the cytoplasm after 24 to 48 h. In contrast, the purified arginine-specific heterodimeric protease (HRgpA) rapidly entered the nucleus within 15 to 30 min. This nuclear targeting (i) was seen with active and Nalpha-p-tosyl-L-lysine chloromethyl ketone (TLCK)-inactivated HRgpA, indicating it was independent of the proteolytic activity; (ii) occurred at both 4 and 37 degrees C; and (iii) failed to occur with the monomeric protease (RgpA(cat)), indicating the importance of the adhesin chain of the HRgpA protease to this process. Rapid cell entry was also observed with recombinant catalytic (alpha) and adhesin (beta) chains, with the latter again targeting the nuclear area. After 48 h of incubation with HRgpA, significant dose-dependent stimulation of metabolic activity was observed (measured by reduction of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide), and a doubling of mitotic activity combined with the presence of apoptotic cells indicated that HRgpA may interfere with cell cycle control mechanisms. These effects were seen with both active and TLCK-inactivated protease, confirming that they were not dependent on proteolytic activity, and thus provide new insights into the functioning of this P. gingivalis protease.

Protein-protein interactions as targets for antiviral chemotherapy

Most cellular and viral processes depend on the coordinated formation of protein-protein interactions. With a better understanding of the molecular biology and biochemistry of human viruses it has become possible to screen for and detect inhibitors with activity against specific viral functions and to develop new approaches for the treatment of viral infections. A novel strategy to inhibit viral replication is based on the disruption of viral protein-protein complexes by peptides that mimic either face of the interaction between subunits. Peptides and peptide mimetics capable of dissociating protein-protein interactions have such exquisite specificity that they hold great promise as the next generation of therapeutic agents. This review is focused on recent developments using peptides and small molecules to inhibit protein-protein interactions between cellular and/or viral proteins with comments on the practicalities of transforming chemical leads into derivatives with the characteristics desired of medicinal compounds.

Enhanced delivery of exogenous peptides into the class I antigen processing and presentation pathway

Current immunization strategies, using peptide or protein antigens, generally fail to elicit cytotoxic-T-lymphocyte responses, since these antigens are unable to access intracellular compartments where loading of major histocompatibility complex class I (MHC-I) molecules occurs. In an attempt to circumvent this, we investigated whether the GM1 receptor-binding B subunit of Escherichia coli heat-labile toxin (EtxB) could be used to deliver class I epitopes. When a class I epitope was conjugated to EtxB, it was delivered into the MHC-I presentation pathway in a GM1-binding-dependent fashion and resulted in the appearance of MHC-I-epitope complexes at the cell surface. Importantly, we show that the efficiency of EtxB-mediated epitope delivery could be strikingly enhanced by incorporating, adjacent to the class I epitope, a 10-amino-acid segment from the C terminus of the DNA polymerase (Pol) of herpes simplex virus. The replacement of this 10-amino-acid segment by a heterologous sequence or the introduction of specific amino acid substitutions within this segment either abolished or markedly reduced the efficiency of class I epitope delivery. If the epitope was extended at its C terminus, EtxB-mediated delivery into the class I presentation pathway was found to be completely dependent on proteasome activity. Thus, by combining the GM1-targeting function of EtxB with the 10-amino-acid Pol segment, highly efficient delivery of exogenous epitopes into the endogenous pathway of class I antigen processing and presentation can be achieved.

New insights into the structure-function relationships and therapeutic applications of cholera-like enterotoxins

Cholera toxin and E. coli heat-labile enterotoxin are structurally homologous proteins comprised of an enzymatically active A-subunit and five B-subunits that bind with high affinity to GM1-ganglioside receptors found on the surface of mammalian cells. The B-subunits have long been thought of simply as trafficking vehicles that trigger entry and subsequent delivery of the 'toxic' A-subunit into cells. Indeed, such is the capacity of the B-subunits to enter cells, that they have been developed as generic carriers for attachment and delivery of a variety of peptides into mammalian cells. However, the B-subunits also appear to possess discrete 'signalling functions', that induce both transcription factor and cell activation. These are thought to be directly responsible for the potent immunomodulatory properties of the B-subunits, and have resulted in their use as adjuvants and as agents to suppress inflammatory immune disorders. The relationship between the signalling properties of the B-subunits and their capacity to act as trafficking vehicles has remained unclear. In an effort to understand the structural requirements for these two functions, a set of mutant B-subunits, with amino acid substitutions at position His-57, have been generated and studied. Importantly, such mutant B-subunits retain an ability to bind with high affinity to GM1 and to traffic into cells, but have entirely lost their capacity to activate immune cell populations. Thus, while binding via GM1 appears to be sufficient to trigger cellular uptake it is not sufficient to activate signal transduction. The His-57 region is therefore speculated to be actively engaged in triggering signalling events, possibly via cognate interaction with other cell surface molecules.

The bacterial toxin toolkit

Pathogenic bacteria and higher eukaryotes have spent a long time together, leading to a precise understanding of one another's way of functioning. Through rapid evolution, bacteria have engineered increasingly sophisticated weapons to hit exactly where it hurts, interfering with fundamental host functions. However, toxins are not only useful to the bacteria - they have also become an essential asset for life scientists, who can now use them as toolkits to explore cellular processes.

Identification of crucial hydrogen-bonding residues for the interaction of herpes simplex virus DNA polymerase subunits via peptide display, mutational, and calorimetric approaches

The catalytic subunit, Pol, of herpes simplex virus DNA polymerase interacts via its extreme C terminus with the processivity subunit, UL42. This interaction is critical for viral replication and thus a potential target for antiviral drug action. To investigate the Pol-binding region on UL42, we engineered UL42 mutations but also used random peptide display to identify artificial ligands of the Pol C terminus. The latter approach selected ligands with homology to residues 171 to 176 of UL42. Substitution of glutamine 171 with alanine greatly impaired binding to Pol and stimulation of long-chain DNA synthesis by Pol, identifying this residue as crucial for subunit interactions. To study these interactions quantitatively, we used isothermal titration calorimetry and wild-type and mutant forms of Pol-derived peptides and UL42. Each of three peptides corresponding to either the last 36, 27, or 18 residues of Pol bound specifically to UL42 in a 1:1 complex with a dissociation constant of 1 to 2 microM. Thus, the last 18 residues suffice for most of the binding energy, which was due mainly to a change in enthalpy. Substitutions at positions corresponding to Pol residue 1228 or 1229 or at UL42 residue 171 abolished or greatly reduced binding. These residues participate in hydrogen bonds observed in the crystal structure of the C terminus of Pol bound to UL42. Thus, interruption of these few bonds is sufficient to disrupt the interaction, suggesting that small molecules targeting the relevant side chains could interfere with Pol-UL42 binding.

Entry of ricin and Shiga toxin into cells: molecular mechanisms and medical perspectives

A large number of plant and bacterial toxins with enzymatic activity on intracellular targets are now known. These toxins enter cells by first binding to cell surface receptors, then they are endocytosed and finally they become translocated into the cytosol from an intracellular compartment. In the case of the plant toxin ricin and the bacterial toxin Shiga toxin, this happens after retrograde transport through the Golgi apparatus and to the endoplasmic reticulum. The toxins are powerful tools to reveal new pathways in intracellular transport. Furthermore, knowledge about their action on cells can be used to combat infectious diseases where such toxins are involved, and a whole new field of research takes advantage of their ability to enter the cytosol for therapeutic purposes in connection with a variety of diseases. This review deals with the mechanisms of entry of ricin and Shiga toxin, and the attempts to use such toxins in medicine are discussed.

Lectin-mediated mucosal delivery of drugs and microparticles

Absorption of drugs and vaccines at mucosal surfaces may be enhanced by conjugation to appropriate bioadhesins which bind to mucosal epithelia. Bioadhesins might also permit cell- and site-selective targeting. One approach is to exploit surface carbohydrates on mucosal epithelial cells for lectin-mediated delivery. We review work supporting the use of lectins as mucosal bioadhesins in the gastrointestinal and respiratory tracts, the oral cavity and the eye. The gastrointestinal tract is particularly favoured for mucosal delivery. Many studies have demonstrated that the antigen sampling intestinal M cells offer a portal for absorption of colloidal delivery vehicles. Evidence is presented that M cell targeting may be achieved using M cell-specific lectins, microbial adhesins or immunoglobulins. While many hurdles must be overcome before mucosal bioadhesins can guarantee consistent, safe, effective mucosal delivery, this is an exciting area of research that has important implications for future drug and vaccine formulation.

Potential use of non-classical pathways for the transport of macromolecular drugs

Since an increasing number of drug delivery strategies utilising proteins and peptides exhibiting 'non-classical' transport activities have been proposed, studies have begun to establish underlying functional relationships between different vectors. These attempts to find common factors have been hampered by a lack of biophysical data for the various potential protein and peptide transporters, as well as by the structural and functional diversity of the group as a whole. We describe the various types of vectors being considered for use and the preliminary therapeutic successes that have been achieved. Additionally, the various models that have been proposed for non-classical import and export are outlined and discussed in relation to therapeutic delivery. Possible future developments are also discussed.

Construction of a multihybrid display system: flagellar filaments carrying two foreign adhesive peptides

A multivalent, bifunctional flagellum carrying two different adhesive peptides in separate flagellin subunits within a filament was constructed in Escherichia coli. The inserted peptides were the fibronectin-binding 115-mer D repeat region of Staphylococcus aureus and the 302-mer collagen-binding region of YadA of Yersinia enterocolitica. Western blotting, immunoelectron microscopy, and adhesion tests with hybrid flagella from an in trans-complemented DeltafliC E. coli strain showed that individual filaments consisted of both recombinant flagellins.

A rapid plate assay for the screening of inhibitors against herpesvirus DNA polymerases and processivity factors

Kaposi's sarcoma-associated herpesvirus (KSHV) is a newly identified human pathogen with tumorigenic potential. The DNA polymerase (Pol-8) and processivity factor (PF-8) of KSHV were cloned recently. It was shown that PF-8 forms specifically a complex with Pol-8 in vitro and allows it to synthesize fully-extended DNA. Since both Pol-8 and PF-8 are apparently essential for viral DNA replication and since they cannot be substituted by any other cellular or viral proteins, they are potentially excellent antiviral targets. The development of a mechanistic plate assay is now described, which is suitable for rapid high-throughput screening of antiviral agents against Pol-8 and PF-8. The assay allows the measurement of not only total DNA synthesis activity (i.e. nucleotide incorporation) but also processivity (i.e. fully-extended DNA product). In this plate assay, any of the screen-compounds with an inhibitory effect against the total DNA synthesis activity and/or the processivity could be potential antiviral agents that target Pol-8 and/or PF-8. Particularly, since PF-8 is highly specific for Pol-8, the discovery of inhibitory agents against PF-8 may lead to specific antiviral therapies with minimal toxicity to host cells. This assay should be suitable for screening for inhibitory compounds against polymerases and processivity factors of other herpesviruses as well.

The catalytic subunit of herpes simplex virus type 1 DNA polymerase contains a nuclear localization signal in the UL42-binding region

The herpes simplex virus type 1 DNA polymerase consists of a catalytic subunit (POL or UL30) and a processivity factor (UL42). The POL/UL42 interaction, which occurs through the extreme C-terminus of POL, is essential for HSV-1 replication and thus represents a valid target for drug inhibition. We recently showed (A. Loregian et al. (1999) Proc. Natl. Acad. Sci. USA 96, 5221-5226) that an oligopeptide corresponding to the 27 C-terminal amino acids of POL, when delivered into herpes simplex virus type 1-infected cells by a protein carrier, was able to localize into the nucleus and to inhibit viral replication by disruption of the POL/UL42 interaction. In this report, to further characterize the 27 mer (Pol peptide), we investigated whether its nuclear localization was due to the presence of a nuclear localization signal. By testing the ability of the Pol peptide to localize the beta-galactosidase, a normally cytoplasmic protein, to the nucleus, we confirmed that the Pol peptide contained a functional nuclear localization signal, corresponding to the RRMLHR motif. This sequence proved not only necessary but also sufficient for nuclear localization, because its substitution with a six-alanine stretch prevented nuclear translocation of the beta-galactosidase-Pol peptide fusion. Site-directed mutagenesis experiments on this revealed that both the three basic arginines and the two hydrophobic residues Met and Leu were crucial for nuclear targeting. Finally, functionally equivalent sequences were also found in the C-terminus of the catalytic subunits of human cytomegalovirus (RRLHL) and of equine herpesvirus-1 DNA polymerase (RRILH).

The crystal structure of an unusual processivity factor, herpes simplex virus UL42, bound to the C terminus of its cognate polymerase

Herpes simplex virus DNA polymerase is a heterodimer composed of a catalytic subunit, Pol, and an unusual processivity subunit, UL42, which, unlike processivity factors such as PCNA, directly binds DNA. The crystal structure of a complex of the C-terminal 36 residues of Pol bound to residues 1-319 of UL42 reveals remarkable similarities between UL42 and PCNA despite contrasting biochemical properties and lack of sequence homology. Moreover, the Pol-UL42 interaction resembles the interaction between the cell cycle regulator p21 and PCNA. The structure and previous data suggest that the UL42 monomer interacts with DNA quite differently than does multimeric toroidal PCNA. The details of the structure lead to a model for the mechanism of UL42, provide the basis for drug design, and allow modeling of other proteins that lack sequence homology with UL42 or PCNA.