Disruption of the Golgi Apparatus and Contribution of the Endoplasmic Reticulum to the SARS-CoV-2 Replication Complex

A variety of immunolabeling procedures for both light and electron microscopy were used to examine the cellular origins of the host membranes supporting the SARS-CoV-2 replication complex. The endoplasmic reticulum has long been implicated as a source of membrane for the coronavirus replication organelle. Using dsRNA as a marker for sites of viral RNA synthesis, we provide additional evidence supporting ER as a prominent source of membrane. In addition, we observed a rapid fragmentation of the Golgi apparatus which is visible by 6 h and complete by 12 h post-infection. Golgi derived lipid appears to be incorporated into the replication organelle although protein markers are dispersed throughout the infected cell. The mechanism of Golgi disruption is undefined, but chemical disruption of the Golgi apparatus by brefeldin A is inhibitory to viral replication. A search for an individual SARS-CoV-2 protein responsible for this activity identified at least five viral proteins, M, S, E, Orf6, and nsp3, that induced Golgi fragmentation when expressed in eukaryotic cells. Each of these proteins, as well as nsp4, also caused visible changes to ER structure as shown by correlative light and electron microscopy (CLEM). Collectively, these results imply that specific disruption of the Golgi apparatus is a critical component of coronavirus replication.

A pocket guide on how to structure SARS-CoV-2 drugs and therapies

The race to identify a successful treatment for COVID19 will be defined by fundamental research into the replication cycle of the SARS-CoV-2 virus. This has identified five distinct stages from which numerous vaccination and clinical trials have emerged alongside an innumerable number of drug discovery studies currently in development for disease intervention. Informing every step of the viral replication cycle has been an unprecedented 'call-to-arms' by the global structural biology community. Of the 20 main SARS-CoV-2 proteins, 13 have been resolved structurally for SARS-CoV-2 with most having a related SARS-CoV and MERS-CoV structural homologue totalling some 300 structures currently available in public repositories. Herein, we review the contribution of structural studies to our understanding of the virus and their role in structure-based development of therapeutics.

Immune Responses During Human Coronavirus Infection: Suggestions for Future Studies

Severe Acute Respiratory human Coronavirus 2 (SARS-hCOV 2) infection which began in December 2019 has rapidly disseminated worldwide due to non-availability of anti-viral treatment or vaccine, no knowledge of virus-human interaction, lack of prognostic factors for stages of illness and ability of hCoV 2 to rapidly mutate and infect multiple cell types. Host inflammation and evasion of host immune responses by viruses are believed to play major roles in disease severity of human Corona viruses (hCoVs), thus uses of anti-inflammatory and immune-boosting agents apart from complete multi-disciplinary approach are suggested to combat the ranvaging SAR-hCOV 2 infection. This paper related the structural proteins and life cycle of CoV with host immune responses to CoV. This is to bring out gaps in knowledge for possible future researches.

Emerging coronaviruses: Genome structure, replication, and pathogenesis

The recent emergence of a novel coronavirus (2019-nCoV), which is causing an outbreak of unusual viral pneumonia in patients in Wuhan, a central city in China, is another warning of the risk of CoVs posed to public health. In this minireview, we provide a brief introduction of the general features of CoVs and describe diseases caused by different CoVs in humans and animals. This review will help understand the biology and potential risk of CoVs that exist in richness in wildlife such as bats.

The pUL37 tegument protein guides alpha-herpesvirus retrograde axonal transport to promote neuroinvasion

A hallmark property of the neurotropic alpha-herpesvirinae is the dissemination of infection to sensory and autonomic ganglia of the peripheral nervous system following an initial exposure at mucosal surfaces. The peripheral ganglia serve as the latent virus reservoir and the source of recurrent infections such as cold sores (herpes simplex virus type I) and shingles (varicella zoster virus). However, the means by which these viruses routinely invade the nervous system is not fully understood. We report that an internal virion component, the pUL37 tegument protein, has a surface region that is an essential neuroinvasion effector. Mutation of this region rendered herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PRV) incapable of spreading by retrograde axonal transport to peripheral ganglia both in culture and animals. By monitoring the axonal transport of individual viral particles by time-lapse fluorescence microscopy, the mutant viruses were determined to lack the characteristic sustained intracellular capsid motion along microtubules that normally traffics capsids to the neural soma. Consistent with the axonal transport deficit, the mutant viruses did not reach sites of latency in peripheral ganglia, and were avirulent. Despite this, viral propagation in peripheral tissues and in cultured epithelial cell lines remained robust. Selective elimination of retrograde delivery to the nervous system has long been sought after as a means to develop vaccines against these ubiquitous, and sometimes devastating viruses. In support of this potential, we find that HSV-1 and PRV mutated in the effector region of pUL37 evoked effective vaccination against subsequent nervous system challenges and encephalitic disease. These findings demonstrate that retrograde axonal transport of the herpesviruses occurs by a virus-directed mechanism that operates by coordinating opposing microtubule motors to favor sustained retrograde delivery of the virus to the peripheral ganglia. The ability to selectively eliminate the retrograde axonal transport mechanism from these viruses will be useful in trans-synaptic mapping studies of the mammalian nervous system, and affords a new vaccination paradigm for human and veterinary neurotropic herpesviruses.

Inner tegument proteins of Herpes Simplex Virus are sufficient for intracellular capsid motility in neurons but not for axonal targeting

Upon reactivation from latency and during lytic infections in neurons, alphaherpesviruses assemble cytosolic capsids, capsids associated with enveloping membranes, and transport vesicles harboring fully enveloped capsids. It is debated whether capsid envelopment of herpes simplex virus (HSV) is completed in the soma prior to axonal targeting or later, and whether the mechanisms are the same in neurons derived from embryos or from adult hosts. We used HSV mutants impaired in capsid envelopment to test whether the inner tegument proteins pUL36 or pUL37 necessary for microtubule-mediated capsid transport were sufficient for axonal capsid targeting in neurons derived from the dorsal root ganglia of adult mice. Such neurons were infected with HSV1-ΔUL20 whose capsids recruited pUL36 and pUL37, with HSV1-ΔUL37 whose capsids associate only with pUL36, or with HSV1-ΔUL36 that assembles capsids lacking both proteins. While capsids of HSV1-ΔUL20 were actively transported along microtubules in epithelial cells and in the somata of neurons, those of HSV1-ΔUL36 and -ΔUL37 could only diffuse in the cytoplasm. Employing a novel image analysis algorithm to quantify capsid targeting to axons, we show that only a few capsids of HSV1-ΔUL20 entered axons, while vesicles transporting gD utilized axonal transport efficiently and independently of pUL36, pUL37, or pUL20. Our data indicate that capsid motility in the somata of neurons mediated by pUL36 and pUL37 does not suffice for targeting capsids to axons, and suggest that capsid envelopment needs to be completed in the soma prior to targeting of herpes simplex virus to the axons, and to spreading from neurons to neighboring cells.

Lettuce necrotic yellows cytorhabdovirus protein localization and interaction map, and comparison with nucleorhabdoviruses

Lettuce necrotic yellows virus (LNYV), Sonchus yellow net virus (SYNV) and Potato yellow dwarf virus (PYDV) are members of the family Rhabdoviridae that infect plants. LNYV is a cytorhabdovirus that replicates in the cytoplasm, while SYNV and PYDV are nucleorhabdoviruses that replicate in the nuclei of infected cells. LNYV and SYNV share a similar genome organization with a gene order of nucleoprotein (N), phosphoprotein (P), putative movement protein (Mv), matrix protein (M), glycoprotein (G) and polymerase (L). PYDV contains an additional predicted gene of unknown function located between N and P. In order to gain insight into the associations of viral structural and non-structural proteins and the mechanisms by which they may function, we constructed protein localization and interaction maps. Subcellular localization was determined by transiently expressing the viral proteins fused to green or red fluorescent protein in leaf epidermal cells of Nicotiana benthamiana. Protein interactions were tested in planta by using bimolecular fluorescence complementation. All three viruses showed Mv to be localized at the cell periphery and the G protein to be membrane associated. Comparing the interaction maps revealed that only the N-P and M-M interactions are common to all three viruses. Associations unique to only one virus include P-M for LNYV, G-Mv for SYNV and M-Mv, M-G and N-M for PYDV. The cognate N-P proteins of all three viruses interacted and exhibited characteristic changes in localization when co-expressed.

Bovine herpesvirus type 1 (BHV-1) UL49.5 luminal domain residues 30 to 32 are critical for MHC-I down-regulation in virus-infected cells

Bovine herpesvirus type 1 (BHV-1) U_L49.5 inhibits transporter associated with antigen processing (TAP) and down-regulates cell-surface expression of major histocompatibility complex (MHC) class I molecules to promote immune evasion. We have constructed a BHV-1 U_L49.5 cytoplasmic tail (CT) null and several U_L49.5 luminal domain mutants in the backbone of wild-type BHV-1 or BHV-1 U_L49.5 CT- null viruses and determined their relative TAP mediated peptide transport inhibition and MHC-1 down-regulation properties compared with BHV-1 wt. Based on our results, the U_L49.5 luminal domain residues 30–32 and U_L49.5 CT residues, together, promote efficient TAP inhibition and MHC-I down-regulation functions. In vitro, BHV-1 U_L49.5 Δ30–32 CT-null virus growth property was similar to that of BHV-1 wt and like the wt U_L49.5, the mutant U_L49.5 was incorporated in the virion envelope and it formed a complex with gM in the infected cells.

Identification and functional characterization of the Varicella zoster virus ORF11 gene product

The deletion of ORF11 severely impaired VZV infection of human skin xenografts. Here, we investigate the characteristics and functions of the ORF11 gene product. ORF11 is expressed as a 118kDa polypeptide in VZV-infected cells; the protein is present in the nucleus and cytoplasm and is incorporated into VZ virions. Although ORF11 had little effect in transactivating VZV gene promoters in transfection assays, deleting ORF11 from the virus was associated with reduced expression of immediate early proteins IE4, IE62 and IE63, and the major glycoprotein, gE. ORF11 was identified as an RNA binding protein and its RNA binding domain was defined. However, disrupting the ORF11 RNA binding domain did not affect skin infection, indicating that RNA binding capacity, conserved among the alphaherpesviruses homologues, is not essential while the contribution of ORF11 to the expression of the IE proteins and gE may be required for VZV pathogenesis in skin in vivo.

The filamentous phages fd and IF1 use different mechanisms to infect Escherichia coli

The filamentous phage fd uses its gene 3 protein (G3P) to target Escherichia coli cells in a two-step process. First, the N2 domain of G3P attaches to an F pilus, and then the N1 domain binds to TolA-C. N1 and N2 are tightly associated, rendering the phage robust but noninfectious because the binding site for TolA-C is buried at the domain interface. Binding of N2 to the F pilus initiates partial unfolding, domain disassembly, and prolyl cis-to-trans isomerization in the hinge between N1 and N2. This activates the phage, and trans-Pro213 maintains this state long enough for N1 to reach TolA-C. Phage IF1 targets I pili, and its G3P contains also an N1 domain and an N2 domain. The pilus-binding N2 domains of the phages IF1 and fd are unrelated, and the N1 domains share a 31% sequence identity. We show that N2 of phage IF1 mediates binding to the I pilus, and that N1 targets TolA. Crystallographic and NMR analyses of the complex between N1 and TolA-C indicate that phage IF1 interacts with the same site on TolA-C as phage fd. In IF1-G3P, N1 and N2 are independently folding units, however, and the TolA binding site on N1 is permanently accessible. Activation by unfolding and prolyl isomerization, as in the case of phage fd, is not observed. In IF1-G3P, the absence of stabilizing domain interactions is compensated for by a strong increase in the stabilities of the individual domains. Apparently, these closely related filamentous phages evolved different mechanisms to reconcile robustness with high infectivity.

Inner tegument protein pUL37 of herpes simplex virus type 1 is involved in directing capsids to the trans-Golgi network for envelopment

Secondary envelopment of herpes simplex virus type 1 has been demonstrated as taking place at the trans-Golgi network (TGN). The inner tegument proteins pUL36 and pUL37 and the envelope glycoproteins gD and gE are known to be important for secondary envelopment. We compared the cellular localizations of capsids from a virus mutant lacking the UL37 gene with those of a virus mutant lacking the genes encoding gD and gE. Although wild-type capsids accumulated at the TGN, capsids of the pUL37⁻ mutant were distributed throughout the cytoplasm and showed no association with TGN-derived vesicles. This was in contrast to capsids from a gD⁻gE⁻ mutant, which accumulated in the vicinity of TGN vesicles, but did not colocalize with them, suggesting that they were transported to the TGN but were unable to undergo envelopment. We conclude that the inner tegument protein pUL37 is required for directing capsids to the TGN, where secondary envelopment occurs.

A herpesvirus encoded deubiquitinase is a novel neuroinvasive determinant

The neuroinvasive property of several alpha-herpesviruses underlies an uncommon infectious process that includes the establishment of life-long latent infections in sensory neurons of the peripheral nervous system. Several herpesvirus proteins are required for replication and dissemination within the nervous system, indicating that exploiting the nervous system as a niche for productive infection requires a specialized set of functions encoded by the virus. Whether initial entry into the nervous system from peripheral tissues also requires specialized viral functions is not known. Here we show that a conserved deubiquitinase domain embedded within a pseudorabies virus structural protein, pUL36, is essential for initial neural invasion, but is subsequently dispensable for transmission within and between neurons of the mammalian nervous system. These findings indicate that the deubiquitinase contributes to neurovirulence by participating in a previously unrecognized initial step in neuroinvasion.

Subsets of herpesviruses, such as herpes simplex virus and pseudorabies virus, are neuroinvasive pathogens. Upon infection, these viruses efficiently target peripheral nervous system tissue and establish a life-long infection for which there is no cure. Very few pathogens are known that invade the nervous system proficiently, and the mechanism by which herpesviruses achieve neuroinvasion is largely unknown. In this study, we demonstrate that a viral protease plays a critical and specific role allowing the virus to cross the threshold of the nervous system, but is dispensable for subsequent replication and encephalitic spread within the brain.

Targeting of pseudorabies virus structural proteins to axons requires association of the viral Us9 protein with lipid rafts

The pseudorabies virus (PRV) Us9 protein plays a central role in targeting viral capsids and glycoproteins to axons of dissociated sympathetic neurons. As a result, Us9 null mutants are defective in anterograde transmission of infection in vivo. However, it is unclear how Us9 promotes axonal sorting of so many viral proteins. It is known that the glycoproteins gB, gC, gD and gE are associated with lipid raft microdomains on the surface of infected swine kidney cells and monocytes, and are directed into the axon in a Us9-dependent manner. In this report, we determined that Us9 is associated with lipid rafts, and that this association is critical to Us9-mediated sorting of viral structural proteins. We used infected non-polarized and polarized PC12 cells, a rat pheochromocytoma cell line that acquires many of the characteristics of sympathetic neurons in the presence of nerve growth factor (NGF). In these cells, Us9 is highly enriched in detergent-resistant membranes (DRMs). Moreover, reducing the affinity of Us9 for lipid rafts inhibited anterograde transmission of infection from sympathetic neurons to epithelial cells in vitro. We conclude that association of Us9 with lipid rafts is key for efficient targeting of structural proteins to axons and, as a consequence, for directional spread of PRV from pre-synaptic to post-synaptic neurons and cells of the mammalian nervous system.

Structure and functions of hepatitis C virus proteins: 15 years after

Since its discovery in 1988, the hepatitis C virus (HCV) has become a hot topic of research by many groups around the world. This globally spread infectious agent is responsible for a large proportion of chronic viral hepatitides. The clue to halting the hepatitis C pandemic may be the detailed understanding of the virus structure, its replication mechanism, and the exact functions of the various proteins. Such understanding could enable the development of new antivirals targeted against hepatitis C virus and possibly an effective vaccine. This review recaps the current knowledge about the HCV genome 15 years after its discovery. The structure and function of particular viral structural (core, E1, E2) and nonstructural (NS2, NS3, NS4, NS5) proteins and noncoding regions known to date are described. With respect to frequent conflicting reports from different research groups, results reproducibly demonstrated by independent investigators are emphasized. Owing to many obstacles and limitations inherent in doing research on this noteworthy virus, the current knowledge is incomplete and the answers to many important questions are to be expected in the future.

A self-excisable infectious bacterial artificial chromosome clone of varicella-zoster virus allows analysis of the essential tegument protein encoded by ORF9

In order to facilitate the generation of mutant viruses of varicella-zoster virus (VZV), the agent causing varicella (chicken pox) and herpes zoster (shingles), we generated a full-length infectious bacterial artificial chromosome (BAC) clone of the P-Oka strain. First, mini-F sequences were inserted into a preexisting VZV cosmid, and the SuperCos replicon was removed. Subsequently, mini-F-containing recombinant virus was generated from overlapping cosmid clones, and full-length VZV DNA recovered from the recombinant virus was established in Escherichia coli as an infectious BAC. An inverted duplication of VZV genomic sequences within the mini-F replicon resulted in markerless excision of vector sequences upon virus reconstitution in eukaryotic cells. Using the novel tool, the role in VZV replication of the major tegument protein encoded by ORF9 was investigated. A markerless point mutation introduced in the start codon by two-step en passant Red mutagenesis abrogated ORF9 expression and resulted in a dramatic growth defect that was not observed in a revertant virus. The essential nature of ORF9 for VZV replication was ultimately confirmed by restoration of the growth of the ORF9-deficient mutant virus using trans-complementation via baculovirus-mediated gene transfer.

Eliminating the Requirement of an Essential Gene Product in an Already Very Small Virus: Scaffolding Protein B-free øX174, B-free

Unlike most viral assembly systems, two scaffolding proteins, B and D, mediate bacteriophage øX174 morphogenesis. The external scaffolding protein D is highly ordered in the atomic structure and proper function is very sensitive to mutation. In contrast, the internal scaffolding protein B is relatively unordered and extensive alterations do not eliminate function. Despite this genetic laxity, protein B is absolutely required for virus assembly. Thus, this system, with its complex arrangements of overlapping reading frames, can be regarded as an example of "irreducible complexity." To address the biochemical functions of a dual scaffolding protein system and the evolution of complexity, progressive and targeted genetic selections were employed to lessen and finally eliminate B protein-dependence. The biochemical and genetic bases of adaptation were characterized throughout the analysis that led to the sextuple mutant with a B-independent phenotype, as evaluated by plaque formation in wild-type cells. The primary adaptation appears to be the over-expression of a mutant external scaffolding protein. Progeny production was followed in lysis-resistant cells. The ability to produce infectious virions does not require all six mutations. However, the lag phase before progeny production is shortened as mutations accumulate. The results suggest that the primary function of the internal scaffolding protein may be to lower the critical concentration of the external scaffolding protein needed to nucleate procapsid formation. Moreover, they demonstrate a novel mechanism by which a stringently required gene product can be bypassed, even in a system encoding only eight strictly essential proteins.

Incorporation of the herpes simplex virus type 1 tegument protein VP22 into the virus particle is independent of interaction with VP16

Herpes simplex virus type 1 (HSV-1) virions contain a proteinaceous layer termed the tegument that lies between the nucleocapsid and viral envelope. The mechanisms underlying tegumentation remain largely undefined for all herpesviruses. Using glutathione S-transferase (GST) pulldowns and coimmunoprecipitation studies, we have identified a domain of the tegument protein VP22 that facilitates interaction with VP16. This region of VP22 (residues 165-225) overlaps the glycoprotein E (gE) binding domain of VP22 (residues 165-270), which is sufficient to mediate VP22 packaging into assembling virus particles. To ascertain the contribution of the VP16 and gE binding activities of VP22 to its virion incorporation, a transfection/infection based virion incorporation assay, using point mutants that discern between the two binding activities, was utilized. Our results suggest that interaction with VP16 is not required for incorporation of VP22 into virus particles and that binding to the cytoplasmic tail of gE is sufficient to facilitate packaging.

Coat protein-mediated resistance to TMV infection of Nicotiana tabacum involves multiple modes of interference by coat protein

Expression of tobacco mosaic virus (TMV) coat protein (CP) restricts virus disassembly and alters the accumulation of the movement protein (MP). To characterize the role of structure of transgenic CP in regulating virus disassembly and production of MP, we generated CPs with mutations at residues Glu50 and Asp77, located in the interface between juxtaposed CP subunits. In transgenic Nicotiana tabacum and BY-2 cells, three categories of coat protein-mediated resistance (CP-MR) levels were identified: wild-type CP-MR; elevated CP-MR; and no CP-MR. Mutant CPs that interfered with the accumulation of virus replication complexes conferred very high levels of protection to TMV, except by CP(E50D) which provided no protection in the systemic host (Xanthi-nn) but high CP-MR in the local lesion host (Xanthi-NN). In transgenic BY-2 cells CP(E50D) strongly reduced accumulation of MP:GFP. In general, there was a strong correlation between the capacity for CP to assemble to pseudovirions and CP-MR, while there was not strong correlation with packaging viral RNA and CP-MR. The data demonstrate that interference with one or more steps in virus infection and replication by wild type and mutant CP determine the degree of CP-MR.

Analysis of the selective advantage conferred by a C-E1 fusion protein synthesized by rubella virus DI RNAs

During serial passaging of rubella virus (RUB) in cell culture, the dominant species of defective-interfering RNA (DI) generated contains an in-frame deletion between the capsid protein (C) gene and E1 glycoprotein gene resulting in production of a C-E1 fusion protein that is necessary for the maintenance of the DI [Tzeng, W.P., Frey, T.K. (2006). C-E1 fusion protein synthesized by rubella virus DI RNAs maintained during serial passage. Virology 356 198-207.]. A BHK cell line stably expressing the RUB structural proteins was established which was used to package DIs into virus particles following transfection with in vitro transcripts from DI infectious cDNA constructs. Packaging of a DI encoding an in-frame C-GFP-E1 reporter fusion protein corresponding to the C-E1 fusion protein expressed in a native DI was only marginally more efficient than packaging of a DI encoding GFP, indicating that the C-E1 fusion protein did not function by enhancing packaging. However, infection with the DI encoding the C-GFP-E1 fusion protein (in the absence of wt RUB helper virus) resulted in formation of clusters of GFP-positive cells and the percentage of GFP-positive cells in the culture following infection remained relatively constant. In contrast, a DI encoding GFP did not form GFP-positive clusters and the percentage of GFP-positive cells declined by roughly half from 2 to 4 days post-infection. Cluster formation and sustaining the percentage of infected (GFP-positive) cells required the C part of the fusion protein, including the downstream but not the upstream of two arginine clusters (both of which are associated with RNA binding and association with mitochondrial p32 protein) and the E1 part through the transmembrane sequence, but not the C-terminal cytoplasmic tail. Among a collection of mutant DI constructs, cluster formation and sustaining infected cell percentage correlated with maintenance during serial passage with wt RUB. We hypothesize that cluster formation and sustaining infected cell percentage increase the likelihood of co-infection by a DI and wt RUB during serial passage thus enhancing maintenance of the DI. Cluster formation and sustaining infected cell percentage were found to be due to a combination of attenuated cytopathogenicity of DIs that express the C-E1 fusion protein and cell-to-cell movement of the DI. In infected cells, the C-GFP-E1 fusion protein was localized to potentially novel vesicular structures that appear to originate from ER-Golgi transport vacuoles. This species of DI expressing a C-E1 fusion protein that exhibits attenuated cytopathogenicity and the ability to increase the number of infected cells through cell-to-cell movement could be the basis for development of an attractive vaccine vector.

Replacement of the respiratory syncytial virus nonstructural proteins NS1 and NS2 by the V protein of parainfluenza virus 5

Paramyxoviruses have been shown to produce proteins that inhibit interferon production and signaling. For human respiratory syncytial virus (RSV), the nonstructural NS1 and NS2 proteins have been shown to have interferon antagonist activity through an unknown mechanism. To understand further the functions of NS1 and NS2, we generated recombinant RSV in which both NS1 and NS2 were replaced by the PIV5 V protein, which has well-characterized IFN antagonist activities (DeltaNS1/2-V). Expression of V was able to partially inhibit IFN responses in DeltaNS1/2-V-infected cells. In addition, the replication kinetics of DeltaNS1/2-V were intermediate between DeltaNS1/2 and wild-type (rA2) in A549 cells. However, expression of V did not affect the ability of DeltaNS1/2-V to activate IRF3 nuclear translocation and IFNbeta transcription. These data indicate that V was able to replace some of the IFN inhibitory functions of the RSV NS1 and NS2 proteins, but also that NS1 and NS2 have functions in viral replication beyond IFN antagonism.

Borna disease virus P protein inhibits nitric oxide synthase gene expression in astrocytes

Borna disease virus (BDV) is one of the potential infectious agents involved in the development of central nervous system (CNS) diseases. Neurons and astrocytes are the main targets of BDV infection, but little is known about the roles of BDV infection in the biological effects of astrocytes. Here we reported that BDV inhibits the activation of inducible nitric oxide synthase (iNOS) in murine astrocytes induced by bacterial LPS and PMA. To determine which protein of BDV is responsible for the regulation of iNOS expression, we co-transfected murine astrocytes with reporter plasmid iNOS-luciferase and plasmid expressing individual BDV proteins. Results from analyses of reporter activities revealed that only the phosphoprotein (P) of BDV had an inhibitory effect on the activation of iNOS. In addition, P protein inhibits nitric oxide production through regulating iNOS expression. We also reported that the nuclear factor kappa B (NF-kappaB) binding element, AP-1 recognition site, and interferon-stimulated response element (ISRE) on the iNOS promoter were involved in the repression of iNOS gene expression regulated by the P protein. Functional analysis indicated that sequences from amino acids 134 to 174 of the P protein are necessary for the regulation of iNOS. These data suggested that BDV may suppress signal transduction pathways, which resulted in the inhibition of iNOS activation in astrocytes.

Sensitivity of rabies virus to type I interferon is determined by the phosphoprotein gene

The growth of a virulent strain of fixed rabies virus, Nishigahara, in mouse neuroblastoma NA cells treated with type I interferon (IFN) was compared with that of a derivative avirulent strain, Ni-CE. Nishigahara strain was slightly sensitive to IFN treatment but still grew more efficiently than did Ni-CE strain in IFN-treated NA cells. Furthermore, a virulent chimeric virus with the phosphoprotein gene from Nishigahara strain in the Ni-CE genome was less sensitive to IFN treatment than was Ni-CE strain, indicating that the IFN sensitivity is determined by the phosphoprotein gene of the virus.

Functional characterization of the major and minor phosphorylation sites of the P protein of Borna disease virus

The phosphoprotein P of Borna disease virus (BDV) is an essential cofactor of the viral RNA-dependent RNA polymerase. It is preferentially phosphorylated at serine residues 26 and 28 by protein kinase C epsilon (PKCepsilon) and, to a lesser extent, at serine residues 70 and 86 by casein kinase II (CKII). To determine whether P phosphorylation is required for viral polymerase activity, we generated P mutants lacking either the PKCepsilon or the CKII phosphate acceptor sites by replacing the corresponding serine residues with alanine (A). Alternatively, these sites were replaced by aspartic acid (D) to mimic phosphorylation. Functional characterization of the various mutants in the BDV minireplicon assay revealed that D substitutions at the CKII sites inhibited the polymerase-supporting activity of P, while A substitutions maintained wild-type activity. Likewise, D substitutions at the PKC sites did not impair the cofactor function of BDV-P, whereas A substitutions at these sites led to increased activity. Interestingly, recombinant viruses could be rescued only when P mutants with modified PKCepsilon sites were used but not when both CKII sites were altered. PKCepsilon mutant viruses showed a reduced capacity to spread in cell culture, while viral RNA and protein expression levels in persistently infected cells were almost normal. Further mutational analyses revealed that substitutions at individual CKII sites were, with the exception of a substitution of A for S86, detrimental for viral rescue. These data demonstrate that, in contrast to other viral P proteins, the cofactor activity of BDV-P is negatively regulated by phosphorylation.

The nucleocytoplasmic rabies virus P protein counteracts interferon signaling by inhibiting both nuclear accumulation and DNA binding of STAT1

Rabies virus P protein inhibits alpha interferon (IFN-alpha)- and IFN-gamma-stimulated Jak-STAT signaling by retaining phosphorylated STAT1 in the cytoplasm. Here, we show that P also blocks an intranuclear step that is the STAT1 binding to the DNA promoter of IFN-responsive genes. As P is a nucleocytoplasmic shuttling protein, we first investigated the effect of the cellular distribution of P on the localization of STAT1 and consequently on IFN signaling. We show that the localization of STAT1 is correlated with the localization of P: in cells expressing a nuclear form of P (the short P3 isoform or the complete P in the presence of the export inhibitor leptomycin B), STAT1 is nuclear, whereas in cells expressing a cytoplasmic form of P, STAT1 is cytoplasmic. However, the expression of nuclear forms of P inhibits the signaling of both IFN-gamma and IFN-alpha, demonstrating that the retention of STAT1 in the cytoplasm is not the only mechanism involved in the inhibition of IFN signaling. Electrophoretic mobility shift analysis indicates that P expression in the cell extracts of infected cells or in stable cell lines prevents IFN-induced DNA binding of STAT1. The loss of the DNA binding of STAT1 and ISGF3 was also observed when purified recombinant P or P3 was added to the extracts of IFN-gamma- or IFN-alpha-treated cells, indicating that P directly affects the DNA binding activity of STAT1. Then products of the rabies virus P gene are able to counteract IFN signaling by creating both cytoplasmic and nuclear blocks for STAT1.

Mutation in the glycosylated gag protein of murine leukemia virus results in reduced in vivo infectivity and a novel defect in viral budding or release

All gammaretroviruses, including murine leukemia viruses (MuLVs), feline leukemia viruses, and gibbon-ape leukemia virus, encode an alternate, glycosylated form of Gag polyprotein (glyco-Gag or gPr80gag) in addition to the polyprotein precursor of the viral capsid proteins (Pr65gag). gPr80gag is translated from an upstream in-frame CUG initiation codon, in contrast to the AUG codon used for Pr65gag. The role of glyco-Gag in MuLV replication has been unclear, since gPr80gag-negative Moloney MuLV (M-MuLV) mutants are replication competent in vitro and pathogenic in vivo. However, reversion to the wild type is frequently observed in vivo. In these experiments, in vivo inoculation of a gPr80gag mutant, Ab-X-M-MuLV, showed substantially lower (2 log) initial infectivity in newborn NIH Swiss mice than that of wild-type virus, and revertants to the wild type could be detected by PCR cloning and DNA sequencing as early as 15 days postinfection. Atomic force microscopy of Ab-X-M-MuLV-infected producer cells or of the PA317 amphotropic MuLV-based vector packaging line (also gPr80gag negative) revealed the presence of tube-like viral structures on the cell surface. In contrast, wild-type virus-infected cells showed the typical spherical, 145-nm particles observed previously. Expression of gPr80gag in PA317 cells converted the tube-like structures to typical spherical particles. PA317 cells expressing gPr80gag produced 5- to 10-fold more infectious vector or viral particles as well. Metabolic labeling studies indicated that this reflected enhanced virus particle release rather than increased viral protein synthesis. These results indicate that gPr80gag is important for M-MuLV replication in vivo and in vitro and that the protein may be involved in a late step in viral budding or release.

The pseudorabies virus VP1/2 tegument protein is required for intracellular capsid transport

Transport of capsids in cells is critical to alphaherpesvirus infection and pathogenesis; however, viral factors required for transport have yet to be identified. Here we provide a detailed examination of capsid dynamics during the egress phase of infection in Vero cells infected with pseudorabies virus. We demonstrate that the VP1/2 tegument protein is required for processive microtubule-based transport of capsids in the cytoplasm. A second tegument protein that binds to VP1/2, UL37, was necessary for wild-type transport but was not essential for this process. Both proteins were also required for efficient nuclear egress of capsids to the cytoplasm.

The multipartite system that mediates entry of herpes simplex virus into the cell

The multipartite entry-fusion system of herpes simplex virus is made of a quartet of glycoproteins-gD, gB, gH.gL-and three alternative gD receptors, herpesvirus entry mediator (HVEM), nectin1 and modified sites on heparan sulphate. This multipartite system recapitulates the basic steps of virus-cell fusion, i.e. receptor recognition, triggering of fusion and fusion execution. Specifically, in addition to serving as the receptor-binding glycoprotein, gD triggers fusion through a specialised domain, named pro-fusion domain (PFD), located C-terminally in the ectodomain. In the unliganded gD the C-terminal region folds around the N-terminal region, such that gD adopts a closed autoinhibited conformation. In HVEM- and nectin1-bound gD the C-terminal region is displaced (opened conformation). gD is the tool for modification of HSV tropism, through insertion of ligands to heterologous tumour-specific receptors. It is discussed whether gD responds to the interaction with the natural and the heterologous receptors by adopting similar conformations, and whether the closed-to-open switch in conformation is a generalised mechanism of activation. A peculiar recombinant highlighted that the central Ig-folded core of gD may not encode executable functions for entry and that the 219-314 aa segment may be sufficient to trigger fusion. With respect to fusion execution, gB appears to be a prospective fusogen based on its coiled-coil trimeric structure, similar to that of another fusion glycoprotein. On the other hand, gH exhibits molecular elements typical of class 1 fusion glycoproteins, in particular heptad repeats and strong tendency to interact with lipids. Whether fusion execution is carried out by gB or gH.gL, or both glycoproteins in complex or sequentially remains to be determined.

Pseudorabies virus particles lacking tegument proteins pUL11 or pUL16 incorporate less full-length pUL36 than wild-type virus, but specifically accumulate a pUL36 N-terminal fragment

Proteins of the virion tegument of alphaherpesviruses are involved in protein-protein interactions, which play important roles in virus morphogenesis. Seven single-gene deletion mutants of Pseudorabies virus were analysed for alterations in the overall composition of the virion beyond the loss of the targeted protein. The UL36 protein (pUL36) was present in equal amounts in wild-type virions and mutants lacking pUL21, pUL49, pUL51, pUS3 or pUS8. Virions lacking pUL11 or pUL16 incorporated less full-length pUL36 than wild-type particles, but contained increased amounts of an N-terminal fragment of pUL36 that is present only in traces in wild-type virus and the other mutants.

[The properties of Ebola virus proteins]

The paper describes the structure and functions of Ebola virus properties. It also presents information on the role of structural (NP, VP40, VP35, GP, VP30, VP24, and L) and secreted (sGP, delta-peptide, GP1, GP(1,2delta), ssGP) proteins in the viral replication cycle and in the pathogenesis of Ebola hemorrhagic fever.

Requirements for the assembly and release of Newcastle disease virus-like particles

Paramyxoviruses, such as Newcastle disease virus (NDV), assemble in and bud from plasma membranes of infected cells. To explore the role of each of the NDV structural proteins in virion assembly and release, virus-like particles (VLPs) released from avian cells expressing all possible combinations of the nucleoprotein (NP), membrane or matrix protein (M), an uncleaved fusion protein (F-K115Q), and hemagglutinin-neuraminidase (HN) protein were characterized for densities, protein content, and efficiencies of release. Coexpression of all four proteins resulted in the release of VLPs with densities and efficiencies of release (1.18 to 1.16 g/cm(3) and 83.8% +/- 1.1%, respectively) similar to those of authentic virions. Expression of M protein alone, but not NP, F-K115Q, or HN protein individually, resulted in efficient VLP release, and expression of all different combinations of proteins in the absence of M protein did not result in particle release. Expression of any combination of proteins that included M protein yielded VLPs, although with different densities and efficiencies of release. To address the roles of NP, F, and HN proteins in VLP assembly, the interactions of proteins in VLPs formed with different combinations of viral proteins were characterized by coimmunoprecipitation. The colocalization of M protein with cell surface F and HN proteins in cells expressing all combinations of viral proteins was characterized. Taken together, the results show that M protein is necessary and sufficient for NDV budding. Furthermore, they suggest that M-HN and M-NP interactions are responsible for incorporation of HN and NP proteins into VLPs and that F protein is incorporated indirectly due to interactions with NP and HN protein.

Analysis of the collar-whisker structure of temperate lactococcal bacteriophage TP901-1

Proteins homologous to the protein NPS (neck passage structure) are widespread among lactococcal phages. We investigated the hypothesis that NPS is involved in the infection of phage TP901-1 by analysis of an NPS- mutant. NPS was determined to form a collar-whisker complex but was shown to be nonessential for infection, phage assembly, and stability.

Generation of filamentous instead of icosahedral particles by repression of African swine fever virus structural protein pB438L

The mechanisms involved in the construction of the icosahedral capsid of the African swine fever virus (ASFV) particle are not well understood at present. Capsid formation requires protein p72, the major capsid component, but other viral proteins are likely to play also a role in this process. We have examined the function of the ASFV structural protein pB438L, encoded by gene B438L, in virus morphogenesis. We show that protein pB438L associates with membranes during the infection, behaving as an integral membrane protein. Using a recombinant ASFV that inducibly expresses protein pB438L, we have determined that this structural protein is essential for the formation of infectious virus particles. In the absence of the protein, the virus assembly sites contain, instead of icosahedral particles, large aberrant tubular structures of viral origin as well as bilobulate forms that present morphological similarities with the tubules. The filamentous particles, which possess an aberrant core shell domain and an inner envelope, are covered by a capsid-like layer that, although containing the major capsid protein p72, does not acquire icosahedral morphology. This capsid, however, is to some extent functional, as the filamentous particles can move from the virus assembly sites to the plasma membrane and exit the cell by budding. The finding that, in the absence of protein pB438L, the viral particles formed have a tubular structure in which the icosahedral symmetry is lost supports a role for this protein in the construction or stabilization of the icosahedral vertices of the virus particle.

Packaging determinants in the UL11 tegument protein of herpes simplex virus type 1

The UL11 gene of herpes simplex virus type 1 encodes a 96-amino-acid tegument protein that is myristylated, palmitylated, and phosphorylated and is found on the cytoplasmic faces of nuclear, Golgi apparatus-derived, and plasma membranes of infected cells. Although this protein is thought to play a role in virus budding, its specific function is unknown. Purified virions were found to contain approximately 700 copies of the UL11 protein per particle, making it an abundant component of the tegument. Moreover, comparisons of cell-associated and virion-associated UL11 showed that packaging is selective for underphosphorylated forms, as has been reported for several other tegument proteins. Although the mechanism by which UL11 is packaged is unknown, previous studies have identified several sequence motifs in the protein that are important for membrane binding, intracellular trafficking, and interaction with UL16, another tegument protein. To ascertain whether any of these motifs are needed for packaging, a transfection/infection-based assay was used in which mutant forms of the protein must compete with the wild type. In this assay, the entire C-terminal half of UL11 was found to be dispensable. In the N-terminal half, the sites of myristylation and palmitylation, which enable membrane-binding and Golgi apparatus-specific targeting, were found to be essential for efficient packaging. The acidic cluster motif, which is not needed for Golgi apparatus-specific targeting but is involved in recycling the protein from the plasma membrane and for the interaction with UL16, was found to be essential, too. Thus, something other than mere localization of UL11 to Golgi apparatus-derived membranes is needed for packaging. The critical factor is unlikely to be the interaction with UL16 because other mutants that fail to bind this protein (due to removal of the dileucine-like motif or substitutions with foreign acidic clusters) were efficiently packaged. Collectively, these results suggest that UL11 packaging is not driven by a passive mechanism but instead requires trafficking through a specific pathway.

Severe acute respiratory syndrome coronavirus 7a accessory protein is a viral structural protein

Severe acute respiratory syndrome coronavirus (SCoV) 7a protein is one of the viral accessory proteins. In expressing cells, 7a protein exhibits a variety of biological activities, including induction of apoptosis, activation of the mitogen-activated protein kinase signaling pathway, inhibition of host protein translation, and suppression of cell growth progression. Analysis of SCoV particles that were purified by either sucrose gradient equilibrium centrifugation or a virus capture assay, in which intact SCoV particles were specifically immunoprecipitated by anti-S protein monoclonal antibody, demonstrated that 7a protein was associated with purified SCoV particles. Coexpression of 7a protein with SCoV S, M, N, and E proteins resulted in production of virus-like particles (VLPs) carrying 7a protein, while 7a protein was not released from cells expressing 7a protein alone. Although interaction between 7a protein and another SCoV accessory protein, 3a, has been reported, 3a protein was dispensable for assembly of 7a protein into VLPs. S protein was not required for the 7a protein incorporation into VLPs, and yet 7a protein interacted with S protein in coexpressing cells. These data established that, in addition to 3a protein, 7a protein was a SCoV accessory protein identified as a SCoV structural protein.

Rinderpest virus blocks type I and type II interferon action: role of structural and nonstructural proteins

Rinderpest virus (RPV) is a paramyxovirus closely related to the human pathogen Measles virus. It causes severe disease in cattle, buffalo, and some wild animals; although it can infect humans, it does not cause disease. Here, we demonstrate that RPV blocks the action of both type I (alpha) and type II (gamma) interferons (IFNs) by blocking the phosphorylation and nuclear translocation of STAT1 and STAT2 and that this block is not related to species specificity. In addition, both wild-type virulent and vaccine strains of the virus blocked IFN action. Unlike the case with some other paramyxoviruses, neither STAT1 nor STAT2 is degraded upon virus infection. STAT1 is bound by both the viral structural protein P, and thereby recruited to concentrations of viral protein in the cell, and the nonstructural protein V. Although both P and V proteins bind to STAT1 and can block IFN action when expressed in transfected cells, the IFN antagonist activity of the P protein is weaker than that of the V protein. The viral C protein also seems to weakly block IFN-induced activation of STAT1 in transfection experiments. However, studies with knockout viruses showed that the viral V protein appears to be the dominant inhibitor of IFN signaling in the context of virus infection, since prevention of viral V expression restored the IFN sensitivity of infected cells. Although a change in the distribution pattern of STAT2 was observed in virus-infected cells, STAT2 was not bound by any viral protein.

Identification of a structural motif crucial for infectivity of hepatitis B viruses

Infectious entry of hepatitis B viruses (HBV) has nonconventional facets. Here we analyzed whether a cell-permeable peptide [translocation motif (TLM)] identified within the surface protein of human HBV is a general feature of all hepadnaviruses and plays a role in the viral life cycle. Surface proteins of all hepadnaviruses contain conserved functional TLMs. Genetic inactivation of the duck HBV TLMs does not interfere with viral morphogenesis; however, these mutants are noninfectious. TLM mutant viruses bind to cells and are taken up into the endosomal compartment, but they cannot escape from endosomes. Processing of surface protein by endosomal proteases induces their exposure on the virus surface. This unmasking of TLMs mediates translocation of viral particles across the endosomal membrane into the cytosol, a prerequisite for productive infection. The ability of unmasked TLMs to translocate processed HBV particles across cellular membranes was shown by confocal immunofluorescence microscopy and by infection of nonpermissive cell lines with HBV processed in vitro with endosomal lysate. Based on these data, we propose an infectious entry mechanism unique for hepadnaviruses that involves virus internalization by receptor-mediated endocytosis followed by processing of surface protein in endosomes. This processing activates the function of TLMs that are essential for viral particle translocation through the endosomal membrane into the cytosol and productive infection.

A simian virus 5 (SV5) P/V mutant is less cytopathic than wild-type SV5 in human dendritic cells and is a more effective activator of dendritic cell maturation and function

Human epithelial cells infected with the parainfluenza virus simian virus 5 (SV5) show minimal activation of host cell interferon (IFN), cytokine, and cell death pathways. In contrast, a recombinant SV5 P/V gene mutant (rSV5-P/V-CPI-) overexpresses viral gene products and is a potent inducer of IFN, proinflammatory cytokines, and apoptosis in these cells. In this study, we have compared the outcomes of wild-type (WT) SV5 and rSV5-P/V-CPI- infections of primary human dendritic cells (DC), important antigen-presenting cells for initiating adaptive immune responses. We have tested the hypothesis that a P/V mutant which activates host antiviral responses will be a more potent inducer of DC maturation and function than WT rSV5, which suppresses host cell responses. Infection of peripheral blood mononuclear cell-derived immature DC with WT rSV5 resulted in high levels of viral protein and progeny virus but very little increase in cell surface costimulatory molecules or secretion of IFN and proinflammatory cytokines. In contrast, immature DC infected with the rSV5-P/V-CPI- mutant produced only low levels of viral protein and progeny virus, but these infected cells were induced to secrete IFN-alpha and other cytokines and showed elevated levels of maturation markers. Unexpectedly, DC infected with WT rSV5 showed extensive cytopathic effects and increased levels of active caspase-3, while infection of DC with the P/V mutant was largely noncytopathic. In mixed-culture assays, WT rSV5-infected DC were impaired in the ability to stimulate proliferation of autologous CD4+ T cells, whereas DC infected with the P/V mutant were very effective at activating T-cell proliferation. The addition of a pancaspase inhibitor to DC infected with WT rSV5 reduced cytopathic effects and resulted in higher surface expression levels of maturation markers. Our finding that the SV5 P/V mutant has both a reduced cytopathic effect in human DC compared to WT SV5 and an enhanced ability to induce DC function has implications for the rational design of novel recombinant paramyxovirus vectors based on engineered mutations in the viral P/V gene.

Crystal structure of infectious bursal disease virus VP2 subviral particle at 2.6A resolution: implications in virion assembly and immunogenicity

The structural protein VP2 of infectious bursal disease virus (IBDV) spontaneously forms a dodecahedral T=1 subviral particle (SVP), and is a primary immunogen of the virus. In this study, the structure of IBDV SVP was determined in a cubic crystal and refined to 2.6A resolution. It contains 20 independent VP2 subunits in a crystallographic asymmetric unit. Each subunit is folded mainly into a shell domain and a protrusion domain, both with the Swiss-roll topology, plus a small helical base domain. Three VP2 subunits constitute a tight trimer, which is the building block of IBDV (sub)viral particles. The structure revealed a calcium ion bound to three pairs of symmetry-related Asp31 and Asp174 to stabilize the VP2 trimer. Our results of treatment of SVP with EGTA, a Ca(2+)-chelating reagent, indicated that the metal-ion may be important not only in maintaining highly stable quaternary structure but also in regulating the swelling and dissociation of the icosahedral particles. A Ca(2+)-dependent assembly pathway was thus proposed, which involves further interactions between the trimers. The 20 independent subunits showed conformational variations, with the surface loops of the protrusion domain being the most diverse. These loops are targets of the neutralizing antibodies. Several common interactions between the surface loops were clearly observed, suggesting a possible major conformation of the immunogenic epitopes.

Biological properties of chimeric West Nile viruses

Recently, we have described a lineage 2 attenuated WN virus suitable for the development of a live WN vaccine. To design vaccine candidates with an improved immunogenicity, we assembled an infectious clone of the NY99 strain and created several chimeric constructs with reciprocal exchanges of structural protein genes between attenuated W956 and virulent NY99 and investigated their biological properties. Our data indicated that, while the growth rates of NY99 and chimeric viruses in tissue culture are determined primarily by properties of the structural proteins, determinants responsible for a highly cytopathic phenotype of NY99 or lack thereof for W956 are located within the nonstructural protein region of the WN genome. The high virulence of NY99 and the attenuated phenotype of W956 were found to be associated with determinants in the nonstructural region. Chimeric viruses carrying the NY99 structural proteins were attenuated in neuroinvasiveness and demonstrated an immunogenicity superior to W956.

Structure of DDB1 in complex with a paramyxovirus V protein: viral hijack of a propeller cluster in ubiquitin ligase

The DDB1-Cul4A ubiquitin ligase complex promotes protein ubiquitination in diverse cellular functions and is reprogrammed by the V proteins of paramyxoviruses to degrade STATs and block interferon signaling. Here we report the crystal structures of DDB1 alone and in complex with the simian virus 5 V protein. The DDB1 structure reveals an intertwined three-propeller cluster, which contains two tightly coupled beta propellers with a large pocket in between and a third beta propeller flexibly attached on the side. The rigid double-propeller fold of DDB1 is targeted by the viral V protein, which inserts an entire helix into the double-propeller pocket, whereas the third propeller domain docks DDB1 to the N terminus of the Cul4A scaffold. Together, these results not only provide structural insights into how the virus hijacks the DDB1-Cul4A ubiquitin ligase but also establish a structural framework for understanding the multiple functions of DDB1 in the uniquely assembled cullin-RING E3 machinery.

Composition of pseudorabies virus particles lacking tegument protein US3, UL47, or UL49 or envelope glycoprotein E

Proteins located in the tegument layer of herpesvirus particles play important roles in the replicative cycle at both early and late times after infection. As major constituents of the virion, they execute important functions in particular during formation of progeny virions. These functions have mostly been elucidated by construction and analysis of mutant viruses deleted in single or multiple tegument protein-encoding genes (reviewed in the work of T. C. Mettenleiter, Virus Res. 106:167-180, 2004). However, since tegument proteins have been shown to be involved in numerous protein-protein interactions, the impact of single protein deletions on the composition of the virus particle is unknown, but they could impair correct interpretation of the results. To analyze how the absence of single virion constituents influences virion composition, we established a procedure to assay relative amounts of virion structural proteins in deletion mutants of the alphaherpesvirus Pseudorabies virus (PrV) in comparison to wild-type particles. The assay is based on the mass spectrometric quantitation of virion protein-derived peptides carrying stable isotope mass tags. After deletion of the US3, UL47, UL49, or glycoprotein E gene, relative amounts of a capsid protein (UL38), a capsid-associated protein (UL25), several tegument proteins (UL36 and UL47, if present), and glycoprotein H were unaffected, whereas the content of other tegument proteins (UL46, UL48, and UL49, if present) varied significantly. In the case of the UL48 gene product, a specific increase in incorporation of a smaller isoform was observed after deletion of the UL47 or UL49 gene, whereas a larger isoform remained unaffected. The cellular protein actin was enriched in virions of mutants deficient in any of the tegument proteins UL47, UL49, or US3. By two-dimensional gel electrophoresis multiple isoforms of host cell-derived heat shock protein 70 and annexins A1 and A2 were also identified as structural components of PrV virions.

Insights into structural proteins of 936-type virulent lactococcal bacteriophages

bIL41 and bIL170, virulent phages of Lactococcus lactis belonging to the 936 group, possess a late gene named l12, coding a putative fiber sharing partial similarity to diverse gene products of dairy phages, including host-range determinants, but whose function is unknown in this group. We observed that the full-size gpl12 gene product is a minor protein constitutive of both phage particles. A derivative of bIL41 deleted for part of this gene was constructed by homologous recombination. The recombinant bIL41DeltaL12 showed normal propagation on strain IL1403 and no altered head and tail structures, demonstrating its non-essential role under our laboratory conditions. bIL170 was investigated for major structural components. Tails were characterized by electron microscopy and image analysis, which indicated that the major repeat unit of the tail occupied a maximum volume of 18.5 nm3, corresponding to a size of 20 kDa for a globular protein. Total protein profiles and head-enriched fractions of bIL170 exhibited a major 38 kDa protein, identified by N-terminal sequence as the product of l13. This result questions some of the functional predictions deduced from synteny relationships assumed for the lambda-supergroup of the family Siphoviridae to which the 936-type phages were proposed to belong.

Intracellular interference of infectious bursal disease virus

A search for dominant-negative mutant polypeptides hampering infectious bursal disease virus (IBDV) replication has been undertaken. We have found that expression of a mutant version of the VP3 structural polypeptide known as VP3/M3, partially lacking the domain responsible for the interaction with the virus-encoded RNA polymerase, efficiently interferes with the IBDV replication cycle. Transformed cells stably expressing VP3/M3 show a significant reduction (up to 96%) in their ability to support IBDV growth. Our findings provide a new tool for the characterization of the IBDV replication cycle and might facilitate the generation of genetically modified chicken lines with a reduced susceptibility to IBDV infection.