Endocytosis and recycling of varicella-zoster virus Fc receptor glycoprotein gE: internalization mediated by a YXXL motif in the cytoplasmic tail

Endocytosis of the Nipah virus glycoproteins

Nipah virus (NiV), a highly pathogenic member of the family Paramyxoviridae, encodes the surface glycoproteins F and G. Since internalization of the NiV envelope proteins from the cell surface might be of functional importance for viral pathogenesis either by regulating cytopathogenicity or by modulating recognition of infected cells by the immune system, we analyzed the endocytosis of the NiV F and G proteins. Interestingly, we found both glycoproteins to be internalized in infected and transfected cells. As endocytosis is normally mediated by tyrosine- or dileucine-dependent signals in the cytoplasmic tails of transmembrane proteins, all potential internalization signals in the NiV glycoproteins were mutated. Whereas the G protein appeared to be constitutively internalized with the bulk flow during membrane turnover, uptake of the F protein was found to be signal mediated. F endocytosis clearly depended on a membrane-proximal YXXPhi motif and was found to be of functional importance for the biological activity of the protein.

Herpesviral Fcgamma receptors: culprits attenuating antiviral IgG?

Production of IgG in response to virus infection is central to antiviral immune effector functions and a hallmark of B cell memory. Antiviral antibodies (Abs) recognising viral glycoproteins or protein antigen displayed on the surface of virions or virus-infected cells are crucial in rendering the virus noninfectious and in eliminating viruses or infected cells, either acting alone or in conjunction with complement. In many instances, passive transfer of Abs is sufficient to protect from viral infection. Herpesviruses (HV) are equipped with a large array of immunomodulatory functions which increase the efficiency of infection by dampening the antiviral immunity. Members of the α- and β-subfamily of the Herpesviridae are distinct in encoding transmembrane glycoproteins which selectively bind IgG via its Fc domain. The Fc-binding proteins constitute viral Fcγ receptors (vFcγRs) which are expressed on the cell surface of infected cells. Moreover, vFcγRs are abundantly incorporated into the envelope of virions. Despite their molecular and structural heterogeneity, the vFcγRs generally interfere with IgG-mediated effector functions like antibody (Ab)-dependent cellular cytolysis, complement activation and neutralisation of infectivity of virions. vFcγRs may thus contribute to the limited therapeutic potency of antiherpesviral IgG in clinical settings. A detailed molecular understanding of vFcγRs opens up the possibility to design recombinant IgG molecules resisting vFcγRs. Engineering IgG with a better antiviral efficiency represents a new therapeutic option against herpesviral diseases.

Incorporation of three endocytosed varicella-zoster virus glycoproteins, gE, gH, and gB, into the virion envelope

The cytoplasmic tails of all three major varicella-zoster virus (VZV) glycoproteins, gE, gH, and gB, harbor functional tyrosine-based endocytosis motifs that mediate internalization. The aim of the present study was to examine whether endocytosis from the plasma membrane is a cellular route by which VZV glycoproteins are delivered to the final envelopment compartment. In this study, we demonstrated that internalization of the glycoproteins occurred in the first 24 h postinfection but was reduced later in infection. Using surface biotinylation of VZV-infected cells followed by a glutathione cleavage assay, we showed that endocytosis was independent of antibody binding to gE, gH, and gB. Subsequently, with this assay, we demonstrated that biotinylated gE, gH, and gB retrieved from the cell surface were incorporated into nascent virus particles isolated after density gradient sedimentation. To confirm and extend this finding, we repeated the above sedimentation step and specifically detected envelopes decorated with Streptavidin-conjugated gold beads on a majority of complete virions through examination by transmission electron microscopy. In addition, a gE-gI complex and a gE-gH complex were found on the virions. Therefore, the above studies established that VZV subsumed a postendocytosis trafficking pathway as one mechanism by which to deliver viral glycoproteins to the site of virion assembly in the cytoplasm. Furthermore, since a recombinant VZV genome lacking only endocytosis-competent gE cannot replicate, these results supported the conclusion that the endocytosis-envelopment pathway is an essential component of the VZV life cycle.

Functions of the C-terminal domain of varicella-zoster virus glycoprotein E in viral replication in vitro and skin and T-cell tropism in vivo

Varicella-zoster virus (VZV) glycoprotein E (gE) is essential for VZV replication. To further analyze the functions of gE in VZV replication, a full deletion and point mutations were made in the 62-amino-acid (aa) C-terminal domain. Targeted mutations were introduced in YAGL (aa 582 to 585), which mediates gE endocytosis, AYRV (aa 568 to 571), which targets gE to the trans-Golgi network (TGN), and SSTT, an "acid cluster" comprising a phosphorylation motif (aa 588 to 601). Substitutions Y582G in YAGL, Y569A in AYRV, and S593A, S595A, T596A, and T598A in SSTT were introduced into the viral genome by using VZV cosmids. These experiments demonstrated a hierarchy in the contributions of these C-terminal motifs to VZV replication and virulence. Deletion of the gE C terminus and mutation of YAGL were lethal for VZV replication in vitro. Mutations of AYRV and SSTT were compatible with recovery of VZV, but the AYRV mutation resulted in rapid virus spread in vitro and the SSTT mutation resulted in higher virus titers than were observed for the parental rOka strain. When the rOka-gE-AYRV and rOka-gE-SSTT mutants were evaluated in skin and T-cell xenografts in SCIDhu mice, interference with TGN targeting was associated with substantial attenuation, especially in skin, whereas the SSTT mutation did not alter VZV infectivity in vivo. These results provide the first information about how targeted mutations of this essential VZV glycoprotein affect viral replication in vitro and VZV virulence in dermal and epidermal cells and T cells within intact tissue microenvironments in vivo.

Conserved cytoplasmic domain sequences mediate the ER export of VZV, HSV-1, and HCMV gB

Glycoprotein B (gB) is conserved among the herpesviruses and participates in both virus entry and cell-cell spread. The ER export of VZV gB is mediated by two cytoplasmic domain regions, aa 818-826, which contains a YXXphi motif, and the C-terminal 17 aa. The current study examines whether related sequences in the cytoplasmic domains of HSV-1 and HCMV gB similarly influence the ER export of their gB homologs. Directed mutations were introduced into the cytoplasmic domains of HSV-1 and HCMV gB, and the efficiencies with which the mutated proteins acquired Golgi-dependent modifications were determined. Sequences homologous to VZV gB aa 818-826 were required for normal ER export of both HSV-1 gB and HCMV gB. However, the C-terminal regions of HSV-1 and HCMV gB had no impact on ER export. Therefore, alpha- and betaherpesvirus gB homologs share conserved ER export signals, but species-specific differences in the ER export of gB also exist.

Internalization of pseudorabies virus glycoprotein B is mediated by an interaction between the YQRL motif in its cytoplasmic domain and the clathrin-associated AP-2 adaptor complex

The cytoplasmic domain of pseudorabies virus (PRV) glycoprotein B (gB) contains three putative internalization motifs. Previously, we demonstrated that the tyrosine-based YQRL motif at positions 902 to 905, but not the YMSI motif at positions 864 to 867 or the LL doublet at positions 887 and 888, is required for correct functioning of gB during antibody-mediated internalization of PRV cell surface-bound glycoproteins. In the present study, we demonstrate that the YQRL motif is also crucial to allow spontaneous internalization of PRV gB, and thus, that spontaneous and antibody-mediated internalizations of PRV gB occur through closely related mechanisms. Furthermore, we found that PRV gB colocalizes with the cellular clathrin-associated AP-2 adaptor complex and that this colocalization depends on the YQRL motif. In addition, by coimmunoprecipitation assays, we found that during both spontaneous and antibody-dependent internalization, PRV gB physically interacts with AP-2, and that efficient interaction between gB and AP-2 required an intact YQRL motif. Collectively, these findings demonstrate for the first time that during internalization of an alphaherpesvirus envelope protein, i.e., PRV gB, a specific amino acid sequence in the cytoplasmic tail of the protein interacts with AP-2 and may constitute a common AP-2-mediated mechanism of internalization of alphaherpesvirus envelope proteins.

Coexpression of UL20p and gK inhibits cell-cell fusion mediated by herpes simplex virus glycoproteins gD, gH-gL, and wild-type gB or an endocytosis-defective gB mutant and downmodulates their cell surface expression

Syncytium formation in cells that express herpes simplex virus glycoprotein B (gB), gD, gH, and gL is blocked by gK (E. Avitabile, G. Lombardi, and G. Campadelli-Fiume, J. Virol. 77:6836-6844, 2003). Here, we report the results of two series of experiments. First, UL20 protein (UL20p) expression weakly inhibited cell-cell fusion. Coexpression of UL20p and gK drastically reduced fusion in a cell-line-dependent manner, with the highest inhibition in BHK cells. Singly expressed UL20p and gK localized at the endoplasmic reticulum and nuclear membranes. When they were coexpressed, both proteins relocalized to the Golgi apparatus. Remarkably, in cells that coexpressed UL20p and gK, the antifusion activity correlated with a downmodulation of gD, gB, gH, and gL cell surface expression. Second, gB(Delta867) has a partial deletion in the cytoplasmic tail that removed endocytosis motifs. Whereas wild-type (wt) gB was internalized in vesicles lined with the endosomal marker Rab5, gB(delta867) was not internalized, exhibited enhanced cell surface expression, and was more efficient in mediating cell-cell fusion than wt gB. The antifusion activity of UL20p and gK was also exerted when gB(delta867) replaced wt gB in the cell fusion assay. These studies show that the gB C tail carries a functional endocytosis motif(s) and that the removal of the motif correlated with increased gB surface expression and increased fusion activity. We conclude that cell-cell fusion in wt-virus-infected cells is negatively controlled by at least two mechanisms. The novel mechanism described here involves the concerted action of UL20p and gK and correlates with a moderate but consistent reduction in the cell surface expression of the fusion glycoproteins. This mechanism is independent of the one exerted through endocytosis-mediated downmodulation of gB from the plasma membrane.

A glycine-rich bovine herpesvirus 5 (BHV-5) gE-specific epitope within the ectodomain is important for BHV-5 neurovirulence

The bovine herpesvirus 5 (BHV-5) gE ectodomain contains a glycine-rich epitope coding region (gE5 epitope), residues 204 to 218, that is significantly different from the corresponding gE region of BHV-1. Deletion of the gE epitope significantly reduced the neurovirulence of BHV-5 in rabbits. Pulse-chase analyses revealed that the epitope-deleted and wild-type gE were synthesized as N-glycosylated endoglycosidase H-sensitive precursors with approximate molecular masses of 85 kDa and 86 kDa, respectively. Like the wild-type gE, epitope-deleted gE complexed with gI and was readily transported from the endoplasmic reticulum. Concomitantly, the epitope-deleted and wild-type gE acquired posttranslational modifications in the Golgi leading to an increased apparent molecular mass of 93-kDa (epitope-deleted gE) and 94-kDa (wild-type gE). The kinetics of mutant and wild-type gE processing were similar, and both mature proteins were resistant to endoglycosidase H but sensitive to glycopeptidase F. The gE epitope-deleted BHV-5 formed wild-type-sized plaques in MDBK cells, and the epitope-deleted gE was expressed on the cell surface. However, rabbits infected intranasally with gE epitope-deleted BHV-5 did not develop seizures, and only 20% of the infected rabbits showed mild neurological signs. The epitope-deleted virus replicated efficiently in the olfactory epithelium. However, within the brains of these rabbits there was a 10- to 20-fold reduction in infected neurons compared with the number of infected neurons within the brains of rabbits infected with the gE5 epitope-reverted and wild-type BHV-5. In comparison, 70 to 80% of the rabbits exhibited severe neurological signs when infected with the gE5 epitope-reverted and wild-type BHV-5. These results indicated that anterograde transport of the gE epitope-deleted virus from the olfactory receptor neurons to the olfactory bulb is defective and that, within the central nervous system, the gE5 epitope-coding region was required for expression of the full virulence potential of BHV-5.

Regulation of varicella-zoster virus-induced cell-to-cell fusion by the endocytosis-competent glycoproteins gH and gE

The gH glycoprotein of varicella-zoster virus (VZV) is a major fusogen. The realigned short cytoplasmic tail of gH (18 amino acids) harbors a functional endocytosis motif (YNKI) that mediates internalization in both VZV-infected and transfected cells (T. J. Pasieka, L. Maresova, and C. Grose, J. Virol. 77: 4194-4202, 2003). During subsequent confocal microscopy studies of endocytosis-deficient gH mutants, we observed that cells transfected with the gH tail mutants exhibited marked fusion. Therefore, we postulated that VZV gH endocytosis served to regulate cell-to-cell fusion. Subsequent analyses of gH+gL transfection fusion assays by the Kolmogorov-Smirnov statistical test demonstrated that expression of the endocytosis-deficient gH mutants resulted in a statistically significant enhancement of cell-to-cell fusion (P < 0.0001) compared to wild-type gH. On the other hand, coexpression of VZV gE, another endocytosis-competent VZV glycoprotein, was able to temper the fusogenicity of the gH endocytosis mutants by facilitating internalization of the mutant gH protein from the cell surface. When the latter results were similarly analyzed, there was no longer any enhanced fusion by the endocytosis-deficient gH mutant protein. In summary, these studies support a role for gH endocytosis in regulating the cell surface expression of gH and thereby regulating gH-mediated fusion. The data also confirm and extend prior observations of a gE-gH interaction during viral glycoprotein trafficking in a VZV transfection system.

Viral and cellular kinases are potential antiviral targets and have a central role in varicella zoster virus pathogenesis

Herpesviruses utilize viral and cellular kinases for replication, and these mediate essential functions that are important for viral pathogenesis. Elucidating the roles of kinases in herpesvirus infections may highlight virus-host interactions that are possible targets for kinase inhibitors with antiviral activity. Varicella zoster virus (VZV) encodes two kinases that phosphorylate viral proteins involved in regulation, assembly, and virulence. VZV infection also induces the activity of host cell cyclin-dependent kinases (cdk4 and cdk2) in nondividing cells, causing a disregulation of the cell cycle. Roscovitine and Purvalanol, kinase inhibitors that target cdks, prevent VZV replication at concentrations with few cytotoxic effects. Cdk inhibitors therefore have potential as antivirals that may extend to a broad range of viruses and have the added advantage that resistance does not arise easily.

Effects of mutations in the cytoplasmic domain of herpes simplex virus type 1 glycoprotein B on intracellular transport and infectivity

Herpes simplex virus type 1 (HSV-1) is a human pathogen of the alphaherpesvirus family which infects and spreads in the nervous system. Glycoproteins play a key role in the process of assembly and maturation of herpesviruses, which is essential for neuroinvasion and transneuronal spread. Glycoprotein B (gB) is a main component of the HSV-1 envelope and is necessary for the production of infectious particles. The cytoplasmic domain of gB, the longest one among HSV-1 glycoproteins, contains several highly conserved peptide sequences homologous to motifs involved in intracellular sorting. To determine the specific roles of these motifs in processing, subcellular localization, and the capacity of HSV-1 gB to complement a gB-null virus, we generated truncated or point mutated forms of a green fluorescent protein (GFP)-tagged gB. GFP-gB with a deletion in the acidic cluster DGDADEDDL (amino acids [aa] 896 to 904) behaved the same as the parental form. Deletion or disruption of the YTQV motif (aa 889 to 892) abolished internalization and reduced complementation by 60%. Disruption of the LL motif (aa 871 to 872) impaired the return of the protein to the trans-Golgi network (TGN) while enhancing its recycling to the plasma membrane. Truncations from residue E 857 abolished transport and processing of the truncated proteins, which had null complementation activity, through the Golgi complex. Altogether, our results favor a model in which HSV-1 gets its final envelope in the TGN, and they suggest that endocytosis, albeit not necessary, might play a role in infectivity.

pH dependence and stoichiometry of binding to the Fc region of IgG by the herpes simplex virus Fc receptor gE-gI

Herpes simplex virus type 1 encodes two glycoproteins, gE and gI, that form a heterodimer on the surface of virions and infected cells. The gE-gI heterodimer has been implicated in cell-to-cell spread of virus and is a receptor for the Fc fragment of IgG. Previous studies localized the gE-gI-binding site on human IgG to a region near the interface between the C(H)2 and C(H)3 domains of Fc, which also serves as the binding site for bacterial and mammalian Fc receptors. Although there are two potential gE-gI-binding sites per Fc homodimer, only one gE-gI heterodimer binds per IgG in gel filtration experiments. Here we report production of recombinant human Fc molecules that contain zero, one, or two potential gE-gI-binding sites and use them in analytical ultracentrifugation experiments to show that two gE-gI heterodimers can bind to each Fc. Further characterization of the gE-gI interaction with Fc reveals a sharp pH dependence of binding, with K(D) values of approximately 340 and approximately 930 nm for the first and second binding events, respectively, at the slightly basic pH of the cell surface (pH 7.4), but undetectable binding at pH 6.0. This strongly pH-dependent interaction suggests a physiological role for gE-gI dissociation from IgG within acidic intracellular compartments, consistent with a mechanism whereby herpes simplex virus promotes intracellular degradation of anti-viral antibodies.

Fcgamma receptor-like activity of hepatitis C virus core protein

We have previously demonstrated that viral particles with the properties of nonenveloped hepatitis C virus (HCV) nucleocapsids occur in the serum of HCV-infected individuals (1). We show here that nucleocapsids purified directly from serum or isolated from HCV virions have FcgammaR-like activity and bind "nonimmune" IgG via its Fcgamma domain. HCV core proteins produced in Escherichia coli and in the baculovirus expression system also bound "nonimmune" IgG and their Fcgamma fragments. Folded conformation was required for IgG binding because the FcgammaR-like site of the core protein was inactive in denaturing conditions. Studies with synthetic core peptides showed that the region spanning amino acids 3-75 was essential for formation of the IgG-binding site. The interaction between the HCV core and human IgG is more efficient in acidic (pH 6.0) than in neutral conditions. The core protein-binding site on the IgG molecule differs from those for C1q, FcgammaRII (CD32), and FcgammaRIII (CD16) but overlaps with that for soluble protein A from Staphylococcus aureus (SpA), which is located in the CH2-CH3 interface of IgG. These characteristics of the core-IgG interaction are very similar to those of the neonatal FcRn. Surface plasmon resonance studies suggested that the binding of an anti-core antibody to HCV core protein might be "bipolar" through its paratope to the corresponding epitope and by its Fcgamma region to the FcgammaR-like motif on this protein. These features of HCV nucleocapsids and HCV core protein may confer an advantage for HCV in terms of survival by interfering with host defense mechanisms mediated by the Fcgamma part of IgG.

Basic residues of the helix six domain of influenza virus M1 involved in nuclear translocation of M1 can be replaced by PTAP and YPDL late assembly domain motifs

Influenza type A virus matrix (M1) protein possesses multiple functional motifs in the helix 6 (H6) domain (amino acids 91 to 105), including nuclear localization signal (NLS) (101-RKLKR-105) involved in translocating M1 from the cytoplasm into the nucleus. To determine the role of the NLS motif in the influenza virus life cycle, we mutated these and the neighboring sequences by site-directed mutagenesis, and influenza virus mutants were generated by reverse genetics. Our results show that infectious viruses were rescued by reverse genetics from all single alanine mutations of amino acids in the H6 domain and the neighboring region except in three positions (K104A and R105A within the NLS motif and E106A in loop 6 outside the NLS motif). Among the rescued mutant viruses, R101A and R105K exhibited reduced growth and small-plaque morphology, and all other mutant viruses showed the wild-type phenotype. On the other hand, three single mutations (K104A, K105A, and E106A) and three double mutations (R101A/K102A, K104A/K105A, and K102A/R105A) failed to generate infectious virus. Deletion (Delta YRKL) or mutation (4A) of YRKL also abolished generation of infectious virus. However, replacement of the YRKL motif with PTAP or YPDL as well as insertion of PTAP after 4A mutation yielded infectious viruses with the wild-type phenotype. Furthermore, mutant M1 proteins (R101A/K102A, Delta YRKL, 4A, PTAP, 4A+PTAP, and YPDL) when expressed alone from cloned cDNAs were only cytoplasmic, whereas the wild-type M1 expressed alone was both nuclear and cytoplasmic as expected. These results show that the nuclear translocation function provided by the positively charged residues within the NLS motif does not play a critical role in influenza virus replication. Furthermore, these sequences of H6 domain can be replaced by late (L) domain motifs and therefore may provide a function similar to that of the L domains of other negative-strand RNA and retroviruses.

A tyrosine motif in the cytoplasmic domain of mason-pfizer monkey virus is essential for the incorporation of glycoprotein into virions

Mason-Pfizer monkey virus (M-PMV) encodes a transmembrane (TM) glycoprotein with a 38-amino-acid-long cytoplasmic domain. After the release of the immature virus, a viral protease-mediated cleavage occurs within the cytoplasmic domain, resulting in the loss of 17 amino acids from the carboxy terminus. This maturational cleavage occurs between a histidine at position 21 and a tyrosine at position 22 in the cytoplasmic domain of the TM protein. We have demonstrated previously that a truncated TM glycoprotein with a 21-amino-acid-long cytoplasmic tail showed enhanced fusogenicity but could not be incorporated into virions. These results suggest that postassembly cleavage of the cytoplasmic domain removes a necessary incorporation signal and activates fusion activity. To investigate the contribution of tyrosine residues to the function of the glycoprotein complex and virus replication, we have introduced amino acid substitutions into two tyrosine residues found in the cytoplasmic domain. The effects of these mutations on glycoprotein biosynthesis and function, as well as on virus infectivity, have been examined. Mutation of tyrosine 34 to alanine had little effect on glycoprotein function. In contrast, substitutions at tyrosine 22 modulated fusion activity in either a positive or negative manner, depending on the substituting amino acid. Moreover, any nonaromatic substitution at this position blocked glycoprotein incorporation into virions and abolished infectivity. These results demonstrate that M-PMV employs a tyrosine signal for the selective incorporation of glycoprotein into budding virions. Antibody uptake studies show that tyrosine 22 is part of an efficient internalization signal in the cytoplasmic domain of the M-PMV glycoprotein that can also be positively and negatively influenced by changes at this site.

Mutational analysis of open reading frames 62 and 71, encoding the varicella-zoster virus immediate-early transactivating protein, IE62, and effects on replication in vitro and in skin xenografts in the SCID-hu mouse in vivo

The varicella-zoster virus (VZV) genome has unique long (U(L)) and unique short (U(S)) segments which are flanked by internal repeat (IR) and terminal repeat (TR) sequences. The immediate-early 62 (IE62) protein, encoded by open reading frame 62 (ORF62) and ORF71 in these repeats, is the major VZV transactivating protein. Mutational analyses were done with VZV cosmids generated from parent Oka (pOka), a low-passage clinical isolate, and repair experiments were done with ORF62 from pOka and vaccine Oka (vOka), which is derived from pOka. Transfections using VZV cosmids from which ORF62, ORF71, or the ORF62/71 gene pair was deleted showed that VZV replication required at least one copy of ORF62. The insertion of ORF62 from pOka or vOka into a nonnative site in U(S) allowed VZV replication in cell culture in vitro, although the plaque size and yields of infectious virus were decreased. Targeted mutations in binding sites reported to affect interaction with IE4 protein and a putative ORF9 protein binding site were not lethal. Single deletions of ORF62 or ORF71 from cosmids permitted recovery of infectious virus, but recombination events repaired the defective repeat region in some progeny viruses, as verified by PCR and Southern hybridization. VZV infectivity in skin xenografts in the SCID-hu model required ORF62 expression; mixtures of single-copy recombinant Oka Delta 62 (rOka Delta 62) or rOka Delta 71 and repaired rOka generated by recombination of the single-copy deletion mutants were detected in some skin implants. Although insertion of ORF62 into the nonnative site permitted replication in cell culture, ORF62 expression from its native site was necessary for cell-cell spread in differentiated human skin tissues in vivo.

A functional YNKI motif in the short cytoplasmic tail of varicella-zoster virus glycoprotein gH mediates clathrin-dependent and antibody-independent endocytosis

The trafficking of varicella-zoster virus (VZV) gH was investigated under both infection and transfection conditions. In initial endocytosis assays performed in infected cells, the three glycoproteins gE, gI, and gB served as positive controls for internalization from the plasma membrane. Subsequently, we discovered that gH in VZV-infected cells was also internalized and followed a similar trafficking pattern. This observation was unexpected because all herpesvirus gH homologues have short endodomains not known to contain trafficking motifs. Further investigation demonstrated that VZV gH, when expressed alone with its chaperone gL, was capable of endocytosis in a clathrin-dependent manner, independent of gE, gI, or gB. Upon inspection of the short gH cytoplasmic tail, we discovered a putative tyrosine-based endocytosis motif (YNKI). When the tyrosine was replaced with an alanine, endocytosis of gH was blocked. Utilizing an endocytosis assay dependent on biotin labeling, we further documented that endocytosis of VZV gH was antibody independent. In control experiments, we showed that gE, gI, and gB also internalized in an antibody-independent manner. Alignment analysis of the VZV gH cytoplasmic tail to other herpesvirus gH homologues revealed two important findings: (i) herpes simplex virus type 1 and 2 homologues lacked an endocytosis motif, while all other alphaherpesvirus gH homologues contained a potential motif, and (ii) the VZV gH and simian varicella virus gH cytoplasmic tails were likely longer in length (18 amino acids) than predicted in the original sequence analyses (12 and 16 amino acids, respectively). The longer tails provided the proper context for a functional endocytosis motif.

Promoter sequences of varicella-zoster virus glycoprotein I targeted by cellular transactivating factors Sp1 and USF determine virulence in skin and T cells in SCIDhu mice in vivo

Varicella-zoster virus (VZV) glycoprotein I is dispensable in cell culture but necessary for infection of human skin and T cells in SCIDhu mice in vivo. The gI promoter contains an activating upstream sequence that binds the cellular transactivators specificity factor 1 (Sp1) and upstream stimulatory factor (USF) and an open reading frame 29 (ORF29)-responsive element (29RE), which mediates enhancement by ORF29 DNA binding protein of immediate-early 62 (IE62)-induced transcription. Recombinants, rOKAgI-Sp1 and rOKAgI-USF, with two base pair substitutions in Sp1 or USF sites, replicated like rOKA in vitro, but infectivity of rOKAgI-Sp1 was significantly impaired in skin and T cells in vivo. A double mutant, rOKAgI-Sp1/USF, did not replicate in skin but yielded low titers of infectious virus in T cells. The repaired protein, rOKAgI:rep-Sp1/USF, was as infectious as rOKA. Thus, disrupting gI promoter sites for cellular transactivators altered VZV virulence in vivo, with variable consequences related to the cellular factor and the host cell type. Mutations in the 29RE of the gI promoter were made by substituting each of four 10-bp blocks in this region with a 10-bp sequence, GATAACTACA, that was predicted to interfere with enhancer effects of the ORF29 protein. One of these mutants, which was designated rOKAgI-29RE-3, had diminished replication in skin and T cells, indicating that ORF29 protein-mediated enhancement of gI expression contributes to VZV virulence. Mutations within promoters of viral genes that are nonessential in vitro should allow construction of recombinant herpesviruses that have altered virulence in specific host cells in vivo and may be useful for designing herpesviral gene therapy vectors and attenuated viral vaccines.

Phosphorylation by the varicella-zoster virus ORF47 protein serine kinase determines whether endocytosed viral gE traffics to the trans-Golgi network or recycles to the cell membrane

Like all alphaherpesviruses, varicella-zoster virus (VZV) infection proceeds by both cell-cell spread and virion production. Virions are enveloped within vacuoles located near the trans-Golgi network (TGN), while in cell-cell spread, surface glycoproteins fuse cells into syncytia. In this report, we delineate a potential role for serine/threonine phosphorylation of the cytoplasmic tail of the predominant VZV glycoprotein, gE, in these processes. The fact that VZV gE (formerly called gpI) is phosphorylated has been documented (E. A. Montalvo and C. Grose, Proc. Natl. Acad. Sci. USA 83:8967-8971, 1986), although respective roles of viral and cellular protein kinases have never been delineated. VZV ORF47 is a viral serine protein kinase that recognized a consensus sequence similar to that of casein kinase II (CKII). During open reading frame 47 (ORF47)-specific in vitro kinase assays, ORF47 phosphorylated four residues in the cytoplasmic tail of VZV gE (S593, S595, T596, and T598), thus modifying the known phosphofurin acidic cluster sorting protein 1 domain. CKII phosphorylated gE predominantly on the two threonine residues. In wild-type-virus-infected cells, where ORF47-mediated phosphorylation predominated, gE endocytosed and relocalized to the TGN. In cells infected with a VZV ORF47-null mutant, internalized VZV gE recycled to the plasma membrane and did not localize to the TGN. The mutant virus also formed larger syncytia than the wild-type virus, linking CKII-mediated gE phosphorylation with increased cell-cell spread. Thus, ORF47 and CKII behaved as "team players" in the phosphorylation of VZV gE. Taken together, the results showed that phosphorylation of VZV gE by ORF47 or CKII determined whether VZV infection proceeded toward a pathway likely involved with either virion production or cell-cell spread.

A Brazilian glycoprotein E-negative bovine herpesvirus type 1.2a (BHV-1.2a) mutant is attenuated for cattle and induces protection against wild-type virus challenge

The authors previously reported the construction of a glycoprotein E-deleted (gE-) mutant of bovine herpesvirus type 1.2a (BHV-1.2a). This mutant, 265gE-, was designed as a vaccinal strain for differential vaccines, allowing the distinction between vaccinated and naturally infected cattle. In order to determine the safety and efficacy of this candidate vaccine virus, a group of calves was inoculated with 265gE-. The virus was detected in secretions of inoculated calves to lower titres and for a shorter period than the parental virus inoculated in control calves. Twenty one days after inoculation, the calves were challenged with the wild type parental virus. Only mild signs of infection were detected on vaccinated calves, whereas non-vaccinated controls displayed intense rhinotracheitis and shed virus for longer and to higher titres than vaccinated calves. Six months after vaccination, both vaccinated and control groups were subjected to reactivation of potentially latent virus. The mutant 265gE- could not be reactivated from vaccinated calves. The clinical signs observed, following the reactivation of the parental virus, were again much milder on vaccinated than on non-vaccinated calves. Moreover, parental virus shedding was considerably reduced on vaccinated calves at reactivation. In view of its attenuation, immunogenicity and protective effect upon challenge and reactivation with a virulent BHV-1, the mutant 265gE- was shown to be suitable for use as a BHV-1 differential vaccine virus.

Investigating the mechanism of acoustically activated uptake of drugs from Pluronic micelles

BACKGROUND

This paper examines the mechanism of ultrasonic enhanced drug delivery from Pluronic micelles. In previous publications by our group, fluorescently labeled Pluronic was shown to penetrate HL-60 cells with and without the action of ultrasound, while drug uptake was increased with the application of ultrasound.

METHODS

In this study, the amount of uptake of two fluorescent probes, Lysosensor Green (a pH-sensitive probe) and Cell Tracker Orange CMTMR (a pH-independent probe), was measured in HL-60 and HeLa cells.

RESULTS

The results of our experiments show that the increase in drug accumulation in the cells as a result of ultrasonication is not due to an increase in endocytosis due to ultrasonication.

CONCLUSIONS

We hypothesize that sonoporation plays an important role in the acoustically activated drug delivery of chemotherapy drugs delivered from Pluronic micelles.

Glycoprotein I of varicella-zoster virus is required for viral replication in skin and T cells

Varicella-zoster virus (VZV) glycoprotein I (gI) is dispensable in cell culture; the SCIDhu model of VZV pathogenesis was used to determine whether gI is necessary in vivo. The parental and repaired viruses grew in human skin and thymus/liver implants, but the gI deletion mutant was not infectious. Thus, gI is essential for VZV infectivity in skin and T cells.

A tyrosine-based motif in the cytoplasmic tail of pseudorabies virus glycoprotein B is important for both antibody-induced internalization of viral glycoproteins and efficient cell-to-cell spread

Pseudorabies virus (PRV), a swine alphaherpesvirus, is capable of causing viremia in vaccinated animals. Two mechanisms that may help PRV avoid recognition by the host immune system during this viremia are direct cell-to-cell spread in tissue and antibody-induced internalization of viral cell surface glycoproteins in PRV-infected blood monocytes, the carrier cells of the virus in the blood. PRV glycoprotein B (gB) is crucial during both processes. Here we show that mutating a tyrosine residue located in a YXXPhi motif in the gB cytoplasmic tail results in decreased efficiency of cell-to-cell spread and a strong reduction in antibody-induced internalization of viral cell surface glycoproteins. Mutating the dileucine motif in the gB tail led to an increased cell-to-cell spread of the virus and the formation of large syncytia.

Antibody-induced internalization of viral glycoproteins in pseudorabies virus-infected monocytes and role of the cytoskeleton: a confocal study

Addition of pseudorabies virus (PrV)-specific polyclonal immunoglobulins to PrV-infected monocytes induces internalization of plasma membrane anchored viral glycoproteins. This process may interfere with antibody-dependent cell lysis and resembles the well-studied physiological endocytosis process. A confocal study was designed to investigate whether the major cellular components, involved in physiological endocytosis (clathrin, actin, dynein and microtubules), play a role in this virological internalization process. In order to visualize the interaction of endosomes, which contain the internalized viral glycoproteins, with clathrin, actin, dynein and microtubules, a double labeling of viral glycoproteins and different cellular proteins was performed. Porcine monocytes were inoculated with the PrV-strain 89V87 at a multiplicity of infection of 50 for 13h. After the addition of FITC-labeled porcine polyclonal PrV-specific antibodies, cells were fixed with para-formaldehyde at different time points and afterwards permeabilized. The different cellular components were visualized with monoclonal antibodies and a Texas Red-conjugate, with the exception of actin, which was stained with phalloidin-Texas Red. The cells were analyzed by confocal microscopy. A clear co-localization was observed between the viral glycoproteins and clathrin and dynein during the internalization process. The microtubules were in close contact with the internalized vesicles. For actin no co-localization could be observed. It can be stated that clathrin, dynein and microtubules, important components during physiological endocytosis, are also of importance during the antibody-induced internalization of viral glycoproteins.

Mutation of the dominant endocytosis motif in human immunodeficiency virus type 1 gp41 can complement matrix mutations without increasing Env incorporation

The human immunodeficiency virus type 1 transmembrane glycoprotein (TM) is efficiently endocytosed in a clathrin-dependent manner. Internalization is mediated by a tyrosine-containing motif within the cytoplasmic domain, and replacement of the cytoplasmic tyrosine by cysteine or phenylalanine increased expression of mutant glycoprotein on the surface of transfected cells by as much as 2.5-fold. Because interactions between the cytoplasmic domain of Env and the matrix protein (MA) have been suggested to mediate incorporation of Env in virus particles, we examined whether perturbation of endocytosis would alter incorporation. Proviruses were constructed to contain the wild-type or mutant Env in conjunction with point mutations in MA that had previously been shown to block Env incorporation. These constructs were used to evaluate the effect of glycoprotein endocytosis on incorporation into virus particles and to test the necessity for a specific interaction between Env and MA to mediate incorporation. Viruses produced from transfected 293T cells were used to infect various cell lines, including MAGI, H9, and CEMx174. Viruses encoding both a disrupted endocytosis motif signal and mutations within MA were significantly more infectious in MAGI cells than their counterparts encoding a mutant MA and wild-type Env. This complementation of infectivity for the MA incorporation mutant viruses was not due to increased glycoprotein incorporation into particles but instead reflected an enhanced fusogenicity of the mutated Env proteins. Our findings further support the concept that a specific interaction between the long cytoplasmic domain of TM and MA is required for efficient incorporation of Env into assembling virions. Alteration of the endocytosis signal of Env, and the resulting increase in cell surface glycoprotein, has no effect on incorporation despite demonstrable effects on fusion, virus entry, and infectivity.

Role of the varicella-zoster virus gB cytoplasmic domain in gB transport and viral egress

To study the function of the varicella-zoster virus (VZV) gB cytoplasmic domain during viral infection, we produced a VZV recombinant virus that expresses a truncated form of gB lacking the C-terminal 36 amino acids of its cytoplasmic domain (VZV gB-36). VZV gB-36 replicates in noncomplementing cells and grows at a rate similar to that of native VZV. However, cells infected with VZVgB-36 form extensive syncytia compared to the relatively small syncytia formed during native VZV infection. In addition, electron microscopy shows that very little virus is present on the surfaces of cells infected with VZV gB-36, while cells infected with native VZV exhibit abundant virions on the cell surface. The C-terminal 36 amino acids of the gB cytoplasmic domain have been shown in transfection-based experiments to contain both an endoplasmic reticulum-to-Golgi transport signal (the C-terminal 17 amino acids) and a consensus YXXphi (where Y is tyrosine, X is any amino acid, and phi is any bulky hydrophobic amino acid) signal sequence (YSRV) that mediates the internalization of gB from the plasma membrane. As predicted based on these data, gB-36 expressed during the infection of cultured cells is transported inefficiently to the Golgi. Despite lacking the YSRV signal sequence, gB-36 is internalized from the plasma membrane; however, in contrast to native gB, it fails to localize to the Golgi. Therefore, the C-terminal 36 amino acids of the VZV gB cytoplasmic domain are required for normal viral egress and for both the pre- and post-Golgi transport of gB.

Involvement of cellular cytoskeleton components in antibody-induced internalization of viral glycoproteins in pseudorabies virus-infected monocytes

Addition of pseudorabies virus (PrV)-specific polyclonal immunoglobulins to PrV-infected monocytes induces internalization of plasma membrane-anchored viral glycoproteins and this may interfere with antibody-dependent cell lysis. We investigated the role of actin, microtubules, clathrin, and dynein, the major cellular components involved in physiological endocytosis during this virological internalization. Porcine monocytes were infected in vitro for 13 h and afterward treated with different concentrations of colchicine, cytochalasin D, latrunculin B, and amantadine-HCl, which inhibit polymerization of microtubules, actin/clathrin, actin, and clathrin, respectively. This resulted in a significant reduction of internalization compared to the nontreated control, indicating that these components are involved in the process. A double labeling was performed during the internalization process and a clear colocalization of actin, microtubules, clathrin, and dynein with the viral glycoproteins was observed at different stages during the internalization process. We conclude that these cellular components are used by PrV to generate the antibody-induced internalization of viral glycoproteins.

Varicella-zoster virus ORF47 protein serine kinase: characterization of a cloned, biologically active phosphotransferase and two viral substrates, ORF62 and ORF63

Varicella-zoster virus (VZV) codes for a protein serine kinase called ORF47; the herpes simplex virus (HSV) homolog is UL13. No recombinant alphaherpesvirus serine kinase has been biologically active in vitro. We discovered that preservation of the intrinsic kinase activity of recombinant VZV ORF47 required unusually stringent in vitro conditions, including physiological concentrations of polyamines. In this assay, ORF47 phosphorylated two VZV regulatory proteins: the ORF62 protein (homolog of HSV ICP4) and the ORF63 protein (homolog of HSV ICP22). Of interest, ORF47 kinase also coprecipitated ORF63 protein from the kinase assay supernatant.

VZV gB endocytosis and Golgi localization are mediated by YXXphi motifs in its cytoplasmic domain

The cytoplasmic domains of many membrane proteins contain sorting signals that mediate their endocytosis from the plasma membrane. VZV gB contains three consensus internalization motifs within its cytoplasmic domain: YMTL (aa 818-821), YSRV (aa 857-860), and LL (aa 841-842). To determine whether VZV gB is internalized from the plasma membrane, and whether these motifs are required for its endocytosis, we compared the internalization of native gB to that of gB containing mutations in each of the predicted internalization motifs. VZV gB present on the surface of transfected cells associated with clathrin and was efficiently internalized to the Golgi apparatus within 60 min at 37 degrees C. VZV gB containing the mutation Y857 failed to be internalized, while gB-Y818A was internalized but did not accumulate in the Golgi. These data indicate that the internalization of VZV gB, and its subsequent localization to the Golgi, is mediated by two tyrosine-based sequence motifs in its cytoplasmic domain.

Immune response to vaccinia virus recombinants expressing glycoproteins gE, gB, gH, and gL of Varicella-zoster virus

Immunogenicity of Varicella-zoster virus glycoproteins gE, gB, gH, and gL expressed by recombinant vaccinia viruses (VV) separately or simultaneously was determined in mice and guinea pigs by ELISA, Western blotting, radioimmunoprecipitation, plaque reduction assay, and skin test. Single VV-gE and VV-gB recombinants and double VV-gH/gL recombinant elicited specific antibodies with VZV neutralizing activity in mice. Co-expression of gE and gB by one recombinant VV resulted in an increased antibody response in comparison with immunization with single recombinants or their mixtures. Unlike anti-gB and anti-gH/gL antibodies, the gE-specific antibodies had no virus neutralizing activity in absence of complement, and when used alone, they even caused considerable increase of VZV infectious units. Moreover, immune sera containing anti-gE antibodies antagonized complement independent virus-neutralizing activity of anti-gB- and anti-gH/gL-positive sera. The ability to induce delayed hypersensitivity reaction to VZV antigens was observed after immunization of guinea pigs with gE- and/or gB-expressing VVs.

Cytoplasmic domain of herpes simplex virus gE causes accumulation in the trans-Golgi network, a site of virus envelopment and sorting of virions to cell junctions

Alphaherpesviruses express a heterodimeric glycoprotein, gE/gI, that facilitates cell-to-cell spread between epithelial cells and neurons. Herpes simplex virus (HSV) gE/gI accumulates at junctions formed between polarized epithelial cells at late times of infection. However, at earlier times after HSV infection, or when gE/gI is expressed using virus vectors, the glycoprotein localizes to the trans-Golgi network (TGN). The cytoplasmic (CT) domains of gE and gI contain numerous TGN and endosomal sorting motifs and are essential for epithelial cell-to-cell spread. Here, we swapped the CT domains of HSV gE and gI onto another HSV glycoprotein, gD. When the gD-gI(CT) chimeric protein was expressed using a replication-defective adenovirus (Ad) vector, the protein was found on both the apical and basolateral surfaces of epithelial cells, as was gD. By contrast, the gD-gE(CT) chimeric protein, gE/gI, and gE, when expressed by using Ad vectors, localized exclusively to the TGN. However, gD-gE(CT), gE/gI, and TGN46, a cellular TGN protein, became redistributed largely to lateral surfaces and cell junctions during intermediate to late stages of HSV infection. Strikingly, gE and TGN46 remained sequestered in the TGN when cells were infected with a gI(-)HSV mutant. The redistribution of gE/gI to lateral cell surfaces did not involve widespread HSV inhibition of endocytosis because the transferrin receptor and gE were both internalized from the cell surface. Thus, gE/gI accumulates in the TGN in early phases of HSV infection then moves to lateral surfaces, to cell junctions, at late stages of infection, coincident with the redistribution of a TGN marker. These results are related to recent observations that gE/gI participates in the envelopment of nucleocapsids into cytoplasmic vesicles (A. R. Brack, B. G. Klupp, H. Granzow, R. Tirabassi, L. W. Enquist, and T. C. Mettenleiter, J. Virol. 74:4004-4016, 2000) and that gE/gI can sort nascent virions from cytoplasmic vesicles specifically to the lateral surfaces of epithelial cells (D. C. Johnson, M. Webb, T. W. Wisner, and C. Brunetti, J. Virol. 75:821-833, 2000). Therefore, gE/gI localizes to the TGN, through interactions between the CT domain of gE and cellular sorting machinery, and then participates in envelopment of cytosolic nucleocapsids there. Nascent virions are then sorted from the TGN to cell junctions.

Herpes simplex virus gE/gI sorts nascent virions to epithelial cell junctions, promoting virus spread

Alphaherpesviruses spread rapidly through dermal tissues and within synaptically connected neuronal circuitry. Spread of virus particles in epithelial tissues involves movement across cell junctions. Herpes simplex virus (HSV), varicella-zoster virus (VZV), and pseudorabies virus (PRV) all utilize a complex of two glycoproteins, gE and gI, to move from cell to cell. HSV gE/gI appears to function primarily, if not exclusively, in polarized cells such as epithelial cells and neurons and not in nonpolarized cells or cells that form less extensive cell junctions. Here, we show that HSV particles are specifically sorted to cell junctions and few virions reach the apical surfaces of polarized epithelial cells. gE/gI participates in this sorting. Mutant HSV virions lacking gE or just the cytoplasmic domain of gE were rarely found at cell junctions; instead, they were found on apical surfaces and in cell culture fluids and accumulated in the cytoplasm. A component of the AP-1 clathrin adapter complexes, mu1B, that is involved in sorting of proteins to basolateral surfaces was involved in targeting of PRV particles to lateral surfaces. These results are related to recent observations that (i) HSV gE/gI localizes specifically to the trans-Golgi network (TGN) during early phases of infection but moves out to cell junctions at intermediate to late times (T. McMillan and D. C. Johnson, J. Virol., in press) and (ii) PRV gE/gI participates in envelopment of nucleocapsids into cytoplasmic membrane vesicles (A. R. Brack, B. G. Klupp, H. Granzow, R. Tirabassi, L. W. Enquist, and T. C. Mettenleiter, J. Virol. 74:4004-4016, 2000). Therefore, interactions between the cytoplasmic domains of gE/gI and the AP-1 cellular sorting machinery cause glycoprotein accumulation and envelopment into specific TGN compartments that are sorted to lateral cell surfaces. Delivery of virus particles to cell junctions would be expected to enhance virus spread and enable viruses to avoid host immune defenses.

Role of the cytoplasmic tails of pseudorabies virus glycoproteins B, E and M in intracellular localization and virion incorporation

The cytoplasmic domains of several herpesviral glycoproteins encompass potential intracellular sorting signals. To analyse the function of the cytoplasmic domains of different pseudorabies virus (PrV) glycoproteins, hybrid proteins were constructed consisting of the extracellular and transmembrane domains of envelope glycoprotein D (gD) fused to the cytoplasmic tails of gB, gE or gM (designated gDB, gDE and gDM), all of which contain putative endocytosis motifs. gD is a type I membrane protein required for binding to and entry into target cells. Localization of hybrid proteins compared to full-length gB, gE and gM as well as carboxy-terminally truncated variants of gD was studied by confocal laser scanning microscopy. The function of gD hybrids was assayed by trans-complementation of a gD-negative PrV mutant. The carboxy-terminal domains of gB and gM directed a predominantly intracellular localization of gDB and gDM, while full-length gD and a tail-less gD mutant (gDc) were preferentially expressed on the cell surface. In contrast gDE, and a gDB lacking the putative gB endocytosis signal (gDB Delta 29), were predominantly located in the plasma membrane. Despite the different intracellular localization, all tested proteins were able to complement infectivity of a PrV gD(-) mutant. Cells which stably express full-length gD and plasma-membrane-associated gD hybrids exhibit a significant resistance to PrV infection, while cells expressing predominantly intracellularly located forms do not. This suggests that the assumed sequestration of receptors by gD, which is supposed to be responsible for the interference phenomenon, occurs at the cell surface.

Essential role played by the C-terminal domain of glycoprotein I in envelopment of varicella-zoster virus in the trans-Golgi network: interactions of glycoproteins with tegument

Varicella-zoster virus (VZV) is enveloped in the trans-Golgi network (TGN). Here we report that glycoprotein I (gI) is required within the TGN for VZV envelopment. Enveloping membranous TGN cisternae were microscopically identified in cells infected with intact VZV. These sacs curved around, and ultimately enclosed, nucleocapsids. Tegument coated the concave face of these sacs, which formed the viral envelope, but the convex surface was tegument-free. TGN cisternae of cells infected with VZV mutants lacking gI (gI(Delta)) or its C (gI(DeltaC))- or N-terminal (gI(DeltaN))-terminal domains were uniformly tegument coated and adhered to one another, forming bizarre membranous stacks. Viral envelopment was compromised, and no virions were delivered to post-Golgi structures. The TGN was not gI-immunoreactive in cells infected with the gI(Delta) or gI(DeltaN) mutants, but it was in cells infected with gI(DeltaC) (because the ectodomains of gI and gE interact). The presence in the TGN of gI lacking a C-terminal domain, therefore, was not sufficient to maintain enveloping cisternae. In cells infected with intact VZV or with gI(Delta), gI(DeltaN), or gI(DeltaC) mutants, ORF10p immunoreactivity was concentrated on the cytosolic face of TGN membranes, suggesting that it interacts with the cytosolic domains of glycoproteins. Because of the gE-gI interaction, cotransfected cells that expressed gE or gI were able to target truncated forms of the other to the TGN. Our data suggest that the C-terminal domain of gI is required to segregate viral and cellular proteins in enveloping TGN cisternae.

Identification of two intracellular mechanisms leading to reduced expression of oncoretrovirus envelope glycoproteins at the cell surface

All retrovirus glycoproteins have a cytoplasmic domain that plays several roles in virus replication. We have determined whether and how the cytoplasmic domains of oncoretrovirus glycoproteins modulate their intracellular trafficking, by using chimeric proteins that combined the alpha-chain of the interleukin-2 receptor with the glycoprotein cytoplasmic domains of five oncoretroviruses: human T-cell leukemia virus type 1 (HTLV-1), Rous sarcoma virus (RSV), bovine leukemia virus (BLV), murine leukemia virus (MuLV), and Mason-Pfizer monkey virus (MPMV). All of these proteins were synthesized and matured in the same way as a control protein with no retrovirus cytoplasmic domain. However, the amounts of all chimeric proteins at the cell surface were smaller than that of the control protein. The protein appearing at and leaving the cell surface and endocytosis were measured in stable transfectants expressing the chimera. We identified two groups of proteins which followed distinct intracellular pathways. Group 1 included chimeric proteins that reached the cell surface normally but were rapidly endocytosed afterwards. This group included the chimeric proteins with HTLV-1, RSV, and BLV cytoplasmic domains. Group 2 included chimeric proteins that were not detected at the cell surface, despite normal intracellular concentrations, and were accumulated in the Golgi complex. This group included the chimeric proteins with MuLV and MPMV cytoplasmic domains. Finally, we verified that the MuLV envelope glycoproteins behaved in the same way as the corresponding chimeras. These results indicate that retroviruses have evolved two distinct mechanisms to ensure a similar biological feature: low concentrations of their glycoproteins at the cell surface.

Glycoprotein E of varicella-zoster virus enhances cell-cell contact in polarized epithelial cells

Varicella-zoster virus (VZV) infection involves the cell-cell spread of virions, but how viral proteins interact with the host cell membranes that comprise intercellular junctions is not known. Madin-Darby canine kidney (MDCK) cells were constructed to express the glycoproteins gE, gI, or gE/gI constitutively and were used to examine the effects of these VZV glycoproteins in polarized epithelial cells. At low cell density, VZV gE induced partial tight junction (TJ) formation under low-calcium conditions, whether expressed alone or with gI. Although most VZV gE was intracellular, gE was also shown to colocalize with the TJ protein ZO-1 with or without concomitant expression of gI. Freeze fracture electron microscopy revealed normal TJ strand morphology in gE-expressing MDCK cells. Functionally, the expression of gE was associated with a marked acceleration in the establishment of maximum transepithelial electrical resistance (TER) in MDCK-gE cells; MDCK-gI and MDCK-gE/gI cells exhibited a similar pattern of early TER compared to MDCK cells, although peak resistances were lower than those of gE alone. VZV gE expression altered F-actin organization and lipid distribution, but coexpression of gI modulated these effects. Two regions of the gE ectodomain, amino acids (aa) 278 to 355 and aa 467 to 498, although lacking Ca(2+) binding motifs, exhibit similarities with corresponding regions of the cell adhesion molecules, E-cadherin and desmocollin. These observations suggest that VZV gE and gE/gI may contribute to viral pathogenesis by facilitating epithelial cell-cell contacts.

Cytoplasmic domain signal sequences that mediate transport of varicella-zoster virus gB from the endoplasmic reticulum to the Golgi

Normal herpesvirus assembly and egress depend on the correct intracellular localization of viral glycoproteins. While several post-Golgi transport motifs have been characterized within the cytoplasmic domains of various viral glycoproteins, few specific endoplasmic reticulum (ER)-to-Golgi transport signals have been described. We report the identification of two regions within the 125-amino-acid cytoplasmic domain of Varicella-Zoster virus gB that are required for its ER-to-Golgi transport. Native gB or gB containing deletions and specific point mutations in its cytoplasmic domain was expressed in mammalian cells. ER-to-Golgi transport of gB was assessed by indirect immunofluorescence and by the acquisition of Golgi-dependent posttranslational modifications. These studies revealed that the ER-to-Golgi transport of gB requires a nine-amino-acid region (YMTLVSAAE) within its cytoplasmic domain. Mutations of individual amino acids within this region markedly impaired the transport of gB from the ER to the Golgi, indicating that this domain functions by a sequence-dependent mechanism. Deletion of the C-terminal 17 amino acids of the gB cytoplasmic domain was also shown to impair the transport of gB from the ER to the Golgi. However, internal mutations within this region did not disrupt the transport of gB, indicating that its function during gB transport is not sequence dependent. Native gB is also transported to the nuclear membrane of transfected cells. gB lacking as many as 67 amino acids from the C terminus of its cytoplasmic domain continued to be transported to the nuclear membrane at apparently normal levels, indicating that the cytoplasmic domain of gB is not required for nuclear membrane localization.

Varicella-zoster virus gE escape mutant VZV-MSP exhibits an accelerated cell-to-cell spread phenotype in both infected cell cultures and SCID-hu mice

Varicella-zoster virus is considered to have one of the most stable genomes of all human herpesviruses. In 1998, we reported the unanticipated discovery of a wild-type virus that had lost an immunodominant B-cell epitope on the gE ectodomain (VZV-MSP); the gE escape mutant virus exhibited an unusual pattern of egress. Further studies have now documented a markedly enhanced cell-to-cell spread by the mutant virus in cell culture. This property was investigated by laser scanning confocal microscopy combined with a software program that allows the measurement of pixel intensity of the fluorescent signal. For this new application of imaging technology, the VZV immediate early protein 62 (IE 62) was selected as the fluoresceinated marker. By 48 h postinfection, the number of IE 62-positive pixels in the VZV-MSP-infected culture was nearly fourfold greater than the number of pixels in a culture infected with a low-passage laboratory strain. Titrations by infectious center assays supported the above image analysis data. Confirmatory studies in the SCID-hu mouse documented that VZV-MSP spread more rapidly than other VZV strains in human fetal skin implants. Generally, the cytopathology and vesicle formation produced by other strains at 21 days postinfection were demonstrable with VZV-MSP at 14 days. To assess whether additional genes were contributing to the unusual VZV-MSP phenotype, approximately 20 kb of the VZV-MSP genome was sequenced, including ORFs 31 (gB), 37 (gH), 47, 60 (gL), 61, 62 (IE 62), 66, 67 (gI), and 68 (gE). Except for a few polymorphisms, as well as the previously discovered mutation within gE, the nucleotide sequences within most open reading frames were identical to the prototype VZV-Dumas strain. In short, VZV-MSP represents a novel variant virus with a distinguishable phenotype demonstrable in both infected cell cultures and SCID-hu mice.

The role of virion membrane protein endocytosis in the herpesvirus life cycle

Endocytosis of cellular surface membrane proteins is a well-characterized, common occurrence. Internalization of cell surface receptors, often with bound ligands, aid in global events, such as cellular metabolism, as well as in specific, directed functions, such as the induction of signal transduction cascades or immune function. Some, but not all, herpesvirus membrane proteins are internalized from the plasma membrane by a process similar to receptor-mediated endocytosis. No known functions, however, have been ascribed to endocytosis of these proteins. In this review, we consider the function of herpesvirus membrane protein endocytosis. We compare and contrast the endocytosis and intracellular trafficking of two pseudorabies virus membrane proteins, the type I glycoprotein, gE, and the type II, tail-anchored membrane protein, Us9, and discuss the possible function of their internalization during the virus life cycle.

Trafficking of varicella-zoster virus glycoprotein gI: T(338)-dependent retention in the trans-Golgi network, secretion, and mannose 6-phosphate-inhibitable uptake of the ectodomain

The trans-Golgi network (TGN) is putatively the site where varicella-zoster virus is enveloped. gE is targeted to the TGN by selective retrieval from the plasmalemma in response to signaling sequences in its endodomain. gI lacks these sequences but forms a complex with gE. We now find that gI is targeted to the TGN and plasma membrane when expressed in Cos-7 cells; nevertheless, surface labeling revealed that gI is not retrieved from the plasma membrane. TGN targeting of gI depended on the T(338) of its endodomain and was lost when T(338) was deleted or mutated to A, S, or D. The endodomain of gI was sufficient, if it contained T(338), to target a fusion protein containing the ectodomain of the human interleukin-2 receptor to the TGN. A truncated protein consisting only of the gI ectodomain was secreted and taken up by nontransfected cells. This uptake of the secreted gI ectodomain was blocked by mannose 6-phosphate. Following cotransfection, both gI and gE were retrieved to the TGN from the plasma membrane in 26.7% of cells, neither gI nor gE was internalized in 18.3%, and gE was retrieved to the TGN while gI remained at the plasma membrane in 55%. We suggest that the T(338) of its endodomain is necessary to retain gI in the TGN; moreover, because gI and gE interact, the signaling sequences of each glycoprotein reinforce one another in ensuring that both glycoproteins are concentrated in the TGN yet remain on the cell surface.

Characterization of interaction of gH and gL glycoproteins of varicella-zoster virus: their processing and trafficking

Varicella-zoster virus (VZV) glycoproteins gH and gL were examined in a recombinant vaccinia virus system. Single expression of glycoprotein gL produced two molecular forms: an 18 kDa form and a 19 kDa form differing in size by one endoglycosidase H-sensitive N-linked oligosaccharide. Coexpression of gL and gH resulted in binding of the 18 kDa gL form with the mature form of gH, while the 19 kDa gL form remained uncomplexed. The glycosylation processing of gL was not dependent on gH; however, gL was required for the conversion of precursor gH (97 kDa) to mature gH (118 kDa). Subsequent analyses indicated that gL (18 kDa) was a more completely processed gL (19 kDa). Screening of the culture media revealed that gH and gL were secreted, but only if coexpressed and complexed together. The secreted form of gL was 18 kDa while that of gH was 114 kDa. The fact that secreted gH was smaller than intracytoplasmic gH suggested a proteolytic processing event prior to secretion. The 19 kDa form of gL was never secreted. These findings support a VZV gL recycling pathway between the endoplasmic reticulum and the cis-Golgi apparatus.

Role of the cytoplasmic tail of pseudorabies virus glycoprotein E in virion formation

Glycoproteins M (gM), E (gE), and I (gI) of pseudorabies virus (PrV) are required for efficient formation of mature virions. The simultaneous absence of gM and the gE/gI complex results in severe deficiencies in virion morphogenesis and cell-to-cell spread, leading to drastically decreased virus titers and a small-plaque phenotype (A. Brack, J. Dijkstra, H. Granzow, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 73:5364-5372, 1999). Serial passaging in noncomplementing cells of a virus mutant unable to express gM, gE, and gI resulted in a reversion of the small-plaque phenotype and restoration of infectious virus formation to the level of a gM(-) mutant. Genetic analyses showed that reversion of the phenotype was accompanied by a genomic rearrangement which led to the fusion of a portion of the gE gene encoding the cytoplasmic domain to the 3' end of the glycoprotein D gene, resulting in expression of a chimeric gD-gE protein. Since this indicated that the intracytoplasmic domain of gE was responsible for the observed phenotypic alterations, the UL10 (gM) gene was deleted in a PrV mutant, PrV-107, which specifically lacked the cytoplasmic tail of gE. Regarding one-step growth, plaque size, and virion formation as observed under the electron microscope, the mutant lacking gM and the gE cytoplasmic tail proved to be very similar to the gE/I/M triple mutant. Thus, our data indicate that it is the cytoplasmic tail of gE which is responsible for the observed phenotypic effects in conjunction with deletion of gM. We hypothesize that the cytoplasmic domain of gE specifically interacts with components of the capsid and/or tegument, leading to efficient secondary envelopment of intracytoplasmic capsids.

Directional transneuronal infection by pseudorabies virus is dependent on an acidic internalization motif in the Us9 cytoplasmic tail

The Us9 gene is conserved among most alphaherpesviruses. In pseudorabies virus (PRV), the Us9 protein is a 98-amino-acid, type II membrane protein found in the virion envelope. It localizes to the trans-Golgi network (TGN) region in infected and transfected cells and is maintained in this compartment by endocytosis from the plasma membrane. Viruses with Us9 deleted have no observable defects in tissue culture yet have reduced virulence and restricted spread to retinorecipient neurons in the rodent brain. In this report, we demonstrate that Us9-promoted transneuronal spread in vivo is dependent on a conserved acidic motif previously shown to be essential for the maintenance of Us9 in the TGN region and recycling from the plasma membrane. Mutant viruses with the acidic motif deleted have an anterograde spread defect indistinguishable from that of Us9 null viruses. Transneuronal spread, however, is not dependent on a dileucine endocytosis motif in the Us9 cytoplasmic tail. Through alanine scanning mutagenesis of the acidic motif, we have identified two conserved tyrosine residues that are essential for Us9-mediated spread as well as two serine residues, comprising putative consensus casein kinase II sites, that modulate the rate of PRV transneuronal spread in vivo.

Human cytomegalovirus pp28 (UL99) localizes to a cytoplasmic compartment which overlaps the endoplasmic reticulum-golgi-intermediate compartment

Although the assembly of herpesviruses has remained an active area of investigation, considerable controversy continues to surround the cellular location of tegument and envelope acquisition. This controversy is particularly evident when the proposed pathways for alpha- and beta-herpesvirus assembly are compared. We have approached this aspect of human cytomegalovirus (HCMV) assembly, specifically, envelopment, by investigating the intracellular trafficking of viral tegument proteins which localize in the cytoplasms of infected cells. In this study we have demonstrated that the virion tegument protein pp28 (UL99), a true late protein, was membrane associated as a result of myristoylation. A mutation in this protein which prevented incorporation of [(3)H]myristic acid also altered the detergent solubility and intracellular distribution of the protein when it was expressed in transfected cells. Using a panel of markers for intracellular compartments, we could localize the expression of wild-type pp28 to an intracellular compartment which colocalized with the endoplasmic reticulum-Golgi-intermediate compartment (ERGIC), a dynamic compartment of the secretory pathway which interfaces with both the ER and Golgi apparatus. The localization of this viral tegument protein within an early secretory compartment of the cell provided further evidence that the assembly of the HCMV tegument likely includes a cytoplasmic phase. Because pp28 has been shown to be localized to a cytoplasmic assembly compartment in HCMV-infected cells, our findings also suggested that viral tegument protein interactions within the secretory pathway may have an important role in the assembly of the virion.

The extracellular domain of herpes simplex virus gE is sufficient for accumulation at cell junctions but not for cell-to-cell spread

Herpes simplex virus (HSV) expresses a number of membrane glycoproteins, including gB, gD, and gH/gL, that function in both entry of virus particles and movement of virus from an infected cell to an uninfected cell (cell-to-cell spread). However, a complex of HSV glycoproteins gE and gI (gE/gI) is required for efficient cell-to-cell spread, especially between cells that form extensive cell junctions, yet it is not necessary for entry of extracellular virions. We previously showed that gE/gI has the capacity to localize specifically to cell junctions; the glycoprotein complex was found at lateral surfaces of cells in contact with other cells but not at those lateral surfaces not forming junctions or at apical surfaces. By virtue of these properties, gE/gI is an important molecular handle on the poorly understood process of cell-to-cell spread. Here, we show that the cytoplasmic domain of gE is important for the proper delivery of gE/gI to lateral surfaces of cells. Without this domain, gE/gI is found on the apical surface of epithelial cells, and more uniformly in the cytoplasm, although incorporation into the virion envelope is unaffected. However, even without proper trafficking signals, a substantial fraction of gE/gI retained the capacity to accumulate at cell junctions. Therefore, the extracellular domain of gE can mediate accumulation of gE/gI at cell junctions, if the glycoprotein can be delivered there, probably through interactions with ligands on the opposing cell. The role of phosphorylation of the cytoplasmic domain of gE was also studied. A second mutant HSV type 1 was constructed in which three serine residues that form a casein kinase II phosphorylation site were changed to alanine residues, reducing phosphorylation by 70 to 80%. This mutation did not affect accumulation at cell junctions or cell-to-cell spread.

Bovine herpesvirus 5 glycoprotein E is important for neuroinvasiveness and neurovirulence in the olfactory pathway of the rabbit

Glycoprotein E (gE) is important for full virulence potential of the alphaherpesviruses in both natural and laboratory hosts. The gE sequence of the neurovirulent bovine herpesvirus 5 (BHV-5) was determined and compared with that of the nonneurovirulent BHV-1. Alignment of the predicted amino acid sequences of BHV-1 and BHV-5 gE open reading frames showed that they had 72% identity and 77% similarity. To determine the role of gE in the differential neuropathogenesis of BHV-1 and BHV-5, we have constructed BHV-1 and BHV-5 recombinants: gE-deleted BHV-5 (BHV-5gEDelta), BHV-5 expressing BHV-1 gE (BHV-5gE1), and BHV-1 expressing BHV-5 gE (BHV-1gE5). Neurovirulence properties of these recombinant viruses were analyzed using a rabbit seizure model (S. I. Chowdhury et al., J. Comp. Pathol. 117:295-310, 1997) that distinguished wild-type BHV-1 and -5 based on their differential neuropathogenesis. Intranasal inoculation of BHV-5 gEDelta and BHV-5gE1 produced significantly reduced neurological signs that affected only 10% of the infected rabbits. The recombinant BHV-1gE5 did not invade the central nervous system (CNS). Virus isolation and immunohistochemistry data suggest that these recombinants replicate and spread significantly less efficiently in the brain than BHV-5 gE revertant or wild-type BHV-5, which produced severe neurological signs in 70 to 80% rabbits. Taken together, the results of neurological signs, brain lesions, virus isolation, and immunohistochemistry indicate that BHV-5 gE is important for efficient neural spread and neurovirulence within the CNS and could not be replaced by BHV-1 gE. However, BHV-5 gE is not required for initial viral entry into olfactory pathway.

The Rous sarcoma virus Env glycoprotein contains a highly conserved motif homologous to tyrosine-based endocytosis signals and displays an unusual internalization phenotype

The cytoplasmic domains of retroviral transmembrane (TM) glycoproteins contain conserved sequence motifs that resemble tyrosine-based (YXXO-type) endocytosis signals. We have previously described a mutant Rous sarcoma virus (RSV) Env protein, Env-mu26, with an L165R mutation in the membrane-spanning domain (MSD) of TM, that exhibited dramatically decreased steady-state surface expression (G. L. Davis and E. Hunter, J. Cell Biol. 105:1191-1203, 1987; P. B. Johnston, J. Y. Dong, and E. Hunter, Virology 206:353-361, 1995). We now demonstrate that the tyrosine of the Y(190)RKM motif in the RSV TM cytoplasmic domain is crucial for the mu26 phenotype and is part of an efficient internalization signal in the context of a mutant MSD. In contrast, despite the presence of the Y(190)RKM motif, wild-type RSV Env is constitutively internalized at a slow rate (1.1%/min) more characteristic of bulk uptake during membrane turnover than of active clustering into endocytic vesicles. The mu26 mutation and two MSD mutations that abrogate palmitoylation of TM resulted in enhanced Env endocytosis indicative of active concentration into coated pits. Surprisingly, an Env-Y190A mutant was apparently excluded from coated pits since its uptake rate of 0.3%/min was significantly below that expected for the bulk rate. We suggest that in RSV Env an inherently functional endocytosis motif is silenced by a counteracting determinant in the MSD that acts to prevent clustering of Env into endocytic vesicles. Mutations in either the cytoplasmic tail or the MSD that inactivate one of the two counteracting signals would thus render the remaining determinant dominant.

An acidic cluster in the cytosolic domain of human cytomegalovirus glycoprotein B is a signal for endocytosis from the plasma membrane

We previously reported that human cytomegalovirus (CMV) glycoprotein B (gB) is transported to apical membranes in CMV-infected polarized retinal pigment epithelial (ARPE-19) cells and in Madin-Darby canine kidney (MDCK) epithelial cells constitutively expressing gB. The cytosolic domain of gB contains a cluster of acidic amino acids, a motif that plays a pivotal role in vectorial trafficking in polarized epithelial cells and may also function as a signal for entry into the endocytic pathway. Here we compared gB internalization and recycling to the plasma membrane in CMV-infected human fibroblasts (HF) and ARPE-19 cells by using antibody-internalization experiments. Immunofluorescence and quantitative assays showed that gB was internalized from the cell surface into clathrin-coated transport vesicles and then recycled to the plasma membrane. gB colocalized with clathrin-coated vesicles containing the transferrin receptor in the early endocytic/recycling pathway, indicating that gB traffics in this pathway. The specific role of the acidic cluster in regulating the sorting of gB-containing vesicles in the early endocytic/recycling pathway was examined in MDCK cells expressing mutated gB derivatives. Immunofluorescence assays showed that derivatives lacking the acidic cluster were impaired in internalization and failed to recycle. These findings, together with our earlier observation that the acidic cluster is a key determinant for targeting gB molecules to apical membranes in epithelial cells, establish that this signal is recognized by cellular proteins that participate in polarized sorting and transport in the early endocytic/recycling pathway.

Life beyond eradication: veterinary viruses in basic science

To some, the focus of research in virology entails the search for solutions of practical problems. By definition then, attention is limited to those viruses that cause disease or to exploitation of some aspect of virology to a practical end (e.g., antiviral drugs or vaccines). Once a disease is cured, or the agent eradicated, it is time to move on to something else. To others, virology offers the opportunity to study fundamental problems in biology. Work on these problems may offer no obvious practical justification; it is an affliction of the terminally curious, perhaps with the outside hope that something "useful" will come of it. To do this so-called "basic science", one must find the most tractable system to solve the problem, not the system that has "relevance" to disease. I have found that veterinary viruses offer a variety of opportunities to study relevant problems at the fundamental level. To illustrate this point, I describe some recent experiments in my laboratory using pseudorabies virus (PRV), a swine herpesvirus.