The bovine herpesvirus 4 Bo10 gene encodes a nonessential viral envelope protein that regulates viral tropism through both positive and negative effects

All gammaherpesviruses encode a glycoprotein positionally homologous to the Epstein-Barr virus gp350 and the Kaposi's sarcoma-associated herpesvirus (KSHV) K8.1. In this study, we characterized the positional homologous glycoprotein of bovine herpesvirus 4 (BoHV-4), encoded by the Bo10 gene. We identified a 180-kDa gene product, gp180, that was incorporated into the virion envelope. A Bo10 deletion virus was viable but showed a growth deficit associated with reduced binding to epithelial cells. This seemed to reflect an interaction of gp180 with glycosaminoglycans (GAGs), since compared to the wild-type virus, the Bo10 mutant virus was both less infectious for GAG-positive (GAG(+)) cells and more infectious for GAG-negative (GAG(-)) cells. However, we could not identify a direct interaction between gp180 and GAGs, implying that any direct interaction must be of low affinity. This function of gp180 was very similar to that previously identified for the murid herpesvirus 4 gp150 and also to that of the Epstein-Barr virus gp350 that promotes CD21(+) cell infection and inhibits CD21(-) cell infection. We propose that such proteins generally regulate virion attachment both by binding to cells and by covering another receptor-binding protein until they are displaced. Thus, they regulate viral tropism both positively and negatively depending upon the presence or absence of their receptor.     

Isolation of coronavirus envelope glycoproteins and interaction with the viral nucleocapsid.

The two envelope glycoproteins and the viral nucleocapsid of the coronavirus A59 were isolated by solubilization of the viral membrane with Nonidet P-40 at 4 degrees C followed by sucrose density gradient sedimentation. Isolated E2 consisted of rosettes of peplomers, whereas E1, the membrane glycoprotein, was irregular and amorphous. Under certain conditions significant interactions occurred between components of Nonidet P-40-disrupted virions. Incubation of the Nonidet P-40-disrupted virus at 37 degrees C resulted in formation of a complex between one of the viral glycoproteins, E1, and the viral nucleocapsid. This was caused by a temperature-dependent conformational change in E1, resulting in aggregation of E1 and interaction with the viral RNA in the nucleocapsid. E1 also bound rRNA. The E1-nucleocapsid complexes can be distinguished on sucrose and Renografin density gradients from native viral nucleocapsids. The separation of the membrane glycoprotein E1 from the peplomeric glycoprotein E2 permitted preparation of antisera against these isolated proteins. A model is proposed for the arrangement of the three major structural proteins in the coronavirus A59 virion in relation to the viral envelope and RNA.     

Quantitative analysis of the interaction between the envelope protein domains and the core protein of human hepatitis B virus

Interaction between preformed nucleocapsids and viral envelope proteins is critical for the assembly of virus particles in infected cells. The pre-S1 and pre-S2 and cytosolic regions of the human hepatitis B virus envelope protein had been implicated in the interaction with the core protein of nucleocapsids. The binding affinities of specific subdomains of the envelope protein to the core protein were quantitatively measured by both ELISA and BIAcore assay. While a marginal binding was detected with the pre-S1 or pre-S2, the core protein showed high affinities to pre-S with apparent dissociation constants (K(D)(app)) of 7.3+/-0.9 and 8.2+/-0.4microM by ELISA and BIAcore assay, respectively. The circular dichroism analysis suggested that conformational change occurs in pre-S through interaction with core protein. These results substantiate the importance of specific envelope domains in virion assembly, and demonstrate that the interaction between viral proteins can be quantitatively measured in vitro.     

Epstein-Barr virus (EBV) antigens processed and presented by B cells, B blasts, and macrophages trigger T-cell-mediated inhibition of EBV-induced B-cell transformation.

The ability of B cells, B blasts, and macrophages to present Epstein-Barr virion antigens to autologous T cells and trigger their capacity to inhibit Epstein-Barr virus-induced B-cell transformation was tested. Macrophages were as efficient as B cells and B blasts in presenting the virus to T lymphocytes. This function required antigen processing, because it was inhibited by chloroquine treatment and by fixation of the antigen-presenting cells immediately after viral exposure but not 18 h later. T cells exposed to the purified Epstein-Barr virus envelope antigen gp350 coupled to immunostimulating complexes also showed inhibitory function. These results suggest that recognition of processed virion antigens elicits the generation of T-cell-mediated inhibition of Epstein-Barr virus-induced B-cell transformation.     

High-resolution footprints of the DNA-binding domain of Epstein-Barr virus nuclear antigen 1.

The DNA-binding domain of Epstein-Barr virus nuclear antigen 1 was found by hydroxyl radical footprinting to protect backbone positions on one side of its DNA-binding site. The guanines contacted in the major groove by the DNA-binding domain of Epstein-Barr virus nuclear antigen 1 were identified by methylation protection. No difference was found in the interaction of the DNA-binding domain of Epstein-Barr virus nuclear antigen 1 with tandemly repeated and overlapping binding sites.     

Type-Common CP-1 Antigen of Herpes Simplex Virus Is Associated with a 59,000-Molecular-Weight Envelope Glycoprotein

The CP-1 antigen of herpes simplex virus type 1 (HSV-1) is a glycoprotein found in the soluble portion of infected cells, in detergent extracts of infected cell membranes, and in the envelope of purified virus. Antisera were prepared against a further purified form of CP-1 prepared from HSV soluble antigen mix; a glycoprotein, gp52, isolated from detergent-treated infected cells; and detergent extracts of purified virus. Each of the antisera reacted with CP-1 to give a single immunoprecipitin band of identity, and each antiserum neutralized the infectivity of HSV-1 and HSV-2. Our results suggested that the type-common determinants involved in the stimulation of neutralizing antibody resided on a 52,000-molecular-weight (52K) glycoprotein. The envelope of HSV contains several glycoproteins: one component at 59K and a complex of two or three components at 130K, none of which corresponds in molecular weight to gp52. Using the antisera as immunological probes, we performed pulse-chase experiments with [(35)S]methionine-labeled HSV-1-infected cells and followed the disposition of the glycoproteins during the infectious cycle. Each antiserum immunoprecipitated a (35)S-labeled 52K protein from lysates of cells pulse-labeled at 5 h after infection. By 10 h, the label was chased into a 59K protein also precipitable by each of the three antisera. The results suggest that gp52 is a precursor of gp59 and that the latter corresponds in molecular weight to one of the major glycoproteins of the virion envelope.     

Molecular mechanisms of virus spread and virion components as tools of virulence. A review

Despite of differences in replication strategy among virus families, some basic principles have remained similar. Analogous mechanisms govern virus entry into cells and the use of enzymes which direct the replication of the virus genome. The function of many cell surface receptors (such as glycosoaminoglycans, glycoproteins, proteins) which interact with viral capsid proteins or envelope glycoproteins has recently been elucidated. The list of cellular receptors (Table I) is still far from being final. The capsid components, similarly as the envelope glycoproteins, may form specific pocket like sites, which interact with the cell surface receptors. Neutralizing antibodies usually react with antigenic domains adjacent to the receptor binding site(s) and hamper the close contact inevitable for virion attachment. In the case of more complex viruses, such as herpes simplex virus, different viral glycoproteins interact with several cellular receptors. At progressed phase of adsorption the virions are engulfed into endocytic vesicles and the virion fusion domain(s) become(s) activated. The outer capsid components of reoviruses which participate in adsorption and fusion may get activated already in the lumen of digestive tract, i.e. before their engulfment by resorptive epithelium cells. Activation of the hydrophobic fusion domain(s) is a further important step allowing to pass through the lipid bilayer when penetrating the cell membrane in order to reach the cytosol. Activation of the virion fusion domain is accomplished by a conformation change, which occurs at acid pH (influenza virus hemagglutinin, sigma 1 protein of the reovirus particle) and/or after protease treatment. The herpes simplex virus fusion factors (gD and gH) undergo conformation changes by a pH-independent mechanism triggered due to interaction with the cell surface receptor(s) and mediated by mutual interactions with the viral envelope glycoproteins. The virion capsid or envelope components participating in the entry and membrane fusion are not the only tools of virulence. The correct function of virus coded proteins, which participate in replication of the viral genome, and/or in the supply of necessary nucleotides, may be very essential. In the case of enteroviruses, which RNA interacts with ribosomes directly, the correct configuration of the non-coding viral RNA sequence is crucial for initiation of translation occurring in the absence of the classical "cap" structure.     

Entry of alphaherpesviruses into cells

Studies on four alphaherpesviruses (herpes simplex virus types 1 and 2, pseudorabies virus and bovine herpesvirus 1) have revealed some common features of their entry into cells. The pathway of entry can be by fusion of the virion envelope with the cell plasma membrane. Receptors for binding and entry include heparan sulphate moieties of cell surface proteoglycans and other as yet unidentified cell surface components. Related glycoproteins specified by each of the four viruses mediate the binding of virus to heparan sulphate and subsequent molecular interactions leading to the penetration of virus into the cell.     

Identification of a new sea urchin vitelline envelope sperm binding glycoprotein

The sea urchin egg vitelline envelope (VE) is composed of eight major glycopolypeptides that are heavily mannosylated and contain fucose and N-acetylglucosamine moieties based on lectin staining. In the present study, the macromolecular composition of the VE and the potential role of a purified VE glycoprotein in initial gamete binding was investigated. The VE components were solubilized from the surface of intact, dejellied eggs with dithiothreitol in divalent cation-free seawater, and analyzed using native, reduced electrophoresis and immunoblotting. Three major VE glycoproteins, VE-A, VE-B and VE-C, and one minor component, VE-D, were identified with antisera against whole VE preparations and against glutaraldehyde-fixed, unfertilized eggs. The electrophoretically purified glycoproteins resolved into a common subunit doublet and one unique subunit each of decreasing size on blots of sodium dodecylsulfate polyacrylamide gels. Lectin affinity chromatography was used for analysis and purification of reduced VE components; a glycoprotein eluted from Con A columns with methyl-mannoside comigrated with VE-B when analyzed by immunoblotting. Whole VE preparations and VE-B obtained from Con A columns were found to inhibit fertilization when preincubated with sperm, thus directly establishing a role for VE-B in gamete binding.     

Identification of a GP64 subdomain involved in receptor binding by budded virions of the baculovirus Autographica californica multicapsid nucleopolyhedrovirus

Enveloped virus entry into host cells is typically initiated by an interaction between a viral envelope glycoprotein and a host cell receptor. For budded virions of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus, the envelope glycoprotein GP64 is involved in host cell receptor binding, and GP64 is sufficient to mediate low-pH-triggered membrane fusion. To better define the role of GP64 in receptor binding, we generated and characterized a panel of antisera against subdomains of GP64. Eight subdomain-specific antisera were generated, and their reactivities with GP64 proteins and neutralization of virus infectivity and binding were examined. Antibodies directed against the N-terminal region of GP64 (amino acids 21 to 159) showed strong neutralization of infectivity and effectively inhibited binding of (35)S-labeled budded virions to Sf9 cells. In addition, we generated virions displaying truncated GP64 constructs. A construct displaying the N-terminal 274 amino acids (residues 21 to 294) of the ectodomain was sufficient to mediate virion binding. Additional studies of antisera directed against small subdomains revealed that an antiserum against a 40-amino-acid region (residues 121 to 160) neutralized virus infectivity. Site-directed mutagenesis was subsequently used for functional analysis of that region. Recombinant viruses expressing GP64 proteins with single amino acid substitutions within amino acids 120 to 124 and 142 to 148 replicated to high titers, suggesting that those amino acids were not critical for receptor binding or other important GP64 functions. In contrast, GP64 proteins with single amino acid substitutions of residues 153 and 156 were unable to substitute for wild-type GP64 and did not rescue a gp64 knockout virus. Further analysis showed that these substitutions substantially reduced binding of recombinant virus to Sf9 cells. Thus, the amino acid region from positions 21 to 159 was identified as a putative receptor binding domain, and amino acids 153 and 156 appear to be important for receptor binding.     

Early steps of the hepatitis C virus life cycle

To replicate its genome, a virus needs to cross the plasma membrane of a host cell and get access to cytosolic and/or nuclear components. For an enveloped virus, this involves binding to the plasma membrane, followed by migration of the virion to a microdomain or an endosomal vesicle where fusion between the virion envelope and a host cell membrane occurs. Increasing evidences indicate that virus entry is a tightly regulated process. Although we are still far from understanding the details of hepatitis C virus (HCV) entry, recent data show that this virus enters into target cells in a slow and complex multistep process involving the presence of several entry factors. Initial attachment of the virion may involve glycosaminoglycans and the low-density lipoprotein receptor, and it is followed by the sequential interaction with the scavenger receptor class B type I, the tetraspanin CD81 and tight junction protein Claudin-1, -6 or -9. Furthermore, the identification of EWI-2wint as a new partner of CD81 which blocks E2–CD81 interaction provides additional evidence of the complexity of the HCV entry process. The current knowledge accumulated on HCV entry is summarized in this review.     

Characterization of the vitelline envelope of the sea urchin Strongylocentrotus purpuratus

The vitelline envelope (VE) is an extremely thin, acellular, proteinaceous coat that surrounds the extracellular surface of sea urchin eggs. Despite previous studies on VE composition, structure and function, our understanding of the envelope is still incomplete at the molecular level. We have isolated VE components from intact, unfertilized Strongylocentrotus purpuratus eggs by reduction with alkaline dithiothreitol-sea water solutions and have characterized the macromolecules by SDS-PAGE. There were eight major glycoprotein bands, including two high molecular weight components at 265 and 300 kDa, and several minor components. We have revealed, by lectin blot analysis, that most components contain mannose, while a subset of glycoproteins contain fucose and N -acetylglucosamine; galactose and sialic acid were also detected. The components in the VE preparations were compared with cell surface complex preparations by immunoblot analysis, using antisera against a VE preparation, a 305 kDa electrophoretically purified VE glycoprotein and an extracellular portion of the sea urchin egg recombinant 350 kDa sperm receptor. Serum against the recombinant sperm receptor reacted with a component of ∼350 kDa on blots, but did not react with the 300 kDa component found in VE preparations. Therefore, we suggest these two glycoproteins are not the same.     

Specific interaction of hepatitis C virus glycoproteins with mannan binding lectin inhibits virus entry

Mannan-binding lectin (MBL) is a soluble innate immune protein that binds to glycosylated targets. MBL acts as an opsonin and activates complement, contributing to the destruction and clearance of infecting microorganisms. Hepatitis C virus (HCV) encodes two envelope glycoproteins E1 and E2, expressed as non-covalent E1/E2 heterodimers in the viral envelope. E1 and E2 are potential ligands for MBL. Here we describe an analysis of the interaction between HCV and MBL using recombinant soluble E2 ectodomain fragment, the full-length E1/E2 heterodimer, expressed in vitro, and assess the effect of this interaction on virus entry. A binding assay using antibody capture of full length E1/E2 heterodimers was used to demonstrate calcium dependent, saturating binding of MBL to HCV glycoproteins. Competition with various saccharides further confirmed that the interaction was via the lectin domain of MBL. MBL binds to E1/E2 representing a broad range of virus genotypes. MBL was shown to neutralize the entry into Huh-7 cells of HCV pseudoparticles (HCVpp) bearing E1/E2 from a wide range of genotypes. HCVpp were neutralized to varying degrees. MBL was also shown to neutralize an authentic cell culture infectious virus, strain JFH-1 (HCVcc). Furthermore, binding of MBL to E1/E2 was able to activate the complement system via MBL-associated serine protease 2. In conclusion, MBL interacts directly with HCV glycoproteins, which are present on the surface of the virion, resulting in neutralization of HCV particles.     

Low-pH-Dependent Fusion of Sindbis Virus with Receptor-Free Cholesterol- and Sphingolipid-Containing Liposomes

There is controversy as to whether the cell entry mechanism of Sindbis virus (SIN) involves direct fusion of the viral envelope with the plasma membrane at neutral pH or uptake by receptor-mediated endocytosis and subsequent low-pH-induced fusion from within acidic endosomes. Here, we studied the membrane fusion activity of SIN in a liposomal model system. Fusion was followed fluorometrically by monitoring the dilution of pyrene-labeled lipids from biosynthetically labeled virus into unlabeled liposomes or from labeled liposomes into unlabeled virus. Fusion was also assessed on the basis of degradation of the viral core protein by trypsin encapsulated in the liposomes. SIN fused efficiently with receptor-free liposomes, consisting of phospholipids and cholesterol, indicating that receptor interaction is not a mechanistic requirement for fusion of the virus. Fusion was optimal at pH 5.0, with a threshold at pH 6.0, and undetectable at neutral pH, supporting a cell entry mechanism of SIN involving fusion from within acidic endosomes. Under optimal conditions, 60 to 85% of the virus fused, depending on the assay used, corresponding to all of the virus bound to the liposomes as assessed in a direct binding assay. Preincubation of the virus alone at pH 5.0 resulted in a rapid loss of fusion capacity. Fusion of SIN required the presence of both cholesterol and sphingolipid in the target liposomes, cholesterol being primarily involved in low-pH-induced virus-liposome binding and the sphingolipid catalyzing the fusion process itself. Under low-pH conditions, the E2/E1 heterodimeric envelope glycoprotein of the virus dissociated, with formation of a trypsin-resistant E1 homotrimer, which kinetically preceded the fusion reaction, thus suggesting that the E1 trimer represents the fusion-active conformation of the viral spike.     

Hepatitis C virus E2 has three immunogenic domains containing conformational epitopes with distinct properties and biological functions

Mechanisms of virion attachment, interaction with its receptor, and cell entry are poorly understood for hepatitis C virus (HCV) because of a lack of an efficient and reliable in vitro system for virus propagation. Infectious HCV retroviral pseudotype particles (HCVpp) were recently shown to express native E1E2 glycoproteins, as defined in part by HCV human monoclonal antibodies (HMAbs) to conformational epitopes on E2, and some of these antibodies block HCVpp infection (A. Op De Beeck, C. Voisset, B. Bartosch, Y. Ciczora, L. Cocquerel, Z. Y. Keck, S. Foung, F. L. Cosset, and J. Dubuisson, J. Virol. 78:2994-3002, 2004). Why some HMAbs are neutralizing and others are nonneutralizing is looked at in this report by a series of studies to determine the expression of their epitopes on E2 associated with HCVpp and the role of antibody binding affinity. Antibody cross-competition defined three E2 immunogenic domains with neutralizing HMAbs restricted to two domains that were also able to block E2 interaction with CD81, a putative receptor for HCV. HCVpp immunoprecipitation showed that neutralizing and nonneutralizing domains are expressed on E2 associated with HCVpp, and affinity studies found moderate-to-high-affinity antibodies in all domains. These findings support the perspective that HCV-specific epitopes are responsible for functional steps in virus infection, with specific antibodies blocking distinct steps of virus attachment and entry, rather than the perspective that virus neutralization correlates with increased antibody binding to any virion surface site, independent of the epitope recognized by the antibody. Segregation of virus neutralization and sensitivity to low pH to specific regions supports a model of HCV E2 immunogenic domains similar to the antigenic structural and functional domains of other flavivirus envelope E glycoproteins.     

Identification and characterization of a novel structural glycoprotein in pseudorabies virus, gL.

Herpesvirus envelope glycoproteins play important roles in the interaction between virions and target cells. In the alphaherpesvirus pseudorabies virus (PrV), seven glycoproteins that all constitute homologs of glycoproteins found in herpes simplex virus type 1 (HSV-1) have been characterized, including a homolog of HSV-1 glycoprotein H (gH). Since HSV-1 gH is found associated with another essential glycoprotein, gL, we analyzed whether PrV also encodes a gL homolog. DNA sequence analysis of a corresponding part of the UL region adjacent to the internal inverted repeat in PrV strains Kaplan and Becker revealed the presence of two open reading frames (ORF). Deduced proteins exhibited homology to uracil-DNA glycosylase encoded by HSV-1 ORF UL2 (54% identity) and gL encoded by HSV-1 ORF UL1 (24% identity), respectively. To identify the PrV UL1 protein, rabbit antisera were prepared against two synthetic oligopeptides that were predicted by computer analysis to encompass antigenic epitopes. Sera against both peptides reacted in Western blots of purified virions with a 20-kDa protein. The specificity of the reaction was demonstrated by peptide competition. Since the PrV UL1 sequence did not reveal the presence of a consensus N-linked glycosylation site, concanavalin A affinity chromatography and enzymatic deglycosylation of virion glycoproteins were used to ascertain that the PrV UL1 product is O glycosylated. Therefore, we designated this protein PrV gL. Analysis of mutant PrV virions lacking gH showed that concomitantly with the absence of gH, gL was also missing in purified virions. In summary, we identified and characterized a novel structural PrV glycoprotein, gL, which represents the eighth PrV glycoprotein described. In addition, we show that virion location of PrV gL is dependent on the presence of PrV gH.     

Depletion of glycoprotein gp85 from virosomes made with Epstein-Barr virus proteins abolishes their ability to fuse with virus receptor-bearing cells.

Entry of an enveloped virus such as Epstein-Barr virus (EBV) into host cells involves fusion of the virion envelope with host cell membranes either at the surface of the cell or within endocytic vesicles. Previous work has indirectly implicated the EBV glycoprotein gp85 in this fusion process. A neutralizing monoclonal antibody to gp85, F-2-1, failed to inhibit binding of EBV to its receptor but interfered with virus fusion as measured with the self-quenching fluorophore octadecyl rhodamine B chloride (R18) (N. Miller and L. M. Hutt-Fletcher, J. Virol. 62:2366-2372, 1988). To test further the hypothesis that gp85 functions as a fusion protein, EBV virion proteins including or depleted of gp85 were incorporated into lipid vesicles to form virosomes. Virosomes were labeled with R18, and those that were made with undepleted protein were shown to behave in a manner similar to that of R18-labeled virus. They bound to receptor-positive but not to receptor-negative cells and fused with Raji cells but not with receptor-positive, fusion-incompetent Molt 4 cells; monoclonal antibodies that inhibited binding or fusion of virus inhibited binding and fusion of virosomes, and virus competed with virosomes for attachment to cells. In contrast, virosomes made from virus proteins depleted of gp85 by immunoaffinity chromatography remained capable of binding to receptor-positive cells but failed to fuse. These results are compatible with the hypothesis that gp85 is actively involved in the fusion of EBV with lymphoblatoid cell lines and suggest that the ability of antibody F-2-1 to neutralize infectivity of EBV represents a direct effect on the function of gp85 as a fusion protein.     

Two Extracellular Matrices From Oocytes of the Marine Shrimp Sicyonia ingentis that Independently Mediate Only Primary or Secondary Sperm Binding

During spawning, female Sicyonia ingentis simultaneously release ova and stored nonmotile sperm and mix them externally to initiate gamete interaction. Sperm bind to a thin vitelline envelope (VE) via their anterior appendage and within seconds are induced to undergo acrosomal exocytosis. The sperm penetrate the VE and become secondarily bound to the surface coat (SC), a glycocalyx on the oocyte surface. In this study, both extracellular matrices were isolated from S. ingentis oocytes. Isolated VEs mediated only primary sperm binding (i.e., before the acrosome reaction), while the isolated SCs mediated only secondary sperm binding (i.e., after acrosomal exocytosis). Isolated S. ingentis VEs were used to characterize primary sperm binding activity. The two extracellular matrices differ morphologically and possess different polypeptide profiles. Soluble fractions of isolated VEs inhibited primary sperm binding in a concentration dependent manner, and immunolocalization of VE components demonstrated highly localized VE binding sites at the tip of the sperm anterior appendage by which sperm bind eggs. Extensive Pronase digestion of VE components did not affect sperm binding activity of solubilized VE components, while complete deglycosylation with trifluoromethanesulfonic acid destroyed sperm binding activity. However, neither alkaline treatment nor enzyme digestion using glycosidases specific for asparagine and serine/ threonine linked oligosaccharides affected sperm binding activity.     

Expanding baculovirus surface display. Modification of the native coat protein gp64 of Autographa californica NPV.

To create a tool for eukaryotic surface display, this approach is aimed at demonstrating a direct modification of the native envelope protein gp64 of Autographa californica NPV without disturbing viral infectivity. Short affinity-tag peptides, the biotin mimic streptagII, and the gp41 amino-acid motif ELDKWA of HIV-1, specific for the human monoclonal antibody 2F5, were engineered into the baculovirus major coat protein gp64 and presented on the viral surface. Two different streptag peptides were inserted at the naturally occurring NotI site at amino-acid 278 of gp64. Additionally, the ten-amino-acid peptide GG-ELDKWA-GG, containing the epitope of mAb 2F5, was introduced into gp64 envelope protein at the same position. In all cases we were able to propagate viable virus-achieving infectious titers in the range of wild-type AcMNPV. Streptag and ELDKWA-epitope surface localization on purified virus particles was demonstrated by flow cytometry and Western blot analysis. We could also show selective retention of mutant viruses by specific interaction between chimeric virions and their target counterparts, recognizing the epitope or the streptag peptide in the viral envelope. These data provide evidence that altering the surface properties of the baculovirus virion could be of value in improving baculovirus display technology and developing new applications.     

Association of human immunodeficiency virus type 1 envelope glycoprotein with particles depends on interactions between the third variable and conserved regions of gp120.

Many regions within the envelope of human immunodeficiency virus type 1 (HIV-1) that affect its structure and function have been identified. We have previously reported that the interaction of the second conserved (C2) and third variable (V3) regions of gp120 influences the ability of HIV-1 to establish a productive infection in susceptible cells. To better understand the basis for this interaction, we have conducted structure-function analyses of envelope expressed from molecular proviral clones of HIV-1 containing defined mutations in C2 and V3 that individually and in combination differentially affect envelope function. The substitution of a glutamine for an asparagine residue (Q-267) at a potential asparagine-linked glycosylation site in C2, which severely impairs virus infectivity, reduces intracellular processing of gp160 into gp120, the association of gp120 with virions, and the ability of gp120 to bind to the HIV-1 cell surface receptor protein, CD4. The change of an arginine to an isoleucine codon in V3 (I-308), in the presence of the Q-267 mutation, restores virus infectivity to near wild-type levels by increasing the amount of gp120 associated with virions as compared with the Q-267 mutant but does not compensate for the Q-267-induced processing defect. The I-308 change in the context of the wild-type HIV-1 has no affect on processing, association, or CD4 binding. These results indicate that the impaired infectivity of the Q-267 mutant virus is due to a marked reduction in the amount of virion gp120 and suggest that the interaction of C2 and V3 stabilizes the association of gp120 with gp41.