Host cell-dependent lateral mobility of viral glycoproteins

Responses of herpes simplex virus type 1-infected cells to the presence of extracellular antibodies: gE-dependent glycoprotein capping and enhancement in cell-to-cell spread

Binding of anti-herpes simplex virus (HSV) immunoglobulin G (IgG) to HSV type 1 (HSV-1)-infected HEL and HEp-2 cells causes changes in surface viral glycoprotein distribution, resulting in a capping of all viral glycoproteins towards one pole of the cell. This occurs in a gE-dependent manner. In HEL cells, low concentrations of anti-HSV IgG also enhance cell-to-cell spread of wild-type HSV-1 but not of gE deletion mutant HSV-1. These observations raised the possibility that gE-dependent mechanisms exist that allow some HSV-1-infected cells to respond to the presence of extracellular antibodies by enhancing the antibody-resistant mode of virus transmission.

Importance of the cytoplasmic tails of the measles virus glycoproteins for fusogenic activity and the generation of recombinant measles viruses

The generation of replication-competent measles virus (MV) depends on the incorporation of biologically active, fusogenic glycoprotein complexes, which are required for attachment and penetration into susceptible host cells and for direct virus spread by cell-to-cell fusion. Whereas multiple studies have analyzed the importance of the ectodomains of the MV glycoproteins hemagglutinin (H) and fusion protein (F), we have investigated the role of the cytoplasmic tails of the F and H proteins for the formation of fusogenic complexes. Deletions in the cytoplasmic tails of transiently expressed MV glycoproteins were found to have varying effects on receptor binding, fusion, or fusion promotion activity. F tail truncation to only three amino acids did not affect fusion capacity. In contrast, truncation of the H cytoplasmic tail was limited. H protein mutants with cytoplasmic tails of <14 residues no longer supported F-mediated cell fusion, predominantly due to a decrease in surface expression and receptor binding. This indicates that a minimal length of the H protein tail of 14 amino acids is required to ensure a threshold local density to have sufficient accumulation of fusogenic H-F complexes. By using reverse genetics, a recombinant MV with an F tail of three amino acids (rMV-FcDelta30), as well as an MV with an H tail of 14 residues (rMV-HcDelta20), could be rescued, whereas generation of viruses with shorter H tails failed. Thus, glycoprotein truncation does not interfere with the successful generation of recombinant MV if fusion competence is maintained.

Inhibition by prostaglandin PGA1 on the multiplication of influenza virus is a dose-dependent effect

Cyclopentenone prostaglandins (PGs), are strong inhibitors of the multiplicative cycle of a wide variety of enveloped RNA and DNA viruses. Their antiviral activity is generally associated with alterations in the synthesis or maturation of specific virus proteins. In this report, we describe the effect of cyclopentenone PGA1 on the replication of influenza A virus Ulster 73 in LLC-MK2 cells. PGA1 was found to inhibit viral replication in a dose-dependent fashion and virus particle yield was reduced at a PGA1 concentration, which did not suppress protein synthesis in mock-infected cells. The kinetic of late viral protein synthesis was delayed in PGA1-treated cells till 10 h post-infection; after that period, viral polypeptide synthesis appeared to be similar in PGA1-treated as well as untreated cells both infected by Ulster 73 virus. This finding suggests that PGA1 might interfere with one or more events in the viral multiplicative cycle such as protein synthesis and assembly, correct insertion of virus polypeptides into the cell membrane and, or maturation of Ulster 73 virion particles. In particular, inhibition of viral replication in LLC-MK2 cells by PGA1 is accompanied by the induction of a cellular polypeptide of 70K molecular weight. We identified this cell protein as a heat shock protein (HSP) related to the inducible isoform of HSP 70, a polypeptide of 72K molecular weight. Induction of this polypeptide by PGA1 was found to be dose-dependent and a substantial accumulation could be seen at a PGA1 concentration that did not inhibit cell protein synthesis in uninfected cells. HSP 70 synthesis started after the beginning of PGA1 treatment and remained at the same level for at least 10 h, leading us to hypothesize that the delay of production of late Ulster 73 proteins could be the consequence of HSP 70 synthesis. These results suggest that HSP 70 could play a role in the antiviral activity of cyclopentenone PGA1 in LLC-MK2 cells.

Effect of the cytoplasmic domain of the simian immunodeficiency virus envelope protein on incorporation of heterologous envelope proteins and sensitivity to neutralization

In addition to the viral envelope (Env) proteins, host cell-derived proteins have been reported to be present in human immunodeficiency virus and simian immunodeficiency virus (SIV) envelopes, and it has been postulated that they may play a role in infection. We investigated whether the incorporation of host cell proteins is affected by the structure and level of incorporation of viral Env proteins. To compare the cellular components incorporated into SIV particles and how this is influenced by the structure of the cytoplasmic domain, we compared SIV virions with full-length and truncated Env proteins. The levels of HLA-I and HLA-II molecules were found to be significantly (15- to 25-fold) higher in virions with full-length Env than in those with a truncated Env. Virions with a truncated Env were also found to be less susceptible to neutralization by specific antibodies against HLA-I or HLA-II proteins. We also compared the level of incorporation into SIV virions of a coexpressed heterologous viral glycoprotein, the influenza virus hemagglutinin (HA) protein. We found that SIV infection of cells expressing influenza virus HA resulted in the production of phenotypically mixed SIV virions containing influenza virus HA as well as SIV envelope proteins. The HA proteins were more effectively incorporated into virions with full-length Env than in virions with truncated Env. The phenotypically mixed particles with full-length Env, containing higher levels of HA, were sensitive to neutralization with anti-HA antibody, whereas virions with truncated Env proteins and containing lower levels of HA were more resistant to neutralization by anti-HA antibody. In contrast, SIV virions with truncated Env proteins were found to be highly sensitive to neutralization by antisera to SIV, whereas virions with full-length Env proteins were relatively resistant to neutralization. These results indicate that the cytoplasmic domain of SIV Env affects the incorporation of cellular as well as heterologous viral membrane proteins into the SIV envelope and may be an important determinant of the sensitivity of the virus to neutralizing antibodies.

Association of influenza virus NP and M1 proteins with cellular cytoskeletal elements in influenza virus-infected cells

We have investigated the association of the influenza virus matrix (M1) and nucleoprotein (NP) with the host cell cytoskeletal elements in influenza virus-infected MDCK and MDBK cells. At 6.5 h postinfection, the newly synthesized M1 was Triton X-100 (TX-100) extractable but became resistant to TX-100 extraction during the chase with a t1/2 of 20 min. NP, on the other hand, acquired TX-100 resistance immediately after synthesis. Significant fractions of both M1 and NP remained resistant to differential detergent (Triton X-114, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate [CHAPS], octylglucoside) extraction, suggesting that M1 and NP were interacting with the cytoskeletal elements. However, the high-molecular-weight form of the viral transmembrane protein hemagglutinin (HA), which had undergone complex glycosylation, also became resistant to TX-100 extraction but was sensitive to octylglucoside detergent extraction, indicating that HA, unlike M1 or NP, was interacting with TX-100-insoluble lipids and not with cytoskeletal elements. Morphological analysis with cytoskeletal disrupting agents demonstrated that M1 and NP were associated with microfilaments in virus-infected cells. However, M1, expressed alone in MDCK or HeLa cells from cloned cDNA or coexpressed with NP, did not become resistant to TX-100 extraction even after a long chase. NP, on the other hand, became TX-100 insoluble as in the virus-infected cells. M1 also did not acquire TX-100 insolubility in ts 56 (a temperature-sensitive mutant with a defect in NP protein)-infected cells at the nonpermissive temperature. Furthermore, early in the infectious cycle in WSN-infected cells, M1 acquired TX-100 resistance very slowly after a long chase and did not acquire TX-100 resistance at all when chased in the presence of cycloheximide. On the other hand, late in the infectious cycle, M1 acquired TX-100 resistance when chased in either the presence or absence of cycloheximide. Taken together, these results demonstrate that M1 and NP interact with host microfilaments in virus-infected cells and that M1 requires other viral proteins or subviral components (possibly viral ribonucleoprotein) for interaction with host cytoskeletal components. The implication of these results for viral morphogenesis is discussed.

Association of the parainfluenza virus fusion and hemagglutinin-neuraminidase glycoproteins on cell surfaces

We previously observed that cell fusion caused by human parainfluenza virus type 2 or type 3 requires the expression of both the fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins from the same virus type, indicating that a type-specific interaction between F and HN is needed for the induction of cell fusion. In the present study we have further investigated the fusion properties of F and HN proteins of parainfluenza virus type 1 (PI1), type 2 (PI2), and type 3 (PI3), Sendai virus (SN), and simian virus 5 (SV5) by expression of their glycoprotein genes in HeLa T4 cells using the vaccinia virus-T7 transient expression system. Consistent with previous results, cell fusion was observed in cells transfected with homotypic F/HN proteins; with one exception, coexpression of any combination of F and HN proteins from different viruses did not result in cell fusion. The only exception was found with the closely related PI1 HN and SN HN glycoproteins, either of which could interact with SN F to induce cell fusion upon coexpression as previously reported. By specific labeling and coprecipitation of proteins expressed on the cell surface, we observed that anti-PI2 HN antiserum coprecipitated PI2 F when the homotypic PI2 F and PI2 HN were coexpressed, but not the F proteins of other paramyxoviruses when heterotypic F genes were coexpressed with PI2 HN, suggesting that the homotypic F and HN proteins are physically associated with each other on cell surfaces. Furthermore, we observed that PI3 F was found to cocap with PI3 HN but not with PI2 HN, also indicating a specific association between the homotypic proteins. These results indicate that the homotypic F and HN glycoproteins are physically associated with each other on the cell surface and suggest that such association is crucial to cell fusion induced by paramyxoviruses.

Role of the cytoplasmic domains of viral glycoproteins in antibody-induced cell surface mobility

We have investigated the role of the cytoplasmic domains of the influenza virus hemagglutinin (HA) and the parainfluenza virus type 3 (PI3) fusion (F) glycoproteins as a determinant of their ability to undergo antibody-induced redistribution on plasma membranes. The viral envelope genes were truncated in their cytoplasmic domains by using oligonucleotide-directed mutagenesis and expressed by using recombinant vaccinia viruses. In HeLa cells, the truncated HA (HAt), like the full-length HA, did not cap in response to specific antibody. In CV-1 cells, HAt showed patchy surface immunofluorescence with few caps, whereas full-length HA exhibited capping in many cells in response to bivalent antibody. Quantitation of cap formation indicated a sevenfold decrease in the frequency of capping of HAt in comparison with full-length HA. Similarly, truncated F also exhibited a significant decrease in cap formation in comparison with full-length F. These results indicate that the ability of influenza virus HA and PI3 F to undergo redistribution in response to bivalent antibody has been altered by truncation of the viral glycoproteins and suggest that capping may involve interactions between the cytoplasmic domain of the viral glycoproteins and host cell components.