A pre-immediate-early role for tegument ICP0 in the proteasome-dependent entry of herpes simplex virus.

Herpes simplex virus (HSV) entry requires host cell 26S proteasomal degradation activity at a postpenetration step. When expressed in the infected cell, the HSV immediate-early protein ICP0 has E3 ubiquitin ligase activity and interacts with the proteasome. The cell is first exposed to ICP0 during viral entry, since ICP0 is a component of the inner tegument layer of the virion. The function of tegument ICP0 is unknown. Deletion of ICP0 or mutations in the N-terminal RING finger domain of ICP0 results in the absence of ICP0 from the tegument. We show here that these mutations negatively influenced the targeting of incoming capsids to the nucleus. Inhibitors of the chymotrypsin-like activity of the proteasome the blocked entry of virions containing tegument ICP0, including ICP0 mutants that are defective in USP7 binding. However, ICP0-deficient virions were not blocked by proteasomal inhibitors and entered cells via a proteasome-independent mechanism. ICP0 appeared to play a postpenetration role in cells that supported either endocytosis or nonendosomal entry pathways for HSV. The results suggest that ICP0 mutant virions are defective upstream of viral gene expression at a pre-immediate-early step in infection. We propose that proteasome-mediated degradation of a virion or host protein is regulated by ICP0 to allow efficient delivery of entering HSV capsids to the nuclear periphery.

Herpes simplex virus tegument ICP0 is capsid associated, and its E3 ubiquitin ligase domain is important for incorporation into virions.

Herpes simplex virus (HSV) immediate-early (IE) protein ICP0 is a multifunctional regulator of HSV infection. ICP0 that is present in the tegument layer has not been well characterized. Protein compositions of wild-type and ICP0 null virions were similar, suggesting that the absence of ICP0 does not grossly impair virion assembly. ICP0 has a RING finger domain with E3 ubiquitin ligase activity that is necessary for IE functions. Virions with mutations in this domain contained greatly reduced levels of tegument ICP0, suggesting that the domain influences the incorporation of ICP0. Virion ICP0 was resistant to removal by detergent and salt and was associated with capsids, features common to inner tegument proteins.

Low pH-induced conformational change in herpes simplex virus glycoprotein B.

Herpesviruses can enter host cells using pH-dependent endocytosis pathways in a cell-specific manner. Envelope glycoprotein B (gB) is conserved among all herpesviruses and is a critical component of the complex that mediates membrane fusion and entry. Here we demonstrate that mildly acidic pH triggers specific conformational changes in herpes simplex virus (HSV) gB. The antigenic structure of gB was specifically altered by exposure to low pH both in vitro and during entry into host cells. The oligomeric conformation of gB was altered at a similar pH range. Exposure to acid pH appeared to convert virion gB into a lower-order oligomer. The detected conformational changes were reversible, similar to those in other class III fusion proteins. Exposure of purified, recombinant gB to mildly acidic pH resulted in similar changes in conformation and caused gB to become more hydrophobic, suggesting that low pH directly affects gB. We propose that intracellular low pH induces alterations in gB conformation that, together with additional triggers such as receptor binding, are essential for virion-cell fusion during herpesviral entry by endocytosis.

Cellular proteasome activity facilitates herpes simplex virus entry at a postpenetration step.

Herpes simplex virus (HSV) entry into cells is a multistep process that engages the host cell machinery. The proteasome is a large, ATP-dependent, multisubunit protease that plays a critical role in the maintenance of cell homeostasis. A battery of assays were used to demonstrate that proteasome inhibitors blocked an early step in HSV entry that occurred after capsid penetration into the cytosol but prior to capsid arrival at the nuclear periphery. Proteasome-dependent viral entry was not reliant on host or viral protein synthesis. MG132, a peptide aldehyde that competitively inhibits the degradative activity of the proteasome, had a reversible inhibitory effect on HSV entry. HSV can use endocytic or nonendocytic pathways to enter cells. These distinct entry routes were both dependent on proteasome-mediated proteolysis. In addition, HSV successfully entered cells in the absence of a functional host ubiquitin-activating enzyme, suggesting that viral entry is ubiquitin independent. We propose that proteasomal degradation of virion and/or host proteins is required for efficient delivery of incoming HSV capsids to the nucleus.

Nectin-2-mediated entry of a syncytial strain of herpes simplex virus via pH-independent fusion with the plasma membrane of Chinese hamster ovary cells.

BACKGROUND: Herpes simplex virus (HSV) can utilize multiple pathways to enter host cells. The factors that determine which route is taken are not clear. Chinese hamster ovary (CHO) cells that express glycoprotein D (gD)-binding receptors are model cells that support a pH-dependent, endocytic entry pathway for all HSV strains tested to date. Fusion-from-without (FFWO) is the induction of target cell fusion by addition of intact virions to cell monolayers in the absence of viral protein expression. The receptor requirements for HSV-induced FFWO are not known. We used the syncytial HSV-1 strain ANG path as a tool to evaluate the complex interplay between receptor usage, membrane fusion, and selection of entry pathway. RESULTS: Inhibitors of endocytosis and endosome acidification blocked ANG path entry into CHO cells expressing nectin-1 receptors, but not CHO-nectin-2 cells. Thus, under these conditions, nectin-2 mediates pH-independent entry at the plasma membrane. In addition, CHO-nectin-2 cells supported pH-dependent, endocytic entry of different strains of HSV-1, including rid1 and HFEM. The kinetics of ANG path entry was rapid (t1/2 of 5-10 min) regardless of entry route. However, HSV-1 ANG path entry by fusion with the CHO-nectin-2 cell plasma membrane was more efficient and resulted in larger syncytia. ANG path virions added to the surface of CHO-nectin-2 cells, but not receptor-negative CHO cells or CHO-nectin-1 cells, induced rapid FFWO. CONCLUSION: HSV-1 ANG path can enter CHO cells by either endocytic or non-endocytic pathways depending on whether nectin-1 or nectin-2 is present. In addition to these cellular receptors, one or more viral determinants is important for the selection of entry pathway. HSV-induced FFWO depends on the presence of an appropriate gD-receptor in the target membrane. Nectin-1 and nectin-2 target ANG path to divergent cellular pathways, and these receptors may have different roles in triggering viral membrane fusion.

Widely Used Herpes Simplex Virus 1 ICP0 Deletion Mutant Strain dl1403 and Its Derivative Viruses Do Not Express Glycoprotein C Due to a Secondary Mutation in the gC Gene.

Herpes simplex virus 1 (HSV-1) ICP0 is a multi-functional phosphoprotein expressed with immediate early kinetics. An ICP0 deletion mutant, HSV-1 dl1403, has been widely used to study the roles of ICP0 in the HSV-1 replication cycle including gene expression, latency, entry and assembly. We show that HSV-1 dl1403 virions lack detectable levels of envelope protein gC, and that gC is not synthesized in infected cells. Sequencing of the gC gene from HSV-1 dl1403 revealed a single amino acid deletion that results in a frameshift mutation. The HSV-1 dl1403 gC gene is predicted to encode a polypeptide consisting of the original 62 N-terminal amino acids of the gC protein followed by 112 irrelevant, non-gC residues. The mutation was also present in a rescuant virus and in two dl1403-derived viruses, D8 and FXE, but absent from the parental 17+, suggesting that the mutation was introduced during the construction of the dl1403 virus, and not as a result of passage in culture.

Defining influenza A virus hemagglutinin antigenic drift by sequential monoclonal antibody selection.

Human influenza A virus (IAV) vaccination is limited by "antigenic drift," rapid antibody-driven escape reflecting amino acid substitutions in the globular domain of hemagglutinin (HA), the viral attachment protein. To better understand drift, we used anti-hemagglutinin monoclonal Abs (mAbs) to sequentially select IAV escape mutants. Twelve selection steps, each resulting in a single amino acid substitution in the hemagglutinin globular domain, were required to eliminate antigenicity defined by monoclonal or polyclonal Abs. Sequential mutants grow robustly, showing the structural plasticity of HA, although several hemagglutinin substitutions required an epistatic substitution in the neuraminidase glycoprotein to maximize growth. Selecting escape mutants from parental versus sequential variants with the same mAb revealed distinct escape repertoires, attributed to contextual changes in antigenicity and the mutation landscape. Since each hemagglutinin mutation potentially sculpts future mutation space, drift can follow many stochastic paths, undermining its unpredictability and underscoring the need for drift-insensitive vaccines.

Role of the UL45 protein in herpes simplex virus entry via low pH-dependent endocytosis and its relationship to the conformation and function of glycoprotein B.

Herpesviruses commandeer distinct cellular pathways to enter target cells. The mechanism by which herpes simplex virus (HSV) selects a pH-dependent, endocytic route or a pH-independent route remains to be elucidated. We investigated the role of the non-glycosylated viral envelope protein UL45 in HSV entry via endocytosis. UL45 plays a role in mediating cell-cell fusion and has been proposed to functionally interact with gB to regulate membrane fusion. Thus, we also probed the impact of UL45 on the structure and function of gB present in virions. A UL45 deletion virus successfully entered cells via low pH, endocytic pathway with wild type kinetics. In the absence or presence of UL45, the antigenic conformation of virion gB appeared unaltered. Antibodies to gB neutralized infection of the UL45-deletion virus and wild type virus to a similar extent, regardless of whether the target cells supported low pH endocytic or non-endocytic entry routes. Lastly, HSV virions were inactivated by low pH regardless of the presence of UL45. The results, together with previous studies, suggest that UL45 plays distinct roles in cell-cell fusion and virus-cell fusion during acid-dependent entry.