Conformational changes in enveloped virus surface proteins during cell entry

Acid-activated structural reorganization of the Rift Valley fever virus Gc fusion protein

The entry of the enveloped Rift Valley fever virus (RVFV) into its host cell is mediated by the viral glycoproteins Gn and Gc. We investigated the RVFV entry process and, in particular, its pH-dependent activation mechanism using our recently developed nonspreading-RVFV-particle system. Entry of the virus into the host cell was efficiently inhibited by lysosomotropic agents that prevent endosomal acidification and by compounds that interfere with dynamin- and clathrin-dependent endocytosis. Exposure of plasma membrane-bound virions to an acidic pH (<pH 6) equivalent to the pH of late endolysosomal compartments allowed the virus to bypass the endosomal route of infection. Acid exposure of virions in the absence of target membranes triggered the class II-like Gc fusion protein to form extremely stable oligomers that were resistant to SDS and temperature dissociation and concomitantly compromised virus infectivity. By targeted mutagenesis of conserved histidines in Gn and Gc, we demonstrated that mutation of a single histidine (H857) in Gc completely abrogated virus entry, as well as acid-induced Gc oligomerization. In conclusion, our data suggest that after endocytic uptake, RVFV traffics to the acidic late endolysosomal compartments, where histidine protonation drives the reorganization of the Gc fusion protein that leads to membrane fusion.

A Role for Sperm Surface Protein Disulfide Isomerase Activity in Gamete Fusion: Evidence for the Participation of ERp57

In mammals, sperm-egg interaction is based on molecular events either unique to gametes or also present in somatic cells. In gamete fusion, it is unknown which features are gamete specific and which are shared with other systems. Conformational changes mediated by thiol-disulfide exchange are involved in the activation of some virus membrane fusion proteins. Here we asked whether that mechanism is also operative in sperm-egg fusion. Different inhibitors of protein disulfide isomerase (PDI) activity were able to inhibit sperm-egg fusion in vitro. While pretreatment of oocytes had no effect, pretreatment of sperm reduced their fusion ability. Some members of the PDI family were detected on the sperm head, and use of specific antibodies and substrates suggested that the oxidoreductase ERp57 has a role in gamete fusion. The results support the idea that thiol-disulfide exchange is a mechanism that may act in gamete fusion to produce conformational changes in fusion-active proteins.

Stable association of herpes simplex virus with target membranes is triggered by low pH in the presence of the gD receptor, HVEM

Using a liposome-binding assay, we investigated the requirements for activation of herpes simplex virus (HSV) into a state capable of membrane interaction. Virions were mixed with liposomes along with the ectodomain of one of three gD receptors (HVEMt, nectin-1t, or nectin-2t) and incubated under different pH and temperature conditions. Virions failed to associate with liposomes in the presence of nectin-1 or nectin-2 at any temperature or pH tested. In contrast, HVEMt triggered association of HSV with liposomes at pH 5.3 or 5.0 when incubated at 37 degrees C, suggesting that HVEM binding and mildly acidic pH at a physiological temperature provide coactivation signals, allowing virus association with membranes. Virions incubated with HVEMt at 37 degrees C without liposomes rapidly lost infectivity upon exposure to pH 5.0, suggesting that these conditions lead to irreversible virus inactivation in the absence of target membranes. Consistent with the idea that soluble receptor molecules provide a trigger for HSV entry, HVEMt promoted virus entry into receptor-deficient CHO K1 cells. However, in B78H1 cells, HVEMt promoted virus entry with markedly lower efficiency. Interestingly, HSV entry into receptor-bearing CHO K1 cells has been shown to proceed via a pH-dependent manner, whereas HSV entry into receptor-bearing B78H1 cells is pH independent. Based on these observations, we propose that the changes triggered by HVEM and mildly acidic pH that allow liposome association are similar or identical to changes that occur during pH-dependent HSV entry.

Paramyxovirus membrane fusion: lessons from the F and HN atomic structures

Paramyxoviruses enter cells by fusion of their lipid envelope with the target cell plasma membrane. Fusion of the viral membrane with the plasma membrane allows entry of the viral genome into the cytoplasm. For paramyxoviruses, membrane fusion occurs at neutral pH, but the trigger mechanism that controls the viral entry machinery such that it occurs at the right time and in the right place remains to be elucidated. Two viral glycoproteins are key to the infection process—an attachment protein that varies among different paramyxoviruses and the fusion (F) protein, which is found in all paramyxoviruses. For many of the paramyxoviruses (parainfluenza viruses 1–5, mumps virus, Newcastle disease virus and others), the attachment protein is the hemagglutinin/neuraminidase (HN) protein. In the last 5 years, atomic structures of paramyxovirus F and HN proteins have been reported. The knowledge gained from these structures towards understanding the mechanism of viral membrane fusion is described.

Structure of the uncleaved ectodomain of the paramyxovirus (hPIV3) fusion protein

Class I viral fusion proteins share common mechanistic and structural features but little sequence similarity. Structural insights into the protein conformational changes associated with membrane fusion are based largely on studies of the influenza virus hemagglutinin in pre- and postfusion conformations. Here, we present the crystal structure of the secreted, uncleaved ectodomain of the paramyxovirus, human parainfluenza virus 3 fusion (F) protein, a member of the class I viral fusion protein group. The secreted human parainfluenza virus 3 F forms a trimer with distinct head, neck, and stalk regions. Unexpectedly, the structure reveals a six-helix bundle associated with the postfusion form of F, suggesting that the anchor-minus ectodomain adopts a conformation largely similar to the postfusion state. The transmembrane anchor domains of F may therefore profoundly influence the folding energetics that establish and maintain a metastable, prefusion state.