Dealing with low pH: entry and exit of alphaviruses and flaviviruses

Abstract

The alphaviruses and flaviviruses include many important human pathogens, such as the dengue, West Nile, and Chikungunya viruses. These enveloped viruses infect cells by a membrane fusion reaction triggered by the low pH in endosomes. Fusion is mediated by viral membrane proteins through their acid-dependent conversion from a dimer on the virus surface to a homotrimer inserted in the host cell membrane. Here we review recent studies on the regulatory mechanisms that silence these fusion proteins during virus exit, and that sense low pH and mediate protein refolding during virus entry. We discuss results using truncated proteins to dissect the fusion reaction, and future research directions including the development of antiviral therapies against these medically important viruses.

Method for measuring the unbinding energy of strongly-bound membrane-associated proteins

“…2 shows a schematic of the gradient layers, along with the corresponding volumes and sample placement before centrifugation. While sE protein binds to membranes at endosomal pH values [6,37,42], the sE protein and liposomes were incubated together for 4 h at pH 3 and 20 °C to achieve a high level of binding while minimizing the amount of protein required. (We observed an increase in liposome-bound protein with decrease in pH from 5.5 to 3.0).…”

“…While it is well known that some proteins undergo large conformational changes upon binding to lipid membranes, more subtle local segmental rearrangements likely take place even when large conformational changes do not occur. Other mechanisms resulting in two-stage protein–membrane interaction are equilibrium binding to the surface of a membrane followed by insertion [27,36] and protein–protein association after membrane binding [10,37,38]. In the case of Dengue sE, we argue below based on sedimentation analysis and also prior work that the protein most likely binds as monomers that subsequently associates to form trimers.…”

Molecular association of herpes simplex virus type 1 glycoprotein E with membrane protein Us9

“…Changes in the stability of protein complexes at low pH are documented for viral proteins involved in membrane fusion [56, 57]. It is possible that pH-dependent dissociation of Us9 from gE may regulate the recycling of Us9 or gE from endosomes to the cell surface or cell junctions that are required for cell-to-cell spread of virus infection [58].…”

Broad Antiviral Activity of Carbohydrate-Binding Agents Against Dengue Virus Infection

“…The low pH triggers dissociation of the E-homodimer, which then leads to the insertion of the fusion peptide into the target cell membrane forming a bridge between the virus and the host. Next, a stable trimer of the E-protein is folded into a hairpin-like structure and forces the target membrane to bend towards the viral membrane and eventually fusion takes place [25,27,28]. The fusion results in the release of viral RNA into the cytoplasm for initiation of replication and translation ( Figure 4).…”