The ins and outs of hepatitis C virus entry and assembly

Abstract

Hepatitis C virus, a major human pathogen, produces infectious virus particles with several unique features, such as an ability to interact with serum lipoproteins, a dizzyingly complicated process of virus entry, and a pathway of virus assembly and release that is closely linked to lipoprotein secretion. Here, we review these unique features, with an emphasis on recent discoveries concerning virus particle structure, virus entry and virus particle assembly and release.

Figure 1

Structural aspects of HCV and related viruses. a. Model of a HCV particle, showing the membrane bilayer envelope, E1-E2 surface glycoproteins, and nucleocapsid containing core protein and viral RNA. HCV-associated lipoproteins are not shown. b. Representative electron micrographs of negatively stained HCVcc particles captured on an affinity electron microscopy (EM) grid (left) or a magnetic bead recognizing the tagged E2 envelope glycoprotein (middle). Cryo-EM image of an HCV virion (right). Scale bars are 50 nm. These EM images, courtesy of M.T. Catanese, demonstrate the lack of regular surface features and pleiomorphic shape of HCV particles. c. Structure of the dengue-2 virus E glycoprotein homodimer as viewed from the top and side views. Domain I is colored red, domain II is yellow, the fusion loop is green and domain III is blue. Rendered from PDB 1OAN. d. Structure of the BVDV-1 E2 glycoprotein homodimer, rendered at the same scale and coloring as in c. Note that no fusion loop has been identified in BVDV-1 E2. Rendered from PDB 4ILD. e. Fitting of the E protein dimer structure into electron density map of purified DENV-2 particles. For clarity, one dimer is highlighted. f. Two-particle model for LVP structure, which proposes that HCV particles and serum lipoproteins transiently interact. Particles are illustrated with approximate relative scales, and apoE is indicated in green. g. Single particle model for LVP structure, illustrating an HCV particle sharing an envelope with an LDL particle.

Figure 2

HCV entry. HCV LVPs attach to the cell surface by interaction with HSPG, LDLR and SR-BI. SR-BI may delipidate HCV-associated lipoproteins and induces conformational changes in E2, exposing the CD81 binding site (step 1). TfR1 plays an unknown, post-CD81 role in HCV entry (not shown). Interaction of E2 with CD81 then activates signal transduction through EGFR and Ras as well as Rho GTPases (step 2). These signaling events promote lateral movement of CD81-HCV complexes to sites of cell-cell contact (step 3), interaction of CD81 with CLDN1, and HCV internalization via clathrin-mediated endocytosis (step 4). The low pH of the endosomal compartment induces HCV fusion (step 5).

Figure 3

HCV assembly and secretion. a. Early events in the HCV lifecycle, including gene viral expression, recruitment of the viral RNA into an RNA replication complex, ER-retention of the E1-E2 glycoprotein and p7-NS2 complexes, and cPLA2-mediated trafficking of core dimers to cLDs, which are formed by the action of DGAT1. b. Late events in the HCV lifecycle. Viral RNA is shifted out of replication and translation and towards virus assembly. The interaction of p7-NS2 with NS3-4A recruits the viral core protein to the site of virus assembly. Virus particles assemble by recruitment of the E1-E2 glycoproteins and budding into the ER. c. The pathways of VLDL and HCV particle secretion, including maturation into low-buoyant density particles, perhaps through specific lipidation steps and/or interaction with maLDs, and the secretion of particles through the p7-buffered secretory pathway in HCV-infected cells.