Unexpected structural features of the hepatitis C virus envelope protein 2 ectodomain

Abstract

Hepatitis C virus (HCV), a member of the family Flaviviridae, is a leading cause of chronic liver disease and cancer. Recent advances in HCV therapeutics have resulted in improved cure rates, but an HCV vaccine is not available and is urgently needed to control the global pandemic. Vaccine development has been hampered by the lack of high-resolution structural information for the two HCV envelope glycoproteins, E1 and E2. Recently, Kong and coworkers (Science 342:1090-1094, 2013, doi:10.1126/science.1243876) and Khan and coworkers (Nature 509[7500]:381-384, 2014, doi:10.1038/nature13117) independently determined the structure of the HCV E2 ectodomain core with some unexpected and informative results. The HCV E2 ectodomain core features a globular architecture with antiparallel β-sheets forming a central β sandwich. The residues comprising the epitopes of several neutralizing and nonneutralizing human monoclonal antibodies were also determined, which is an essential step toward obtaining a fine map of the human humoral response to HCV. Also clarified were the regions of E2 that directly bind CD81, an important HCV cellular receptor. While it has been widely assumed that HCV E2 is a class II viral fusion protein (VFP), the newly determined structure suggests that the HCV E2 ectodomain shares structural and functional similarities only with domain III of class II VFPs. The new structural determinations suggest that the HCV glycoproteins use a different mechanism than that used by class II fusion proteins for cell fusion.

FIG 1

Structure of the ectodomain core of HCV envelope protein 2. (A) Ribbon diagram of the construct used to determine the structure of the HCV E2 ectodomain core by Kong et al. (33). β-Sheets are represented by arrows, and helices are represented by cylinders. Glycosylation sites are numbered from the N terminus. Dicysteine bonds are indicated by finely dashed lines. Due to constraints of representing a three-dimensional structure in two dimensions, the length of loops may not correspond to the length in the loops in the HCV E2 ectodomain core structure. Numbering of sheets, helices, and glycosylation sites is from Kong et al. (33). Blue structures represent the central β sandwich, a structural motif present in class II viral fusion proteins. The red loop is the putative CD81 binding domain. Gray structures represent amino acids not present in the construct. Gold structures were present in the construct, but disordered. Glycosylation sites 4 and 9 were mutated. Hypervariable region 2 (HVR2) is deleted from the construct but had an inserted linker (GSSG), which was disordered. Amino acids delineating structural and other features are numbered from the start of the polyprotein. (B) Ribbon diagram of the construct used to determine the structure of the HCV E2 ectodomain core by Khan et al. (34). Symbols and the color scheme are as in panel A, except that glycosylation was mostly removed enzymatically. (C) X-ray crystallographic structure of the HCV E2 ectodomain core by Kong et al. (33). Symbols and the color scheme are as in panel A, except that cysteines are represented as yellow sticks. The dashed circle indicates the fusion loop postulated by Krey et al. (36). (D) X-ray crystallographic structure of the HCV E2 ectodomain core by Khan et al. (34). Symbols and the color scheme are as in panel C.

FIG 2

Comparison of the architecture of the dengue virus E/M fusion protein complex with the proposed architecture of the HCV E1/E2 complex. (A) The domain architecture of DENV E is indicated in green (domain I), yellow (domain II), and blue (domain III). The stem and anchor helices of E are violet and blue, respectively. The M protein is depicted in orange. Panel A is reprinted from Nature Structural Biology (106) with permission of the publisher. (B) Model in which the E2 ectodomain structure is combined with a schematic diagram of the ectodomain, stem, and transmembrane domain proteins, based on the presentation by Zhang et al. (106). Also depicted is a speculative model in which HCV E1 contains a fusion loop. Proposed analogous regions of E1 and E2 are colored according to their similarity to corresponding regions of DENV E. This model indicates that it is unlikely that either E1 or E2 folds back on itself to yield the canonical class II postfusion complex. (C) Alternative model for the HCV envelope glycoproteins in which neither E1 nor E2 contains a classical fusion loop.