Functional analysis of cell surface-expressed hepatitis C virus E2 glycoprotein

Hepatitis C virus (HCV) glycoproteins El and E2, when expressed in eukaryotic cells, are retained in the endoplasmic reticulum (ER). C-terminal truncation of E2 at residue 661 or 715 (position on the polyprotein) leads to secretion, consistent with deletion of a proposed hydrophobic transmembrane anchor sequence. We demonstrate cell surface expression of a chimeric glycoprotein consisting of E2 residues 384 to 661 fused to the transmembrane and cytoplasmic domains of influenza A virus hemagglutinin (HA), termed E2(661)-HA(TMCT). The E2(661)-HA(TMCT) chimeric glycoprotein was able to bind a number of conformation-dependent monoclonal antibodies and a recombinant soluble form of CD81, suggesting that: it was folded in a manner comparable to "native" E2. Furthermore, cell surface-expressed E2(661)-HA(TMCT) demonstrated pH-dependent changes in antigen conformation, consistent with an acid-mediated fusion mechanism, However, E2(661)-HA(TMCT) was unable to induce cell fusion of CD81-positive HEK cells after neutral- or low-pH treatment. We propose that a stretch of conserved, hydrophobic amino acids within the El glycoprotein, displaying similarities to flavivirus and paramyxovirus fusion peptides, may constitute the HCV fusion peptide. We demonstrate that influenza virus can incorporate E2(661)-HA(TMCT) into particles and discuss experiments to address the relevance of the E2-CD81 interaction for HCV attachment and entry.