Viral determinants: envelope glycoproteins E1 and E2 − Rakyat Biologi

The HCV genome encodes a single precursor polyprotein of about 3,000 amino acids that is cleaved co- and post-translationally into functional structural and non-structural proteins by host and viral proteases including three structural proteins: the core protein forming the viral nucleocapsid and two envelope glycoproteins, E1 and E2. HCV particles have a size of about 55-60 nm in diameter (7-9). In analogy to other members of the Flaviviridae family, HCV is thought to adopt a classical icosahedral scaffold in which the two envelope glycoproteins E1 and E2 are anchored to the host cell-derived double-layer lipid envelope (10). E1 and E2 are type I transmembrane glycoproteins containing up to 6 and 11 potential glycosylation sites, respectively (11) and forming noncovalent heterodimers. The nucleocapsid is probably composed of multiple copies of the core protein in complex with the viral genome and lies underneath the envelope (10).

Studies of the HCV life cycle have long been hampered by the lack of an efficient cell culture system to generate infectious virus in vitro. Several model systems have thus been developed for the study of defined aspects of the HCV life cycle such as viral entry, replication, assembly and release (for review see (12)). Recombinant HCV envelope glycoproteins (13), HCV-like particles (HCV-LPs) (14-16) and retroviral HCV pseudotypes (HCVpp) (17, 18) have been successfully used to analyze virus attachment and entry. Most recently, efficient in vitro model systems for the production of infectious recombinant virions (HCVcc) have been described (9, 19, 20). Using these model systems, it could be demonstrated that envelope glycoproteins E1 and E2 are crititcal for host cell entry. In fact, HCVpp assembled with either E1 or E2 glycoproteins were significantly less infective than HCVpp containing both envelope glycoproteins (17). Furthermore, HCVcc generated from a construct with an in-frame-deletion of the HCV envelope protein coding sequence are not infectious (9).

Monoclonal or polyclonal antibodies targeting both linear and conformational epitopes of envelope glycoprotein E2 have been shown to inhibit cellular binding of HCV-LP binding, entry of HCVpp and infection of HCVcc (9, 14-20) suggesting that envelope glycoprotein E2 plays a key role for host cell surface interaction. Within the E2 envelope glycoprotein sequence hypervariable regions have been identified. These amino acids differ by up to 80% among HCV genotypes, and even among different subtypes of the same genotype. The N-terminal 27 residues of E2 (aa 384-410) show a very high degree of variation and this portion of the sequence has been termed hypervariable region 1 (HVR-1) (21). This region plays a critical role in HCV interaction with host cells as HVR-1-delted HCVpp demonstrate reduced infectivity (22, 23). The important role of HVR-1 in HCV infectivity is further supported by studies demonstrating that antibodies targeting regions within HVR-1 inhibit cellular recombinant E2 (24, 25) and HCV-LP binding (15, 26) as well as HCVpp entry into target cells (18, 27). The exact role of E1 still remains unknown. E1 may directly interact with cell surface molecules and/or contribute to proper folding and processing of E2. Interestingly, antibodies targeting the N-terminal region of E1 have been shown to inhibit HCV-LP binding (15, 28) as well as HCV infection of a B-cell-derived cell line (29) suggesting that E1-cell surface interaction may contribute to viral binding and entry. In addition, HCV envelope proteins E1 and E2 are thought to induce fusion between the viral envelope and host cell membranes (30).