title: Hepatitis C virus is primed by CD81 protein for low pH-dependent fusion.
abstract: Hepatitis C virus (HCV) entry into permissive cells is a complex process that involves interactions with at least four co-factors followed by endocytosis and low pH-dependent fusion with endosomes. The precise sequence of receptor engagement and their roles in promoting HCV E1E2 glycoprotein-mediated fusion are poorly characterized. Because cell-free HCV tolerates an acidic environment, we hypothesized that binding to one or more receptors on the cell surface renders E1E2 competent to undergo low pH-induced conformational changes and promote fusion with endosomes. To test this hypothesis, we examined the effects of low pH and of the second extracellular loop (ECL2) of CD81, one of the four entry factors, on HCV infectivity. Pretreatment with an acidic buffer or with ECL2 enhanced infection through changing the E1E2 conformation, as evidenced by the altered reactivity of these proteins with conformation-specific antibodies and stable association with liposomes. However, neither of the two treatments alone permitted direct fusion with the cell plasma membrane. Sequential HCV preincubation with ECL2 and acidic buffer in the absence of target cells resulted in a marked loss of infectivity, implying that the receptor-bound HCV is primed for low pH-dependent conformational changes. Indeed, soluble receptor-pretreated HCV fused with the cell plasma membrane at low pH under conditions blocking an endocytic entry pathway. These findings suggest that CD81 primes HCV for low pH-dependent fusion early in the entry process. The simple triggering paradigm and intermediate conformations of E1E2 identified in this study could help guide future vaccine and therapeutic efforts to block HCV infection.
authors: Division of Pediatric Infectious Diseases, Emory University Children's Center, Atlanta, Georgia 30322.
authors: Nishi R
authors: NR
authors: Sharma
authors: Division of Infectious Diseases, Department of Medicine, Lyon, F-69007, France.
authors: Guaniri
authors: G
authors: Mateu
authors: Université de Lyon, UCB-Lyon1, IFR128, Lyon, F-69007, France; INSERM, U758, Lyon, F-69364, France; Ecole Normale Supérieure de Lyon, Lyon, F-69364, France.
authors: Marlene
authors: M
authors: Dreux
authors: Division of Infectious Diseases, Department of Medicine, Lyon, F-69007, France.
authors: Arash
authors: A
authors: Grakoui
authors: Université de Lyon, UCB-Lyon1, IFR128, Lyon, F-69007, France; INSERM, U758, Lyon, F-69364, France; Ecole Normale Supérieure de Lyon, Lyon, F-69364, France.
authors: François-Loïc
authors: FL
authors: Cosset
authors: Division of Pediatric Infectious Diseases, Emory University Children's Center, Atlanta, Georgia 30322; Children's Healthcare of Atlanta, Atlanta, Georgia 30322. Electronic address: gmeliki@emory.edu.
authors: Gregory B
authors: GB
authors: Melikyan
mesh_terms: D000911:Antibodies, Monoclonal / Q000737:chemistry; D015703:Antigens, CD / Q000378:metabolism*; D002021:Buffers; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002474:Cell-Free System; D004705:Endocytosis; D016174:Hepacivirus / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008081:Liposomes / Q000737:chemistry; D011487:Protein Conformation; D011993:Recombinant Fusion Proteins / Q000737:chemistry; D060225:Tetraspanin 28; D014764:Viral Proteins / Q000737:chemistry
chemical_list: D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D002021:Buffers; C554798:CD81 protein, human; D008081:Liposomes; D011993:Recombinant Fusion Proteins; D060225:Tetraspanin 28; D014764:Viral Proteins