Secretion of HCV Glycoproteins Figure 10. Model of TRL assembly and formation of HCV envelope glycoprotein hybrid TRL (adapted from Shelness [41]). During its translation at the rough ER, the nascent apoB peptide is translocated within the ER lumen (step 1) where a lipid cavity is formed. This lipid cavity is closed by MTP allowing the recruitment of a small amount of phospholipids and triglycerides (TG) (step 2). Continued apoB translation and MTP- mediated TG transfer form a neutral lipid core and conversion to a spheric particle (step 3) that is then released into the ER lumen after translation completion. This first precursor then moves to a distal compartment of the secretory pathway (step 4). In the smooth ER, a second precursor is formed from the ER membrane and MTP-mediated TG enrichment (step 5) and then traffics to the secretory pathway (step 6) where the two precursors meet and fuse to form mature TRL (fusion step). Several factors are required for the formation of the second precursor and the fusion between the two precursors (see text). HCV envelope glycoproteins are retained in the ER membrane after their maturation and clivages by ER peptidases. We hypothesise that E1E2 dimers might diffuse from the ER membrane constituted of two phospholipid monolayers to the budding second precursor. Transition from a two leaflets membrane to the phospholipid monolayer membrane of the precursor might induce conformation changes favouring the formation of E1E2 trimers triggered by the trimerisation of E1 as recently proposed [34]. The glycoprotein loaded second precursor can then fuse with the first precursor leading to hybrid TRL as observed in Caco-2 and HepG2 but not in Huh-7 cells (see text). doi:10.1371/journal.pone.0004233.g010 Acknowledgments We are grateful to Martine Carreras for her skillful technical help in performing some experiments. We acknowledge Didier Ne`gre and Franc¸ois Loic Cosset and the ‘‘Plateau de Vectorologie’’ de l’IFR 128 BioSciences Lyon-Gerland for help in preparing the lentiviral vectors and Birke Bartosch for the HCV genotype 1a envelope sequence. We are grateful to George Luo, Department of Microbiology, University of Kentucky College of Medicine, Lexington, Kentucky for the gift of the References 1. Choo QL, Kuo G, Weiner AJ, Overby LR, Bradley DW, et al. (1989) Isolation of a cDNA clone derived from a blood-borne non-A, non-B viral hepatitis genome. Science 244: 359–362. 2. Robertson B, Myers G, Howard C, Brettin T, Bukh J, et al. (1998) Classification, nomenclature, and database development for hepatitis C virus (HCV) and related viruses: proposals for standardization. International Committee on Virus Taxonomy. Arch Virol 143: 2493–2503. 3. Appel N, Schaller T, Penin F, Bartenschlager R (2006) From structure to function: new insights into hepatitis C virus RNA replication. J Biol Chem 281: 9833–9836. 4. Moradpour D, Penin F, Rice CM (2007) Replication of hepatitis C virus. Nat Rev Microbiol 5: 453–463. 5. Rey FA (2003) Dengue virus envelope glycoprotein structure: new insight into its interactions during viral entry. Proc Natl Acad Sci U S A 100: 6899–6901. MTP inhibitor and to Jean Dubuisson, Institut Pasteur, Lille, France for the envelope specific monoclonal antibodies. Author Contributions Conceived and designed the experiments: VI OD PA. Performed the experiments: VI OD. Analyzed the data: VI OD CS LPC CR RB FP VL PA. Contributed reagents/materials/analysis tools: CS LPC CR RB FP VL. Wrote the paper: PA. 6. Bressanelli S, Stiasny K, Allison SL, Stura EA, Duquerroy S, et al. (2004) Structure of a flavivirus envelope glycoprotein in its low-pH-induced membrane fusion conformation. Embo J 23: 728–738. 7. Mukhopadhyay S, Kim BS, Chipman PR, Rossmann MG, Kuhn RJ (2003) Structure of West Nile virus. Science 302: 248. 8. Thomssen R, Bonk S, Propfe C, Heermann KH, Kochel HG, et al. (1992) Association of hepatitis C virus in human sera with beta-lipoprotein. Med Microbiol Immunol (Berl) 181: 293–300. 9. Thomssen R, Bonk S, Thiele A (1993) Density heterogeneities of hepatitis C virus in human sera due to the binding of beta-lipoproteins and immunoglob- ulins. Med Microbiol Immunol (Berl) 182: 329–334. 10. Andre P, Komurian-Pradel F, Deforges S, Perret M, Berland JL, et al. (2002) Characterization of low- and very-low-density hepatitis C virus RNA-containing particles. J Virol 76: 6919–6928. PLoS ONE | www.plosone.org 12 January 2009 | Volume 4 | Issue 1 | e4233