Receptor Complementation and Mutagenesis Reveal SR-BI as an EssentialHCV Entry Factor and Functionally Imply Its Intra- and Extra-Cellular Domains

摘要

HCV entry into cells is a multi-step and slow process. It is believed that the initial capture of HCV particles by glycosaminoglycans and/or lipoprotein receptors is followed by coordinated interactions with the scavenger receptor class B type I (SR-BI), a major receptor of high-density lipoprotein (HDL), the CD81 tetraspanin, and the tight junction protein Claudin-1, ultimately leading to uptake and cellular penetration of HCV via low-pH endosomes. Several reports have indicated that HDL promotes HCV entry through interaction with SR-BI. This pathway remains largely elusive, although it was shown that HDL neither associates with HCV particles nor modulates HCV binding to SR-BI. In contrast to CD81 and Claudin-1, the importance of SR-BI has only been addressed indirectly because of lack of cells in which functional complementation assays with mutant receptors could be performed. Here we identified for the first time two cell types that supported HCVpp and HCVcc entry upon ectopic SR-BI expression. Remarkably, the undetectable expression of SR-BI in rat hepatoma cells allowed unambiguous investigation of human SR-BI functions during HCV entry. By expressing different SR-BI mutants in either cell line, our results revealed features of SR-BI intracellular domains that influence HCV infectivitywithout affecting receptor binding and stimulation of HCV entry induced byHDL/SR-BI interaction. Conversely, we identified positions of SR-BI ectodomainthat, by altering HCV binding, inhibit entry. Finally, we characterizedalternative ectodomain determinants that, by reducing SR-BI cholesterol uptakeand efflux functions, abolish HDL-mediated infection-enhancement. Altogether, wedemonstrate that SR-BI is an essential HCV entry factor. Moreover, our resultshighlight specific SR-BI determinants required during HCV entry andphysiological lipid transfer functions hijacked by HCV to favor infection.