The mechanism of entry of hepatitis C virus (HCV) through interactions between the envelope glycoproteins and specific cell surface receptors remains unclear at this time. We have previously shown with the vesicular stomatitis virus (VSV)/HCV pseudotype model that the hypervariable region 1 of the HCV E2 envelope glycoprotein helps in binding with glycosaminoglycans present on the cell surface. In this study, we have examined the binding of HCV envelope glycoproteins with chemically modified derivatives of heparin. Furthermore, we have determined the functional relevance of the interaction of heparin derivatives with HCV envelope glycoproteins for infectivity by using a human immunodeficiency virus (HIV)/HCV pseudotype, a VSV/HCV pseudotype, and cell culture-grown HCV genotype 1a. Taken together, our results suggest that the HCV envelope glycoproteins rely upon O-sulfated esters of a heparin homologue to facilitate entry into mammalian cells.We first reported the generation of a vesicular stomatitis virus (VSV)/hepatitis C virus (HCV) pseudotype to understand the role of the ectodomains of HCV envelope glycoproteins in relation to the initiation of virus infection (20). Since VSV particles assemble at the plasma membrane (1), we expressed chimeric HCV envelope glycoproteins (E1G and E2G) by appending the transmembrane domain and cytoplasmic tail of VSV glycoprotein at their C termini to provide membrane anchor signals. This modification allowed for the localization of the glycoproteins to the cell surface for efficient incorporation onto VSV pseudotype particles. Pseudotypes generated from either E1G or E2G which had been expressed individually displayed a low level of infectivity, while the presence of both envelope glycoproteins significantly increased the infectious titer of the pseudotype (26). Interestingly, each HCV envelope protein is suggested to have class I and/or class II fusion peptide motifs (10,11,12,31), which may contribute to the infectivity of the parent virus. Murine leukemia virus (MuLV)-and human immunodeficiency virus (HIV)-derived pseudotypes were subsequently generated from human embryonic kidney epithelial cells (293T) by the expression of an unmodified HCV envelope genomic region (5, 15). Although there are apparent differences, the overall observations from MuLV-and HIV-derived pseudotypes were similar to observations from the VSV/HCV pseudotype (8, 26). We have previously observed that the E2 glycoprotein binds to cell surface glycosaminoglycans (GAGs) (7,24). Further studies suggested that the hypervariable region 1 (HVR1) of E2 binds with heparin, and the infectivity of the VSV/HCV pseudotype is diminished in the presence of soluble GAGs or decreased after the enzymatic removal of heparinlike GAGs from the cell surface (7). A different group of investigators have suggested that HCV particles can be efficiently purified from patient sera by using heparin, and GAG binding sites are located within E2 (27, 32).The binding of E2 with a heparin-like molecule appears to facilitate VSV-deri...