The formation and the activity of the membranous web are still poorly understood. NS4B and
NS5A play a major role in the induction of membrane rearrangements. The expression of NS4B
protein alone induces membrane alterations similar to membranous webs of infected cells [88,89] and
the expression of NS5A induces the formation of double membrane vesicles [94]. Both morphological
and biochemical data led to the proposal that HCV replication complexes are derived from the ER
membrane [88,89,91,94,102]. However, several endosomal markers were also reported as being
colocalized with replication complexes and/or functionally involved in RNA replication [103–105].
Moreover, it has also been shown that RNA replication occurs in detergent-resistant membranes [106].
This indicates that the membranes of HCV replication complexes are enriched in cholesterol and
sphingolipids, two lipids underrepresented in the ER membrane. Therefore, these lipids have to be
transported to the replication complexes, or the replication complexes form in sub-domains of the ER
membrane locally enriched in cholesterol and sphingolipids. All these data suggest that the membranes
of the HCV replicase could be derived from the ER membrane and biochemically modified to create a
particular lipid environment required for the replicative activity of the complexes. The formation of HCV
replication complexes from the membrane of the ER would therefore involve coordinated biochemical
and morphological changes.