Biology Reference
In-Depth Information
conserved feature among different HCV isolates and among other
members of the
Flaviviridae
. Evidence indicating an involvement of
NS5A in the viral RNA replication is now accumulating. A hot spot
of the cell culture-adaptive mutations that increase replication effi-
ciency of HCV RNA is located in the central region of NS5A.
14-16
The
membrane association of NS5A through its N-terminal transmem-
brane domain
55
and the interaction between NS5A and 5B
56
are
essential for the RNA replication. Several cellular proteins interacting
with NS5A have been identified, and human vesicle-associated mem-
brane protein-associated protein A (h-VAP-A) is likely to play a key
role in the RNA replication through the interacting with NS5A, as
demonstrated by experiments using RNA interference and dominant-
negative protein fragments.
49
In addition to its function related to the
viral replication, NS5A appears to be implicated in resistance of HCV-
infected cells to the antiviral effect of IFN. At least for some HCV iso-
lates, NS5A interacts structurally and functionally with an
IFN-induced protein kinase PKR, leading to the inhibition of the
kinase function and hence blocking the translation reduction in the
IFN-treated cells.
57,58
NS5B is an RNA-dependent RNA polymerase (RdRp), a key
enzyme involved in viral replication.
27,59-64
The RdRp activity of
NS5B as well as its three-dimensional structure have been demon-
strated using recombinant gene products prepared from a variety
of expression systems. The optimal temperature, pH requirements
and concentration of bivalent cations for the activity were found to
be similar to those for the poliovirus 3D polymerase.
61
The X-ray
studies have reported that while the enzyme has the typical right-
handed “finger-palm-thumb” domains of the polymerase, extensive
interactions of the fingers and thumb lead to a more fully enclosed
active site tunnel, unlike other RdRp.
65-67
NS5B contains a
hydrophobic domain at its C terminal 21 residues, which is a trans-
membrane segment. Recent studies have demonstrated that the
C-terminal domain of NS5B appears to serve dual functions in the
viral RNA replication, both through its role as a membrane anchor
and through its involvement in RNA synthesis in a sequence-specific
manner.
68,69
