Biology Reference
In-Depth Information
activity, the helicase and nucleotide triphosphatase (NTPase) activities
reside in the C-terminal 500 residues of the NS3 protein.
31-35
Helicase/NTPase activities are essential for the replication of the
HCV genome. The helicase activity is presumed to be involved in
unwinding a putative double-stranded replication intermediate or to
remove regions of secondary structure so that the RNA-dependent
RNA polymerase of NS5B (see below) can copy both strands of
the viral RNA. It is likely that the NTPase activity is coupled with the
helicase function, supplying the energy required for disrupting RNA
duplexes.
NS4A functions as a cofactor of the NS3 serine protease and is
required for efficient polyprotein processing. There are significant
differences in the stability and activity of the NS3 protease in the
presence and absence of NS4A. NS3 protein is relatively unstable
when expressed in cells in the absence of NS4A.
36
Structural studies
by NMR and X-ray methods indicate that the NS3-4A complex has
a more highly ordered N-terminal domain, and NS4A binding leads
the NS3 protease to a rearrangement of the active site triad to a
canonical conformation.
37
It has been predicted that the N-terminus
of NS4A forms a transmembrane helix, which presumably anchors
the NS3-4A complex to the cellular membrane.
38
NS4B is a hydropho-
bic, membrane-associated protein, which colocalizes predominantly
with ER markers.
39-42
Although relatively little is known to date
about functions of NS4B in the HCV life cycle, inhibitory activities
in the translation
43,44
and modulation of NS5B enzymatic function
45
have been reported. It has also been shown that NS4B protein can
induce a membranous web consisting of small vesicles embedded in
a membranous matrix,
46
and that the newly synthesized HCV RNA
exists in these membrane webs and speckles.
47,48
NS4B may play
an important role in the formation of the HCV RNA replication
complex.
49
NS5A is a phosphoprotein, which is mainly phophorylated on ser-
ine residues
50
mediated by one or more cellular serine-threonine
kinases.
51-54
The role of NS5A phosphorylation in HCV replication is
so far not clear; however, it has been thought that the protein is
important for the viral lifecycle since phosphorylation of NS5A is a
