Abstract: | Early events leading to the establishment of hepatitis C virus (HCV)
infection are not completely understood. We show that intact and dynamic
microtubules play a key role in the initiation of productive HCV infection.
Microtubules were required for virus entry into cells, as evidenced using
virus pseudotypes presenting HCV envelope proteins on their surface. Studies
carried out using the recent infectious HCV model revealed that microtubules
also play an essential role in early, postfusion steps of the virus cycle.
Moreover, low concentrations of vinblastin and nocodazol,
microtubule-affecting drugs, and paclitaxel, which stabilizes microtubules,
inhibited infection, suggesting that microtubule dynamic instability and/or
treadmilling mechanisms are involved in HCV internalization and early
transport. By protein chip and direct core-dependent pull-down assays,
followed by mass spectrometry, we identified β- and α-tubulin as
cellular partners of the HCV core protein. Surface plasmon resonance analyses
confirmed that core directly binds to tubulin with high affinity via amino
acids 2-117. The interaction of core with tubulin in vitro promoted
its polymerization and enhanced the formation of microtubules. Immune electron
microscopy showed that HCV core associates, at least temporarily, with
microtubules polymerized in its presence. Studies by confocal microscopy
showed a juxtaposition of core with microtubules in HCV-infected cells. In
summary, we report that intact and dynamic microtubules are required for virus
entry into cells and for early postfusion steps of infection. HCV may exploit
a direct interaction of core with tubulin, enhancing microtubule
polymerization, to establish efficient infection and promote virus transport
and/or assembly in infected cells.HCV5 infection is a
major cause of chronic liver disease, which frequently progresses to cirrhosis
and hepatocellular carcinoma. HCV represents a global public health problem,
with 130 million people infected worldwide. There is currently no vaccine
directed against HCV and the available antiviral treatments eliminate the
virus in 40-80% of patients, depending on the virus genotype (for review, see
Ref. 1).HCV has a single-stranded, positive-sense RNA genome of ∼9.6 kilobases
encoding a large polyprotein that is processed by both host and viral
proteases to produce three structural proteins (core protein and the envelope
glycoproteins E1 and E2), p7, and six nonstructural proteins, which are
involved in polyprotein processing and replication of the virus genome (for
review, see Ref. 2).HCV core is a basic protein, synthesized as the most N-terminal component
of the polyprotein, and is followed by the signal sequence of the E1 envelope
glycoprotein (3). The
polypeptide is cleaved by signal peptidase and signal peptide peptidase,
resulting in the release of core from the endoplasmic reticulum membrane and
its trafficking to lipid droplets
(3-5).
Mature core protein forms the viral nucleocapsid
(6) and consists of two
domains, D1 and D2. D1 lies at the protein N terminus, is composed of about
117 amino acids (aa), and is involved in RNA binding
(7). D2 is relatively
hydrophobic, has a length of about 55 aa, and targets HCV core to lipid
droplets (8).Microtubules (MTs) are ubiquitous cytoskeleton components that play a key
role in various cellular processes relating to cell shape and division,
motility, and intracellular trafficking
(9). MTs are dynamic, polarized
polymers composed of α/β-tubulin heterodimers that undergo
alternate phases of growth and shrinkage, dependent on so-called
“dynamic instability”
(10). Active transport by MTs
is bidirectional and involves both plus and minus end-directed motors: kinesin
and dynein (11,
12).Another mechanism of cytosolic transport on MTs, called
“treadmilling”
(13,
14) involves polymerization at
the plus end and depolymerization at the minus end after severing of MTs by
cellular katenin (15).MTs have important functions in the life cycle of most viruses
(13,
16,
17). Cytoplasmic transport on
MTs provides viruses with the means to reach sites of replication or enables
progeny virus to leave the infected cell. Some viruses, such as Ebola virus
(18) or reovirus
(19), are transported on MTs
within membranous compartments, whereas other viruses like herpes simplex
virus type 1 (20), murine
polyoma virus (21), human
cytomegalovirus (22), or
adenovirus (23) interact with
MT motors or MT-associated proteins to allow their transport along
microtubules.Previous studies have established that the cell cytoskeleton is involved in
HCV replication, since HCV replication complexes are subjected to
intracellular transport and their formation is closely linked to the dynamic
organization of endoplasmic reticulum, actin filaments, and the microtubule
network
(24-26).
In addition, intact microtubules are essential for viral morphogenesis and the
secretion of progeny virus from infected cells
(27). The role of microtubules
in HCV cell entry and the initiation of productive HCV infection has not yet
been addressed.In this study, we provide evidence that the MT network plays a key role in
HCV cell entry and postfusion steps of the virus cycle that lead to the
establishment of productive HCV infection. The initial steps of the viral
cycle are sensitive to MT-affecting drugs that inhibit MT formation or
depolymerize or stabilize microtubules. We also show a unique property of the
HCV core protein, its capacity to directly bind to tubulin and to enhance MT
polymerization in vitro. Our findings suggest that HCV could exploit
the MT network by polymerization-related mechanisms to productively infect its
target cell. Thus, microtubules may provide a novel target for therapeutic
interventions against HCV infection. |