Download Studying the epstein barr virus

Studying the epstein barr virus
Epstein-Barr Virus (EBV) is an enveloped virus that uses a receptor-mediated
approach to gain entry into its target cells, which include B cells and the
epithelial cells of the nasopharynx and oropharynx (7). EBV has an affinity for
the CD21 receptor that these cells express; CD21 is an important receptor for
the complement cascade, and thus only a select few cells display this protein
on their surface (7). As a result, EBV has a narrow cell tropism and
preferentially attaches to these aforementioned B cells (7). From here, the
virus fuses with the target cell employing glycoproteins B, H, and L which are
located on the virion’s surface (4). These proteins undergo an as of yet
unidentified conformational change that aids in fusion (4). From here, how
EBV specifically uncoats and enters the nucleus is uncertain; however, its
method may be similar to that of other gammaherpesviruses (5). These
viruses use microtubules and manipulation of the cell’s signaling pathways to
direct the virion to the nucleus where it can then inject its genome (5).
Once the viral DNA is inside of the nucleus, the virus can either enter a latent
phase or a lytic phase (9). During a lytic infection gene expression occurs,
and the host cell’s DNA-dependent RNA polymerase is used for
circularization of the the linear DNA and for transcription (9). EBV undergoes
three phases of gene expression: the intermediate-early phase, the early
phase, and the late phase (10). The intermediate-early phase controls genes
such as BZLF1 and BRLF1, which initiates a productive infection by encoding
for the ZEBRA protein (10). An origin binding protein, ZEBRA also activates
transcription, thereby initiating gene expression and moving the virus from the
latent phase, if it is in one, into the lytic cycle (2,10). In order to perform this
function, the protein must bind to specific sequences of DNA called ZEBRA
response elements (2). ZEBRA also uses these sequences to initiate viral
replication; it is currently unknown how this protein can differentiate between
when to initiate transcription and when to initiate viral replication (2).
In all, EBV’s genome encodes for six important proteins needed for viral
replication, including ZEBRA (6). Early gene BMRF1 encodes for a
polymerase processivity factor, and early gene BALF5 encodes for a catalytic
subunit; once produced, these proteins interact to form the DNA polymerase
(2,6). Other early genes include BALF2, which encodes for a single-stranded
DNA-binding protein, BSLF1, which encodes for a primase, and BBLF4,
which encodes for a helicase (6). Furthermore, EBV expresses a protein
known as EB2 that exports mRNA from the nucleus to the cytoplasm where
translation by host ribosomes can occur (1). Once all of these necessary
proteins are synthesized, DNA replication can begin. EBV uses a rolling circle
mechanism for the synthesis of new DNA (6). Initiation of replication begins
with ZEBRA recognizing a lengthy stretch of DNA termed oriLYt (2). This
origin of replication contains seven ZEBRA-binding sites which the protein
must recognize for replication to occur (2). The polymerase, helicase, and
primase all work in conjunction at the origin of replication to produce progeny
DNA in an intermediate concatemer form before being cleaved into several
progeny viral genomes (6,10). The method by which this cleavage occurs has
yet to be determined (9).
Once viral replication has begun, late genes are expressed (10). These late
genes include BLRFR, which encodes for a protein to be packaged in the
tegument, BcLF1, which encodes for the major capsid protein, and BFRF3,
which encodes for a small capsid element (3). Other important late genes
transcribed and subsequently translated include BdRF1, which encodes for
the scaffolding protein, and BVRF2, which encodes for a protease used to
achieve maturation of the virus (3). Late genes are also expressed to lead to
the production of glycoproteins, such as the ones mentioned earlier that aid in
fusion (4,10). Again, the expression of these late genes seems to be tied to
the ability of ZEBRA to commence the lytic cycle; once replication has
occurred, these genes are immediately expressed (2,10). Assembly of the
progeny virions occurs in the nucleus of infected cells; therefore structural
proteins must make specific associations with each other in order to be
moved to this location (3). For example, it is the scaffolding protein’s task to
transport the major capsid protein into the nucleus, and, subsequently, when
these two proteins are associated with each other, the small capsid protein
can enter the nucleus as well (3). Once inside, EBV structural proteins selfassemble through physical interactions (3). The small and major capsid
proteins and the scaffolding protein form the procapsid; this shell interacts
with the DNA packaging complex in the nucleus and is then filled with the
progeny DNA (3). Once the genome is inside of the capsid, EBV uses a
protease to dispose of the scaffolding protein, and the progeny virion is now
mature and ready to exit the cell (3). The mechanism by which EBV obtains
its envelope and exits the cell is still uncertain (8). One study, which looks at
gammaherpesviruses in general, states that the virus may receive an initial
envelope from budding off of the inner nuclear membrane followed by a deenvelopment stage where the virus fuses with the outer nuclear membrane
(8). As a result, the naked virus is released into the cytoplasm where it
obtains its tegument proteins (8). The virus acquires these proteins while in
proximity to the Golgi apparatus; as a result, numerous vesicles are readily
available to swallow the virions and carry them to the plasma membrane
where egress occurs through exocytosis (8).