Epstein BARR Virus Replication Biology Essay
Even though the Epstein-Barr virus is a member of the Herpesviridae household, its life rhythm has some alone features that are non found in other members of the household. After transmittal in the spit to an clean individual, it is believed that the virus infects naif B-lymphocytic cells in the oropharynx ( 1 ) . Some research workers hypothesize that this B-cell infection occurs secondary to a primary lytic infection of the mucosal epithelial tissue, but every bit yet this procedure has non been proven ( 2 ) . As portion of the immune system, B-cells are an clever host for a virus that sets up a life-long latent infection. The mechanisms by which the virus preferentially infects and transforms resting memory B-cells are under probe as are the mechanisms for passage to the lytic life rhythm.
When the virus comes across a mark B-cell, viral protein gp350/220 attaches to the cellular receptor CD21, conveying the enveloped virus in close propinquity to the plasma membrane ( 3 ) . Additionally, gp42 binds to human leucocyte antigen ( HLA ) Class II molecules ( 2 ) . This viral fond regard induces endocytosis via non-clathrin coated cysts. The viral envelope and the cell membrane fuse, using viral proteins gB, gH, gL, and gp42. Structurally correspondent to proteins found in other herpes viruses, EBV sarin has been shown to take portion in viral assembly every bit good as merger. The other three proteins are closely associated with each other and organize a complex that is required for merger ( 4 ) . Once across the plasma membrane the virus needs to acquire to the karyon which is likely accomplished by travel along the cytoplasmatic microtubules in a mode similar to the herpes simplex virus 1. This hypothesis is based on similarities between EBV mirid bug protein BFRF3 and its HSV homologue UL35 which is known to consort with dynein visible radiation ironss ( 5 ) .
The procedure of uncoating for EBV can merely be speculated on, but based once more on structural comparings, it is believed that BVRF1, an EBV protein, may interact with the atomic pore composite ( NPC ) as another HSV1 protein UL25 does. If this is true, the mirid bug likely stays localized to the microtubules after let go ofing the viral Deoxyribonucleic acid to the karyon ( 5 ) .
Once in the karyon, Epstein-Barr viral DNA could conceivably ensue in two separate results. A lytic infection would bring forth immense Numberss of virus in a comparatively short period of clip, but for EBV infections what occurs in about every instance is latent infection of the B-cells. Immediately upon entry into the karyon, EBV linear DNA circularizes. This episome, or plasmid, will stay in the karyon of septic cells and be replicated with the host cell constituents until a lytic infection is stimulated ( 6 ) . Transformation of the B-cell to an immortalized province is dependent on the Epstein-Barr virus atomic antigen 2 ( EBNA-2 ) moving as a transcriptional activator for both host and viral cistrons ( 4 ) .
Latent EBV infection of B-lymphocytes involves care of the episome in the karyon of septic cells, incentive of proliferation, and turning away of host cell programmed cell death. Host cell machinery is responsible for the production of EBV proteins and reproduction of the genome during latency. Of the many possible cistron merchandises, merely a limited figure are latently expressed, including six atomic proteins and three membrane proteins ( 6 ) . The atomic proteins ( EBNA-1, EBNA-2, EBNA-3A, EBNA-3B, EBNA-3C, and EBNA-LP ) have critical functions in keeping the episomal DNA and in latent viral cistron written text ( 4 ) . Strictly talking, EBNA-1 is the lone one of these proteins that is perfectly required to keep and retroflex the viral episome ( 6 ; 7 ) . It binds straight to the two functional spheres of the beginning of plasmid reproduction ( OriP ) , the part of the EBV genome believed to be responsible for latent reproduction. In latency, the EBV genome is capable to the host cell rhythm, regulated by host cell proteins, and is replicated merely one time during S stage. The origin acknowledgment composite ( ORC ) is merely one of the host protein composites that is required for OriP replicative activity ( 4 ) . The round composing of the EBV genome in latency allows for the viral genome to be replicated right alongside the host chromosomal DNA and so segregated into the girl cells ( 6 ) .
While EBV can keep a womb-to-tomb latent infection, it has besides been observed that on occasion the virus will transition to a lytic province, ensuing in fecund viral sloughing and cell decease. While the stimulation to do this passage in vivo has non been proven, lytic EBV infection relies much more to a great extent on viral proteins, two of which are immediate-early ( IE ) cistron merchandises BZLF1 and BRLF1 ( 6 ) . BZLF1, besides called ZEBRA or Zta, is a needed written text factor that binds to the EBV lytic beginning of DNA reproduction ( OriLyt ) and has been speculated to be portion of the induction of the lytic stage. BRLF1 is believed to complex with BZLF1 as a transcriptional activator ( 8 ) . A regulated cascade of viral cistron look ensues. BALF5 encodes for the catalytic part of the viral DNA polymerase while BMRF1 codifications for the accessary fractional monetary unit. The BALF2 cistron merchandise is a single-stranded DNA-binding protein that acts to destabilise the two-base hit stranded spiral. In add-on, a composite of three proteins is coded for by BBLF4, BSLF1, and BBLF2/3. These proteins are believed to move at the reproduction forks to transcribe the viral genome ( 6 ; 8 ) . The exact mechanisms for DNA reproduction are still under probe, but one proposed theoretical account holds that the BALF2 single-stranded DNA binding protein works with a composite of BZLF1-BBLF4/BSLF1/BBLF2/3 to open up the dual spiral at OriLyt and synthesise RNA primers. If this theoretical account proves to be true, lytic reproduction is about wholly accomplished by viral proteins ( 6 ) .
Lytic reproduction takes topographic point in the karyon, in specialised reproduction compartments ( 6 ) . It is likely that reproduction is accomplished via a rolling-circle mechanism and one time replicated, the now additive genome is prepared for atomic issue by boxing it into virion atoms ( 4 ) . Late cistron look of the nucleocapsid proteins that make these virion atoms is done in the cytol and the proteins are transported into the karyon. There are six structural proteins coded for by EBV. BdRF1 is a scaffolding protein that has been shown to transport the major mirid bug protein BcLF1 into the karyon. BORF1 and BDLF1 are both ternary proteins that travel together into the karyon. A little mirid bug protein, BFRF3, and BVRF2, a peptidase, besides localize to the karyon. The scaffold protein forms a spherical procapsid which is cleaved by the peptidase after the major mirid bug proteins, ternary proteins, and little mirid bug proteins self-assemble into a viral mirid bug incorporating DNA ( 9 ) . It is believed that the mirid bug so buds through the atomic membrane, geting a impermanent envelope which is lost in the cytol and so another envelope is gained, perchance from the trans-golgi web. The freshly enveloped virus is so transported to the plasma membrane for exocytosis. This procedure has non been confirmed, but has been hypothesized based on surveies of the alphaherpesviruses ( 10 ) .