The EBOV genome is approximately 19 kb in length. It encodes seven structural proteins: nucleoprotein (NP), polymerase cofactor (VP35), (VP40), GP, transcription activator (VP30), VP24, and RNA polymerase (L).[3]
Structure[edit]
Electron micrographs of EBOV show them to have the characteristic threadlike structure of a filovirus.[4] EBOV VP30 is around 288 amino acids long.[5] The virions are tubular in general form but variable in overall shape and may appear as the classic shepherd's crook or eyebolt, as a U or a 6, or coiled, circular, or branched; laboratory techniques, such as centrifugation, may be the origin of some of these formations.[6] Virions are generally 80 nm in diameter with a lipid bilayer anchoring the glycoprotein which projects 7 to 10 nm long spikes from its surface.[7] They are of variable length, typically around 800 nm, but may be up to 1000 nm long. In the center of the virion is a structure called nucleocapsid, which is formed by the helically wound viral genomic RNA complexed with the proteins NP, VP35, VP30, and L.[8] It has a diameter of 80 nm and contains a central channel of 20–30 nm in diameter. Virally encoded glycoprotein (GP) spikes 10 nm long and 10 nm apart are present on the outer viral envelope of the virion, which is derived from the host cell membrane. Between envelope and nucleocapsid, in the so-called matrix space, the viral proteins VP40 and VP24 are located.[9]
Genome[edit]
Each virion contains one molecule of linear, single-stranded, negative-sense RNA, 18,959 to 18,961 nucleotides in length. The 3′ terminus is not polyadenylated and the 5′ end is not capped. It was found that 472 nucleotides from the 3' end and 731 nucleotides from the 5' end are sufficient for replication.[10] It codes for seven structural proteins and one non-structural protein. The gene order is 3′ – leader – NP – VP35 – VP40 – GP/sGP – VP30 – VP24 – L – trailer – 5′; with the leader and trailer being non-transcribed regions, which carry important signals to control transcription, replication, and packaging of the viral genomes into new virions. The genomic material by itself is not infectious, because viral proteins, among them the RNA-dependent RNA polymerase, are necessary to transcribe the viral genome into mRNAs because it is a negative sense RNA virus, as well as for replication of the viral genome. Sections of the NP and the L genes from filoviruses have been identified as endogenous in the genomes of several groups of small mammals.[11]
Entry[edit]
Niemann–Pick C1 (NPC1) appears to be essential for Ebola infection. Two independent studies reported in the same issue of Nature showed that Ebola virus cell entry and replication requires the cholesterol transporter protein NPC1.[12][13] When cells from Niemann Pick Type C patients were exposed to Ebola virus in the laboratory, the cells survived and appeared immune to the virus, further indicating that Ebola relies on NPC1 to enter cells. This might imply that genetic mutations in the NPC1 gene in humans could make some people resistant to one of the deadliest known viruses affecting humans. The same studies described similar results with Ebola's cousin in the filovirus group, Marburg virus, showing that it too needs NPC1 to enter cells.[12][13] Furthermore, NPC1 was shown to be critical to filovirus entry because it mediates infection by binding directly to the viral envelope glycoprotein.[13] A later study confirmed the findings that NPC1 is a critical filovirus receptor that mediates infection by binding directly to the viral envelope glycoprotein and that the second lysosomal domain of NPC1 mediates this binding.[14]
In one of the original studies, a small molecule was shown to inhibit Ebola virus infection by preventing the virus glycoprotein from binding to NPC1.[13][15] In the other study, mice that were heterozygous for NPC1 were shown to be protected from lethal challenge with mouse adapted Ebola virus.[12] Together, these studies suggest NPC1 may be potential therapeutic target for an Ebola anti-viral drug.
Replication[edit]
Being acellular, viruses do not grow through cell division; instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell.[8]
The virus attaches to host receptors through the glycoprotein (GP) surface peplomer and is endocytosed into macropinosomes in the host cell [16]
Viral membrane fuses with vesicle membrane, nucleocapsid is released into the cytoplasm
Encapsidated, negative-sense genomic ssRNA is used as a template for the synthesis (3' – 5') of polyadenylated, monocistronic mRNAs
Using the host cell's machinery, translation of the mRNA into viral proteins occurs
Viral proteins are processed, glycoprotein precursor (GP0) is cleaved to GP1 and GP2, which are heavily glycosylated. These two molecules assemble, first into heterodimers, and then into trimers to give the surface peplomers. Secreted glycoprotein (sGP) precursor is cleaved to sGP and delta peptide, both of which are released from the cell.
As viral protein levels rise, a switch occurs from translation to replication. Using the negative-sense genomic RNA as a template, a complementary +ssRNA is synthesized; this is then used as a template for the synthesis of new genomic (-)ssRNA, which is rapidly encapsidated.
The newly formed nucleocapsids and envelope proteins associate at the host cell's plasma membrane; budding occurs, destroying the cell.
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