Molluscum contagiosum virus (MCV) is a species of DNA poxvirus that causes the human skin infection molluscum contagiosum. Molluscum contagiosum affects about 200,000 people a year, about 1% of all diagnosed skin diseases. Diagnosis is based on the size and shape of the skin lesions and can be confirmed with a biopsy, as the virus cannot be routinely cultured. Molluscum contagiosum virus is the only species in the genus Molluscipoxvirus. MCV is a member of the subfamily Chordopoxvirinae of family Poxviridae. Other commonly known viruses that reside in the subfamily Chordopoxvirinae are Variola virus (the cause of smallpox) and Monkeypox virus. The poxvirus family uniquely contains both non-enveloped particles (mature virions) and enveloped particles (extracellular virions). The structure of the virions is consistent with that of others in the poxvirus family: they are composed of a nucleocapsid, core envelope, lateral body, and an extracellular envelope. Like other poxviruses, MCV is a DNA virus that replicates in the cytoplasm instead of the nucleus. Because of this, the virus must bring all necessary enzymes for replication with it or encode the enzymes in its genome.
Structure The molluscum contagiosum virus virion is described as oval-shaped and has the dimensions of approximately 320 nm × 250 nm × 200 nm. The virus has two distinct infectious particles called the mature virion (MV) and the enveloped virion (EV), which differ in that the EV contains a second outer cellular membrane. Poxviridae is the only virus family that has both enveloped and non-enveloped infectious particles. Other structures of the EV and MV virion include the nucleocapsid, core wall, and two lateral bodies. Before the virion is released into the cytosol, the lateral bodies are associated with the virion core through binding interactions. However, during virion release into the cytoplasm, the virion core wall expands and forces the lateral bodies to dissociate. The lateral bodies function to transport one or more vital virion proteins needed for genome replication or expression.
Genome The genome consists of a linear double-stranded DNA molecule that is approximately 190 kilobases in length. The genome is unique in that the two ends of the double-stranded DNA sequence are inverted repeats of each other. This inverted terminal sequence is 4.7 kilobases in length, but can vary from 0.7–12 kilobases among poxviruses. There are 182 genes that are encoded by molluscum contagiosum virus. Over 100 of these genes are conserved in other viruses from the poxvirus family, such as Variola virus and Vaccinia virus. The inverted terminal sequence of MCV differs from the sequences of others in the poxvirus family because it contains genus-specific host response evasion genes. The genome contains 64% GC bases, and thus encodes a lesser amount of the stop codons UAA, UGA, and UAG compared to other poxviruses. Further gene analysis has shown the MCV genome contains many long and overlapping open reading frames.
Replication cycle
Entry into cell Molluscum contagiosum virus, similar to all poxviruses, produces two infectious particles: mature virions (MV) and extracellular virions (EV), with the EV differing from the MV in that they possess an extra cellular membrane. To enter the cell, the membrane of MV fuses to the plasma membrane, specifically glycosaminoglycans, of the host cell and then enters via macropinocytosis. This process is initiated by the presence of phosphatidylserine molecules exposed on the MV cellular membrane. Similarly, the outer membrane of EV fuses to the plasma membrane, specifically glycosaminoglycans, of the host cell and also enters via macropinocytosis. After macropinocytosis, H+ is pumped inside the internalized vacuole containing EV, and this acidification breaks down the outer membrane, exposing an MV-like particle. For both MV and EV, the cellular membranes then fuse with the vacuole allowing the release of the virus core into the cytosol. The virion is then uncoated, exposing the DNA to commence replication.
Replication and transcription Molluscum contagiosum virus, like other poxviruses, replicates entirely in the cytoplasm of the host cell. This is a property unique to poxviruses, as all other DNA viruses replicate in the nucleus. Therefore, because the host cell proteins for DNA replication are present inside the nucleus, this virus must bring or encode all the proteins needed for replication. Each virion sets up a region in the cytoplasm, called a 'viral factory' where DNA replication, transcription, and translation all occur sequentially. There are three phases of DNA transcription. During the early phase, genes that encode for transcription factors, viral DNA, and RNA polymerases, and proteins that stimulate host cell mitosis are transcribed by DNA-dependent RNA polymerase that the virion carries with it. The mRNA produced in the early phase is then translated by viral transcriptase that the virion also carries with it. Then, during the intermediate phase, the proteins encoded in the early phase are used to replicate the viral DNA. Additionally, more transcription factors are produced to transcribe the late-phase mRNA. During the late phase, the genes encoding for structural proteins and enzymes needed for future infection are transcribed and then translated. Gene expression is sequential from early to intermediate to late phase of transcription, and it is temporally regulated.
Assembly and release The virion cytoplasmic factories serve as the place where mature virions are assembled for future infection. Mature virions are released via cell lysis and aid in host-to-host transmission of the virus. Extracellular virions are made when the MV acquires a second membrane via the Golgi apparatus and then buds out of the cell. Extracellular virions aid in spreading the virus within the epithelial tissue.
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