Poxviridae
- Baltimore Group I - dsDNA linear
- Enveloped, complex shape
- Only DNA viruses to replicate in cytoplasm - must provide their own mRNa & DNA enzymes
- Small pox, cowpox - inflection or disease
Name the two subfamilies in Poxviridae
- Entomopoxivirinae (invertebrates) 3 Genera A,B & C
- Chordopoxvidinae (vertebrates) 8 Genera: One genus called Orthopoxvirus
What is the name of the live virus in small pox vaccine?
Vaccina - is a type of poxvirus that is used as a vaccine to protect against smallpox.
What is the origin of vaccinia?
- Exact origin is unknown.
- Does not appear to be related to any other knwon pox virus
Characteristics of Vaccine
- Very large (300+nm) Complex Capsid
- Linear dsDNA genome (190kb: 250 genes)
- Replicates in cytoplasm of host
- Outer membrane
- “core” envelope and nucleoprotein/caspid
- Viral enzymes
True or False: Viruses have cytoplamic and metabolic activity
False, viruses have no cytoplasm or metabolic activity.
What are the infectious forms of vaccina that are produced during replication?
- intracellular mature virion (IMV)
- intracellular enveloped virion (IEV)
- cell-associated enveloped virion (CEV)
- extracellular enveloped virion (EEV)
IMV is most infectious form
Describe vaccinia genome organisation
- ds DNA genome, strands covalently linked by hairpin loops
- 190kb coding for >180 proteins (Fully sequenced)
- flanked by multiple terminal repeat sequences (TR)
- Coding sequences are densely packed, may be in TR different reading frames (overlap) and be on either strand
Where does vaccinia replication occur?
- occurs in cytoplasm
- replication may occur in enucleated cells but maturation is blocked
What receptors are on vaccinia?
Receptors: unknown but probably >1 on different cell types
How does gene expression work in vaccinia replication?
Gene Expression: viral enzymes & divided into three phases
– Early gene: ~50% genome, expressed before genome replication
- Intermediate: genome replication & late gene proteins produced
- Late genes: expressed after genome replication & late promoters are dependent on DNA replication for activity
Early Phase
- Early proteins cause cells to divide, block immunity disassemble core induce intermediate phase
- Early proteins: viral DNA polymerase and viral thymidine kinase (TK)
Intermediate Phase
- Int. proteins induce late phase genes DNA replication
Late Phase
- Late proteins= Virus structural proteins Assembly of immature virion in cis Golgi Maturation in endosome
- Late proteins = structural proteins, enzymes that turn off early viral protein synthesis and promote virion production
- Mature Virus disseminates to external environment or neighbouring cells
Viral entry and uncoating
- The outer viral membrane of the vaccinia virus fuses with the host cell’s plasma membrane, releasing the virus into the cell’ cytoplasm.
- Following release, the virus undergoes a second uncoating step to release nucleoproteins and the DNA.
- Within minutes after infection, viral enzymes that enter the cell with the vaccinia virus cores initiate transcription, the copying of messenger RNA from the DNA viral chromosome.
- Host DNA, RNA and protein synthesis are switched off almost immediately by viral factors carried into the cell with the virion.
Early Transcription and Early Proteins
- Early viral transcription is transcribed by a multisubunit viral RNA polymerase. About one half the genome are transcribed before the DNA is replicated.
- Major protein products are viral DNA polymerase and viral thymidine kinase (TK). Early viral mRNAs are translated on cellular ribosomes soon after they are transcribed.
Vaccinia Virus DNA Replication
- The DNA of poxviruses (a linear, doublestranded, DNA genome) is replicated in the cytoplasm of the host cell by the virus DNA polymerase.
- As most proteins involved in replicating vaccinia virus DNA are virus enzymes to produce a new viral chromosome.
Late Transciption and Late Proteins
- There are two classes of proteins produces in late vaccinia virus infection: an immediate DNA synthesis and a delayed– late class transcribed directly after late class.
- Proteins produced during both these stages include most of the structural proteins of the virus particles, some of which are processed or cleaved during virion assembly, as well as certain viral enzymes that turn off early viral protein synthesis and lock the infection into virion production mode.
Virus Assembly, Maturation and Release
- Immature viral particles appear circular or spherical in three dimensions in special virion assembly areas within the cytoplasm of the cell.
- Mature vaccinia virus virions are moved out of assembly areas and transported to the cell membrane.
- Mature virions are drawn to the tips of specialized, actincontaining microvilli and are released.
- The doublelayer membrane surrounding virions is thought to be derived from the Golgi apparatus. Seven viral glycoproteins stud the viral envelope.
What is an Interferon ?
- Inteferon (IFN) is a signalling protein released by presence of virus. a host cell in response to the IFN is a cytokine that triggers the immune system to help eradicate the virus.
- IFNs are named for their ability to “interfere” with viral replication by protecting cells against viral infections.
VV Evasion of host
- VV shows considerable resistance to antiviral effects of interferons
- an early gene of VV, K3L, action of PKR
- E3L, inhibits the protein binds to dsRNA and sequesters it & so it inhibits dsRNA mediated activation of PKR
- other vaccinia proteins interfere with the actions of the complement system & IL1
