Circovirus (taxid:39725)

VIRION

Non-enveloped, round, T=1 icosahedral symmetry, about 17 nm in diameter. The capsid consists of 12 pentagonal trumpet-shaped pentamers.

GENOME

Monopartite, circular, ssDNA genome of about 1800 to 2000bp.
The genome is replicated through double-stranded intermediates. The replication (Rep) protein initiates and terminates rolling circle replication, the host DNA polymerase being used for DNA replication itself. There is a potential stem-loop structure in the intergenic region that includes a conserved nonanucleotide sequence (AGTATTAC) where ssDNA synthesis is initiated.

GENE EXPRESSION

The encapsidated viral DNA and the complementary DNA of the replicative intermediate encode two mRNAs for the Rep and capsid proteins.
Rep mRNA can undergo alternative splicing in some strains to produce Rep' protein.

ENZYMES

• Cress-type Endonuclease [Rep]

REPLICATION

NUCLEAR

1. Virus penetrates into the host cell probably by clathrin- and caveolae-independent endocytosis.
2. Uncoating, the viral ssDNA genome penetrates into the nucleus.
3. Viral ssDNA is converted into dsDNA with the participation of cellular factors. dsDNA transcription gives rise to viral mRNAs.
4. Viral mRNAs are translated to produce viral proteins.
5. Replication may be mediated by a Rep protein, and would occur by rolling circle producing ssDNA genomes.
6. These newly synthesized ssDNA can either
   a) be converted to dsDNA and serve as a template for transcription/replication
   b) be encapsidated by capsid protein and form virions released by cell lysis.

Host-virus interaction

Autophagy modulation

Cap protein of PCV2 induces activation of host autophagy in order to enhance its own replication .

Apoptosis modulation

PCV2 ORF3-encoded protein induces apoptosis by activating caspase-8 and caspase-3 pathways . This protein may be a candidate for PML bodies inhibition which is essential for nuclear virus replication in mammals.