Betaherpesvirinae

VIRION

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Enveloped, spherical to pleomorphic, 150-200 nm in diameter, T=16 icosahedral symmetry. Capsid consists of 162 capsomers and is surrounded by an amorphous tegument. Glycoproteins complexes are embeded in the lipid envelope.

GENOME

Monopartite, linear, dsDNA genome of 140-240 kb. The genome contains terminal and internal reiterated sequences.

GENE EXPRESSION

Each viral transcript usually encodes a single protein and has a promoter/regulatory sequence, a TATA box, a transcription initiation site, a 5' leader sequence of 30-300 bp (not translated), a 3' nontranslated sequence of 10-30 bp, and a poly A signal. There are many gene overlaps. There are only few spliced genes. Some of the expressed ORFs are antisense to each other. Some ORFs can be accessed from more than one promoter. There are some non-coding genes.

REPLICATION

NUCLEAR

Lytic replication:

  1. Attachment of the viral glycoproteins to host receptors mediates endocytosis of the virus into the host cell.
  2. Fusion with the plasma membrane to release the core and the tegument proteins into the host cytoplasm.
  3. The capsid is transported to the nuclear pore where the viral DNA is released into the nucleus.
  4. Transcription of immediate early genes which promote transcription of early genes and protect the virus against innate host immunity.
  5. Transcription of early viral mRNA by host polymerase II, encoding proteins involved in replication of the viral DNA.
  6. A first round of circular genome amplification occurs by bidirectional replication
  7. Synthesis of linear concatemer copies of viral DNA by rolling circle.
  8. Transcription of late mRNAs by host polymerase II, encoding structural proteins.
  9. Assembly of the virus in nuclear viral factories and budding through the inner lamella of the nuclear membrane which has been modified by the insertion of herpes glycoproteins, throughout the Golgi and final release at the plasma membrane.

Latent replication : replication of circular viral episome in tandem with the host cell DNA using the host cell replication machinery.

Host-virus interaction

Adaptive immune response inhibition

Beta-herpesviruses have evolved different strategies to inhibit the host adaptive immune response. For example, HCMV US2 destroys two components of the MHC class II pathway, HLA-DR-alpha and DM-alpha, preventing recognition by CD4+ T cells. Mouse cytomegalovirus (MCMV) uses m152 and m06 encoded proteins to inhibit surface expression of MHC I molecules .

Apoptosis modulation

Apoptosis is very often modulated (and usually inhibited) by beta-herpesviruses. Human cytomegalovirus (HCMV) viral inhibitor of caspase-8-induced apoptosis (from gene UL36) plays a role in the inhibition of apoptosis by interacting with the pro-domain of pro-caspase-8/CASP8 and thus preventing its activation . Murine cytomegalovirus m38.5 protein interacts with host Bax in infected cells and thus inhibits Bax-mediated cell death .

Autophagy modulation

Several beta-herpesviruses are able to inhibit host autophagy process, such as HCMV protein TRS1 via its interaction with host Beclin 1 .

Cell-cycle modulation

The UL24 protein that is present in all herpesvirus subfamilies (alpha, beta and gamma-herpesviruses) induces a cell cycle arrest at G2/M transition through inactivation of the host cyclinB/cdc2 complex .

Innate immune response inhibition

Herpes viruses inhibit the cascade leading to production of interferon-beta by mainly targeting the host IRF3 protein. For example, HCMV targets host IRF3 protein with the viral pp65 protein .

Host splicing inhibition

Modulates the host mRNA expression by exporting unspliced mRNA, thereby inducing alternative splicing .