ICP

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HHV Infected Cell Polypeptide 0

Human Herpes Virus (HHV) Infected Cell Polypeptide 0 (ICP0) is a protein, encoded by the DNA of herpes viruses. It is produced by herpes viruses during the earliest stage of infection, when the virus has recently entered the host cell; this stage is known as the immediate-early or α ("alpha") phase of viral gene expression. During these early stages of infection, ICP0 protein is synthesized and transported to the nucleus of the infected host cell. Here, ICP0 promotes transcription from viral genes, disrupts structures in the nucleus known as nuclear dots or promyelocytic leukemia (PML) nuclear bodies, and alters the expression of host and viral genes in combination with a neuron specific protein. At later stages of cellular infection, ICP0 relocates to the cell cytoplasm to be incorporated into new virion particles.

History and background

ICP0 was identified as an immediate-early polypeptide product of Herpes simplex virus-1 (HSV-1) infection in 1976. The gene, in HSV-1, from which ICP0 is produced is known as HSV-1 α0 ("alpha zero"), Immediate Early (IE) gene 1, or simply as the HSV-1 ICP0 gene. The HSV-1 ICP0 gene was characterized and sequenced in 1986. This sequence predicted a 775 amino acid sequence with a molecular weight of 78.5 KDa. At the time of gene isolation, ICP0 was known as IE110 as gel electrophoresis experiments performed prior to obtaining the gene sequence indicated the ICP0 protein weighed 110 kDa. Post-translational modifications, such as phosphorylation or sumoylation, were presumed to account for the actual protein size appearing 30 kDa larger than that of the predicted amino acid sequence.

ICP8

ICP8, the herpes simplex virus type-1 single-strand DNA-binding protein, is one of seven proteins encoded in the viral genome of HSV-1 that is required for HSV-1 DNA replication. It is able to anneal to single-stranded DNA (ssDNA) as well as melt small fragments of dsDNA; its role is to destabilize duplex DNA during initiation of replication. It differs from helicases because it is ATP- and Mg²⁺-independent. In cells infected with HSV-1, the DNA in those cells become colocalized with ICP8.

ICP8 is required in late gene transcription, and has found to be associated with cellular RNA polymerase II holoenzyme.

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