Infected cell protein 34.5 also ICP-34.5 or ICP34.5 is a protein expressed by the ɣ34.5 gene in viruses such as herpes simplex virus; it blocks a cellular stress response to viral infection.
When a cell is infected with a virus, protein kinase R is activated by the virus' double-stranded RNA,. Protein kinase R then phosphorylates a protein called eukaryotic initiation factor-2A (eIF-2A), which inactivates eIF-2A. EIF-2A is required for translation so by shutting down eIF-2A, the cell prevents the virus from hijacking its own protein-making machinery. Viruses in turn evolved ICP34.5 to defeat the defense; it activates protein phosphatase-1A which dephosphorylates eIF-2A, allowing translation to occur again. A herpesvirus lacking the ɣ34.5 gene will not be able to replicate in normal cells because it cannot make proteins.