Activation
IRES are often used by viruses as a means to ensure that viral translation is active during periods of time when host translation is inhibited. These mechanisms of host translation inhibition are varied, and can be initiated by both virus and host, depending on the type of virus in question. However, in the case of most picornaviruses, this is accomplished by the viral protease cleaving eIF-4G so that it cannot interact with eIF-4E. Interaction between these two initiation factors is necessary for mRNA 5'cap to 3'poly-A-tail loop formation, which is usually a necessary event for initiation of translation. The virus may even use the eIF-4G to aid in initiation of IRES-mediated translation.
The cell may also use IRES to increase translation of certain proteins during mitosis and programmed cell death. In mitosis, the cell dephosphorylates eIF-4E so that it has little affinity for the 5'cap. As a result, the pre-initiation mRNA loop is not formed, and the translational machinery is diverted to IRES within the mRNA. Many proteins involved in mitosis are encoded by IRES mRNA. In programmed cell death, cleavage of eIF-4G, such as performed by viruses, decreases translation. Lack of essential proteins contributes to the death of the cell, as does translation of IRES mRNA sequences coding proteins involved in controlling cell death.
Read more about this topic: Internal Ribosome Entry Site