Loss of function of the influenza A virus NS1 protein promotes apoptosis but this is not due to a failure to activate phosphatidylinositol 3-kinase (PI3K).

A panel of influenza A viruses encoding mutant NS1 proteins was created in which a number of NS1 functions, including interactions with dsRNA, PI3K, CPSF30 and PKR, were inhibited. Surprisingly, given previous reports that NS1 activates PI3K to prevent apoptosis, the mutant viruses rUd-Y89F and rUd-P164/7A that fail to activate PI3K did not induce any more apoptosis than wild-type virus in MRC-5 and A549 cells, even though these cells are highly sensitive to inducers of apoptosis. Induction of cell death by the apoptogenic rUd-184-8(P) virus could not be prevented by serum-mediated activation of PI3K/Akt. Neither infection of MRC-5 or A549 cells with wild-type virus nor constitutive expression of NS1 prevented cell death caused by apoptosis inducers, suggesting that NS1 is not directly anti-apoptotic. Our data suggest that the loss of a functionally intact NS1 protein promotes apoptosis, but this is not due to an inability to activate PI3K.

CDK/ERK-mediated phosphorylation of the human influenza A virus NS1 protein at threonine-215.

Posttranslational modification of viral proteins by cellular enzymes is a feature of many virus replication strategies. Here, we report that during infection the multifunctional human influenza A virus NS1 protein is phosphorylated at threonine-215. Substitution of alanine for threonine at this position reduced early viral propagation, an effect apparently unrelated to NS1 antagonizing host interferon responses or activating phosphoinositide 3-kinase signaling. In vitro, a subset of cellular proline-directed kinases, including cyclin dependent kinases (CDKs) and extracellular signal-regulated kinases (ERKs), potently phosphorylated NS1 protein at threonine-215. Our data suggest that CDK/ERK-mediated phosphorylation of NS1 at threonine-215 is important for efficient virus replication.