SH3 binding motif 1 in influenza a virus NS1 protein is essential for PI3K/Akt signaling pathway activation

Recent studies have demonstrated that influenza A virus infection activates the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway by binding of influenza NS1 protein to the p85 regulatory subunit of PI3K. Our previous study proposed that two polyproline motifs in NS1 (amino acids 164 to 167 [PXXP], SH3 binding motif 1, and amino acids 213 to 216 [PPXXP], SH3 binding motif 2) may mediate binding to the p85 subunit of PI3K. Here we performed individual mutational analyses on these two motifs and demonstrated that SH3 binding motif I contributes to the interactions of NS1 with p85 beta, whereas SH3 binding motif 2 is not required for this process. Mutant viruses carrying NS1 with mutations in SH3 binding motif I failed to interact with p85 beta and induce the subsequent activation of PI3K/Akt pathway. Mutant virus bearing mutations in SH3 binding motif 2 exhibited similar phenotype as the wild-type (WT) virus. Furthermore, viruses with mutations in SH3 binding motif 1 induced more severe apoptosis than did the WT virus. Our data suggest that SH3 binding motif I in NS1 protein is required for NS1-p85 beta interaction and PI3K/Akt activation. Activation of PI3K/Akt pathway is beneficial for virus replication by inhibiting virus induced apoptosis through phosphorylation of caspase-9.