Transport across the nuclear envelop has recently been suggested as a virus–cell interaction barrier for cross-species Rucaparib transmission of influenza virus [112]. Nuclear transport of influenza virus vRNP is mediated by importin-α proteins, which recognize vRNP nuclear localization signals, as part of the classical nuclear import pathway. Six isoforms of importin-α have been described in humans. The nuclear transport of vRNP of HPAIV H7N7 (SC35) and H7N1 subtypes was shown to be mediated by importin-α1 and importin-α3 in mammalian cells. In contrast, the nuclear transport of vRNP of a mouse-adapted variant of the H7N7 virus (SC35M), of HPAIV H5N1 isolated from

a fatal human case, and of seasonal influenza virus H3N2 was mediated by importin-α1 and importin-α7 [112]. D701N substitution in the PB2 protein and N319K substitution in the NP protein of the H7N7 virus were associated with increased binding to importin-α1 and switch from importin-α3 to importin-α7

dependency, resulting in increased nuclear transport, transcription and viral replication in mammalian cells (Table 2) [112], [113], [114] and [115]. Another key amino-acid associated with increased polymerase activity and viral replication in mammalian cells is that at position 627 in the PB2 protein (Table 2) [111]. Most avian influenza viruses have a glutamic acid residue at SKI-606 manufacturer position 627 of the PB2 protein while human influenza viruses typically have a lysine residue at that position. E627K substitution has been shown to increase viral replication and expand tissue tropism in mice, and is acquired rapidly upon adaptation

of influenza virus in this species. Conversely, the presence of a glutamic acid at this position severely reduces viral replication efficiency in mice (for a review see Ref. [111]). PB2 627E residue contributes to the temperature sensitivity of avian virus replication in mammalian cells [116]. Viral replication of a strain of HPAIV H5N1 with substitution E627K was improved in vitro at 33 °C, which is the temperature unless of the upper respiratory tract of mammals. Accordingly, this substitution led to increased viral titers of HPAIV H5N1 in the nasal turbinates of infected mice [117]. The mechanism behind improved replication associated with PB2 E627K substitution has recently been partly elucidated. PB2 protein with a glutamic acid at position 627 was shown to be selectively and potently restricted by a dominant inhibitory activity in human cells, and failed to bind to NP proteins and assemble into vRNP, resulting in decreased transcription, replication and viral production [118]. The necessary compatibility between PB2 protein with 627K residue and the NP protein has further been demonstrated for HPAIV H5N1 clade 2.2 [119].