Interactions between the West Nile virus capsid protein and the host cell-encoded phosphatase inhibitor, I2PP2A.

The West Nile virus (WNV) capsid protein functions in virus assembly to package genomic RNA into nucleocapsid structures. It is becoming clear, that in addition to their structural roles, capsid proteins of RNA viruses have non-structural functions. For example, the WNV capsid protein has been implicated as a pathogenic determinant. Presumably, many, if not all, of the non-structural functions of this protein involve interactions with host cell-encoded proteins. In the present study, we used affinity purification to isolate human proteins that bind to the WNV capsid protein. One of the capsid binding proteins is I(2)(PP2A), a previously characterized inhibitor of the serine/threonine phosphatase PP2A. Mapping studies revealed that capsid binding site overlaps with the region of I(2)(PP2A) that is required for inhibition of PP2A activity. Moreover, expression of the WNV capsid protein resulted in significantly increased PP2A activity and expected downstream events, such as inhibition of AP1-dependent transcription. Infected cells treated with I(2)(PP2A)-specific siRNAs produced less infectious virus than control siRNA-transfected cells, but this difference was minimal. Together, our data indicate that interactions between WNV capsid and I(2)(PP2A) result in increased PP2A activity. Given the central role of this phosphatase in cellular physiology, capsid/I(2)(PP2A) interactions may yet prove to be important for viral pathogenesis.