Distribution and phosphorylation of the basic protein P6.9 of Autographa californica nucleopolyhedrovirus

A protamine-like protein named P6.9 is thought to play a role in the condensation of genomes of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) during an infection. Previous studies have shown that P6.9 is phosphorylated immediately upon synthesis and dephosphorylated upon the entry of the P6.9-DNA complex into the capsid. Here, we investigate the dynamic distribution of P6.9 in AcMNPV-infected Spodoptera frugiperda cells using an influenza virus hemagglutinin (HA)-tagged P6.9. Although a portion of P6.9-HA localized to the virogenic stroma, which is the center of viral DNA replication, transcription, and nucleocapsid assembly, the majority of P6.9-HA was distributed near the inner nuclear membrane throughout the course of infection. Antiserum against P6.9 detected specific phosphorylated forms of P6.9 at the edge of, but not within, the electron-dense matte regions of the virogenic stroma. Further analysis using immunoblotting revealed that at least 11 different phosphorylated forms of P6.9, as well as dephosphorylated P6.9, were present in association with occlusion-derived virions, although only dephosphorylated P6.9 was associated with budded virions.