The heptad repeats region is essential for AcMNPV P10 filament formation and not the proline-rich or the C-terminus basic regions.

Baculovirus P10 protein is a small conserved protein and is expressed as bundles of filaments in the host cell during the late phase of virus infection. So far the published results on the domain responsible for filament structural formation have been contradictory. Electron microscopy revealed that the C-terminus basic region was involved in filament structural formation in the Autographa californica multiple nucleocapsid nucleopolyhedrovirus (AcMNPV) [van Oers, M.M., Flipsen, J.T., Reusken, C.B., Sliwinsky, E.L., Vlak, J.M., 1993. Functional domains of the p10 protein of Autographa californica nuclear polyhedorsis virus. J. Gen. Virol. 74, 563-574.]. While in the Helicoverpa armigera nucleopolyhedrovirus (HearNPV), the heptad repeats region but not the C-terminus domain was proven to be responsible for filament formation [Dong, C., Li, D., Long, G., Deng, F., Wang, H., Hu, Z., 2005. Identification of functional domains required for HearNPV P10 filament formation. Virology 338, 112-120.]. In this manuscript, fluorescence confocal microscopy was applied to study AcMNPV P10 filament formation. A set of plasmids containing different P10 structural domains fused with a fluorescent protein were constructed and transfected into Sf-9 cells. The data indicated that the heptad repeats region, but not the proline-rich region or the C-terminus basic region, is essential for AcMNPV P10 filament formation. Co-transfection of P10s tagged with different fluorescent revealed that P10s with defective heptad repeats region could not interact with intact heptad repeats region or even full-length P10s to form filament structure. Within the heptad repeats region, deletion of the three amino acids spacing of AcMNPV P10 appeared to have no significant impact on the formation of filament structures, but the content of the heptad repeats region appeared to play a role in the morphology of the filaments.