Penetration of Autographa californica nuclear polyhedrosis virus nucleocapsids into IPLB Sf 21 cells induces actin cable formation.

The budded form of Autographa californica nuclear polyhedrosis virus functionally enters cells by adsorptive endocytosis. During the period of virus uptake into Spodoptera frugiperda IPLB Sf21 cells (1 to 4 hr postinfection), filamentous F-actin cables, visualized by fluorescence microscopy, were formed within the cytoplasm of infected cells. Cable formation appeared to be a direct effect of viral inoculum in that the numbers of observed cables increased with an increase in multiplicity of infection, and cable formation was not dependent on protein synthesis. Cable induction was first apparent around 30 min postinfection, although uptake of virus into endocytic vesicles began much earlier. Chloroquine, a chemical that prevents release of nucleocapsids from endosomes, inhibited cable formation. Similarly, when virus adsorbed to the exterior of cells was exposed to neutralizing antibody, release of nucleocapsids from endosomes and subsequent cable induction were prevented. Careful observation of co-labeled F-actin cables and nucleocapsids revealed a single nucleocapsid at an end of most cables. These results suggested that nucleocapsids or nucleocapsid-associated proteins of budded virus induced the polymerization or bundling of actin following their penetration into the cytoplasm from endocytic vesicles.