Autographa californica multiple nucleopolyhedrovirus ac75 is required for egress of nucleocapsids from the nucleus and formation of de novo intranuclear membrane microvesicles

In this study, Autographa californica multiple nucleopolyhedrovirus ac75 was functionally characterized. Ac75 has homologs in all sequenced genomes of alphabaculoviruses, betabaculoviruses, and gammabaculoviruses. It was determined to encode a protein that is associated with the nucleocapsid of budded virus and with both envelope and nucleocapsids of occlusion-derived virus. Sf9 cells transfected by an ac75-knockout bacmid resulted in the infection being restricted to single cells. No budded virus were detected although viral DNA replication and late gene expression were unaffected. Electron microscopy revealed that the virogenic stroma, nucleocapsids and occlusion bodies appeared normal in the cells transfected by an ac75-knockout bacmid. However, the nucleocapsids were unenveloped, the occlusion bodies did not contain any virions or nucleocapsids, and no nucleocapsids were found outside the nucleus or spanning the nuclear membrane. In addition, de novo intranuclear membrane microvesicles that are the precursor of occlusion-derived virus envelopes were absent in the nuclei of transfected cells. Confocal microscopy showed that AC75 protein appeared in the cytoplasm as early as 6 hours post infection. It localized to the ring zone at the periphery of the nucleus from 15 to 24 hours post infection and demonstrated light blocky cloud-like distribution in the center of the nucleus. AC75 was found to co-immunoprecipitate with BV and ODV associated envelope protein ODV-E25. The data from this study suggest that ac75 is essential for induction of the intranuclear membrane microvesicles, it appears to be required for the intranuclear envelopment of nucleocapsids, and is also essential for egress of nucleocapsids from the nuclei, in infected cells.