Baculovirus infection of nondividing mammalian cells: Mechanisms of entry and nuclear transport of capsids

We have studied the infection pathway of Autographa californica multinuclea r polyhedrosis virus (baculovirus) in mammalian cells. By titration with a baculovirus containing a green fluorescent protein cassette, we found that several, but not all, mammalian cell types can be infected efficiently. In contrast to previous suggestions, our data show that the asialoglycoprotein receptor is not required for efficient infection. We demonstrate for the f irst time that this baculovirus can infect nondividing mammalian cells, whi ch implies that the baculovirus is able to transport its genome across the nuclear membrane of mammalian cells. Our data further show that the virus e nters via endocytosis, followed by an acid-induced fusion event, which rele ases the nucleocapsid into the cytoplasm. Cytochalasin D strongly reduces t he infection efficiency but not the delivery of nucleocapsids to the cytopl asm, suggesting involvement of actin filaments in cytoplasmic transport of the capsids. Electron microscopic analysis shows the cigar-shaped nucleocap sids located at nuclear pores of nondividing cells. Under these conditions, we observed the viral genome, major capsid protein, and electron-dense cap sids inside the nucleus. This suggests that the nucleocapsid is transported through the nuclear pore. This mode of transport seems different from viru ses with large spherical capsids, such as herpes simplex virus and adenovir us, which are disassembled before nuclear transport of the genome. The impl ications for the application of baculovirus or its capsid proteins in gene therapy are discussed.