The cell biology of receptor-mediated virus entry

Grove, Joe; Marsh, Mark

For an invading virus, the endosomal lumen is a dynamic labyrinth of vesicles and tubules. The sorting function of the early and recycling endosomes can potentially return virusreceptor complexes to the cell surface. Alternatively, maturation to late endosomes and lysosomes renders the endosomal lumen a potentially hazardous environment (Dikic, 2006) . Thus, many viruses fuse/penetrate at mildly acid pH (approximately pH 6.0) in early endosomes to avoid these fates (Fig. 3 B) , whereas others exploit the changing environment within endosomes to precisely regulate the timing or cellular location of fusion/penetration. Endosomal maturation to late endosomes so-called actin comets to propel virus particles during viral egress is well documented for Vaccinia virus (Taylor et al., 2011) . However, the intracellular transport strategy of insect baculoviruses is unique in their capacity to induce comet formation during entry. Upon reaching the cytoplasm, the baculovirus Autographa californica P78/83 capsid protein acts as a nucleation site for Arp2/3-dependent actin polymerization that drives the virion through the cell interior. As capsids reach the nucleus, they are held against the nuclear membrane by continuing actin polymerization, promoting their ultimate invasion through nuclear pores (Ohkawa et al., 2010) .