title: Vaccinia extracellular virions enter cells by macropinocytosis and acid-activated membrane rupture.
abstract: Vaccinia virus (VACV), the model poxvirus, produces two types of infectious particles: mature virions (MVs) and extracellular virions (EVs). EV particles possess two membranes and therefore require an unusual cellular entry mechanism. By a combination of fluorescence and electron microscopy as well as flow cytometry, we investigated the cellular processes that EVs required to infect HeLa cells. We found that EV particles were endocytosed, and that internalization and infection depended on actin rearrangements, activity of Na(+)/H(+) exchangers, and signalling events typical for the macropinocytic mechanism of endocytosis. To promote their internalization, EVs were capable of actively triggering macropinocytosis. EV infection also required vacuolar acidification, and acid exposure in endocytic vacuoles was needed to disrupt the outer EV membrane. Once exposed, the underlying MV-like particle presumably fused its single membrane with the limiting vacuolar membrane. Release of the viral core into the host cell cytosol allowed for productive infection.
authors.affiliation: Institute of Biochemistry, ETH Zurich, Switzerland.
authors.forename: Florian Ingo
authors.initials: FI
authors.lastname: Schmidt
authors.forename: Christopher Karl Ernst
authors.initials: CK
authors.lastname: Bleck
authors.forename: Ari
authors.initials: A
authors.lastname: Helenius
authors.forename: Jason
authors.initials: J
authors.lastname: Mercer
mesh_terms: D000199:Actins / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D010873:Pinocytosis; D015398:Signal Transduction; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism; D014616:Vaccinia virus / Q000502:physiology*; D014617:Vacuoles / Q000378:metabolism; D014771:Virion / Q000502:physiology*; D053586:Virus Internalization
chemical_list: D000199:Actins; D017923:Sodium-Hydrogen Exchangers