Abstract
Biological membranes create compartments, and are usually formed by lipid bilayers. However, in hyperthermophilic archaea that live optimally at temperatures above 80°C the membranes are monolayers which resemble fused bilayers. Many double-stranded DNA viruses which parasitize such hosts, including the filamentous virus AFV1 of Acidianus hospitalis, are enveloped with a lipid-containing membrane. Using cryo-EM, we show that the membrane in AFV1 is a ~2 nm-thick monolayer, approximately half the expected membrane thickness, formed by host membrane-derived lipids which adopt a U-shaped 'horseshoe' conformation. We hypothesize that this unusual viral envelope structure results from the extreme curvature of the viral capsid, as 'horseshoe' lipid conformations favor such curvature and host membrane lipids that permit horseshoe conformations are selectively recruited into the viral envelope. The unusual envelope found in AFV1 also has many implications for biotechnology, since this membrane can survive the most aggressive conditions involving extremes of temperature and pH.
Highlights
Many viruses, including some of the most devastating human pathogens such as Ebola virus, are enveloped with a lipid membrane
Determining the structure of Acidianus filamentous virus 1 (AFV1) was complicated by the fact that the virions are significantly more flexible, both with respect to bending as well as extension and compression, than those of SIRV2
Because we have combined multiple experimental approaches with computational modeling to analyze the AFV-1 envelope, we briefly recapitulate the lines of evidence for each major finding before going on to discuss some of the important implications of this previously unappreciated envelope structure
Summary
Many viruses, including some of the most devastating human pathogens such as Ebola virus, are enveloped with a lipid membrane. One example is provided by hyperthermophilic archaeal viruses of the order Ligamenvirales (Prangishvili and Krupovic, 2012) which contain non-enveloped, rigid rod-shaped viruses of the family Rudiviridae and enveloped, flexible filamentous viruses of the family Lipothrixviridae. Viruses from these two families have many homologous genes and build their virions using structurally similar major capsid proteins.
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