title: Budded baculovirus particle structure revisited.
abstract: Baculoviruses are a group of enveloped, double-stranded DNA insect viruses with budded (BV) and occlusion-derived (ODV) virions produced during their infection cycle. BVs are commonly described as rod shaped particles with a high apical density of protein extensions (spikes) on the lipid envelope surface. However, due to the fragility of BVs the conventional purification and electron microscopy (EM) staining methods considerably distort the native viral structure. Here, we use cryo-EM analysis to reveal the near-native morphology of two intensively studied baculoviruses, Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Spodoptera exigua MNPV (SeMNPV), as models for BVs carrying GP64 and F as envelope fusion protein on the surface. The now well-preserved AcMNPV and SeMNPV BV particles have a remarkable elongated, ovoid shape leaving a large, lateral space between nucleocapsid (NC) and envelope. Consistent with previous findings the NC has a distinctive cap and base structure interacting tightly with the envelope. This tight interaction may explain the partial retaining of the envelope on both ends of the NC and the disappearance of the remainder of the BV envelope in the negative-staining EM images. Cryo-EM also reveals that the viral envelope contains two layers with a total thickness of ≈ 6-7 nm, which is significantly thicker than a usual biological membrane (<4 nm) as measured by X-ray scanning. Most spikes are densely clustered at the two apical ends of the virion although some envelope proteins are also found more sparsely on the lateral regions. The spikes on the surface of AcMNPV BVs appear distinctly different from those of SeMNPV. Based on our observations we propose a new near-native structural model of baculovirus BVs.
authors.affiliation: Laboratory of Virology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands; Virology Division, Department of Infectious Disease and Immunology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
authors.forename: Qiushi
authors.initials: Q
authors.lastname: Wang
authors.affiliation: Virology Division, Department of Infectious Disease and Immunology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
authors.forename: Berend-Jan
authors.initials: BJ
authors.lastname: Bosch
authors.affiliation: Laboratory of Virology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands.
authors.forename: Just M
authors.initials: JM
authors.lastname: Vlak
authors.affiliation: Laboratory of Virology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands.
authors.forename: Monique M
authors.initials: MM
authors.lastname: van Oers
authors.affiliation: Virology Division, Department of Infectious Disease and Immunology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Rottier
authors.affiliation: Laboratory of Virology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands. Electronic address: jan.vanlent@wur.nl.
authors.forename: Jan W M
authors.initials: JWM
authors.lastname: van Lent
mesh_terms: D020285:Cryoelectron Microscopy; D059646:Genome Size; D016679:Genome, Viral; D019251:Nucleocapsid / Q000648:ultrastructure; D017924:Nucleopolyhedroviruses / Q000235:genetics / Q000502:physiology / Q000648:ultrastructure*; D014771:Virion / Q000648:ultrastructure*
keywords: Baculovirus; Budded virus; Cryo-EM; Spike structure; Ultrastructure