The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress

ABSTRACT Many viruses migrate between different cellular compartments for successive stages of assembly. The HSV-1 capsid assembles in the nucleus and then transfers into the cytoplasm. First, the capsid buds through the inner nuclear membrane, becoming coated with nuclear egress complex (NEC) prote...

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Autores principales:	William W. Newcomb, Juan Fontana, Dennis C. Winkler, Naiqian Cheng, J. Bernard Heymann, Alasdair C. Steven
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Lenguaje:	EN
Publicado:	American Society for Microbiology 2017
Materias:	HSV capsid cryo-electron microscopy cryo-electron tomography nuclear egress nuclear egress complex nuclear envelope Microbiology QR1-502
Acceso en línea:	https://doaj.org/article/d721955c8359428484177c7422185897
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spelling	oai:doaj.org-article:d721955c8359428484177c74221858972021-11-15T15:51:30ZThe Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress10.1128/mBio.00825-172150-7511https://doaj.org/article/d721955c8359428484177c74221858972017-07-01T00:00:00Zhttps://journals.asm.org/doi/10.1128/mBio.00825-17https://doaj.org/toc/2150-7511ABSTRACT Many viruses migrate between different cellular compartments for successive stages of assembly. The HSV-1 capsid assembles in the nucleus and then transfers into the cytoplasm. First, the capsid buds through the inner nuclear membrane, becoming coated with nuclear egress complex (NEC) protein. This yields a primary enveloped virion (PEV) whose envelope fuses with the outer nuclear membrane, releasing the capsid into the cytoplasm. We investigated the associated molecular mechanisms by isolating PEVs from US3-null-infected cells and imaging them by cryo-electron microscopy and tomography. (pUS3 is a viral protein kinase in whose absence PEVs accumulate in the perinuclear space.) Unlike mature extracellular virions, PEVs have very few glycoprotein spikes. PEVs are ~20% smaller than mature virions, and the little space available between the capsid and the NEC layer suggests that most tegument proteins are acquired later in the egress pathway. Previous studies have proposed that NEC is organized as hexamers in honeycomb arrays in PEVs, but we find arrays of heptameric rings in extracts from US3-null-infected cells. In a PEV, NEC contacts the capsid predominantly via the pUL17/pUL25 complexes which are located close to the capsid vertices. Finally, the NEC layer dissociates from the capsid as it leaves the nucleus, possibly in response to pUS3-mediated phosphorylation. Overall, nuclear egress emerges as a process driven by a program of multiple weak interactions. IMPORTANCE On its maturation pathway, the newly formed HSV-1 nucleocapsid must traverse the nuclear envelope, while respecting the integrity of that barrier. Nucleocapsids (125 nm in diameter) are too large to pass through the nuclear pore complexes that conduct most nucleocytoplasmic traffic. It is now widely accepted that the process involves envelopment/de-envelopment of a key intermediate—the primary enveloped virion. In wild-type infections, PEVs are short-lived, which has impeded study. Using a mutant that accumulates PEVs in the perinuclear space, we were able to isolate PEVs in sufficient quantity for structural analysis by cryo-electron microscopy and tomography. The findings not only elucidate the maturation pathway of an important human pathogen but also have implications for cellular processes that involve the trafficking of large macromolecular complexes.William W. NewcombJuan FontanaDennis C. WinklerNaiqian ChengJ. Bernard HeymannAlasdair C. StevenAmerican Society for MicrobiologyarticleHSV capsidcryo-electron microscopycryo-electron tomographynuclear egressnuclear egress complexnuclear envelopeMicrobiologyQR1-502ENmBio, Vol 8, Iss 3 (2017)
institution	DOAJ
collection	DOAJ
language	EN
topic	HSV capsid cryo-electron microscopy cryo-electron tomography nuclear egress nuclear egress complex nuclear envelope Microbiology QR1-502
spellingShingle	HSV capsid cryo-electron microscopy cryo-electron tomography nuclear egress nuclear egress complex nuclear envelope Microbiology QR1-502 William W. Newcomb Juan Fontana Dennis C. Winkler Naiqian Cheng J. Bernard Heymann Alasdair C. Steven The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress
description	ABSTRACT Many viruses migrate between different cellular compartments for successive stages of assembly. The HSV-1 capsid assembles in the nucleus and then transfers into the cytoplasm. First, the capsid buds through the inner nuclear membrane, becoming coated with nuclear egress complex (NEC) protein. This yields a primary enveloped virion (PEV) whose envelope fuses with the outer nuclear membrane, releasing the capsid into the cytoplasm. We investigated the associated molecular mechanisms by isolating PEVs from US3-null-infected cells and imaging them by cryo-electron microscopy and tomography. (pUS3 is a viral protein kinase in whose absence PEVs accumulate in the perinuclear space.) Unlike mature extracellular virions, PEVs have very few glycoprotein spikes. PEVs are ~20% smaller than mature virions, and the little space available between the capsid and the NEC layer suggests that most tegument proteins are acquired later in the egress pathway. Previous studies have proposed that NEC is organized as hexamers in honeycomb arrays in PEVs, but we find arrays of heptameric rings in extracts from US3-null-infected cells. In a PEV, NEC contacts the capsid predominantly via the pUL17/pUL25 complexes which are located close to the capsid vertices. Finally, the NEC layer dissociates from the capsid as it leaves the nucleus, possibly in response to pUS3-mediated phosphorylation. Overall, nuclear egress emerges as a process driven by a program of multiple weak interactions. IMPORTANCE On its maturation pathway, the newly formed HSV-1 nucleocapsid must traverse the nuclear envelope, while respecting the integrity of that barrier. Nucleocapsids (125 nm in diameter) are too large to pass through the nuclear pore complexes that conduct most nucleocytoplasmic traffic. It is now widely accepted that the process involves envelopment/de-envelopment of a key intermediate—the primary enveloped virion. In wild-type infections, PEVs are short-lived, which has impeded study. Using a mutant that accumulates PEVs in the perinuclear space, we were able to isolate PEVs in sufficient quantity for structural analysis by cryo-electron microscopy and tomography. The findings not only elucidate the maturation pathway of an important human pathogen but also have implications for cellular processes that involve the trafficking of large macromolecular complexes.
format	article
author	William W. Newcomb Juan Fontana Dennis C. Winkler Naiqian Cheng J. Bernard Heymann Alasdair C. Steven
author_facet	William W. Newcomb Juan Fontana Dennis C. Winkler Naiqian Cheng J. Bernard Heymann Alasdair C. Steven
author_sort	William W. Newcomb
title	The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress
title_short	The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress
title_full	The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress
title_fullStr	The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress
title_full_unstemmed	The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress
title_sort	primary enveloped virion of herpes simplex virus 1: its role in nuclear egress
publisher	American Society for Microbiology
publishDate	2017
url	https://doaj.org/article/d721955c8359428484177c7422185897
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The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress

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