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Internal Proteins of the Procapsid and Mature Capsids of Herpes Simplex Virus 1 Mapped by Bubblegram Imaging.

ABSTRACT: The herpes simplex virus 1 (HSV-1) capsid is a huge assembly, ?1,250 Å in diameter, and is composed of thousands of protein subunits with a combined mass of ?200 MDa, housing a 100-MDa genome. First, a procapsid is formed through coassembly of the surface shell with an inner scaffolding shell; then the procapsid matures via a major structural transformation, triggered by limited proteolysis of the scaffolding proteins. Three mature capsids are found in the nuclei of infected cells. A capsids are empty, B capsids retain a shrunken scaffolding shell, and C capsids-which develop into infectious virions-are filled with DNA and ostensibly have expelled the scaffolding shell. The possible presence of other internal proteins in C capsids has been moot as, in cryo-electron microscopy (cryo-EM), they would be camouflaged by the surrounding DNA. We have used bubblegram imaging to map internal proteins in all four capsids, aided by the discovery that the scaffolding protein is exceptionally prone to radiation-induced bubbling. We confirmed that this protein forms thick-walled inner shells in the procapsid and the B capsid. C capsids generate two classes of bubbles: one occupies positions beneath the vertices of the icosahedral surface shell, and the other is distributed throughout its interior. A likely candidate is the viral protease. A subpopulation of C capsids bubbles particularly profusely and may represent particles in which expulsion of scaffold and DNA packaging are incomplete. Based on the procapsid structure, we propose that the axial channels of hexameric capsomers afford the pathway via which the scaffolding protein is expelled.In addition to DNA, capsids of tailed bacteriophages and their distant relatives, herpesviruses, contain internal proteins. These proteins are often essential for infectivity but are difficult to locate within the virion. A novel adaptation of cryo-EM based on detecting gas bubbles generated by radiation damage was used to localize internal proteins of HSV-1, yielding insights into how capsid maturation is regulated. The scaffolding protein, which forms inner shells in the procapsid and B capsid, is exceptionally bubbling-prone. In the mature DNA-filled C capsid, a previously undetected protein was found to underlie the icosahedral vertices: this is tentatively assigned as a storage form of the viral protease. We also observed a capsid species that appears to contain substantial amounts of scaffolding protein as well as DNA, suggesting that DNA packaging and expulsion of the scaffolding protein are coupled processes.

SUBMITTER: Wu W

PROVIDER: S-EPMC4859710 | biostudies-literature | 2016 May

REPOSITORIES: biostudies-literature

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Internal Proteins of the Procapsid and Mature Capsids of Herpes Simplex Virus 1 Mapped by Bubblegram Imaging.

Wu Weimin W Newcomb William W WW Cheng Naiqian N Aksyuk Anastasia A Winkler Dennis C DC Steven Alasdair C AC

Journal of virology 20160429 10

<h4>Unlabelled</h4>The herpes simplex virus 1 (HSV-1) capsid is a huge assembly, ∼1,250 Å in diameter, and is composed of thousands of protein subunits with a combined mass of ∼200 MDa, housing a 100-MDa genome. First, a procapsid is formed through coassembly of the surface shell with an inner scaffolding shell; then the procapsid matures via a major structural transformation, triggered by limited proteolysis of the scaffolding proteins. Three mature capsids are found in the nuclei of infected c ...[more]

PMID: 26984725

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Project description:UnlabelledThe herpes simplex virus 1 (HSV-1) capsid is a massive particle (~200 MDa; 1,250-Å diameter) with T=16 icosahedral symmetry. It initially assembles as a procapsid with ~4,000 protein subunits of 11 different kinds. The procapsid undergoes major changes in structure and composition as it matures, a process driven by proteolysis and expulsion of the internal scaffolding protein. Assembly also relies on an external scaffolding protein, the triplex, an α2β heterotrimer that coordinates neighboring capsomers in the procapsid and becomes a stabilizing clamp in the mature capsid. To investigate the mechanisms that regulate its assembly, we developed a novel isolation procedure for the metastable procapsid and collected a large set of cryo-electron microscopy data. In addition to procapsids, these preparations contain maturation intermediates, which were distinguished by classifying the images and calculating a three-dimensional reconstruction for each class. Appraisal of the procapsid structure led to a new model for assembly; in it, the protomer (assembly unit) consists of one triplex, surrounded by three major capsid protein (MCP) subunits. The model exploits the triplexes' departure from 3-fold symmetry to explain the highly skewed MCP hexamers, the triplex orientations at each 3-fold site, and the T=16 architecture. These observations also yielded new insights into maturation.ImportanceThis paper addresses the molecular mechanisms that govern the self-assembly of large, structurally complex, macromolecular particles, such as the capsids of double-stranded DNA viruses. Although they may consist of thousands of protein subunits of many different kinds, their assembly is precise, ranking them among the largest entities in the biosphere whose structures are uniquely defined to the atomic level. Assembly proceeds in two stages: formation of a precursor particle (procapsid) and maturation, during which major changes in structure and composition take place. Our analysis of the HSV procapsid by cryo-electron microscopy suggests a hierarchical pathway in which multisubunit "protomers" are the building blocks of the procapsid but their subunits are redistributed into different subcomplexes upon being incorporated into a nascent procapsid and are redistributed again in maturation. Assembly is a highly virus-specific process, making it a potential target for antiviral intervention.

Project description:Herpes simplex virus replicates in the nucleus, where new capsids are assembled. It produces procapsids devoid of nucleic acid but containing the preVP22a scaffold protein. These thermo-unstable particles then mature into A-, B- or C-nuclear icosahedral capsids, depending on their ability to shed the proteolytically processed scaffold and incorporation of the viral genome. To study how these viral capsids differ, we performed proteomics studies of highly enriched HSV-1 A-, B- and C-nuclear capsids, relying in part on a novel and powerful flow virometry approach to purify C-capsids. We found that the viral particles contained the expected capsid components and identified several tegument proteins in the C-capsid fraction (pUL21, pUL36, pUL46, pUL48, pUL49, pUL50, pUL51 and pUS10). Moreover, numerous ribosomal, hnRNPs and other host proteins, absent from the uninfected controls, were detected on the capsids with some of them seemingly specific to C-capsids (glycogen synthase, four different keratin-related proteins, fibronectin 1 and PCBP1). A subsequent proteomics analysis was performed to rule out the presence of protein complexes that may share similar density as the viral capsids but do not otherwise interact with them. Using pUL25 or VP5 mutant viruses incapable of assembling C-nuclear or all nuclear capsids, respectively, we confirmed the bulk of our initial findings. Naturally, it will next be important to address the functional relevance of these proteins.IMPORTANCE Much is known about the biology of herpesviruses. This includes their unique ability to traverse the two nuclear envelopes by sequential budding and fusion steps. For HSV-1, this implies the pUL31/pUL34 and pUL17/pUL25 complexes that may favor C-capsid egress. However, this selection process is not clear, nor are all the differences that distinguish A-, B- and C-capsids. The present study probes what proteins compose these capsids, including host proteins. This should open up new research avenues to clarify the biology of this most interesting family of viruses. It also reiterates the use of flow virometry as an innovative tool to purify viral particles.

Dataset Information

Internal Proteins of the Procapsid and Mature Capsids of Herpes Simplex Virus 1 Mapped by Bubblegram Imaging.

Publications

Internal Proteins of the Procapsid and Mature Capsids of Herpes Simplex Virus 1 Mapped by Bubblegram Imaging.

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