title: The large tegument protein pUL36 is essential for formation of the capsid vertex-specific component at the capsid-tegument interface of herpes simplex virus 1.
abstract: Herpesviruses have a characteristic particle structure comprising an icosahedral capsid, which contains the DNA genome and is, in turn, surrounded by a proteinaceous tegument layer and a lipid envelope. In herpes simplex virus, the interaction between the capsid and tegument is limited to the capsid vertices and involves two minor capsid proteins, pUL17 and pUL25, and the large inner tegument protein pUL36. pUL17 and pUL25 form a heterodimeric structure, the capsid vertex-specific component (CVSC), that lies on top of the peripentonal triplexes, while pUL36 has been reported to connect the CVSC to the penton. In this study, we used virus mutants with deletions in the genes for pUL36 and another inner tegument protein, pUL37, to analyze the contributions of these proteins to CVSC structure. Using electron cryomicroscopy and icosahedral reconstruction of mutants that express pUL17 and pUL25 but not pUL36, we showed that in contrast to accepted models, the CVSC is not formed from pUL17 and pUL25 on their own but requires a contribution from pUL36. In addition, the presence of full-length pUL36 results in weak density that extends the CVSC toward the penton, suggesting either that this extra density is formed directly by pUL36 or that pUL36 stabilizes other components of the vertex-tegument interface.
  
  
  OBJECTIVE
  Herpesviruses have complex particles that are formed as a result of a carefully controlled sequence of assembly steps. The nature of the interaction between two of the major particle compartments, the icosahedral capsid and the amorphous tegument, has been extensively studied, but the identity of the interacting proteins and their roles in forming the connections are still unclear. In this study, we used electron microscopy and three-dimensional reconstruction to analyze virus particles formed by mutants that do not express particular interacting proteins. We show that the largest viral protein, pUL36, which occupies the layer of tegument closest to the capsid, is essential for formation of structurally normal connections to the capsid. This demonstrates the importance of pUL36 in the initial stages of tegument addition and provides new insights into the process of virus particle assembly.
authors.affiliation: MRC-University of Glasgow Centre for Virus Research, Glasgow, United Kingdom.
authors.forename: Wan H
authors.initials: WH
authors.lastname: Fan
authors.affiliation: MRC-University of Glasgow Centre for Virus Research, Glasgow, United Kingdom.
authors.forename: Ashley P E
authors.initials: AP
authors.lastname: Roberts
authors.affiliation: MRC-University of Glasgow Centre for Virus Research, Glasgow, United Kingdom.
authors.forename: Marion
authors.initials: M
authors.lastname: McElwee
authors.affiliation: MRC-University of Glasgow Centre for Virus Research, Glasgow, United Kingdom.
authors.forename: David
authors.initials: D
authors.lastname: Bhella
authors.affiliation: MRC-University of Glasgow Centre for Virus Research, Glasgow, United Kingdom frazer.rixon@glasgow.ac.uk.
authors.forename: Frazer J
authors.initials: FJ
authors.lastname: Rixon
authors.affiliation: MRC-University of Glasgow Centre for Virus Research, Glasgow, United Kingdom.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Lauder
mesh_terms: D000818:Animals; D002213:Capsid / Q000378:metabolism; D036022:Capsid Proteins / Q000378:metabolism*; D002460:Cell Line; D020285:Cryoelectron Microscopy; D017353:Gene Deletion; D018259:Herpesvirus 1, Human / Q000502:physiology*; D007091:Image Processing, Computer-Assisted; D014764:Viral Proteins / Q000235:genetics / Q000502:physiology*; D019065:Virus Assembly
chemical_list: D036022:Capsid Proteins; C078035:UL36 protein, Human herpesvirus 1; D014764:Viral Proteins