title: The Essential Human Cytomegalovirus Proteins pUL77 and pUL93 Are Structural Components Necessary for Viral Genome Encapsidation.
abstract: Several essential viral proteins are proposed to participate in genome encapsidation of human cytomegalovirus (HCMV), among them pUL77 and pUL93, which remain largely uncharacterized. To gain insight into their properties, we generated an HCMV mutant expressing a pUL77-monomeric enhanced green fluorescent protein (mGFP) fusion protein and a pUL93-specific antibody. Immunoblotting demonstrated that both proteins are incorporated into capsids and virions. Conversely to data suggesting internal translation initiation sites within the UL93 open reading frame (ORF), we provide evidence that pUL93 synthesis commences at the first start codon. In infected cells, pUL77-mGFP was found in nuclear replication compartments and dot-like structures, colocalizing with capsid proteins. Immunogold labeling of nuclear capsids revealed that pUL77 is present on A, B, and C capsids. Pulldown of pUL77-mGFP revealed copurification of pUL93, indicating interaction between these proteins, which still occurred when capsid formation was prevented. Correct subnuclear distribution of pUL77-mGFP required pUL93 as well as the major capsid protein (and thus probably the presence of capsids), but not the tegument protein pp150 or the encapsidation protein pUL52, demonstrating that pUL77 nuclear targeting occurs independently of the formation of DNA-filled capsids. When pUL77 or pUL93 was missing, generation of unit-length genomes was not observed, and only empty B capsids were produced. Taken together, these results show that pUL77 and pUL93 are capsid constituents needed for HCMV genome encapsidation. Therefore, the task of pUL77 seems to differ from that of its alphaherpesvirus orthologue pUL25, which exerts its function subsequent to genome cleavage-packaging.
  
  
  
  The essential HCMV proteins pUL77 and pUL93 were suggested to be involved in viral genome cleavage-packaging but are poorly characterized both biochemically and functionally. By producing a monoclonal antibody against pUL93 and generating an HCMV mutant in which pUL77 is fused to a fluorescent protein, we show that pUL77 and pUL93 are capsid constituents, with pUL77 being similarly abundant on all capsid types. Each protein is required for genome encapsidation, as the absence of either pUL77 or pUL93 results in a genome packaging defect with the formation of empty capsids only. This distinguishes pUL77 from its alphaherpesvirus orthologue pUL25, which is enriched on DNA-filled capsids and exerts its function after the viral DNA is packaged. Our data for the first time describe an HCMV mutant with a fluorescent capsid and provide insight into the roles of pUL77 and pUL93, thus contributing to a better understanding of the HCMV encapsidation network.
authors.affiliation: Institute for Virology, Hannover Medical School, Hannover, Germany borst.eva@mh-hannover.de messerle.martin@mh-hannover.de.
authors.forename: Eva Maria
authors.initials: EM
authors.lastname: Borst
authors.affiliation: Institute for Cell Biology, Hannover Medical School, Hannover, Germany.
authors.forename: Rudolf
authors.initials: R
authors.lastname: Bauerfeind
authors.affiliation: Institute for Virology, Hannover Medical School, Hannover, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Binz
authors.affiliation: Institute for Virology, Hannover Medical School, Hannover, Germany.
authors.forename: Thomas Min
authors.initials: TM
authors.lastname: Stephan
authors.affiliation: Institute for Virology, Hannover Medical School, Hannover, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Neuber
authors.affiliation: Institute for Virology, Hannover Medical School, Hannover, Germany.
authors.forename: Karen
authors.initials: K
authors.lastname: Wagner
authors.affiliation: Institute for Virology, Hannover Medical School, Hannover, Germany.
authors.forename: Lars
authors.initials: L
authors.lastname: Steinbrück
authors.affiliation: Institute for Virology, Hannover Medical School, Hannover, Germany.|German Center for Infection Research (DZIF), Partner Site Hannover-Braunschweig, Germany.
authors.forename: Beate
authors.initials: B
authors.lastname: Sodeik
authors.affiliation: Department of Histology and Embryology, Faculty of Medicine, University of Rijeka, Rijeka, Croatia.
authors.forename: Tihana
authors.initials: T
authors.lastname: Lenac Roviš
authors.affiliation: Department of Histology and Embryology, Faculty of Medicine, University of Rijeka, Rijeka, Croatia.
authors.forename: Stipan
authors.initials: S
authors.lastname: Jonjić
authors.affiliation: Institute for Virology, Hannover Medical School, Hannover, Germany borst.eva@mh-hannover.de messerle.martin@mh-hannover.de.|German Center for Infection Research (DZIF), Partner Site Hannover-Braunschweig, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Messerle
mesh_terms: D002213:Capsid / Q000378:metabolism*; D036022:Capsid Proteins / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D003587:Cytomegalovirus / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D004279:DNA, Viral / Q000235:genetics / Q000378:metabolism*; D016679:Genome, Viral; D049452:Green Fluorescent Proteins; D006801:Humans; D014764:Viral Proteins / Q000378:metabolism*; D019065:Virus Assembly
chemical_list: D036022:Capsid Proteins; D004279:DNA, Viral; D014764:Viral Proteins; D049452:Green Fluorescent Proteins