Introduction

Human cytomegalovirus (HCMV) is a significant human pathogen that infects the majority of the world's population. Viral infection causes birth defects and severe disease in patients with suppressed immune function and is associated with age-related immunosenescence, cancer, and cardiovascular disease (Mocarski

R.F. Kalejta

Institute for Molecular Virology and McArdle Laboratory for Cancer Research, University of Wisconsin-Madison, Madison, WI 53706-1596, USA

[email protected]

T.E. Shenk and M.F. Stinski (eds.), Human Cytomegalovirus. Current Topics in Microbiology and Immunology 325. © Springer-Verlag Berlin Heidelberg 2008

et al. 2007). Within mature virions, the HCMV genome is housed in an icosahedral protein capsid that is surrounded by a layer of proteins called the tegument, which in turn is enclosed within a lipid membrane termed the envelope. Virally encoded glycoproteins in the envelope function as mediators of viral entry through a membrane fusion event (see the chapter by M.K. Isaacson et al., this volume) that releases both the DNA-containing capsids and the tegument proteins into the cell (Fig. 1).

As many as 59 viral proteins have been found in the viral tegument, although only about 35 are incorporated at significant levels (Baldick et al. 1996; Varnum et al. 2004). Virions also contain a sampling of cellular proteins (Varnum et al. 2004), as well as viral and cellular RNA molecules (Terhune et al. 2004). Bioinformatic and experimental approaches have failed to detect a tegument localization signal (i.e., a sequence necessary and sufficient to direct macromolecules into the tegument) on either proteins or RNAs. The process of assembling the tegument upon viral egress, as well as the disassembly of the tegument upon viral entry into cells, is poorly understood. Likewise, the structure of the tegument within the virion is not known. Although mostly amorphous, there appears to be some structuring of tegument proteins that are closely associated with the capsid (Chen et al. 1999; Trus et al. 1999). Many tegument proteins are phosphorylated (Irmiere and

Fig. 1 Postfusion, preimmediate early events during lytic replication of human cytomegalovirus. Schematic representation of delivery of viral genomes and tegument proteins to the nucleus (1.1-1.4), the generation of a silencing complex (PML-NB) on infecting viral genomes (1.5-1.8), and the initial step in the destruction of that complex by tegument-delivered pp71 (1.9-1.11). See the text for further details

Fig. 1 Postfusion, preimmediate early events during lytic replication of human cytomegalovirus. Schematic representation of delivery of viral genomes and tegument proteins to the nucleus (1.1-1.4), the generation of a silencing complex (PML-NB) on infecting viral genomes (1.5-1.8), and the initial step in the destruction of that complex by tegument-delivered pp71 (1.9-1.11). See the text for further details

Gibson 1983), but the significance of this or other posttranslational modifications to these proteins remains largely unexplored.

Table 1 Human cytomegalovirus tegument proteins with known or predicted functions

Gene, protein

Phenotype

Function(s)

References

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