Herpesvirus assembly: An update

Abstract

The order Herpesvirales contains viruses infecting animals from molluscs to men with a common virion morphology which have been classified into the families Herpesviridae, Alloherpesviridae and Malacoherpesviridae. Herpes virions are among the most complex virus particles containing a multitude of viral and cellular proteins which assemble into nucleocapsid, envelope and tegument. After autocatalytic assembly of the capsid and packaging of the newly replicated viral genome, a process which occurs in the nucleus and resembles head formation and genome packaging in the tailed double-stranded DNA bacteriophages, the nucleocapsid is translocated to the cytoplasm by budding at the inner nuclear membrane followed by fusion of the primary envelope with the outer nuclear membrane. Viral and cellular proteins are involved in mediating this ‘nuclear egress’ which entails substantial remodeling of the nuclear architecture. For final maturation within the cytoplasm tegument components associate with the translocated nucleocapsid, with themselves, and with the future envelope containing viral membrane proteins in a complex network of interactions resulting in the formation of an infectious herpes virion. The diverse interactions between the involved proteins exhibit a striking redundancy which is still insufficiently understood. In this review, recent advances in our understanding of the molecular processes resulting in herpes virion maturation will be presented and discussed as an update of a previous contribution [Mettenleiter, T.C., 2004. Budding events in herpesvirus morphogenesis. Virus Res. 106, 167–180].

Introduction

The newly established order Herpesvirales (Davison et al., 2009) encompasses large DNA viruses exhibiting common virion morphology with a genome-containing nucleocapsid and a lipid envelope separated by a proteinaceous matrix called the tegument (Pellett and Roizman, 2007; Fig. 1). Herpesviruses infecting reptiles, birds and mammals are grouped together within the family Herpesviridae in the subfamilies Alphaherpesvirinae, Betaherpesvirinae and Gammaherpesvirinae. Their genomic sequence and gene arrangement exhibit significant homology allowing the identification of ca. 40 conserved ‘core’ genes which may execute related functions in the replication cycle (Fig. 2) of the respective virus (McGeoch et al., 2006, Pellett and Roizman, 2007). Phylogenetically more distant are herpesviruses infecting amphibia and fish included in the new family Alloherpesviridae. The single herpesvirus identified in oyster constitutes currently the only member of the Malacoherpesviridae (Davison et al., 2005, McGeoch et al., 2006). Despite limited to nearly undetectable nucleotide or amino acid homology in genes and deduced proteins between the three different herpesvirus families, their common virion morphology suggests that the underlying mechanisms of virion formation are comparable (Davison and Davison, 1995; Fig. 1). However, very little is known about these stages beyond the Herpesviridae.

Detailed mass spectrometric analyses of purified extracellular herpes virions identified numerous viral and cellular proteins as constituents of the mature virus particle (reviewed in Maxwell and Frappier, 2007). For the prototypic alphaherpesvirus herpes simplex virus type 1 (HSV-1), in addition to eight capsid, and intimately capsid-associated polypeptides, 13 viral glycoproteins, 23 potential viral tegument proteins and up to 49 distinct host proteins were identified in the virion (Loret et al., 2008). In the betaherpesvirus human cytomegalovirus (HCMV), 71 viral and more than 70 host proteins were detected (Varnum et al., 2004). In the gammaherpesviruses 23 viral and 6 cellular proteins were found in alcelaphine herpesvirus 1 (Dry et al., 2008), 35 viral proteins and several cellular proteins in Epstein-Barr Virus (Johannsen et al., 2004), and 24 viral and 21 cellular proteins in Kaposi's sarcoma associated herpesvirus (KSHV) (Zhu et al., 2005). 33 viral proteins were found in purified rhesus monkey rhadinovirus of which 17 are supposed to be tegument proteins and 9 envelope proteins (O’Connor and Kedes, 2006). The most frequently found, and most abundant cellular proteins in herpesvirus particles are actin, hsp70, hsp90, enolase, ezrin/moesin, β-tubulin and annexin (Maxwell and Frappier, 2007). So far, it is unclear whether packaging of these proteins has an impact on infection. Besides proteins, RNAs of viral and cellular origin have also been detected in purified herpesvirus preparations (Bresnahan and Shenk, 2000, Greijer et al., 2000, Terhune et al., 2004). Viral transcripts, which corresponded in size to full-length cytoplasmic mRNAs as well as microRNAs were detected in KHSV and murine gammaherpesvirus 68 virions (Bechtel et al., 2005, Cliffe et al., 2009). Because these RNAs are present in abundance in infected cells, they may find their way into the nascent herpes virion by chance (Terhune et al., 2004) or can be specifically incorporated (Cliffe et al., 2009, Leisenfelder et al., 2008). The formation of these complex herpesvirus particles, which contain a multitude of different components, is still a largely unresolved jigsaw puzzle but recent findings have shed more light on crucial steps in herpes virion morphogenesis.