Nuclear Egress of Herpesviruses

Abstract

During viral replication, herpesviruses utilize a unique strategy, termed nuclear egress, to translocate capsids from the nucleus into the cytoplasm. This initial budding step transfers a newly formed capsid from within the nucleus, too large to fit through nuclear pores, through the inner nuclear membrane to the perinuclear space. The perinuclear enveloped virion must then fuse with the outer nuclear membrane to be released into the cytoplasm for further maturation, undergoing budding once again at the trans-Golgi network or early endosomes, and ultimately exit the cell non-lytically to spread infection. This first budding process is mediated by two conserved viral proteins, UL31 and UL34, that form a heterodimer called the nuclear egress complex (NEC). This review focuses on what we know about how the NEC mediates capsid transport to the perinuclear space, including steps prior to and after this budding event. Additionally, we discuss the involvement of other viral proteins in this process and how NEC-mediated budding may be regulated during infection. Introduction: overview of nuclear egressHerpesviruses replicate their double-stranded-DNA genomes and package them into capsids within the host cell nucleus. These progeny capsids must then exit the nucleus to form mature virions within the cytoplasm (Figure 1). The nucleus is caister.com/cimb 125 caister.com/cimb 126 Curr. Issues Mol. Biol. Vol. 41 Nuclear Egress DraganovaThis envelopment/de-envelopment model of nuclear egress is the most widely accepted model for capsid escape from the nucleus and is backed up by substantial data. First, enveloped virions in the PNS have been observed in cells infected with herpesvirus for several decades (Fuchs et al., 2002;Granzow et al., 1997;Reynolds et al., 2002;Stackpole, 1969). The phenotypes of viral gene deletions likewise provide compelling evidence in favor of the envelopment/deenvelopment model. Deletion of the US3 kinase gene from either HSV-1 or from the closely related pseudorabies virus (PRV) results in the accumulation of PEVs (Klupp et al., 2001;Reynolds et al., 2002;Ryckman and Roller, 2004;Sehl et al., 2020), which points to budding into the PNS being an intermediate stage in nuclear egress. Second, PEVs differ from mature virions in their morphology and protein composition. For example, UL31 and UL34, two viral proteins critical for nuclear egress, are absent from mature HSV-1 and PRV virions (Fuchs et al., 2002;Reynolds et al., 2002). Conversely, most tegument proteins found in mature virions are not found in the nucleus or in PEVs (Gershon et al., 1994;Klupp et al., 2000;Skepper et al., 2001). Furthermore, mature virions are studded with glycoprotein spikes and have a thick tegument while PEVs have a smooth envelope and thin tegument (Gershon et al., 1994;Granzow et al., 2001), consistent with the two types of particles being formed during different budding events. Third, deletions of particular tegument proteins, such as HSV-1 UL36 and UL37, or the glycoproteins gD and gE from either HSV-1 or PRV result ...

Properties of Oligomeric Interaction of the Cytomegalovirus Core Nuclear Egress Complex (NEC) and Its Sensitivity to an NEC Inhibitory Small Molecule

“…In previous studies, the 3D crystal structure of the globular domain of the HCMV core NEC was resolved, also revealing a higher-order assembly of the pUL50-pUL53 heterodimers in the form of hexameric ring-like structures [7][8][9], which were similarly detected for other herpesviruses [10][11][12][13][14]. The 3D structural image of the HCMV core NEC strongly suggested a binding principle, termed hook-into-groove interaction since it is based on an N-terminal hook-like pUL53 protrusion that embraces an α-helical pUL50 binding groove.…”

“…Cellular protein expression patterns were investigated in cells transfected with empty vector (Figure S3B), constructs for coexpression of pUL50 and pUL53 ( Figure S3C), constructs for single expression of pUL50 or pUL53 ( Figure S3D), or pUL50::pUL53 fusion constructs ( Figure S3E). Pronounced colocalization signals between the nuclear rim staining of viral NEC proteins and the cellular factors were obtained for emerin ( Figure S3C, panels 1-5), p32/gC1qR (panels 6-10 and in part for PKCα (panels [16][17][18][19][20], whereas CDK1 showed very little or no signals of NEC colocalization (panels [11][12][13][14][15]. This colocalization pattern was similarly found using either coexpressed pUL50-pUL53 ( Figure S3C), a single expression of pUL50 or pUL53 ( Figure S3D) or the pUL50::pUL53 fusion construct ( Figure S3E).…”

Virus-derived peptide inhibitors of the herpes simplex virus type 1 nuclear egress complex

“…The spread of infection to uninfected tissues and hosts relies on the ability of the virus to effectively replicate 2,7 . During replication, viral capsids are assembled within the nucleus and must translocate into the cytoplasm to form infectious virions but are too large to traverse nuclear pores, and instead use an unusual route termed nuclear egress.…”

“…During replication, viral capsids are assembled within the nucleus and must translocate into the cytoplasm to form infectious virions but are too large to traverse nuclear pores, and instead use an unusual route termed nuclear egress. The first step in this process is the budding of a capsid at the inner nuclear membrane (INM), which is mediated by the conserved nuclear egress complex (NEC), a heterodimer of viral proteins UL31 and UL34 2,7,8 . The NEC is anchored to the INM by the single C-terminal transmembrane helix of UL34 9 .…”

“…The (EAEKAAK) 7 EAEK peptide DNA sequence was obtained as a gBlock gene fragment (IDT) and subjected to restriction digest cloning with BamHI and XhoI into the pGEX-6P-1 vector (the gene fragment sequence is listed inSupplementary Table S2) to create the pED39 [(EAEKAAK)7 EAEK peptide] plasmid.…”