A Physical Link between the Pseudorabies Virus Capsid and the Nuclear Egress Complex

Abstract

Following their assembly, herpesvirus capsids exit the nucleus by budding at the inner nuclear membrane. Two highly conserved viral proteins are required for this process, pUL31 and pUL34. In this report, we demonstrate that the pUL31 component of the pseudorabies virus nuclear egress complex is a conditional capsid-binding protein that is unmasked in the absence of pUL34. The interaction between pUL31 and capsids was confirmed through fluorescence microscopy and Western blot analysis of purified intranuclear capsids. Three viral proteins were tested for their abilities to mediate the pUL31-capsid interaction: the minor capsid protein pUL25, the portal protein pUL6, and the terminase subunit pUL33. Despite the requirement for each protein in nuclear egress, none of these viral proteins were required for the pUL31-capsid interaction. These findings provide the first formal evidence that a herpesvirus nuclear egress complex interacts with capsids and have implications for how DNA-containing capsids are selectively targeted for nuclear egress.Although the herpesviridae constitute a diverse array of viral agents, all share a related structure that begins as an icosahedral protein shell that is assembled and packaged with a linear double-stranded DNA (dsDNA) genome in the nucleus of the host cell. Capsid assembly and genome encapsidation are complex processes with a reasonably high failure rate; defective capsid species are readily purified from infected cells in culture and are visible in electron micrographs of infected cells and tissues. The modest fidelity of these processes appears to be compensated for by a quality control step operating at the nuclear membrane that selects capsids containing genomes for egress from the nucleus to the cytosol (16,65,79,87). Viral proteins resident in the inner nuclear membrane constitute a nuclear egress complex (NEC). How the NEC selects for packaged capsids is not understood, and in fact, interactions between the capsid and NEC remain to be defined.Herpesvirus assembly begins with the formation of fragile procapsid icosahedral shells that are built upon protein scaffolds (15,51,53,68,83). The encapsidation of the viral genome occurs through a dodecameric portal ring that occupies one of the 12 vertices of the procapsid shell and is encoded by the UL6 gene (11,12,19,52,84). The terminase complex, consisting of the products of the UL15, UL28, and UL33 genes, is the molecular motor that facilitates the packaging of the genome through the portal (2,8,60,90,94). The initial stabilization of the capsid occurs concomitantly with the cleavage of the internal scaffolding by the VP24 protease (32), with additional structural rigidity gained at a subsequent step that may be coincident with the release of the proteolyzed scaffold and genome packaging (70). From the progenitor procapsid, three stable capsid species that can be isolated by differential sedimentation in continuous sucrose gradients are produced: A, B, and C capsids (9, 26, 58). The production of A capsids, which lack DNA an...

The Essential Human Cytomegalovirus Proteins pUL77 and pUL93 Are Structural Components Necessary for Viral Genome Encapsidation

“…Likewise, the nuclear egress complex (NEC) components pUL50 and pUL53 neither coprecipitated with pUL77-mGFP nor were detected in nuclear capsids in substantial amounts. In contrast, for alphaherpesviruses, an association of the HCMV pUL53 homologue pUL31 with capsids was reported (65)(66)(67). Interaction of herpesvirus capsids with the NEC is transient and probably of low affinity, because the capsids have to dissociate from the NEC to get released into the cytosol.…”

“…Upon overexpression of isolated proteins, interactions among the CVSCs of different herpesviruses and terminase or NEC components were seen (68)(69)(70)(71). To assess whether during infection pUL77 or pUL93 is required for NEC binding, a similar approach to that described for HSV-1 by Leelawong et al (66) could be applied, using mutants lacking either UL50 or UL53. This would lead to inhibition of nuclear egress, stabilizing the capsid association with the remaining component of the NEC and thus facilitating the detection of the interaction.…”

Nuclear Egress of Pseudorabies Virus Capsids Is Enhanced by a Subspecies of the Large Tegument Protein That Is Lost upon Cytoplasmic Maturation

“…Using this method, three types of capsids are isolated: A (empty), B (scaffold containing), and C (DNA containing) (8,25,57). A and B capsids sediment closely together from PRV-infected cells, and in these initial experiments the A and B capsids were harvested together (40). The isolated A/B and C capsids were each spotted onto coverslips and imaged by fluorescence microscopy (Fig.…”

“…Intranuclear A, B, and C capsids were isolated from PK15 or Vero cells at 18 hpi following infection at an MOI of 10, as previously described (40). The capsid gradients were collected from top to bottom either as 18 continuous fractions or as 3 fractions specific to the A, B, and C light-scattering bands.…”

“…The mechanism of nuclear egress selectivity is a topic of much discussion given that the obvious difference between the three capsid species is hidden inside the capsid shell. The HSV-1 and PRV pUL31 component of the nuclear egress complex binds to all three capsid species indiscriminately, further raising the question of how C capsids are selected for egress (40,92). The prevailing model is that a C capsid-specific component must tag the exterior of capsids that are ready for egress (86).…”