The nuclear egress complex of MHV-68 is not essential for nuclear egress but mediates C-capsid specificity
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Herpesvirus capsids must exit the nucleus to undergo additional maturation steps in the cytoplasm, such as secondary envelopment. This process is orchestrated by the nuclear egress complex (NEC), a conserved heterodimer that deforms the inner nuclear membrane and facilitates capsid egress. While NEC proteins are critical for the release of capsids into the cytoplasm and, therefore, efficient replication in ɑ-, β-, and γ-herpesviruses, residual infectivity has been observed in NEC-deficient mutants of certain herpesviruses, including the ɑ-herpesvirus Pseudorabies virus (PrV).
To investigate the role of the NEC in murine gammaherpesvirus 68 (MHV-68), a mutant virus lacking most of the nucleoplasmic NEC component ORF69 (ΔORF69) was generated. Unlike previous studies relying on transfection-based systems, this study demonstrates that NEC-deficient MHV-68 remains replication-competent, albeit with a significant replication defect. Despite the absence of ORF69, the mutant virus produced infectious progeny in non-complementing cells. Quantitative electron microscopy revealed regular nuclear capsid assembly and DNA packaging. However, while the parental virus primarily exported mature, genome-filled C-capsids, the ΔORF69 mutant exhibited cytoplasmic and extracellular emergence of all capsid forms.
These findings indicate that the MHV-68 NEC functions as a quality control checkpoint, selectively facilitating the nuclear egress of mature capsids. In the absence of ORF69, some capsids still egress, likely through alternative, less efficient pathways. Inhibition of cyclin-dependent kinases reduced parental and mutant virus spread, suggesting that γ-herpesviruses exploit host-dependent mechanisms for an alternative nuclear egress route.
Importance
Gammaherpesviruses, such as Epstein-Barr virus and Kaposi’s sarcoma-associated herpesvirus, establish lifelong infections and can contribute to cancer. A crucial step in the viral life cycle depends on the nuclear egress of capsids, a process mediated by the viral nuclear egress complex (NEC). Surprisingly, murine gammaherpesvirus 68 (MHV-68) can translocate capsids from the nucleus to the cytoplasm even when a key NEC component, nucleoplasmic ORF69, is disrupted. Deleting most of ORF69 reduced viral replication but did not entirely block nuclear egress. Instead, the mutant virus allowed both mature and immature capsids to leave the nucleus, suggesting that the NEC functions as a quality control checkpoint for mature capsids. Furthermore, host cell kinases seem to enable alternative routes for egress that do not rely on the NEC. These findings further support the notion that NEC is not strictly necessary and suggest the potential exploitation of host-virus interactions for antiviral approaches.
