Mechanism of Nuclear Lamina Disruption and the Role of pUS3 in Herpes Simplex Virus 1 Nuclear Egress

Abstract

ABSTRACT Herpes simplex virus (HSV) capsid envelopment at the nuclear membrane is coordinated by nuclear egress complex (NEC) proteins pUL34 and pUL31 and is accompanied by alteration of the nuclear architecture and local disruption of the nuclear lamina. Here, we examined the role of capsid envelopment in the changes of the nuclear architecture by characterizing HSV-1 recombinants that do not form capsids. Typical changes in nuclear architecture and disruption of the lamina were observed in the absence of capsids, suggesting that disruption of the nuclear lamina occurs prior to capsid envelopment. Surprisingly, in the absence of capsid envelopment, lamin A/C becomes concentrated at the nuclear envelope in a pUL34-independent and cell type-specific manner, suggesting that ongoing nuclear egress may be required for the dispersal of lamins observed in wild-type virus infection. Mutation of the virus-encoded protein kinase pUS3 on a wild-type virus background has been shown to cause accumulation of perinuclear enveloped capsids, formation of NEC aggregates, and exacerbated lamina disruption. We observed that mutation of US3 in the absence of capsids results in identical NEC aggregation and lamina disruption phenotypes, suggesting that they do not result from accumulation of perinuclear virions. Transmission electron microscopy analysis revealed that, in the absence of capsids, NEC aggregates correspond to multifolded nuclear membrane structures, suggesting that pUS3 may control NEC self-association and membrane deformation. To determine the significance of the pUS3 nuclear egress function for virus growth, the replication of single and double UL34 and US3 mutants was measured, showing that the significance of pUS3 nuclear egress function is cell type specific. IMPORTANCE The nuclear lamina is an important player in infection by viruses that replicate in the nucleus. Herpesviruses alter the structure of the nuclear lamina to facilitate transport of capsids from the nucleus to the cytoplasm and use both viral and cellular effectors to disrupt the protein-protein interactions that maintain the lamina. Here, we explore the role of capsid envelopment and the virus-encoded protein kinase pUS3 in disruption of the lamina structure. We show that capsid envelopment is not necessary for the lamina disruption or for US3 mutant phenotypes, including exaggerated lamina disruption, that accompany nuclear egress. These results clarify the mechanisms behind alteration of the nuclear lamina structure and support a function for pUS3 in regulating the aggregation state of the nuclear egress machinery.