Differentiating the Roles of UL16, UL21 and Us3 in the Nuclear Egress of Herpes Simplex Virus Capsids

Abstract

Previous studies from our laboratory established that pUL16 and pUL21 are required for efficient nuclear egress of herpes simplex type 2 (HSV-2) capsids. To better understand the role of these proteins in nuclear egress, we wished to establish whether nuclear egress complex (NEC) localization and/or function was altered in the absence of either pUL16 or pUL21. We used antiserum raised against HSV-2 NEC components pUL31 and pUL34 to examine NEC localization by immunofluorescence microscopy. NEC localization in cells infected with pUL16 deficient viruses was indistinguishable from that observed in cells infected with wild type viruses. By contrast, NEC localization was found to be aberrant in cells infected with pUL21 deficient virus and, instead, showed some similarity to the aberrant NEC localization pattern observed in cells infected with pUs3 deficient virus. These results indicated that pUL16 plays a role in nuclear egress that is distinct from that of pUL21 and pUs3. Higher resolution examination of nuclear envelope ultrastructure in cells infected with pUL21 deficient viruses by transmission electron microscopy showed different types of nuclear envelope perturbations, including some that were not observed in cells infected with pUs3 deficient virus. The formation of the nuclear envelope perturbations observed in pUL21 deficient virus infections was found to be dependent on a functional NEC, revealing a novel role for pUL21 in regulating NEC activity. The results of comparisons of nuclear envelope ultrastructure in cells infected with viruses lacking pUs3, pUL16 or both pUs3 and pUL16 were consistent with a role for pUL16 upstream of primary capsid envelopment and shed new light on how pUs3 functions in nuclear egress. Author summaryThe membrane deformation activity of the herpesvirus nuclear egress complex (NEC), allows viral capsids to transit from their site of assembly in the nucleus through both nuclear membranes into the cytoplasm. The timing, extent and directionality of NEC activity must be precisely controlled during viral infection, yet our knowledge of how NEC activity is controlled is incomplete. To determine how pUL16 and pUL21, two viral proteins required for nuclear egress of herpes simplex virus type 2 (HSV-2) capsids, function to promote nuclear egress, we examined how the lack of each protein impacted NEC localization. These analyses revealed a function of pUL16 in nuclear egress that is distinct from that of pUL21, uncovered a novel role for pUL21 in regulating NEC activity and shed new light on how a viral kinase, pUs3, regulates nuclear egress. Nuclear egress of viral capsids is a common feature of the replicative cycle of all herpesviruses. A complete understanding of all aspects of nuclear egress, including how viral NEC activity is controlled, may yield strategies to disrupt this process that could be applied to the development of herpes-specific antiviral drugs.