The role of HSV-2 UL16 in nuclear egress of capsids
The herpes simplex virus (HSV) UL16 gene is conserved throughout the Herpesviridae and the herpes simplex virus type 1 (HSV-1) UL16 protein forms complex with another virus protein called UL21. Previous studies demonstrated that, unlike HSV-1, herpes simplex virus type 2 (HSV-2) UL21 was essential for virus propagation and the translocation of capsids from the nucleus to the cytoplasm - a critical process in virus morphogenesis called nuclear egress. We hypothesized that if a UL16/UL21 complex were required for nuclear egress then HSV-2 lacking UL16 would have a similar phenotype as HSV-2 lacking UL21. Deletion of HSV-2 UL16 resulted in a 200 to 4,000-fold reduction in virus propagation, depending on the cell type, and like UL21, UL16 was critical for the nuclear egress of capsids. Our findings related to HSV-2 UL16 were in stark contrast to what has been observed with UL16 mutants of HSV-1 and pseudorabies virus (PRV) where roughly 10-fold replication deficiencies were reported that were accompanied by defects in the secondary envelopment of cytoplasmic capsids. To resolve these discrepancies, multiple UL16 null mutants were constructed in different HSV strains and their phenotypes characterized side-by-side. This analysis showed that all the HSV-2 UL16 mutants had 50 to 100-fold replication deficiencies that were accompanied by defects in the nuclear egress of capsids as well as defects in the envelopment of cytoplasmic capsids. By contrast, most HSV-1 UL16 mutants had 10-fold replication deficiencies that were accompanied by defects in envelopment of cytoplasmic capsids. These findings indicated that UL16 has HSV species-specific functions. Nuclear egress requires the disruption of the nuclear lamina and proper localization of nuclear egress complex (NEC) components, UL31 and UL34. To investigate the requirements for UL16 and UL21 in nuclear egress we examined the disruption of the nuclear lamina and the localization of NEC in infected cells. Similar to UL21, UL16 was not involved in the disruption of nuclear lamina integrity. Interestingly, HSV-2 UL21 is required for proper localization of the NEC while UL16 is not, suggesting that UL16 and UL21 play distinct roles in the nuclear egress of HSV-2 capsids.