UL20 protein functions precede and are required for the UL11 functions of herpes simplex virus type 1 cytoplasmic virion envelopment.

Egress of herpes simplex virus type 1 (HSV-1) from the nucleus of the infected cell to extracellular spaces involves a number of distinct steps, including primary envelopment by budding into the perinuclear space, de-envelopment into the cytoplasm, cytoplasmic reenvelopment, and translocation of enveloped virions to extracellular spaces. UL20/gK-null viruses are blocked in cytoplasmic virion envelopment and egress, as indicated by an accumulation of unenveloped or partially enveloped capsids in the cytoplasm. Similarly, UL11-null mutants accumulate unenveloped capsids in the cytoplasm. To assess whether UL11 and UL20/gK function independently or synergistically in cytoplasmic envelopment, recombinant viruses having either the UL20 or UL11 gene deleted were generated. In addition, a recombinant virus containing a deletion of both UL20 and UL11 genes was constructed using the HSV-1(F) genome cloned into a bacterial artificial chromosome. Ultrastructural examination of virus-infected cells showed that both UL20- and UL11-null viruses accumulated unenveloped capsids in the cytoplasm. However, the morphology and distribution of the accumulated capsids appeared to be distinct, with the UL11-null virions forming aggregates of capsids having diffuse tegument-derived material and the UL20-null virus producing individual capsids in close juxtaposition to cytoplasmic membranes. The UL20/UL11 double-null virions appeared morphologically similar to the UL20-null viruses. Experiments on the kinetics of viral replication revealed that the UL20/UL11 double-null virus replicated in a manner similar to the UL20-null virus. Additional experiments revealed that transiently expressed UL11 localized to the trans-Golgi network (TGN) independently of either gK or UL20. Furthermore, virus infection with the UL11/UL20 double-null virus did not alter the TGN localization of transiently expressed UL11 or UL20 proteins, indicating that these proteins did not interact. Taken together, these results show that the intracellular transport and TGN localization of UL11 is independent of UL20/gK functions, and that UL20/gK are required and function prior to UL11 protein in virion cytoplasmic envelopment.