Complex formation between the UL16 and UL21 tegument proteins of pseudorabies virus

The products of the UL16 and UL21 genes represent tegument proteins which are conserved throughout the mammalian herpesviruses. To identify and functionally characterize the respective proteins in the alphaherpesvirus pseudorabies virus, monospecific antisera against bacterially expressed fusion proteins were generated. In immunoblots the UL16 antiserum detected a ca. 40-kDa protein in infected cells and purified virion preparations, whereas the anti-UL21 serum recognized a protein of approximately 60 kDa. Interestingly, in immunoprecipitations using either antiserum, both proteins were coprecipitated, demonstrating the formation of a physical complex. To investigate protein function, viruses lacking either UL16, UL21, or both were constructed. Mutant viruses could be propagated on noncomplementing cells, indicating that these proteins, either alone or in combination, are not required for viral replication in cell culture. However, plaque sizes and viral titers were reduced. Electron microscopy showed only slight alterations in cytoplasmic virion morphogenesis, whereas intranuclear maturation stages were not affected. Similar results were obtained with a triple mutant simultaneously lacking the three conserved tegument proteins UL11, UL16, and UL21. In summary, our results uncover a novel interaction between conserved herpesvirus tegument proteins that increases the complexity of the intricate network of protein-protein interactions involved in herpesvirus morphogenesis.