Packaging determinants in the UL11 tegument protein of herpes simplex virus type 1

The UL11 gene of herpes simplex virus type 1 encodes a 96-amino-acid tegument protein that is myristylated, palmitylated, and phosphorylated and is found on the cytoplasmic faces of nuclear, Golgi apparatus-derived, and plasma membranes of infected cells. Although this protein is thought to play a role in virus budding, its specific function is unknown. Purified virions were found to contain approximately 700 copies of the UL11 protein per particle, making it an abundant component of the tegument. Moreover, comparisons of cell-associated and virion-associated UL11 showed that packaging is selective for underphosphorylated forms, as has been reported for several other tegument proteins. Although the mechanism by which UL11 is packaged is unknown, previous studies have identified several sequence motifs in the protein that are important for membrane binding, intracellular trafficking, and interaction with UL16, another tegument protein. To ascertain whether any of these motifs are needed for packaging, a transfection/infection-based assay was used in which mutant forms of the protein must compete with the wild type. In this assay, the entire C-terminal half of UL11 was found to be dispensable. In the N-terminal half, the sites of myristylation and palmitylation, which enable membrane-binding and Golgi apparatus-specific targeting, were found to be essential for efficient packaging. The acidic cluster motif, which is not needed for Golgi apparatus-specific targeting but is involved in recycling the protein from the plasma membrane and for the interaction with UL16, was found to be essential, too. Thus, something other than mere localization of UL11 to Golgi apparatus-derived membranes is needed for packaging. The critical factor is unlikely to be the interaction with UL16 because other mutants that fail to bind this protein (due to removal of the dileucine-like motif or substitutions with foreign acidic clusters) were efficiently packaged. Collectively, these results suggest that UL11 packaging is not driven by a passive mechanism but instead requires trafficking through a specific pathway.