The product of the U L 11 gene of herpes simplex virus type 1 (HSV-1) is a 96-amino-acid tegument protein that accumulates on the cytoplasmic face of internal membranes. Although it is thought to be important for nucleocapsid envelopment and egress, the actual function of this protein is unknown. Previous studies focused on the characterization of sequence elements within the UL11 protein that function in membrane binding and trafficking to the Golgi apparatus. Binding was found to be mediated by two fatty acyl groups (myristate and palmitate), while an acidic cluster and a dileucine motif were identified as being important for the recycling of UL11 from the plasma membrane to the Golgi apparatus. The goal of the experiments described here was to identify and characterize binding partners (viral or cellular) of UL11. Using both immunoprecipitation and glutathione S-transferase (GST) pull-down assays, we identified a 40-kDa protein that specifically associates with UL11 from infected Vero cells. Mutational analyses revealed that the acidic cluster and the dileucine motif are required for this association, whereas the entire second half of UL11 is not. In addition, UL11 homologs from pseudorabies and Marek's disease herpesviruses were also found to be capable of binding to the 40-kDa protein from HSV-1-infected cells, suggesting that the interaction is conserved among alphaherpesviruses. Purification and analysis of the 40-kDa protein by mass spectrometry revealed that it is the product of the U L 16 gene, a virion protein reported to be involved in nucleocapsid assembly. Cells transfected with a UL16-green fluorescent protein expression vector produced a protein that was of the expected size, could be pulled down with GST-UL11, and accumulated in a Golgi-like compartment only when coexpressed with UL11, indicating that the interaction does not require any other viral products. These data represent the first steps toward elucidating the network of tegument proteins that UL11 links to membranes.During herpes simplex virus type 1 (HSV-1) assembly, over 30 different proteins come together to form three major structures: the nucleocapsid, the glycoprotein-containing envelope, and the collection of proteins located between the capsid and the envelope, known as the tegument (33). As with other herpesviruses, the most recent model for HSV-1 envelopment suggests that assembled nucleocapsids are shuttled out of the nucleus by budding and fusion events on the inner and outer nuclear membranes, respectively, and then travel through the cytoplasm until reaching trans-Golgi network (TGN)-derived vesicles (reviewed in reference 27). At this site, nucleocapsids are thought to acquire their final lipid bilayer in a process that also results in the acquisition of the tegument and glycoproteins. The mature virions subsequently follow the secretory pathway out to the cell surface, where they are released into the extracellular medium. Although several lines of evidence support this model, the specific molecular mechanisms by which t...

Growing evidence indicates that herpes simplex virus type 1 (HSV-1) acquires its final envelope in the trans-Golgi network (TGN). During the envelopment process, the viral nucleocapsid as well as the envelope and tegument proteins must arrive at this site in order to be incorporated into assembling virions. To gain a better understanding of how these proteins associate with cellular membranes and target to the correct compartment, we have been studying the intracellular trafficking properties of the small tegument protein encoded by the U L 11 gene of HSV-1. This 96-amino-acid, myristylated protein accumulates on the cytoplasmic face of internal membranes, where it is thought to play a role in nucleocapsid envelopment and egress. When expressed in the absence of other HSV-1 proteins, the UL11 protein localizes to the Golgi apparatus, and previous deletion analyses have revealed that the membrane-trafficking information is contained within the first 49 amino acids. The goal of this study was to map the functional domains required for proper Golgi membrane localization. In addition to N-terminal myristylation, which allows for weak membrane binding, UL11 appears to be palmitylated on one or more of three consecutive N-terminal cysteines. Using membranepelleting experiments and confocal microscopy, we show that palmitylation of UL11 is required for both Golgi targeting specificity and strong membrane binding. Furthermore, we found that a conserved acidic cluster within the first half of UL11 is required for the recycling of this tegument protein from the plasma membrane to the Golgi apparatus. Taken together, our results demonstrate that UL11 has highly dynamic membranetrafficking properties, which suggests that it may play multiple roles on the plasma membrane as well as on the nuclear and TGN membranes.During herpes simplex virus type 1 (HSV-1) assembly, over 30 different proteins come together to form three major structures: the nucleocapsid, the glycoprotein-containing envelope, and the collection of proteins located between the capsid and envelope known as the tegument (30). While it is generally accepted that capsid assembly and genome packaging occur in the nucleus, the compartment(s) in which tegument and envelope are acquired is less well defined (30). As with other herpesviruses (e.g., pseudorabies virus, varicella-zoster virus, and human cytomegalovirus), the most recent model for HSV-1 envelopment suggests that assembled nucleocapsids are shuttled out of the nucleus by a budding-fusion event on the inner and outer nuclear membranes (respectively) and then travel through the cytoplasm as unenveloped capsids until reaching a trans-Golgi network (TGN)-derived vesicle (7, 12-14, 18, 31, 40, 43, 47). While at this site, nucleocapsids are thought to acquire their final lipid bilayer in a process that also results in the acquisition of the tegument and glycoproteins. The mature virions subsequently follow the secretory pathway out to the cell surface, where they are released into the extracellular medium.Alth...

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