The Herpes Simplex Virus 1 UL51 Gene Product Has Cell Type-Specific Functions in Cell-to-Cell Spread

Abstract

The herpes simplex virus 1 (HSV-1) UL51 gene encodes a 244-amino-acid (aa) palmitoylated protein that is conserved in all herpesviruses. The alphaherpesvirus UL51 (pUL51) protein has been reported to function in nuclear egress and cytoplasmic envelopment. No complete deletion has been generated because of the overlap of the UL51 coding sequence 5= end with the UL52 promoter sequences, but partial deletions generated in HSV and pseudorabies virus (PrV) suggest an additional function in epithelial cell-to-cell spread. Here we show partial uncoupling of the replication, release, and cell-to-cell spread functions of HSV-1 pUL51 in two ways. Viruses in which aa 73 to 244 were deleted from pUL51 or in which a conserved YXX⌽ motif near the N terminus was altered showed cell-specific defects in spread that cannot be accounted for by defects in replication and virus release. Also, a cell line that expresses C-terminally enhanced green fluorescent protein (EGFP)-tagged pUL51 supported normal virus replication and release into the medium but the formation of only small plaques. This cell line also failed to support normal localization of gE to cell junctions. gE and pUL51 partially colocalized in infected cells, and these two proteins could be coimmunoprecipitated from infected cells, suggesting that they can form a complex during infection. The cell-to-cell spread defect associated with the pUL51 mutation was more severe than that associated with gE-null virus, suggesting that pUL51 has gE-independent functions in epithelial cell spread. IMPORTANCEHerpesviruses establish and reactivate from lifelong latency in their hosts. When they reactivate, they are able to spread within their hosts despite the presence of a potent immune response that includes neutralizing antibody. This ability is derived in part from a specialized mechanism for virus spread between cells. Cell-to-cell spread is a conserved property of herpesviruses that likely relies on conserved viral genes. An understanding of their function may aid in the design of vaccines and therapeutics. Here we show that one of the conserved viral genes, UL51, has an important role in cell-to-cell spread in addition to its previously demonstrated role in virus assembly. We find that its function depends on the type of cell that is infected, and we show that it interacts with and modulates the function of another viral spread factor, gE.

The Interaction between Herpes Simplex Virus 1 Tegument Proteins UL51 and UL14 and Its Role in Virion Morphogenesis

“…As described above, UL7 has been reported to interact with UL51 (29), and therefore we focused on UL14 in this study. We noted that viral gE coimmunoprecipitated with both FEM-tagged UL51 and FEM-tagged UL11 (data not shown), which was in agreement with previous reports (25,48).…”

“…Taken together, all of these results suggested that UL51 and UL14 formed a complex in HSV-1-infected cells, and this complex regulated viral secondary envelopment for efficient viral replication. The UL51-UL14 complex likely anchors the cytoplasmic membrane and interacts with viral envelope glycoprotein gE and capsid proteins VP26 and VP19C (25,52,53); therefore, the UL51-UL14 complex may facilitate viral secondary envelopment by bridging the nucleocapsid and the cytoplasmic membrane via these interactions. Since this conclusion was based on experiments with the UL51LIY/AAA mutation, we cannot eliminate the possibility that UL14 regulated viral secondary envelopment upstream or downstream of UL51 in the same pathway and/or that the phenotype observed with the UL51LIY/AAA mutation was due to global misfolding of UL51 caused by the amino acid substitutions that may have impaired the function of UL51 rather than preventing UL51 interaction with UL14.…”

“…In earlier reports, the effects of UL51 and UL14 on HSV-1 replication were investigated using different HSV-1 strains (16,25). To clarify the role of the interaction between UL51 and UL14 in HSV-1 replication in infected cells, we simultaneously examined the effect of the UL14-null, UL51-null, and UL51LIY/AAA mutations on progeny virus yields in Vero cells infected with wild-type HSV-1(F), YK5015 (⌬UL14), YK5016 (⌬UL14-repair), YK5011 (⌬UL51), YK5012 (⌬UL51-repair), YK5013 (UL51LIY/AAA), YK5014 (UL51LIY/AAA-repair), YK5017 (⌬UL51/⌬UL14), or YK5018 (⌬UL51/⌬UL14-repair) at an MOI of 5 or 0.01 for 18 h or 48 h, respectively.…”

“…Although HSV-1 UL51 was shown to interact with UL7 (29) and UL7 homologs in PRV and HCMV were shown to facilitate viral secondary envelopment (30,31), it remained to be determined whether HSV-1 UL51 and UL7 are involved in viral secondary envelopment. HSV-1 UL51 has also been reported to interact with viral envelope glycoprotein E (gE) (25). However, the biological significance of these interactions in HSV-1 replication remains to be addressed.…”

“…Like HSV-1 UL51, UL51 homologs in other members of the Alpha-, Beta-, and Gammaherpesvirinae subfamilies were also shown to be incorporated into the tegument of virions (2,(21)(22)(23)(24), suggesting that UL51 homologs are conserved tegument proteins in herpesviruses. UL51 is an important positive regulator for HSV-1 replication in cell cultures based on the observation that recombinant HSV-1 UL51-null mutants show significantly impaired plaque formation and have a significant reduction in progeny virus titers in cell cultures (25). Palmitoylation of UL51 has been suggested to have a role in UL51 association with cytoplasmic membranes, with UL51 topology indicating that it should be displayed on the exterior surface of cytoplasmic membranes and, therefore, on the interior surface of virions (26).…”