Deletion of the Herpes Simplex Virus 1 U
            <sub>L</sub>
            49 Gene Results in mRNA and Protein Translation Defects That Are Complemented by Secondary Mutations in U
            <sub>L</sub>
            41

UL16 is a tegument protein of herpes simplex virus (HSV) that is conserved among all members of the Herpesviridae, but its function is poorly understood. Previous studies revealed that UL16 is associated with capsids in the cytoplasm and interacts with the membrane protein UL11, which suggested a "bridging" function during cytoplasmic envelopment, but this conjecture has not been tested. To gain further insight, cells infected with UL16-null mutants were examined by electron microscopy. No defects in the transport of capsids to cytoplasmic membranes were observed, but the wrapping of capsids with membranes was delayed. Moreover, clusters of cytoplasmic capsids were often observed, but only near membranes, where they were wrapped to produce multiple capsids within a single envelope. Normal virion production was restored when UL16 was expressed either by complementing cells or from a novel position in the HSV genome. When the composition of the UL16-null viruses was analyzed, a reduction in the packaging of glycoprotein E (gE) was observed, which was not surprising, since it has been reported that UL16 interacts with this glycoprotein. However, levels of the tegument protein VP22 were also dramatically reduced in virions, even though this gE-binding protein has been shown not to depend on its membrane partner for packaging. Cotransfection experiments revealed that UL16 and VP22 can interact in the absence of other viral proteins. These results extend the UL16 interaction network beyond its previously identified binding partners to include VP22 and provide evidence that UL16 plays an important function at the membrane during virion production. Infectious herpesviruses contain approximately 40 viral proteins and are produced when their DNA-containing capsids are wrapped with a cell-derived membrane in the cytoplasm (1). This envelopment process is driven by complex interactions that are still poorly understood but is known to involve bridging interactions provided by a growing list of tegument proteins, which provide linkages between the capsid and viral membrane proteins (1-3). The UL16 tegument protein of herpes simplex virus (HSV) is remarkable for its numerous interactions with several other viral proteins, namely, tegument protein UL21 (4, 5), membrane protein UL11 (4, 6-8), membrane glycoprotein E (gE) (4, 9), and an unidentified protein(s) that is associated with the capsid (10-12). UL16 is conserved among all the alpha-, beta-, and gammaherpesvirinae (2, 13, 14), but its actual function remains unknown.There are several reasons for suggesting a role for UL16 in HSV envelopment. The earliest study showed that a U L 16-null mutant (here named the ⌬U L 16B mutant) produces infectious virions at a level only one-tenth that of the wild-type virus (15). Also, UL16 has been shown to be bound in some manner to cytoplasmic capsids (10, 16, 17), and thus, its direct interactions with membrane proteins UL11 (8) and gE (9) suggest that UL16 might provide bridging functions that help drive virion production, as first pro...

Section: Ultrastructural Characterization Of Ul16-null Mutantssupporting

confidence: 61%

mentioning

confidence: 99%

Elucidation of the Block to Herpes Simplex Virus Egress in the Absence of Tegument Protein UL16 Reveals a Novel Interaction with VP22

Starkey

Han

Chadha

et al. 2014

“…This led the authors to conclude that VHS is lethal for the virus in the absence of VP22. Other studies on a BAC-recovered ⌬22 virus have also identified a spontaneous secondary frameshift mutation resulting in a truncated VHS (12,13).…”

mentioning

confidence: 99%

Mode of Virus Rescue Determines the Acquisition of VHS Mutations in VP22-Negative Herpes Simplex Virus 1

Ebert

Depledge

Breuer

et al. 2013

c It has been proposed that herpes simplex virus 1 with VP22 deleted requires secondary mutation of VHS for viability. Here we show that a replication-competent ⌬22 virus constructed by homologous recombination maintains a wild-type (Wt) VHS gene and has no other gross mutations. By contrast, ⌬22 viruses recovered from a bacterial artificial chromosome contain multiple amino acid changes within a conserved region of VHS. Hence, the mode of virus rescue influences the acquisition of secondary mutations.

“…First, at least 23 different viral tegument proteins may be present in mature extracellular virions (8). Second, the tegument is involved in many facets of the viral life cycle, including the migration of capsids on microtubules (9)(10)(11)(12)(13)(14), the anchorage of the capsids to nuclear pores (15)(16)(17)(18)(19)(20), the transactivation of viral genes (21), the modulation of host protein expression (22,23), viral latency (24), and the regulation of the immune response (13,(25)(26)(27). Finally, many tegument proteins also interact with each other and/or with viral glycoproteins and accumulate at the trans-Golgi network (TGN), where they altogether delineate the likely final envelopment site (4,5,28).…”

mentioning

confidence: 99%

Analysis of the Early Steps of Herpes Simplex Virus 1 Capsid Tegumentation

Henaff

Rémillard-Labrosse

Loret

et al. 2013

Herpes simplex virus type 1 particles are multilayered structures with a DNA genome surrounded by a capsid, tegument, and envelope. While the protein content of mature virions is known, the sequence of addition of the tegument and the intracellular compartments where this occurs are intensely debated. To probe this process during the initial stages of egress, we used two approaches: an in vitro nuclear egress assay, which reconstitutes the exit of nuclear capsids to the cytoplasm, and a classical nuclear capsid sedimentation assay. As anticipated, in vitro cytoplasmic capsids did not harbor U L 34, U L 31, or viral glycoproteins but contained U S 3. In agreement with previous findings, both nuclear and in vitro capsids were positive for ICP0 and ICP4. Unexpectedly, nuclear C capsids and cytoplasmic capsids produced in vitro without any cytosolic viral proteins also scored positive for U L 36 and U L 37. Immunoelectron microscopy confirmed that these tegument proteins were closely associated with nuclear capsids. When cytosolic viral proteins were present in the in vitro assay, no additional tegument proteins were detected on the capsids. As previously reported, the tegument was sensitive to high-salt extraction but, surprisingly, was stabilized by exogenous proteins. Finally, some tegument proteins seemed partially lost during egress, while others possibly were added at multiple steps or modified along the way. Overall, an emerging picture hints at the early coating of capsids with up to 5 tegument proteins at the nuclear stage, the shedding of some viral proteins during nuclear egress, and the acquisition of others tegument proteins during reenvelopment.