M94 Is Essential for the Secondary Envelopment of Murine Cytomegalovirus

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: Discussionmentioning

confidence: 99%

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

“…Although their interaction is also conserved (7,10,11,13,27,31), its functional significance remains unclear.…”

Section: Discussionmentioning

confidence: 99%

The Human Cytomegalovirus (HCMV) Tegument Protein UL94 Is Essential for Secondary Envelopment of HCMV Virions

Phillips

Bresnahan

2012

Human cytomegalovirus (HCMV) virions are structurally complex, and the mechanisms by which they are assembled are poorly understood. However, several tegument proteins are known to be essential for proper particle assembly and maturation. Despite intense investigation, the function of many tegument proteins remains unknown. The HCMV UL94 gene is conserved among all herpesviruses and encodes a virion protein of unknown function. We demonstrate here that UL94 is a tegument protein that is expressed with true-late kinetics and localizes to the viral assembly complex during infection. To elucidate the function of UL94, we constructed a UL94-null mutant, designated UL94stop. This mutant is completely defective for replication, demonstrating that UL94 is essential. Phenotypic analysis of the UL94stop mutant shows that in the absence of UL94, viral gene expression and genome synthesis occur at wild-type levels. However, analysis of the localization of viral proteins to the cytoplasmic assembly complex shows that the essential tegument protein UL99 (pp28) exhibits aberrant localization in cells infected with the UL94stop mutant. Finally, we show that there is a complete block in secondary envelopment in the absence of UL94. Taken together, our data suggest that UL94 functions late in infection to direct UL99 to the assembly complex, thereby facilitating secondary envelopment of virions.

“…UL16 is found in both the nucleus and cytoplasm of infected cells (51,55,58), but a stable association with capsids has been found only in the cytoplasm (51). UL16-null mutants have a 10-fold reduction in virus titers (3,35), and for various other herpesviruses, such mutants have defects in cytoplasmic budding, based on the accumulation of capsids in the cytoplasm (24,48,61). Curiously, the UL16-capsid interaction is weakened within extracellular virions (51), indicating that tegument assembly is a dynamic process.…”

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confidence: 99%

“…UL16 is a 373-aa tegument protein that is also conserved among all herpesviruses (24,34,48,55,58,73). It contains 20 cysteines (51, 75), 8 of which are conserved.…”

mentioning

confidence: 99%

Regulated Interaction of Tegument Proteins UL16 and UL11 from Herpes Simplex Virus

Chadha

Han

Starkey

et al. 2012

It is well known that proteins in the tegument (located between the viral capsid and envelope proteins) play critical roles in the assembly and budding of herpesviruses. Tegument proteins UL16 and UL11 of herpes simplex virus (HSV) are conserved among all the Herpesviridae. Although these proteins directly interact in vitro, UL16 was found to colocalize poorly with UL11 in cotransfected cells. To explain this discrepancy, we hypothesized that UL16 is initially made in an inactive form and is artificially transformed to the binding-competent state when cells are disrupted. Consistent with a regulated interaction, UL16 was able to fully colocalize with UL11 when a large C-terminal segment of UL16 was removed, creating mutant UL16 (1-155). Moreover, membrane flotation assays revealed a massive movement of this mutant to the top of sucrose gradients in the presence of UL11, whereas both the full-length UL16 and the C-terminal fragment (residues 156 to 373) remained at the bottom. Further evidence for the presence of a C-terminal regulatory domain was provided by single-amino-acid substitutions at conserved cysteines (C269S, C271S, and C357S), which enabled the efficient interaction of full-length UL16 with UL11. Lastly, the binding site for UL11 was further mapped to residues 81 to 155, and to our surprise, the 5 Cys residues within UL16(1-155) are not required, even though the modification of free cysteines in UL16 with N-ethylmaleimide does in fact prevent binding. Collectively, these results reveal a regulatory function within the C-terminal region of UL16 that controls an N-terminal UL11-binding activity.