Functional Comparison of Herpes Simplex Virus 1 (HSV-1) and HSV-2 ICP27 Homologs Reveals a Role for ICP27 in Virion Release

Abstract

Numerous studies have focused on the regulatory functions of ICP27, an immediate-early (IE) protein of herpes simplex virus 1 (HSV-1). However, its homolog in HSV-2, termed ICP27t2, has been little studied. Here, we used two different approaches to functionally compare ICP27t2 and ICP27. In transfection-based assays, ICP27t2 closely resembled ICP27 in its capacity to enhance HSV-1 late gene expression, suppress the splicing of a viral intron, and complement the growth of an HSV-1 ICP27 null mutant. To study ICP27t2 in the context of viral infection, we engineered K2F1, an HSV-1 mutant that encodes ICP27t2 in place of ICP27. In Vero cells, K2F1 replicated with wild-type (WT) kinetics and yields, expressed delayed-early and late proteins normally, and was fully capable of activating several cellular signal transduction pathways that are ICP27 dependent. Thus, we conclude that ICP27t2 and ICP27 are functionally very similar and that ICP27t2 can mediate all ICP27 activities that are required for HSV-1 replication in cell culture. Surprisingly, however, we found that K2F1 forms plaques that are morphologically different from those of WT HSV-1. Investigation of this trait demonstrated that it results from the decreased release of progeny virions into the culture medium. This appears to be due to a reduction in the detachment of K2F1 progeny from the extracellular surface of the infected cell. We identified two HSV-1 ICP27 amino-terminal deletion mutants with a similar release defect. Together, these results demonstrate that ICP27 plays a heretofore-unappreciated role in modulating the efficiency of progeny virion release. IMPORTANCEICP27 is an essential, multifunctional regulatory protein that has a number of critical roles in the HSV-1 life cycle. Although ICP27 homologs are encoded by all known members of the Herpesviridae, previous work with several of these homologs has shown that they cannot substitute for ICP27 in the context of HSV-1-infected cells. Here, we identify ICP27t2 as the first homolog that can efficiently replace ICP27 in HSV-1 infection. Unexpectedly, our results also reveal that the sequence of the ICP27 gene can affect the release of HSV-1 progeny virions from the infected cell. Thus, our comparative study has revealed a novel function for ICP27 in the regulation of virus release. Herpes simplex virus 1 (HSV-1), a human alphaherpesvirus, is widespread in the human population, with greater than 57% of U.S. adults seropositive (1). The interaction of HSV-1 with the human host has been well studied (reviewed in reference 2). The virus is transmitted to new hosts via body secretions such as saliva, with the primary site of infection often being the oral mucosa. Following replication in epithelial cells, HSV-1 infects sensory neurons, where it establishes a lifelong latent infection in which the viral genome persists in the trigeminal ganglion as a semiquiescent episome. Periodically, the virus reactivates and returns to the oral cavity, where it can cause lesions (cold sores) and be transmitte...

A Conserved Mechanism of APOBEC3 Relocalization by Herpesviral Ribonucleotide Reductase Large Subunits

“…The HSV-1 strains used were wild-type strain KOS1.1 [72], K26GFP [51], ICP6 deletion mutant ICP6Δ, and the ICP4 deletion mutant d120 [53]. HSV-1 infections were done at a multiplicity of infection of 5 PFU per cell, as previously described [73]. Titers of viral stocks were determined by plaque assay on either Vero cells (KOS1.1, K26GFP, and ICP6Δ) or ICP4-complementing E5-Vero cells [74].…”

Shutoff of Host Gene Expression in Influenza A Virus and Herpesviruses: Similar Mechanisms and Common Themes

“…This effect is due to the fact that ICP27 interferes with splicing of unprocessed intron-containing pre-mRNAs into mature mRNAs [ 100 ], and spliced mRNAs are exported more efficiently than unspliced mRNAs [ 101 ] ( Figure 3 ). Although most of the studies have focused on HSV-1 ICP27, recent data shows a similar effect of HSV-2 ICP27 on splicing of host and viral mRNAs [ 102 , 103 ]. Formation of the spliceosome complex is strongly inhibited in the presence of ICP27 or HSV-1 splicing extracts [ 104 , 105 ], and ICP27 interacts with and regulates multiple splicing factors, including SRp20 [ 106 ] and SAP145 [ 105 ].…”

Herpes simplex virus ICP27 regulates alternative pre-mRNA polyadenylation and splicing in a sequence-dependent manner

“…3B). ICP27 induced retention of four introns (16)(17)(18)(19) near the 3′ end of ATXN2L (ataxin-2-like), a regulator of stress granules that is also implicated in neurodegenerative disorders (26) (Fig. 3C).…”

“…For example, splicing of alpha-globin is inhibited by ICP27 when ICP4, another viral IE gene, is present (15). ICP27 also promotes expression of the full-length glycoprotein C protein (16,17) and a truncated form of HSV-2 ICP34.5 (18,19), the major viral neurovirulence factor, by inhibiting splicing of these genes. ICP27 inhibits splicing of only introns 7a and 8 of promyelocytic leukemia protein (PML) (20).…”