Abstract
ABSTRACTHuman cytomegalovirus (HCMV) immediate-early 2 (IE2) protein is the major transactivator for viral gene expression and is required for lytic replication. In addition to transcriptional activation, IE2 is known to mediate transcriptional repression of promoters, including the major immediate-early (MIE) promoter and a bidirectional promoter within the lytic origin of replication (oriLyt). The activity of IE2 is modulated by another viral protein, UL84. UL84 is multifunctional and is proposed to act as the origin-binding protein (OBP) during lytic replication. UL84 specifically interacts with IE2 to relieve IE2-mediated repression at the MIE and oriLyt promoters. Originally, UL84 was thought to be indispensable for viral replication, but recent work demonstrated that some strains of HCMV (TB40E and TR) can replicate independently of UL84. This peculiarity is due to a single amino acid change of IE2 (UL122 H388D). Here, we identified that a UL84-dependent (AD169) Δ84 viral mutant had distinct IE2 localization and was unable to synthesize DNA. We also demonstrated that a TB40E Δ84 IE2 D388H mutant containing the reversed IE2 amino acid switch adopted the phenotype of AD169 Δ84. Further functional experiments, including chromatin-immunoprecipitation sequencing (ChIP-seq), suggest distinct protein interactions and transactivation function at oriLyt between strains. Together, these data further highlight the complexity of initiation of HCMV viral DNA replication.IMPORTANCE Human cytomegalovirus (HCMV) is a significant cause of morbidity and mortality in immunocompromised individuals and is also the leading viral cause of congenital birth defects. After initial infection, HCMV establishes a lifelong latent infection with periodic reactivation and lytic replication. During lytic DNA synthesis, IE2 and UL84 have been regarded as essential factors required for initiation of viral DNA replication. However, previous reports identified that some isolates of HCMV can replicate in a UL84-independent manner due to a single amino acid change in IE2 (H388D). These UL84-independent strains are an important consideration, as they may have implications for HCMV disease and research. This has prompted renewed interest into the functional roles of IE2 and UL84. The work presented here focuses on the described functions of UL84 and ascertains if those required functions are fulfilled by IE2 in UL84-independent strains.
Highlights
Human cytomegalovirus (HCMV) immediate-early 2 (IE2) protein is the major transactivator for viral gene expression and is required for lytic replication
To generate the complementing cell line expressing UL84, life-expanded telomerase human fibroblast cells (T-HF) were transduced with a lentivirus containing the UL84 gene expressed from the eF1a promoter
This complex consists of the replication fork machinery that directly participates in DNA synthesis
Summary
Human cytomegalovirus (HCMV) immediate-early 2 (IE2) protein is the major transactivator for viral gene expression and is required for lytic replication. UL84 was thought to be indispensable for viral replication, but recent work demonstrated that some strains of HCMV (TB40E and TR) can replicate independently of UL84 This peculiarity is due to a single amino acid change of IE2 (UL122 H388D). Further functional experiments, including chromatin-immunoprecipitation sequencing (ChIP-seq), suggest distinct protein interactions and transactivation function at oriLyt between strains Together, these data further highlight the complexity of initiation of HCMV viral DNA replication. HCMV encodes a set of six core proteins required for DNA synthesis These proteins are: UL54 (DNA polymerase), UL44 (polymerase processivity factor), UL105 (helicase), UL70 (primase), UL102 (primase-associated factor), and UL57 (single-stranded DNA-binding protein). The transient-transfection replication assay is a method in which a plasmid containing the cis-acting oriLyt is cotransfected with plasmids that encode viral factors required for origin-dependent DNA replication. UL84 and IE2 were factors seemingly required for oriLyt-dependent replication, presumably by acting as initiator proteins or origin-binding proteins (OBP) [16, 17]
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