ICP4 Binding Research Articles

DNA binding by the herpes simplex virus type 1 ICP4 protein is necessary for efficient down regulation of the ICP0 promoter

The herpes simplex virus type 1 ICP4 and ICP0 polypeptides are immediate-early proteins that positively and negatively regulate expression of other viral genes in trans. ICP4 has recently been shown to bind DNA bearing the consensus sequence 5'-ATCGTCNNNN(T/C)CG(A/G)C-3', present upstream of a number of viral genes. To test the hypothesis that this DNA-binding activity is involved in ICP4-mediated gene regulation, site-specific mutagenesis was employed to mutate the version of this sequence in the promoter of the ICP0 gene. The mutation eliminated detectable binding of ICP4 to the promoter as measured in vitro by a gel electrophoresis band shift assay. The ability of the mutated ICP0 promoter to direct synthesis of a reporter gene was also investigated in a transient transfection assay. Whereas ICP4 was found to transactivate the wild-type ICP0 promoter two- to threefold, the mutated promoter was transactivated seven- to ninefold. In assays containing the ICP0 transactivator gene, ICP4 down regulated the wild-type promoter far more efficiently than the mutated promoter. Finally, both the wild-type and mutated ICP0 promoters exhibited a similar response to ICP4 in transfections that included a vector expressing the viral transactivator protein VP16. These experiments suggest that the sequence-specific DNA-binding activity of ICP4 is an essential element of its role as a negative regulator of gene expression.

Role for DNA-protein interaction in activation of the herpes simplex virus glycoprotein D gene.

On the basis of experiments with mutant virus and transfection with isolated genes, the herpes simplex virus immediate-early gene product ICP4 is known to positively regulate the transcription of viral early and late genes and negatively regulate expression from its own promoter. Binding of ICP4 to DNA sequences in several viral genes has been reported, yet the significance of ICP4-DNA interaction in transcriptional activation remains unclear. We have studied this problem by using the early glycoprotein D (gD) gene, which possesses a binding site at approximately -100 relative to the RNA initiation site. We linked this promoter and various mutant constructs to the chloramphenicol acetyltransferase gene in order to measure promoter activity in transient transfections both in the presence and in the absence of an ICP4-encoding plasmid. The natural promoter was activated 3.3-fold, and a deletion construct lacking the binding site was activated minimally (1.7-fold). Constructs containing multiple tandem repeats of the binding site (three or five inserts) demonstrated higher expression in the presence of ICP4 than did the natural promoter while retaining low levels of expression when unstimulated. Gel mobility shift assays and DNase I footprinting analyses indicated that ICP4 associated with multiple binding sites. In vitro transcription from a gD promoter construct containing multiple binding sites showed increased RNA synthesis in the presence of partially purified ICP4. These data provide the first direct evidence that binding of ICP4 to a specific DNA sequence in the gD gene contributes to activation of transcription.

Association of herpes simplex virus regulatory protein ICP4 with sequences spanning the ICP4 gene transcription initiation site.

The HSV gene encoding ICP4 is negatively regulated and the HSV gene encoding thymidine kinase is positively regulated by ICP4 in vivo. We report that ICP4 is a component of a stable complex that contains protein and a sequence of approximately 28 nucleotides that span the ICP4 gene transcription initiation site. The association of ICP4 with DNA sequences between positions -103 and +32 relative to the ICP4 mRNA start site was demonstrated by DNA binding immunoassays. DNase footprinting revealed that nucleotides between positions -8 and +20 are protected by ICP4. In contrast, binding of ICP4 to sequences flanking the mRNA start site in the thymidine kinase gene was not observed. Models for ICP4-mediated positive or negative regulation of HSV gene transcription are discussed.

Binding of the herpes simplex virus immediate-early gene product ICP4 to its own transcription start site.

A gel electrophoresis DNA-binding assay was used to detect proteins from herpes simplex virus type 1-infected and uninfected cells that specifically bind the upstream region of immediate-early (IE) gene 3. The assay is based on the altered electrophoretic mobility of DNA-protein complexes relative to that of free DNA in native gels. A series of end-labeled overlapping DNA fragments spanning a region from -272 to +27 (relative to the 5' terminus of the IE gene 3 mRNA) were used as probes. Two complexes were identified (referred to as A and B) which were driven by different protein factors. Formation of the A complex required infected-cell proteins extracted at any time from 2 to 16 h postinfection; a 0.5 to 1 M NaCl extract of infected cells, and a DNA probe that contained the sequences from -4 to +27 (relative to the 5' terminus of IE gene 3 mRNA). The protein that drove the formation of the A complex is not related to transcription factors TFIIIA or Sp1 or their cognate binding domains since neither the 5S RNA gene nor the GC box of simian virus 40 could compete for proteins that induced formation of the A complex. Through the use of monoclonal antibodies, the complex was shown to contain the IE gene 3 product, ICP4. A more detailed localization of the DNA-binding site in vitro by using chemical footprinting revealed that binding occurs over the sequence from -10 to +3 relative to the mRNA terminus. The binding of ICP4 to its own transcription start site may explain the repression of IE gene transcription which attends the onset of early (beta) gene expression and suggests an autoregulatory mechanism for gene control in herpes simplex virus type 1. The B complex was readily detected in uninfected cells (of a number of different cell lines), as well as in infected cells, with a probe containing the IE consensus sequence TAATGARATTC (where R is a purine) and two nested copies of the Sp1 binding motif GGGCGG; however, complexes were also detected with probes that lack the IE consensus sequence but contain Sp1 sites. These data suggest that the B complex contains the promoter-specific factor Sp1, and competition experiments with the clustered Sp1 binding domains from simian virus 40 confirmed this idea.