A truncated herpes simplex virus origin binding protein which contains the carboxyl terminal origin binding domain binds to the origin of replication but does not alter its conformation

Structural and biophysical characterization of the proteins interacting with the herpes simplex virus 1 origin of replication

The C terminus of the herpes simplex virus type 1 origin-binding protein, UL9ct, interacts directly with the viral single-stranded DNA-binding protein ICP8. We show that a 60-amino acid C-terminal deletion mutant of ICP8 (ICP8DeltaC) also binds very strongly to UL9ct. Using small angle x-ray scattering, the low resolution solution structures of UL9ct alone, in complex with ICP8DeltaC, and in complex with a 15-mer double-stranded DNA containing Box I of the origin of replication are described. Size exclusion chromatography, analytical ultracentrifugation, and electrophoretic mobility shift assays, backed up by isothermal titration calorimetry measurements, are used to show that the stoichiometry of the UL9ct-dsDNA15-mer complex is 2:1 at micromolar protein concentrations. The reaction occurs in two steps with initial binding of UL9ct to DNA (Kd approximately 6 nM) followed by a second binding event (Kd approximately 0.8 nM). It is also shown that the stoichiometry of the ternary UL9ct-ICP8DeltaC-dsDNA15-mer complex is 2:1:1, at the concentrations used in the different assays. Electron microscopy indicates that the complex assembled on the extended origin, oriS, rather than Box I alone, is much larger. The results are consistent with a simple model whereby a conformational switch of the UL9 DNA-binding domain upon binding to Box I allows the recruitment of a UL9-ICP8 complex by interaction between the UL9 DNA-binding domains.

Herpes simplex virus type 1 C-terminal variants of the origin binding protein (OBP), OBPC-1 and OBPC-2, cooperatively regulate viral DNA levels in vitro, and OBPC-2 affects mortality in mice

Two in-frame, C-terminal isoforms of the herpes simplex virus type 1 (HSV-1) origin binding protein (OBP), OBPC-1 and OBPC-2, and a unique C-terminal transcript, UL8.5, are specified by HSV-1 DNA. As the first isoform identified, OBPC-1 was initially assumed to be the product of the UL8.5 transcript. Recent evidence has demonstrated, however, that OBPC-1 is a cathepsin B-mediated cleavage product of OBP, suggesting that OBPC-2 is the product of the UL8.5 transcript. Because both OBPC-1 and -2 contain the majority of the OBP DNA binding domain, we hypothesized that both may be involved in regulating origin-dependent, OBP-mediated viral DNA replication. In this paper, we demonstrate that OBPC-2 is, indeed, the product of the UL8.5 transcript. The translational start site of OBPC-2 was mapped, and a virus (M571A) that does not express this protein efficiently was constructed. Using M571A, we have shown that OBPC-2 is able to bind origin DNA, even though it lacks seven N-terminal amino acid residues of the previously mapped OBP DNA binding domain, resulting in a revision of the limits of the OBP DNA binding domain. Consistent with their proposed roles in regulating viral DNA replication, OBPC-1 and -2 act together to down-regulate viral DNA replication in vitro. During functional studies in vivo, OBPC-2 was identified as a factor that increases mortality in the mouse ocular model of HSV-1 infection.

The interaction of herpes simplex type 1 virus origin-binding protein (UL9 protein) with Box I, the high affinity element of the viral origin of DNA replication

The herpes simplex type 1 (HSV-1) origin binding protein, the UL9 protein, exists in solution as a homodimer of 94-kDa monomers. It binds to Box I, the high affinity element of the HSV-1 origin, Oris, as a dimer. The UL9 protein also binds the HSV-1 single strand DNA-binding protein, ICP8. Photocross-linking studies have shown that although the UL9 protein binds Box I as a dimer, only one of the two monomers contacts Box I. It is this form of the UL9 homodimer that upon interaction with ICP8, promotes the unwinding of Box I coupled to the hydrolysis of ATP to ADP and Pi. Photocross-linking studies have also shown that the amount of UL9 protein that interacts with Box I is reduced by its interaction with ICP8. Antibody directed against the C-terminal ten amino acids of the UL9 protein inhibits its Box I unwinding activity, consistent with the requirement for interaction of the C terminus of the UL9 protein with ICP8. Inhibition by the antibody is enhanced when the UL9 protein is first bound to Box I, suggesting that the C terminus of the UL9 protein undergoes a conformational change upon binding Box I.

The herpes simplex virus type 1 origin binding protein. Specific recognition of phosphates and methyl groups defines the interacting surface for a monomeric DNA binding domain in the major groove of DNA

The UL9 gene of herpes simplex virus type 1 (HSV-1) encodes an origin binding protein (OBP). It is an ATP-dependent DNA helicase and a sequence-specific DNA-binding protein. The latter function is carried out by the C-terminal domain of OBP (DeltaOBP). We have now performed a quantitative analysis of the interaction between DeltaOBP and its recognition sequence, GTTCGCAC, in oriS. Initially optimal conditions for binding were carefully determined. We observed that complexes with different electrophoretic mobilities were formed. A cross-linking experiment demonstrated that nonspecific complexes containing 2 or more protein monomers per DNA molecule were formed at high protein concentrations. The specific complex formed at low concentrations of DeltaOBP had an electrophoretic mobility corresponding to a 1:1 complex. We then demonstrated that the methyl groups of thymine in the major groove were essential for high affinity binding. Changes in the minor groove had considerably smaller effects. Ethylation interference experiments indicated that specific contacts were made between OBP and three phosphates in the recognition sequence. Finally, these observations were used to present a model of the surface of DNA that interacts with DeltaOBP in a sequence-specific manner.

Identification of residues within the herpes simplex virus type 1 origin-binding protein that contribute to sequence-specific DNA binding

Gene UL9 of herpes simplex virus type 1 encodes an 851-amino-acid protein which is essential for viral DNA synthesis and functions as a sequence-specific origin-binding protein and DNA helicase. We generated monoclonal antibodies against purified UL9 protein and identified one such antibody (MAb 13924) that can block the interaction of the UL9 C-terminal DNA-binding domain (amino acids 534-851) with its recognition sequence. MAb 13924 interacted with immobilized peptides containing residues 780-786 of UL9. Although the corresponding region of the homologous protein encoded by varicell-azoster virus differs at only a single position it was not recognized by MAb 13924. Site-directed mutagenesis experiments confirmed that residues within this region contribute to the epitope recognized by MAb 13924 and may be involved in sequence-specific DNA binding. In addition, all eight lysine residues within the DNA-binding domain were separately changed to alanine and the DNA-binding properties of the mutated proteins were examined. The results showed that lysine residues that are located close to the peptide recognized by MAb 13924 or lie within the region of the DNA-binding domain most highly conserved among homologous alphaherpesvirus proteins play a role in sequence-specific DNA binding. Moreover, alteration of a lysine residue 18 amino acids from the recognized peptide prevented the interaction of MAb 13924 with the UL9 C-terminal DNA-binding domain. Three helical segments are predicted to occur within the region containing mutations that affect sequence-specific binding and interaction with MAb 13924.

Herpes simplex virus DNA replication

The Herpesviridae comprise a large class of animal viruses of considerable public health importance. Of the Herpesviridae, replication of herpes simplex virustype-1 (HSV-1) has been the most extensively studied. The linear 152-kbp HSV-1 genome contains three origins of DNA replication and approximately 75 open-reading frames. Of these frames, seven encode proteins that are required for originspecific DNA replication. These proteins include a processive heterodimeric DNA polymerase, a single-strand DNA-binding protein, a heterotrimeric primosome with 5'-3' DNA helicase and primase activities, and an origin-binding protein with 3'-5' DNA helicase activity. HSV-1 also encodes a set of enzymes involved in nucleotide metabolism that are not required for viral replication in cultured cells. These enzymes include a deoxyuridine triphosphatase, a ribonucleotide reductase, a thymidine kinase, an alkaline endo-exonuclease, and a uracil-DNA glycosylase. Host enzymes, notably DNA polymerase alpha-primase, DNA ligase I, and topoisomerase II, are probably also required. Following circularization of the linear viral genome, DNA replication very likely proceeds in two phases: an initial phase of theta replication, initiated at one or more of the origins, followed by a rolling-circle mode of replication. The latter generates concatemers that are cleaved and packaged into infectious viral particles. The rolling-circle phase of HSV-1 DNA replication has been reconstituted in vitro by a complex containing several of the HSV-1 encoded DNA replication enzymes. Reconstitution of the theta phase has thus far eluded workers in the field and remains a challenge for the future.

Use of transdominant mutants of the origin-binding protein (UL9) of herpes simplex virus type 1 to define functional domains

UL9, the origin-binding protein of herpes simplex virus type 1, contains six sequence motifs conserved in a large superfamily of RNA and DNA helicases. Single-amino-acid substitution mutations in these motifs inactivate UL9 function in vivo (R. Martinez, L. Shao, and S. K. Weller, J. Virol. 66:6735-6746, 1992). Overexpression of wild-type UL9 is inhibitory to plaque formation in a transfection assay which measures viral plaque formation by infectious herpes simplex virus type 1 DNA. Constructs containing mutations in motif I, II, or VI exhibit even stronger inhibitory effects in the same assay and thus can be considered strong transdominant inhibitors of plaque formation by the wild-type virus. The transdominant phenotype can be relieved by introducing a second mutation in the DNA-binding domain or by deleting the N-terminal 35 amino acids of the protein. The inhibitory effects of wild-type UL9 can also be partially relieved by deletion of amino acids 292 to 404. We propose that the N-terminal 35 amino acids of UL9 and residues 292 to 404 may define new functional domains of the UL9 protein.

Sindbis virus vectors for expression in animal cells

Sindbis virus and other alphavirus gene expression vectors have recently been used to express and study the functions of proteins and RNA, to evaluate classical vaccine and novel antiviral approaches, and for nucleic acid immunization. The vectors will likely attract continuing, innovative applications that exploit their useful features: rapid and efficient gene expression, wide host range, and RNA genomes.

Properties of the novel herpes simplex virus type 1 origin binding protein, OBPC

We have recently identified a novel 53-kDa herpes simplex virus type 1 (HSV-1) protein encoded by, and in frame with, the 3' half of the UL9 open reading frame, designated OBPC (K. Baradaran, C. Dabrowski and P. A. Schaffer, J. Virol. 68:4251-4261, 1994). Here we show that OBPC is a nuclear protein synthesized at both early and late times postinfection. In gel-shift assays in vitro-synthesized OBPC bound to oriS site I DNA to form a complex identical in mobility to complex A, generated with infected cell extracts and site I DNA. OBPC inhibited both plaque formation and viral DNA replication in transient assays, consistent with its ability to bind to site I DNA and its limited ability to interact with other essential DNA replication proteins. These properties suggest that OBPC may play a role in the initiation, elongation, or packaging of viral DNA.

The stoichiometry of binding of the herpes simplex virus type 1 origin binding protein, UL9, to OriS

A number of studies have demonstrated that the herpes simplex virus type 1 (HSV-1) UL9 protein, which is a homodimer in solution, binds to two high affinity binding sites in each origin of replication. Interaction between the proteins bound at the two sites leads to the formation of a complex nucleoprotein structure. The simplest models for this binding interaction predict two possible binding stoichiometries: 1) one UL9 dimer is bound at each site; or 2) one UL9 monomer is bound at each site so that one UL9 dimer occupies both sites. Two recent papers have addressed this issue by using indirect methods to measure the binding stoichiometry. Martin et al. (Martin, D. W., Muñoz, R. M., Oliver, D., Subler, M. A., and Deb, S. (1994) Virology 198, 71-80) reported that a monomer of UL9 binds to a single high affinity site, and Stabell and Olivo (Stabell, E. C., and Olivo, P.D. (1993) Nucleic Acids Res. 21, 5203-5211) concluded that a dimer of UL9 binds to a single high affinity site. We have directly measured the stoichiometry of binding of the carboxyl-terminal DNA binding domain of UL9 (t-UL9) to the origin of replication using a double-label gel shift assay. Using a short synthetic double-stranded oligonucleotide containing a single UL9 binding site, one protein-DNA complex was detected in the gel shift assay, and the molar ratio of UL9 DNA binding domains to DNA binding sites in this complex was determined to be 2.0 +/- 0.1 (n = 13). Using the minimal origin sequence excised from plasmid DNA, two protein-DNA complexes were detected. The binding stoichiometry of the faster migrating complex was 1.8 +/- 0.1 (n = 15), and the stoichiometry of the more slowly migrating band was 3.7 +/- 0.4 (n = 15). The simplest explanation for these data is that UL9 binds to the origin of replication as a homodimer with one dimer bound at both high affinity sites.

Alphavirus expression systems

Alphavirus vectors are newcomers in the field of heterologous gene expression. Nevertheless, they have rapidly become popular and are now being used in a wide range of applications. During the past year, new vectors and new methods for their use have improved levels of gene expression. As alphaviruses are capable of infecting humans, biosafety was an important issue during early work with these vectors. The construction of a conditional lethal helper system has now largely overcome this problem, and should further increase the utility of these types of vector in animal cell systems.

Transcriptional analysis of the region of the herpes simplex virus type 1 genome containing the UL8, UL9, and UL10 genes and identification of a novel delayed-early gene product, OBPC

The region of the UL component of the herpes simplex virus type 1 genome between nucleotides 17,793 and 25,150 includes three open reading frames that code for the protein products of the UL8, UL9, and UL10 genes (D.J. McGeogh, M.A. Dalrymple, A.J. Davison, A. Dolan, M.C. Frame, D. McNab, L.J. Perry, J.E. Scott, and P. Taylor, J. Gen. Virol. 69:1531-1574, 1988). We have mapped and characterized the overlapping transcripts in this region and have found that, in addition to the low-abundance UL8 and UL9 transcripts and the abundant UL10 transcript, at least two additional transcription units, designated UL8.5 and UL9.5, are specified by this region of the genome. The 5' ends of the UL8, UL8.5, and UL9 transcripts were mapped to nucleotides 20,682, 22,351, and 23,381, respectively. The 5' terminus of the UL9.5 transcript has not yet been mapped. The 3' ends of the UL8, UL8.5, UL9, and UL9.5 transcripts are coterminal at nucleotide 18,197. The 5' end of the UL10 mRNA, which is transcribed from the strand opposite that specifying the UL8, UL8.5, UL9, and UL9.5 transcripts, lies within the UL9 open reading frame at nucleotide 22,944, while the 3' terminus was mapped to nucleotide 24,666. Time course studies demonstrated that the UL8 and UL9 transcripts are members of the early kinetic class, the UL8.5 mRNA is a delayed-early transcript, and the UL9.5 and UL10 transcripts belong to the true-late kinetic class. Examination of the nucleotide sequence of the UL8.5 transcript revealed a potential open reading frame that overlaps and is in frame with the C-terminal half of the open reading frame encoding the origin-binding protein (OBP), the product of the UL9 gene. In vitro translation of the UL8.5 transcript demonstrated that it encodes a protein with an apparent molecular mass of 53 kDa. This protein was recognized by antibody directed against the C-terminal region of OBP and has thus been designated OBPC. A protein with an identical apparent molecular mass was also recognized by this antibody in infected-cell lysates, indicating that OBPC is synthesized during viral infection.