Crystallization and preliminary X-ray crystallographic studies on the herpes simplex virus 1 single-stranded DNA binding protein

Evidence for a direct interaction between HSV-1 ICP27 and ICP8 proteins

Herpes simplex virus 1 (HSV-1) ICP27 and ICP8 proteins have both been implicated in the transcription of late genes and regulation of viral gene expression. We showed previously that ICP27 and ICP8 associate with the RNAP II holoenzyme (Zhou and Knipe, J. Virol. 76, 5893-5904). Here, we demonstrate that ICP27 and ICP8 coprecipitate from lysates of HSV-1-infected HEp2 cells and from lysates of insect cells expressing ICP27 and ICP8, the latter being in the absence of other HSV-1 proteins. By expressing and purifying hexahistidine-tagged ICP8 (His-ICP8) and maltose binding protein (MBP)-tagged ICP27 (MBP-27) proteins and performing in vitro immunoprecipitation and pull-down assays, we also demonstrate that ICP27 and ICP8 coprecipitate in the absence of other viral or cellular proteins. Taken together, these data provide evidence that ICP27 and ICP8 interact directly in vitro and in infected cells. We hypothesize that the ICP27-ICP8 interaction plays a role in the stimulation of late gene transcription.

The crystal structure of the herpes simplex virus 1 ssDNA-binding protein suggests the structural basis for flexible, cooperative single-stranded DNA binding

All organisms including animal viruses use specific proteins to bind single-stranded DNA rapidly in a non-sequence-specific, flexible, and cooperative manner during the DNA replication process. The crystal structure of a 60-residue C-terminal deletion construct of ICP8, the major single-stranded DNA-binding protein from herpes simplex virus-1, was determined at 3.0 A resolution. The structure reveals a novel fold, consisting of a large N-terminal domain (residues 9-1038) and a small C-terminal domain (residues 1049-1129). On the basis of the structure and the nearest neighbor interactions in the crystal, we have presented a model describing the site of single-stranded DNA binding and explaining the basis for cooperative binding. This model agrees with the beaded morphology observed in electron micrographs.

Two-dimensional crystallization of herpes simplex virus type 1 single-stranded DNA-binding protein, ICP8, on a lipid monolayer

Herpes simplex virus type 1 single-stranded DNA-binding protein (ICP8) has been crystallized on a positively charged lipid monolayer. The crystals belong to the planar group p2 with a=39 nm, b=23.2 nm and gamma=87.2 degrees. The projected map of ICP8 crystals calculated at a resolution of 3.9 nm shows four ICP8 monomers per unit cell with the crystals formed by a parallel arrangement of 16.2 nm helical ICP8 filaments. This novel filamentous form has not been reported before. The ICP8 monomers show different appearances in projection, suggesting that they may adopt different orientations, probably reflecting the strong intermolecular and lipid-filament interactions in the crystal. When the 23 nm diameter filaments formed by ICP8 in solution at low temperature in the presence of magnesium were generated and then layered on the phospholipid monolayer, highly ordered arrays of an 8.5 nm filament with a shallow 31.2 nm pitch were observed and reconstruction revealed a double-helical structure.

Conformational changes in the herpes simplex virus ICP8 DNA-binding protein coincident with assembly in viral replication structures

The herpes simplex virus (HSV) single-stranded DNA-binding protein, ICP8, is required for viral DNA synthesis. Before viral DNA replication, ICP8 colocalizes with other replication proteins at small punctate foci called prereplicative sites. With the onset of viral genome amplification, these proteins become redistributed into large globular replication compartments. Here we present the results of immunocytochemical and biochemical analysis of ICP8 showing that various antibodies recognize distinct forms of ICP8. Using these ICP8-specific antibodies as probes for ICP8 structure, we detected a time-dependent appearance and disappearance of ICP8 epitopes in immunoprecipitation assays. Immunofluorescence staining of ICP8 in cells infected with different HSV mutant viruses as well as cells transfected with a limited number of viral genes demonstrated that these and other antigenic changes occur coincident with ICP8 assembly at intranuclear replication structures. Genetic analysis has revealed a correlation between the ability of various ICP8 mutant proteins to form the 39S epitope and their ability to bind to DNA. These results support the hypothesis that ICP8 undergoes a conformational change upon binding to other HSV proteins and/or to DNA coincident with assembly into viral DNA replication structures.

C-terminal region of herpes simplex virus ICP8 protein needed for intranuclear localization

The herpes simplex virus single-stranded DNA-binding protein, ICP8, localizes initially to structures in the nucleus called prereplicative sites. As replication proceeds, these sites mature into large globular structures called replication compartments. The details of what signals or proteins are involved in the redistribution of viral and cellular proteins within the nucleus between prereplicative sites and replication compartments are poorly understood; however, we showed previously that the dominant-negative d105 ICP8 does not localize to prereplicative sites and prevents the localization of other viral proteins to prereplicative sites (J. Virol. 74 (2000) 10122). Within the residues deleted in d105 (1083 to 1168), we identified a region between amino acid residues 1080 and 1135 that was predicted by computer models to contain two alpha-helices, one with considerable amphipathic nature. We used site-specific and random mutagenesis techniques to identify residues or structures within this region that are required for proper ICP8 localization within the nucleus. Proline substitutions in the predicted helix generated ICP8 molecules that did not localize to prereplicative sites and acted as dominant-negative inhibitors. Other substitutions that altered the charged residues in the predicted alpha-helix to alanine or leucine residues had little or no effect on ICP8 intranuclear localization. The predicted alpha-helix was dispensable for the interaction of ICP8 with the U(L)9 origin-binding protein. We propose that this C-terminal alpha-helix is required for localization of ICP8 to prereplicative sites by binding viral or cellular factors that target or retain ICP8 at specific intranuclear sites.

The 60-residue C-terminal region of the single-stranded DNA binding protein of herpes simplex virus type 1 is required for cooperative DNA binding

ICP8 is the major single-stranded DNA (ssDNA) binding protein of the herpes simplex virus type 1 and is required for the onset and maintenance of viral genomic replication. To identify regions responsible for the cooperative binding to ssDNA, several mutants of ICP8 have been characterized. Total reflection X-ray fluorescence experiments on the constructs confirmed the presence of one zinc atom per molecule. Comparative analysis of the mutants by electrophoretic mobility shift assays was done with oligonucleotides for which the number of bases is approximately that occluded by one protein molecule. The analysis indicated that neither removal of the 60-amino-acid C-terminal region nor Cys254Ser and Cys455Ser mutations qualitatively affect the intrinsic DNA binding ability of ICP8. The C-terminal deletion mutants, however, exhibit a total loss of cooperativity on longer ssDNA stretches. This behavior is only slightly modulated by the two-cysteine substitution. Circular dichroism experiments suggest a role for this C-terminal tail in protein stabilization as well as in intermolecular interactions. The results show that the cooperative nature of the ssDNA binding of ICP8 is localized in the 60-residue C-terminal region. Since the anchoring of a C- or N-terminal arm of one protein onto the adjacent one on the DNA strand has been reported for other ssDNA binding proteins, this appears to be the general structural mechanism responsible for the cooperative ssDNA binding by this class of protein.