Identification of a dimerization domain in the C-terminal segment of the IE110 transactivator protein from herpes simplex virus.

The 775-amino-acid IE110 (or ICP0) phosphoprotein of herpes simplex virus (HSV) functions as an accessory transcription factor during the lytic cycle and plays a critical role in reactivation from latent infection. By immunofluorescence analysis, IE110 localizes in a novel pattern consisting of several dozen spherical punctate granules in the nuclei of DNA-transfected cells. We constructed a hybrid version of IE110 that contained an epitope-tagged domain from the N terminus of the HSV IE175 protein and lacked the IE110 N-terminal domain that confers punctate characteristics. This hybrid IE175(N)/IE110(C) protein gave an irregular nuclear diffuse pattern on its own but was redistributed very efficiently into spherical punctate granules after cotransfection with the wild-type HSV-1 IE110 protein. Similar colocalization interactions occurred with internally deleted forms of IE110 that lacked the zinc finger region or large segments from the center of the protein, including both cytoplasmic and elongated punctate forms, but C-terminal truncated versions of IE110 did not interact. In all such interactions, the punctate phenotype was dominant. Evidence that C-terminal segments of IE110 could also form stable mixed-subunit oligomers in vitro was obtained by coimmunoprecipitation of in vitro-translated IE110 polypeptides with different-size hemagglutinin epitope-tagged forms of the protein. This occurred only when the two forms were cotranslated, not when they were simply mixed together. An in vitro-synthesized IE110 C-terminal polypeptide also gave immunoprecipitable homodimers and heterodimers when two different-size forms were cross-linked with glutaraldehyde and reacted specifically with a bacterial glutathione S-transferase/IE110 C-terminal protein in far-Western blotting experiments. The use of various N-terminal and C-terminal truncated forms of IE110 in the in vivo assays revealed that the outer boundaries of the interaction domain mapped between codons 617 and 711, although inclusion of adjacent codons on either side increased the efficiency severalfold in some assays. We conclude that the C-terminal region of IE110 contains a high-affinity self-interaction domain that leads to stable dimer and higher-order complex formation both in DNA-transfected cells and in in vitro assays. This segment of IE110 is highly conserved between HSV-1 and HSV-2 and appears to have the potential to play an important role in the interaction with the IE175 protein, as well as in correct intracellular localization, but it is not present in the equivalent proteins from varicella-zoster virus, pseudorabies virus, or equine abortion virus.