Herpes simplex virus alpha protein ICP27 possesses separable positive and negative regulatory activities

The HSV-1 alpha (immediate-early) protein ICP27 expressed in transfected cells can activate the expression of certain HSV-1 promoters as well as inhibit the transactivated expression of others. We constructed a set of plasmids encoding mutant ICP27 molecules truncated at their carboxyl termini and used transfection assays to determine the functional properties of the mutant proteins. A polypeptide containing the amino-terminal 263 amino acid residues of ICP27 retained partial ability to activate gene expression but was unable to inhibit transactivation. Mutant proteins possessing 406 or 504 amino acids of ICP27 were unable to activate gene expression but retained full ability to inhibit transactivation. These results define two separable regulatory activities of ICP27, one positive and one negative, which can modulate gene expression in transfected cells. Immunoblot and immunofluorescence experiments were used to study the immunological reactivities and intracellular localizations of the mutant proteins. All proteins possessing the amino-terminal 263 amino acids of ICP27 reacted with an ICP27-specific monoclonal antibody and were localized to the cell nucleus. The mutant proteins, however, exhibited a number of phenotypes with regard to intranuclear localization. A mutant possessing 504 residues of ICP27 was similar to the wild-type protein in apparently localizing to all regions of the nucleus. A mutant containing 406 residues of ICP27, on the other hand, was mostly excluded from the nucleolar regions, while a 263-residue mutant was localized predominantly in the nucleoli. Thus, some aspect of ICP27 structure or function can dramatically affect its intranuclear distribution.