MERIT Award Recipient: Bernard Roizman, Sc.D.
Dissection of the Functions of Herpes Simplex Virus ICP0
Herpes simplex virus 1 genome encodes 84 known proteins. Of this number, several regulatory proteins are of particular interest, because each performs several functions. One of these, the infected cell protein No.0 (ICP0) enables the virus to multiply efficiently, cause disease, and persist. In uninfected cells, this protein activates or at least enhances the expression of any gene that is introduced into the cell by transfection or infection. ICP0 does not bind DNA and does not behave like a typical transcriptional factor. In the first step designed to dissect the functions of ICP0, the Roizman Laboratory identified several cellular proteins with which it interacts. These cellular proteins include a protein known a P60, a transcriptional factor (BMAL1), cyclin D3, and the translation elongation factor ef-1delta. Another laboratory identified still another protein known as ubiquitin specific protease No.7. The interactive sites of ICP0 with these proteins have been mapped.
In an attempt to determine why ICP0 interacts with these proteins, the Roizman Laboratory discovered that ICP0 acts as a very unusual ubiquitin ligase--an enzyme that plays a key role in targeting cellular protein for degradation. ICP0 was the first viral protein shown to be an ubiquitin ligase, but it is also very unusual in that it contains two such enzymatic sites designed to degrade different proteins. One of the targeted proteins is cdc34, an enzyme responsible for degradation of D cyclins. By degrading cdc34, the virus blocks the degradation of cyclin D3. The second site appears to be involved in the degradation of a set of cellular proteins modified by sumoylation that are involved in the host defenses against infection.
These studies raised several important questions that are the objective of the Roizman Laboratory's research for the next 5 years. Thus, (i) Given the many functions of ICP0, what role does the ubiquitin ligase enzyme play in the biology of the virus? (ii) Are the functions mapped to ICP0 capable of accounting for the observed role of ICP0 in the context of the infected cells and by itself, when introduced into the uninfected cell? (iii) They have accounted for some, but not all of the amino acid sequences of the virus; are there novel, hitherto unaccounted functions encoded in the unmapped portions of the protein? The exciting aspects of these studies stem from the observation that ICP0 is an exceptional analytical tool for the study of cellular functions as well as an important contributor to the pathology caused by the virus. ICP0 also exemplifies the complexities of function of many biologically important, multifunctional proteins. Analyses of the structure of ICP0 will contribute to an important goal: modular design of proteins for gene therapy.