ICP0 Induces the Accumulation of Colocalizing Conjugated Ubiquitin

Abstract

Herpes simplex virus type 1 (HSV-1) immediate-early protein ICP0 is a general activator of viral gene expression which stimulates the initiation of lytic infection and reactivation from quiescence and latency. The importance of ICP0 to the biology of HSV-1 infection has stimulated interest in its mode of action. Previous studies have reported its interactions with other viral regulatory molecules, with the translation apparatus, with cyclin D3, and with a ubiquitin-specific protease. It has been demonstrated that ICP0 is able to induce the proteasome-dependent degradation of a number of cellular proteins, including components of centromeres and small nuclear substructures known as ND10 or PML nuclear bodies. ICP0 has a RING finger zinc-binding domain which is essential for its functions. In view of several recent examples of other RING finger proteins which modulate the stability of specific target proteins by acting as components of E3 ubiquitin ligase complexes, this study has explored whether ICP0 might operate via a similar mechanism. Evidence that the foci of accumulated ICP0 in transfected and infected cells contain enhanced levels of conjugated ubiquitin is presented. This effect was dependent on the RING finger region of ICP0, and comparison of the properties of a number of ICP0 mutants revealed an excellent correlation between previously established functions of ICP0 and its ability to induce concentrations of colocalizing conjugated ubiquitin. These results strongly support the hypothesis that a major factor in the mechanism by which ICP0 influences virus infection is its ability to induce the degradation of specific cellular targets by interaction with the ubiquitin-proteasome pathway.A definitive feature of herpes simplex virus type 1 (HSV-1) is the contrast between episodic lytic infection in the epithelia and periods of latency when the viral genome resides in a mainly quiescent state in neurons (for general reviews, see reference 22). Lytic infection involves the initial synthesis of immediate-early (IE) regulatory proteins: ICP4 functions at the level of transcription by interacting with components of the basal transcription apparatus (4), ICP27 is a multifunctional protein which acts at both the transcriptional and posttranscriptional levels (reviewed in reference 45), ICP22 enhances levels of late gene expression in selected cell types (49), and ICP0 enhances gene expression from all classes of viral genes (reviewed in references 11 and 13). The combined activities of the IE proteins lead to transcription of the early and late classes of genes throughout the whole viral genome. In contrast, abundant transcription during latency is limited to the family of latency-associated transcripts of uncertain function (reviewed in reference 55).Attempts to understand the basis of the contrasting patterns of viral transcription during lytic and latent infection have concentrated on the properties of the IE regulatory proteins. ICP0 was originally defined as a promiscuous activator of gene expression in tra...

A RING Finger Ubiquitin Ligase Is Protected from Autocatalyzed Ubiquitination and Degradation by Binding to Ubiquitin-specific Protease USP7

“…ICP0 includes a zinc-binding RING finger domain in its N-terminal portion, and in its C-terminal third lie a nuclear localization signal and motifs required for self-multimerization and efficient localization at specific nuclear substructures known as ND10 or PML nuclear bodies. Consistent with its ability to induce the degradation of certain cellular proteins, the RING finger domain of ICP0 has been shown to possess the ubiquitin E3 ligase activity that is typical of RING finger family proteins both in vitro and in vivo (13)(14)(15)(16)(17).…”

Herpes simplex virus 1-infected cell protein 0 contains two E3 ubiquitin ligase sites specific for different E2 ubiquitin-conjugating enzymes

“…Thus, ICP0 differs from other RING finger E3 ligases in that the domain containing the RING finger binds E2, whereas the ligase activity maps to a different region of the protein. In vivo, ICP0 demonstrates attributes of an E3 ligase in that it mediates the proteasome-dependent degradation of multiple cellular proteins, including SUMOylated promyelocytic leukemia protein and other unidentified SUMOylated proteins (13), the catalytic subunit of DNA-dependent protein kinase (19,20), centromeric proteins C and A (21,22), and Sp100 (23,24), and that foci of ICP0 observed in infected and transfected cells contain enhanced levels of conjugated Ub (25,26).…”

HSV vector-mediated transduction and GDNF secretion from adipose cells

“…[30][31][32] Additionally, ICP0 possesses ubiquitin ligase activity and has been shown to target particular cellular functions for proteosome degradation as a prelude to viral replication. 13 Mutant viruses that fail to express the IE genes including ICP0 are noncytotoxic, but their genomes are strongly repressed in standard fibroblast cultures and transgenes are rapidly silenced. 11 In contrast to the situation in nonneuronal cells, ICP0 is rapidly degraded in neurons, which may contribute to the ability of HSV to establish latency in these cells.…”

“…In QOZHGtransduced human adipocytes differentiated in vitro, ICP0 was detected in the nucleus as early as 9 h postinfection and staining of the entire cell was seen from day 1 (Figure 3f) until day 4 post-infection. Although ICP0 is known to have toxic effects on dividing cells, 13 it is likely that this viral function mediates cytotoxic effects on both dividing preadipocytes and quiescent adipocytes.…”