Non-canonical regulation of the reactivation of an oncogenic herpesvirus by the OTUD4-USP7 deubiquitinases

Ubiquitin-dependent proteasomal degradation of small hepatitis B virus surface antigen mediated by TRIM21 and antagonized by OTUD4

The small hepatitis B surface antigen (SHBs) is the most abundant hepatitis B virus (HBV) protein in individuals infected with HBV, and clearance of HBV surface antigen, which is primarily composed of SHBs, is considered a surrogate biomarker for achieving a functional cure of chronic HBV. Understanding SHBs degradation is crucial for its elimination and targeted eradication strategies. This study demonstrates that SHBs undergoes degradation via a ubiquitin/proteasome pathway, primarily through K48-linked ubiquitination, with K122 as the critical ubiquitination site. Utilizing immunoprecipitation and mass spectrometry, we identified TRIM21 (an E3 ubiquitin ligase) and OTUD4 (a deubiquitinase) as key regulators of SHBs. We verified the direct interaction between SHBs and TRIM21's coiled-coil domain, as well as the N-terminal amino acids 1-180 of OTUD4, using coimmunoprecipitation and glutathione S-transferase (GST) pull-down assays in both in vivo and in vitro settings. TRIM21 was observed to reduce SHBs stability and abundance by promoting its polyubiquitination, whereas OTUD4 acted to negate the effects of TRIM21-induced polyubiquitination, thereby stabilizing and increasing the levels of SHBs. Notably, TRIM21-mediated degradation of SHBs substantially impaired subviral particle and virion production and its biological activities such as migratory and angiogenic capabilities, opposite to the effect produced by the introduction of OTUD4. These findings suggest that TRIM21 and OTUD4 modulate SHBs protein stability and function through a ubiquitination-dependent proteasomal pathway, offering new insights into clearing SHBs and intervening in the progression of HBV-related liver diseases.IMPORTANCEThe small hepatitis B surface antigen (SHBs) is a key structural component of the hepatitis B virus (HBV) virion and subviral particles and is the most abundant HBV protein in individuals with chronic infection. Gaining a better understanding of its degradation pathways may help inform strategies to reduce SHBs levels and potentially support the design of targeted therapies. However, the specific mechanisms and processes involved in the degradation of SHBs remain largely unexplored. This study reveals that SHBs is degraded via the ubiquitin/proteasome pathway, specifically through K48-linked ubiquitination at the K122 site. TRIM21 promotes SHBs degradation by enhancing its polyubiquitination, while OTUD4 stabilizes SHBs by counteracting TRIM21's effects. TRIM21 reduces SHBs stability, subviral particle and virion production, and its related biological activities, whereas OTUD4 stabilizes SHBs, promoting its accumulation. These findings highlight the roles of TRIM21 and OTUD4 in regulating SHBs stability and function, offering new insights into potential interventions for HBV-related liver diseases.

Disrupting the OTUD4-USP7 deubiquitinase complex to suppress herpesvirus replication: a novel antiviral strategy

The development of effective and broad-spectrum antiviral therapies remains an unmet need. Current virus-targeted antiviral strategies are often limited by narrow spectrum of activity and the rapid emergence of resistance. As a result, there is increasing interest in alternative approaches that target host cell factors critical for viral replication. One promising strategy is the targeting of deubiquitinases (DUBs), enzymes that regulate key host and viral proteins involved in viral reactivation and replication. In this study, we explore the potential of targeting a DUB complex for antiviral therapy based on our previous study. Our previous work revealed that the OTUD4-USP7 DUB complex plays a crucial role in KSHV lytic reactivation. Here, we developed a peptide, p8, which effectively disrupts the interaction between OTUD4 and USP7, leading to decreased abundance of the key viral transcription factor, RTA, and suppression of murine herpesvirus replication in vivo. These findings underscore the OTUD4-USP7 DUB complex as a promising host-targeting antiviral therapeutic target for the treatment of KSHV-associated malignancies. Moreover, our study highlights the potential of DUB-targeting therapies as a novel and effective strategy for the development of broad-spectrum antiviral agents.

Palmitoylation of KSHV pORF55 is required for Golgi localization and efficient progeny virion production

Kaposi's sarcoma-associated herpesvirus (KSHV) is a double-stranded DNA virus etiologically associated with multiple malignancies. Both latency and sporadic lytic reactivation contribute to KSHV-associated malignancies, however, the specific roles of many KSHV lytic gene products in KSHV replication remain elusive. In this study, we report that ablation of ORF55, a late gene encoding a tegument protein, does not impact KSHV lytic reactivation but significantly reduces the production of progeny virions. We found that cysteine 10 and 11 (C10 and C11) of pORF55 are palmitoylated, and the palmytoilation is essential for its Golgi localization and secondary envelope formation. Palmitoylation-defective pORF55 mutants are unstable and undergo proteasomal degradation. Notably, introduction of a putative Golgi localization sequence to these palmitoylation-defective pORF55 mutants restores Golgi localization and fully reinstates KSHV progeny virion production. Together, our study provides new insight into the critical role of pORF55 palmitoylation in KSHV progeny virion production and offers potential therapeutic targets for the treatment of related malignancies.