HSPBP1 facilitates cellular RLR-mediated antiviral response by inhibiting the K48-linked ubiquitination of RIG-I

The RNA-binding protein ZFP36 strengthens innate antiviral signaling by targeting RIG-I for K63-linked ubiquitination

Innate immunity is the first line of defense against infections, which functions as a significant role in resisting pathogen invasion. Rapid immune response is initiated by pattern recognition receptors (PRRs) quickly distinguishing "self" and "non-self." Upon evolutionarily conserved pathogen-associated molecular pattern (PAMP) is recognized by PRRs, innate immune response against infection is triggered via an orchestration of molecular interaction, cytokines cascades, and immune cells. RIG-I plays a critical role in type I interferon (IFN-I) production by direct recognition of cytoplasmic double-stranded viral RNA. However, the activation mechanism of RIG-I is incompletely understood. In this study, we reported RNA-binding protein ZFP36 as a positive regulator of RIG-I-mediated IFN-I production. ZFP36 is a member of Zinc finger proteins (ZFPs) characterized by the zinc finger (ZnF) motif, which broadly involved gene transcription and signal transduction. However, its role in regulating antiviral innate immune signaling is still unclear. We found that ZFP36 associates with RIG-I and potentiates the FN-β production induced by SeV. Mechanistically, ZFP36 promotes K63-linked polyubiquitination of RIG-I, mostly at K154/K164/K172, thereby facilitating the activation of RIG-I during infection. While the mutant ZFP36 (C118S/C162S) failed to increase polyubiquitination of RIG-I and SeV induced FN-β. Our findings collectively demonstrated that ZFP36 acts as a positive regulator in antiviral innate immunity by targeting RIG-I for K63-linked ubiquitination, thus improving our understanding of the activation mechanism of RIG-I.

Heat shock protein: A double-edged sword linking innate immunity and hepatitis B virus infection

Heat shock proteins (HSPs), which have a variety of functions, are one of the stress protein families. In recent years, They have been reported to play a dual role in hepatitis B virus (HBV) which as persistent infection which is associated with, cirrhosis and liver cancer. In this article, we have summarized the regulatory mechanisms between HSPs and viruses, especially HBV and associated diseases based on HSP biological functions of in response to viral infections. In view of their potential as broad-spectrum antiviral targets, we have also discuss current progress and challenges in drug development based on HSPs, as well as the potential applications of agents that have been evaluated clinically in HBV treatment.

Identification of 5' upstream sequence involved in HSPBP1 gene transcription and its downregulation during HIV-1 infection

The Human Immunodeficiency Virus-1 (HIV-1) is known to modulate the host environment for successful replication and propagation like other viruses. The virus utilises its proteins to interact with or modulate host factors and host signalling pathways that may otherwise restrict the virus. A previous study from our lab has shown that the host heat shock protein 70 (HSP70) binding protein (HSPBP1) is a co-chaperone that inhibits viral replication. We have also shown that the virus downregulates HSPBP1 during infection. However, the mechanism of downregulation remains to be elucidated. In the present study, we hypothesized that the HSPBP1 promoter may be repressed during infection leading to its downmodulation at the RNA and protein levels. The 5' upstream region of the HSPBP1 gene was first mapped and it was identified that a fragment comprising of a ∼600 bp upstream region of the transcription start site show the highest promoter-like activity. Further, the Sp1 transcription factor was shown to be essential for normal promoter activation. Our results further demonstrate that HIV-1 downregulates the activity of the identified promoter. It was seen that the viral transactivator protein, Tat, was responsible for the downmodulation of the HSPBP1 promoter. HIV-1 Tat is known to bind and regulate several cellular promoters during infection, thereby making the environment conducive for establishment of the virus. Our results further show that Tat is recruited to the HSPBP1 promoter and in the presence of Tat, recruitment of Sp1 on HSPBP1 promoter was decreased, which explains the suppression of HSPBP1 during HIV-1 infection. Therefore, this study further adds to the list of cellular promoters that are modulated by Tat during HIV-1 infection either directly or indirectly.

BAG6 negatively regulates the RLR signaling pathway by targeting VISA/MAVS

The virus-induced signaling adaptor protein VISA (also known as MAVS, ISP-1, Cardif) is a critical adaptor protein in the innate immune response to RNA virus infection. Upon viral infection, VISA self-aggregates to form a sizeable prion-like complex and recruits downstream signal components for signal transduction. Here, we discover that BAG6 (BCL2-associated athanogene 6, formerly BAT3 or Scythe) is an essential negative regulator in the RIG-I-like receptor signaling pathway. BAG6 inhibits the aggregation of VISA by promoting the K48-linked ubiquitination and specifically attenuates the recruitment of TRAF2 by VISA to inhibit RLR signaling. The aggregation of VISA and the interaction of VISA and TRAF2 are enhanced in BAG6-deficient cell lines after viral infection, resulting in the enhanced transcription level of downstream antiviral genes. Our research shows that BAG6 is a critical regulating factor in RIG-I/VISA-mediated innate immune response by targeting VISA.