Molecular Mechanisms Of The Inhibition Of Host Cellular Apoptosis And RNA Silencing By Virus Capsid Related Proteins

Antiviral mechanisms are triggered immediately after pathogen attack by host defense systems that have been evolutionally conserved across a wide range of eukaryotes. After virus infection, host defense responses are activated to stimulate transcription of multiple effecter genes involved in apoptosis, proinflammation, cell proliferation and DNA damage repair that orchestrate inhibition of virus replication, maintenance of genetic stability and cell transformation which is regarded as no specific antiviral immunity. In the past decade, it has been discovered that virus infection in diverse eukaryotic hosts also induces the production of virus-derived small RNAs. These virus derived small RNAs share features with host endogenous small interfering RNAs (siRNAs), microRNAs (miRNAs) or Piwi-interacting RNAs (piRNAs) and so similar to host small RNAs, they can potentially mediate RNA interference (RNAi) and related RNA silencing pathways resulting in specific antiviral immunity. As a countermeasure, viruses have evolved strategies to interfere with these immune signaling events and facilitate its persistent infection. Virus always harbor small genomes with limited gene encoding ability especially for RNA virus and small DNA virus like Hepatitis B Virus (HBV). Virus encoding proteins are of two types: structural proteins which located in virions participate in progeny virus assembly, maturation, invasion and genome replication and non-structural proteins called regulatory proteins play roles in genome replication regulation and Immune pathogenesis. Previous studies about virus strategies by which antagonize hose defense systems mainly center on regulatory proteins, while paying too little attention to structural proteins. All viruses possess nucleocapsid proteins which have genome binding activities. In our study, we choose two important medical viruses capsid protein as our research object, Hepatitis B Virus capsid protein (core) and Severe acute Respiratory Syndrome Coronavirus (SARS coronavirus, SARS-CoV) Nucleocapsid protein (N). We found that HBV Core precursor precore and its variants could inhibit virus induced cell apoptosis and cellular p53 stability, while SARS-CoV N could inhibit RNA silencing pathway by which preventing genomes from degradation. These findings in our work reveal the importance of viral structural protein or its precursors in virus antagonizing host antiviral responses and pathogenesis, moreover, providing new knowledge for us to further understand the mechanisms in virus replication and pathogenesis.1. Molecular mechanism of HBV precore and its variants inhibit apoptosis and p53 stability.Hepatitis B virus can cause acute and chronic infection and is one of the major causes of hepatocellular carcinoma (HCC). Loss of p53 protein activity is one of the most common steps in hepatocarcinogenesis. In this study we plan to investigate the molecular function of HBV Core and its precursor precore in antagonizing the host antiviral response and causing diseases. By use of the Yeast two hybrid systems and various biochemistry and in vivo cell biological method we proved that HBV precore interact with Numb. Meanwhile, this interaction could influence p53 transcriptional activity, promote hepatoma cell proliferation and inhibit p53 dependent apoptosis. Furthermore, the level of p53 stability was markedly reduced in the precore variants stably-transfected HepG2 cells. Finally, we revealed the molecular mechanisms of HBV precore regulating p53 activity and hepatoma cell growth:HBV precore disrupts the tricomplex MDM2-p53-Numb balance and promotes MDM2-dependent p53 polyubiquition, thereby, resulting in decreased p53 stability through proteasomol degradation pathway.2. Study of Nucleocapsid protein of Severe acute Respiratory Syndrome Coronavirus acts as an RNA Silencing SuppressorViral infection in both plant and invertebrate hosts requires a virus-encoded RNA silencing suppressor function to block the RNA silencing antiviral defense. RNA silencing suppressors (RSSs) are well studied for plant viruses but are not well defined to date for animal viruses. Here, we report that the Nucleocapsid protein N of SARS-CoV acts as a potent RSS. We found that the SARS-CoV N protein inhibited RNA silencing induced by short hairpin RNAs (shRNAs). Furthermore, siRNA induced RNA silencing was inhibited by N protein. This viral counter-defense strategy that targets the silencing antiviral pathway may be essential for virus genome transcription and replication and protecting SARS with such a large RNA genome.