The Deubiquitinase USP2a Sustains Interferons Antiviral Activity Through Restricting Ubiquitination Of Activated STAT1 In The Nucleus

The interferons(IFNs) family of cytokines plays a critical role in the defense against viral infection and regulating innate immunity. Three classes of IFNs(type I, II, and III) have been identified, which exert function through activating a signaling cascade comprised of the Janus kinase(Jak) and signal transducers and activators of transcription(STAT). Amongst seven members of STAT family, STAT1 regulates all three classes of IFNs signaling.STAT1 becomes activated and forms a transcriptional complex, which then translocates into the nucleus to induce ISGs expression through binding to the IFN-stimulated response element(ISRE) within the promoters of ISGs. STAT1 activation is largely dependent on phosphorylation at tyrosine 701 site of STAT1(pY701-STAT1). Maintaining sufficient p Y701-STAT1 levels in the nucleus is essential to sustain efficient interferons(IFNs) signaling and antiviral functions.The previous studies about the regulation of STAT1 signaling have been mostly focused on total STAT1. Different posttranslational modifications of total STAT1, such as methylation, acetylation, sumoylation and ubiquitination, have been suggested as the regulatory means of STAT1 signaling. However, the detailed mechanisms remain poorly defined. In regard of ubiquitination modification, how cytokines induce STAT1 ubiquitination and what roles STAT1 ubiquitination plays in regulating cytokines signaling have remained elusive.Total STAT1 proteins are relatively stable even if undergoing cytokines stimulation for a couple of hours. Thus, cytokines(such as IFNs) initiated STAT1 signaling at early stages may not be significantly affected through changes of total STAT1 ubiquitination and levels. Therefore, it is important to clarify how pY701-STAT1 levels are positively regulated in the nucleus. Here we find that pY701-STAT1 is the major form of ubiquitinated-STAT1 induced by Interferon(IFNs). Unlike total STAT1 which keeps relatively stable at early stage of IFNs signaling, pY701-STAT1 can be rapidly downregulated by ubiquitin-proteasome system. Moreover, ubiquitinated pY701-STAT1 is mostly located in the nucleus, and inhibiting nuclear import of pY701-STAT1 significantly blocks ubiquitination and downregulation of pY701-STAT1.Furthermore, we identify the deubiquitinase USP2 a which translocates into the nucleus and binds to pY701-STAT1, and inhibits K48-linked ubiquitination and degradation of pY701-STAT1. Importantly, USP2 a sustains IFNs-induced pY701-STAT1 levels and enhances all three classes of IFNs-mediated signaling and antiviral function.To our knowledge, this is the first identified deubiquitinase targetting activated pY701-STAT1. These findings uncover a positive mechanism by which IFNs execute efficient antiviral signaling and function, and may provide potential targets for improving IFNs antiviral therapy.