ORF3a Protein of Severe Acute Respiratory Syndrome Coronavirus 2 Inhibits Interferon-Activated Janus Kinase/Signal Transducer and Activator of Transcription Signaling via Elevating Suppressor of Cytokine Signaling 1

Abstract

Coronavirus disease 2019 (COVID-19) has caused a crisis to global public health since its outbreak at the end of 2019. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the pathogen of COVID-19, appears to efficiently evade the host immune responses, including interferon (IFN) signaling. Several SARS-CoV-2 viral proteins are believed to involve in the inhibition of IFN signaling. In this study, we discovered that ORF3a, an accessory protein of SARS-CoV-2, inhibited IFN-activated Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling via upregulating suppressor of cytokine signaling 1 (SOCS1), a negative regulator of cytokine signaling. ORF3a induced SOCS1 elevation in a dose- and time-dependent manner. RNAi-mediated silencing of SOCS1 efficiently abolished ORF3a-induced blockage of JAK/STAT signaling. Interestingly, we found that ORF3a also promoted the ubiquitin-proteasomal degradation of Janus kinase 2 (JAK2), an important kinase in IFN signaling. Silencing of SOCS1 by siRNA distinctly blocked ORF3a-induced JAK2 ubiquitination and degradation. These results demonstrate that ORF3a dampens IFN signaling via upregulating SOCS1, which suppressed STAT1 phosphorylation and accelerated JAK2 ubiquitin-proteasomal degradation. Furthermore, analysis of ORF3a deletion constructs showed that the middle domain of ORF3a (amino acids 70–130) was responsible for SOCS1 upregulation. These findings contribute to our understanding of the mechanism of SARS-CoV-2 antagonizing host antiviral response.

A live-attenuated SARS-CoV-2 vaccine candidate with accessory protein deletions

“…Thus, the deletion of ORF3a conferred SARS-CoV-2 more susceptible to type-I interferon suppression. Our results are supported by previous reports that expression of SARS-CoV-2 ORF3a alone antagonized type-I interferon signaling 19 , 35 . The ORF3a protein was recently shown to form a dimer in the cell membrane with an ion channel activity 36 , which may account for its role in cell membrane rearrangement, inflammasome activation, and apoptosis 37 – 39 .…”

Individual and Synergistic Anti-Coronavirus Activities of SOCS1/3 Antagonist and Interferon α1 Peptides

“…We have shown a direct induction of SOCS1 and SOCS3 by HuCoV-OC43, which is in agreement with previous reports of SOCS induction by beta-coronaviruses spike protein ( 42 ). ORF3a protein encoded by SARS-CoV-2 was reported to inhibit IFN signaling by inducing SOCS1 ( 43 ). The presence of SOCS1 and/or SOCS3 reduces the induction of various types of IFNs ( 30 , 31 , 44 ).…”

Interferon antagonists encoded by SARS-CoV-2 at a glance

“…The ORF3 locus encodes multiple proteins, ORF3a being the longest with smaller products, like ORF3b and ORF3c, produced from downstream start codons. ORF3a was suggested to interfere with proper activation of JAKs by inducing Suppressor of Cytokine Signalling 1 (SOCS1), thus attenuating IFN signaling [ 43 ]. ORF3b was reported to inhibit IFN production through its C-terminus [ 44 ].…”