The Membrane Protein of Severe Acute Respiratory Syndrome Coronavirus Functions as a Novel Cytosolic Pathogen-Associated Molecular Pattern To Promote Beta Interferon Induction via a Toll-Like-Receptor-Related TRAF3-Independent Mechanism

Wang, Yi; Liu, Li

It still remains elusive which cytosolic PRR is responsible for M-mediated IFN-␤ induction. The M protein might be a multifaceted molecule that physically interacts with diverse intracellular sensors and signaling factors (34, 42) . Our data indicate that the TLR-related signaling pathway rather than the RIG-I signaling cascade is responsible for M-mediated IFN-␤ induction. Interestingly, we observed consistent TRAF3 reductions in both HEK293T and J2-M cells as tested by the transiently intracellular overexpression of the M gene. TRAF3 is one of the key signaling molecules specifically responsible for IFN-I induction (28) . Data from the work of Siu and colleagues have shown that M(V68A) excludes the TRAF3 inclusion in the TRAF3.TANK.TBK1/IKK complex (39) . Although M(V68A)-mediated TRAF3 exclusion inhibits IFN-␤ induction, our study showed that M-mediated TRAF3 exclusion is independent of ligand stimulation and the disassociated TRAF3 and TBK1 could not complex with M protein, indicating that M protein might modulate the functions of TBK1 complex indirectly. Interestingly, we demonstrated that M-mediated IFN-␤ induction was associated with the upregulation of the adaptor proteins MyD88, TIRAP, and TICAM2 but not TRIF, which are all involved in the initiating phase of TLR signaling pathways. TRIF is required for both TLR3-and TLR4mediated IFN-␤ induction. In the activation of TLR3, TRIF is directly recruited to the TIR domain of dimerized TLR3, while in the activation of TLR4, TRIF is recruited to dimerized TLR4 indirectly through another TIR-containing adaptor protein, TICAM2 (also called TRAM). If TRIF is not required for M-mediated IFN-␤ induction, M-mediated IFN-␤ induction might be activated via a noncanonical TLR4-related signaling cascade independently of TRIF and TRAF3.