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

The membrane-associated PRRs, such as TLR2 and TLR4, can sense not only bacterial components but also viral coat proteins (37) . Kurt-Jones and colleagues first demonstrated that the innate immune response to the fusion (F) protein of respiratory syncytial virus (RSV) is mediated by TLR4 plus its cofactor CD14 on the plasma membrane, indicating that nonbacterial components can serve as extracellular PAMPs (24) . Although some viral structural proteins, such as the nucleocapsid (N) from measles virus (38) and viral ribonucleoprotein from vesicular stomatitis virus (39) , can activate IRF3 and TBK1/IKK␦, respectively, it remains to be determined how these viral products can function as PAMPs and where the driving forces for their activation come from. It has been shown that the SARS-CoV M protein alone not only can form virus-like particles (VLPs) in a viral RNA-independent manner (40) but also can induce cell apoptosis by inhibiting some key survival signal pathways, such as the Akt signaling pathway (41) . Thus, the M proteins derived from these infected and apoptotic cells might be eventually secreted and released into the extracellular compartments where the TLRs (such as TLR2 and TLR4) on the cell surface can be potentially recognized and subsequently activated by these extracellular PAMPs for the induction of the IFN-I response. To clarify this possibility, we employed several approaches to test whether the driving force for M proteinmediated IFN-␤ production is generated from inside or outside the cells. We used two types of inhibitors, BFA and OxPAPC, to block either the intracellular transport of M protein or the extracellular binding of M protein on TLR2 and TLR4, respectively. Our results reveal that M-mediated IFN-␤ induction is independent of M protein secretion as well as the extracellular stimulation of TLR2/TLR4 signaling, indicating that the driving force for the M-mediated IFN-␤ induction is likely generated from inside the cells.