Abstract
Middle East respiratory syndrome coronavirus (MERS-CoV) causes severe respiratory infections that can be life-threatening. To establish an infection and spread, MERS-CoV, like most other viruses, must navigate through an intricate network of antiviral host responses. Besides the well-known type I interferon (IFN-α/β) response, the protein kinase R (PKR)-mediated stress response is being recognized as an important innate response pathway. Upon detecting viral dsRNA, PKR phosphorylates eIF2α, leading to the inhibition of cellular and viral translation and the formation of stress granules (SGs), which are increasingly recognized as platforms for antiviral signaling pathways. It is unknown whether cellular infection by MERS-CoV activates the stress response pathway or whether the virus has evolved strategies to suppress this infection-limiting pathway. Here, we show that cellular infection with MERS-CoV does not lead to the formation of SGs. By transiently expressing the MERS-CoV accessory proteins individually, we identified a role of protein 4a (p4a) in preventing activation of the stress response pathway. Expression of MERS-CoV p4a impeded dsRNA-mediated PKR activation, thereby rescuing translation inhibition and preventing SG formation. In contrast, p4a failed to suppress stress response pathway activation that is independent of PKR and dsRNA. MERS-CoV p4a is a dsRNA binding protein. Mutation of the dsRNA binding motif in p4a disrupted its PKR antagonistic activity. By inserting p4a in a picornavirus lacking its natural PKR antagonist, we showed that p4a exerts PKR antagonistic activity also under infection conditions. However, a recombinant MERS-CoV deficient in p4a expression still suppressed SG formation, indicating the expression of at least one other stress response antagonist. This virus also suppressed the dsRNA-independent stress response pathway. Thus, MERS-CoV interferes with antiviral stress responses using at least two different mechanisms, with p4a suppressing the PKR-dependent stress response pathway, probably by sequestering dsRNA. MERS-CoV p4a represents the first coronavirus stress response antagonist described.
Highlights
Innate antiviral responses represent the first line of defense against invading viral pathogens
In the past 15 years, the world has witnessed the emergence of two coronaviruses with high mortality rates in humans; severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002 and Middle East respiratory syndrome coronavirus (MERS-CoV) in 2012, both originating from animal reservoirs
Successful infection of a host depends on the presence of an appropriate receptor and on the ability of a virus to evade innate antiviral host responses, which constitute the first line of defense against invading viruses
Summary
Innate antiviral responses represent the first line of defense against invading viral pathogens. Host cells are equipped with multiple mechanisms to detect and respond to non-self, pathogen-associated molecular patterns (PAMPs). One of these PAMPs, viral cytosolic RNA, can be detected by RIG-I-like receptors (RLRs), such as melanoma differentiation-associated protein 5 (MDA5) and retinoic acid inducible gene 1 (RIG-I). Together with nucleating factors like G3BP1, G3BP2, TIA-1 and many translation initiation factors like eIF3, form cytoplasmic aggregates, which are called stress granules (SGs) The role of these SGs remains controversial, but growing evidence points to a role of these SGs as a platform for antiviral signal transduction [3,4,5]
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