PABPC4 Broadly Inhibits Coronavirus Replication by Degrading Nucleocapsid Protein through Selective Autophagy.

Emerging coronaviruses (CoVs) can cause severe diseases in humans and animals, and, as of yet, none of the currently available broad-spectrum drugs or vaccines can effectively control these diseases. Host antiviral proteins play an important role in inhibiting viral proliferation. One of the isoforms of cytoplasmic poly(A)-binding protein (PABP), PABPC4, is an RNA-processing protein, which plays an important role in promoting gene expression by enhancing translation and mRNA stability. However, its function in viruses remains poorly understood. Here, we report that the host protein, PABPC4, could be regulated by transcription factor SP1 and broadly inhibits the replication of CoVs, covering four genera (Alphacoronavirus, Betacoronavirus, Gammacoronavirus, and Deltacoronavirus) of the Coronaviridae family by targeting the nucleocapsid (N) protein through the autophagosomes for degradation. PABPC4 recruited the E3 ubiquitin ligase MARCH8/MARCHF8 to the N protein for ubiquitination. Ubiquitinated N protein was recognized by the cargo receptor NDP52/CALCOCO2, which delivered it to the autolysosomes for degradation, resulting in impaired viral proliferation. In addition to regulating gene expression, these data demonstrate a novel antiviral function of PABPC4, which broadly suppresses CoVs by degrading the N protein via the selective autophagy pathway. This study will shed light on the development of broad anticoronaviral therapies. IMPORTANCE Emerging coronaviruses (CoVs) can cause severe diseases in humans and animals, but none of the currently available drugs or vaccines can effectively control these diseases. During viral infection, the host will activate the interferon (IFN) signaling pathways and host restriction factors in maintaining the innate antiviral responses and suppressing viral replication. This study demonstrated that the host protein, PABPC4, interacts with the nucleocapsid (N) proteins from eight CoVs covering four genera (Alphacoronavirus, Betacoronavirus, Gammacoronavirus, and Deltacoronavirus) of the Coronaviridae family. PABPC4 could be regulated by SP1 and broadly inhibits the replication of CoVs by targeting the nucleocapsid (N) protein through the autophagosomes for degradation. This study significantly increases our understanding of the novel host restriction factor PABPC4 against CoV replication and will help develop novel antiviral strategies.

Replicative capacity of four porcine enteric coronaviruses in LLC-PK1 cells.

Enteric coronaviruses (CoVs) are major pathogens that cause diarrhea in piglets. To date, four porcine enteric CoVs have been identified: transmissible gastroenteritis virus (TGEV), porcine epidemic diarrhea virus (PEDV), porcine deltacoronavirus (PDCoV), and HKU2-like porcine enteric alphacoronavirus (PEAV). In this study, we investigated the replicative capacity of these four enteric CoVs in LLC-PK1 cells, a porcine kidney cell line. The results showed that LLC-PK1 cells are susceptible to all four enteric CoVs, particularly to TGEV and PDCoV infections, indicating that LLC-PK1 cells can be applied to porcine enteric CoV research in vitro, particularly for coinfection studies.

Nucleocapsid proteins from other swine enteric coronaviruses differentially modulate PEDV replication.

Porcine epidemic diarrhea virus (PEDV), transmissible gastroenteritis virus (TGEV) and porcine deltacoronavirus (PDCoV) share tropism for swine intestinal epithelial cells. Whether mixing of viral components during co-infection alters pathogenic outcomes or viral replication is not known. In this study, we investigated how different coronavirus nucleocapsid (CoV N) proteins interact and affect PEDV replication. We found that PDCoV N and TGEV N can competitively interact with PEDV N. However, the presence of PDCoV or TGEV N led to very different outcomes on PEDV replication. While PDCoV N significantly suppresses PEDV replication, overexpression of TGEV N, like that of PEDV N, increases production of PEDV RNA and virions. Despite partial interchangeability in nucleocapsid oligomerization and viral RNA synthesis, endogenous PEDV N cannot be replaced in the production of infectious PEDV particles. Results from this study give insights into functional compatibilities and evolutionary relationship between CoV viral proteins during viral co-infection and co-evolution.