Abstract
African swine fever (ASF) is a highly pathogenic infectious disease that poses a serious threat to the pig farming industry worldwide. Currently, there are no effective vaccines and drugs. The 5' cap structure of messenger RNAs (mRNAs) is a critical modification that is found in all eukaryotic organisms and many viruses. This modification improves mRNA stability by protecting it from cleavage by 5' to 3' exonuclease, thus facilitating efficient translation. The African swine fever virus (ASFV) NP868R protein (pNP868R) has 5'-triphosphatase (TPase), guanylyltransferase (GTase), and guanine-N7-methyltransferase (MTase) activities, endowing the viral transcripts with a 7-methylguanosine RNA cap (m(7)GpppN, cap-0). In this study, we found that the EP424R gene encodes the 2'-O-ribose methyltransferase (2'-O-MTase) of ASFV, which confers a m(7)GpppNmp-RNA (cap-1) structure on the viral transcripts. Furthermore, we determined the crystal structures of the EP424R monomer and the complexes of EP424R with S-adenosyl-L-methionine (AdoMet, SAM), S-adenosyl-L-homocysteine (AdoHcy, SAH), and Sinefungin (SFG). Notably, we identified the key catalytic residues (KDKE) of EP424R and clarified its enzymatic mechanism. Additionally, overexpression experiments using the eukaryotic expression plasmid of EP424R, along with its mutant plasmid, and specific small interfering RNA (siRNA) interference assays in wild boar lung (WSL) cells demonstrated that this gene is critical for replication of the ASFV. Furthermore, this role is partially mediated by its methyltransferase activity. In summary, our structural and biochemical studies elucidated the process of ASFV mRNA capping, and subsequent virological investigations confirmed the significance of this gene in ASFV replication, highlighting its potential as an antiviral target. IMPORTANCE: 2'-O-ribose methyltransferases (2'-O-MTases) play a vital role in cellular processes of eukaryotes and in the replication of many viruses. However, these enzymes in the African swine fever virus (ASFV) remain uncharacterized. Our study revealed that the EP424R protein (pEP424R) of ASFV exhibits 2'-O-MTase activity during viral replication and is crucial for ASFV replication. Notably, this function is partially dependent on its methyltransferase activity. These findings increase our understanding of the transcriptional capping mechanism of ASFV and provide new insights for developing antiviral drugs targeting African swine fever (ASF).