The E301R protein of African swine fever virus functions as a sliding clamp involved in viral genome replication.

African swine fever virus (ASFV) is a complex nucleocytoplasmic, large DNA virus that infects both domestic pigs and wild boar, but little is known about the process of genomic replication. The E301R protein (pE301R) from ASFV was previously predicted as a proliferating cell nuclear antigen (PCNA)-like protein through clamping DNA polymerase to the DNA duplex, but its exact structure and functions remain uncharacterized. Here, the crystal structure of pE301R revealed that it is composed of structurally similar head and tail domains and shares significant structural similarities to the DNA polymerase processivity factors, including sliding clamp and eukaryotic PCNA. More specifically, we demonstrated that pE301R exhibited multiple oligomeric states (with dimers and tetramers dominant), and the tetramers are consistent with the ring-shaped homotetramers in a head-to-tail manner generated by crystal packing. We also showed that pE301R interacted with the ASFV genome and viral DNA polymerase O174L. Furthermore, knockdown of E301R by specific small interfering RNAs (siRNAs) significantly decreased the virus genomic replication. Interestingly, the downregulation of PCNA by siRNAs significantly decreased the cell viability, whereas the inhibitory effect was reversed by pE301R overexpression. Notably, we demonstrated that overexpression of PCNA partially restored ASFV replication upon transfection of siRNAs targeting E301R . More importantly, T2 amino alcohol, a PCNA-specific inhibitor, markedly inhibited ASFV replication at the stage of viral genome replication. Taken together, we revealed that pE301R functions as a sliding clamp in ASFV genomic replication and can be used as a potential antiviral target. IMPORTANCE Sliding clamp is a highly conserved protein in the evolution of prokaryotic and eukaryotic cells. The sliding clamp is required for genomic replication as a critical co-factor of DNA polymerases. However, the sliding clamp analogs in viruses remain largely unknown. We found that the ASFV E301R protein (pE301R) exhibited a sliding clamp-like structure and similar functions during ASFV replication. Interestingly, pE301R is assembled into a unique ring-shaped homotetramer distinct from sliding clamps or proliferating cell nuclear antigens (PCNAs) from other species. Notably, the E301R gene is required for viral life cycle, but the pE301R function can be partially restored by the porcine PCNA. This study not only highlights the functional role of the ASFV pE301R as a viral sliding clamp analog, but also facilitates the dissection of the complex replication mechanism of ASFV, which provides novel clues for developing antivirals against ASF.