Structural and functional studies of PCNA from African swine fever virus.

Proliferating cell nuclear antigen (PCNA) belongs to the DNA sliding clamp family. Via interacting with various partner proteins, PCNA plays critical roles in DNA replication, DNA repair, chromatin assembly, epigenetic inheritance, chromatin remodeling, and many other fundamental biological processes. Although PCNA and PCNA-interacting partner networks are conserved across species, PCNA of a given species is rarely functional in heterologous systems, emphasizing the importance of more representative PCNA studies. Here, we report two crystal structures of PCNA from African swine fever virus (ASFV), which is the only member of the family. Compared to the eukaryotic and archaeal PCNAs and the sliding clamp structural homologs from other viruses, PCNA possesses unique sequences and/or conformations at several regions, such as the J-loop, interdomain-connecting loop (IDCL), P-loop, and C-tail, which are involved in partner recognition or modification of sliding clamps. In addition to double-stranded DNA binding, we also demonstrate that PCNA can modestly enhance the ligation activity of the LIG protein. The unique structural features of PCNA can serve as a potential target for the development of ASFV-specific inhibitors and help combat the deadly virus. IMPORTANCE Two high-resolution crystal structures of African swine fever virus proliferating cell nuclear antigen (PCNA) are presented here. Structural comparison revealed that PCNA is unique at several regions, such as the J-loop, the interdomain-connecting loop linker, and the P-loop, which may play important roles in ASFV-specific partner selection of PCNA. Unlike eukaryotic and archaeal PCNAs, PCNA possesses high double-stranded DNA-binding affinity. Besides DNA binding, PCNA can also modestly enhance the ligation activity of the LIG protein, which is essential for the replication and repair of ASFV genome. The unique structural features make PCNA a potential target for drug development, which will help combat the deadly virus.