Ubiquitination-dependent degradation of nucleolin mediated by porcine circovirus type 3 capsid protein

Porcine circovirus type 3 (PCV3) is a virus that causes a wide range of illnesses, including porcine dermatitis nephropathy syndrome, reproductive failure, and multisystemic inflammation. Nucleolin is predominantly localized in the nucleus and is important in viral replication. Mass spectrometry has revealed that the PCV3 capsid (Cap) protein interacts with nucleolin. However, the effects of nucleolin on PCV3 replication remain unclear. This study confirmed that PCV3 Cap interacts with nucleolin and that the Cap nuclear location signal domain and nucleolin amino acids 1–110 are essential for the Cap-nucleolin interaction. Further examinations indicated that lysine residue 102 of nucleolin is conjugated to K48-linked polyubiquitination chains by the E3 ligase RNF34, which is transported by PCV3 Cap from the cytoplasm to the nucleolus, resulting in nucleolin degradation. PCV3 replication was increased by small interfering RNA-mediated nucleolin knockdown and was decreased by nucleolin overexpression. Inhibition of PCV3 replication by nucleolin overexpression was associated with nucleolin-promoted release of interferon β. These findings suggest that nucleolin functions as an antiviral protein to combat PCV3 infection by activating innate immunity. The findings provide important insights concerning the prevention and treatment of PCV3 infection. IMPORTANCE Porcine circovirus type 3 (PCV3) is an emerging pathogen that causes multisystem disease in pigs and poses a severe threat to the swine industry. However, the mechanisms of how PCV3 uses host proteins to regulate its own life cycle are not well understood. In this study, we found that PCV3 capsid protein interacts with nucleolin and degrades it. Degradation of nucleolin by the PCV3 capsid protein requires recruitment of the enzyme RNF34, which is transported to the nucleolus from the cytoplasm in the presence of the PCV3 capsid protein. Nucleolin also decreases PCV3 replication by promoting the release of interferon β. These findings clarify the mechanism by which nucleolin modulates PCV3 replication in cells, thereby facilitating to provide an important strategy for preventing and controlling PCV3 infection.