Porcine Circovirus Type 2 Hijacks Host IPO5 to Sustain the Intracytoplasmic Stability of Its Capsid Protein

Circovirus is the smallest virus to cause immune suppression in pigs. The capsid protein (Cap) is the only viral structural protein that is closely related to viral infection. ABSTRACT Nuclear entrance and stability of porcine circovirus type 2 (PCV2), the smallest virus in mammals, are crucial for its infection and replication. However, the mechanisms are not fully understood. Here, we found that the PCV2 virion maintains self-stability via the host importin 5 (IPO5) during infection. Coimmunoprecipitation combined with mass spectrometry and glutathione S-transferase pulldown assays showed that the capsid protein (Cap) of PCV2 binds directly to IPO5. Fine identification demonstrated that the N-terminal residue arginine(24) of Cap is the most critical to efficient binding to the proline(709) residue of IPO5. Detection of replication ability further showed that IPO5 supports PCV2 replication by promoting the nuclear import of incoming PCV2 virions. Knockdown of IPO5 delayed the nuclear transport of incoming PCV2 virions and significantly decreased the intracellular levels of overexpressed PCV2 Cap, which was reversed by treatment with a proteasome inhibitor or by rescuing IPO5 expression. Cycloheximide treatment showed that IPO5 increases the stability of the PCV2 Cap protein. Taken together, our findings demonstrated that during infection, IPO5 facilitates PCV2 replication by directly binding to the nuclear localization signal of Cap to block proteasome degradation. IMPORTANCE Circovirus is the smallest virus to cause immune suppression in pigs. The capsid protein (Cap) is the only viral structural protein that is closely related to viral infection. The nuclear entry and stability of Cap are necessary for PCV2 replication. However, the molecular mechanism maintaining the stability of Cap during nuclear trafficking of PCV2 is unknown. Here, we report that IPO5 aggregates within the nuclear periphery and combines with incoming PCV2 capsids to promote their nuclear entry. Concurrently, IPO5 inhibits the degradation of newly synthesized Cap protein, which facilitates the synthesis of virus proteins and virus replication. These findings highlight a mechanism whereby IPO5 plays a dual role in PCV2 infection, which not only enriches our understanding of the virus replication cycle but also lays the foundation for the subsequent development of antiviral drugs.

Automated summary

Circovirus is the smallest virus to cause immune suppression in pigs. The nuclear entry and stability of the capsid protein (Cap) of porcine circovirus type 2 (PCV2) are crucial for its infection and replication. IPO5 acts as a host factor that maintains self-stability of PCV2 virions and promotes their nuclear import. By directly binding to Cap, IPO5 blocks proteasome degradation and facilitates PCV2 replication.