hnRNP K regulates mitochondrial apoptosis induced by porcine circovirus type 3 capsid protein

Porcine circovirus type 3 (PCV3) is a globally emerging circovirus affecting pigs and other animals. The capsid protein (Cap) is the sole structural protein of PCV, with a crucial role in virus evolution and pathogenesis. Through interactions with host factors, Cap enables viral entry, transport, and replication while modifying various cellular processes. Cap protein-induced apoptosis has important implications for viral pathogenesis, but remains poorly defined. Herein, we demonstrated for the first time that PCV3 Cap induced cell cycle arrest of PK-15 cells in S-phase and initiated apoptosis via a mitochondrial Caspase-9-dependent pathway. Truncation analysis localized the apoptotic determinant to the N-terminal 1-34 aa of PCV3 Cap and heterogeneous nuclear ribonucleoprotein K (hnRNP K) was identified as a host protein that binds to PCV3 Cap. Overexpression of hnRNP K reduced PCV3 Cap-induced release of Cyt-c into the cytoplasm, implying a regulatory role in apoptosis. Based on structural modelling and molecular docking, amino acids at sites 24 and 27 of Cap from PCV3 variants, which define genotypes (PCV3a/b/c), affected binding with hnRNP K. Specifically, PCV3c Cap (V24/K27 and V24/R27) had higher affinity than PCV3a Cap (A24/R27) or PCV3b Cap (A24/K27), consistent with its superior apoptosis-inducing capacity compared to PCV3a/b variants, highlighting the importance of Cap interactions with hnRNP K. In summary, we identified novel molecular determinants of PCV3 pathogenesis that will inform development of vaccines and diagnostics.