Y-box-binding protein 1 stabilizes the capsid protein of porcine circovirus type 2 to promote viral replication.

Porcine circovirus type 2 (PCV2), a major causative agent of PCV2-associated diseases, poses a serious threat to the global swine industry. The stability of PCV2 capsid (Cap) protein is critical for viral replication; however, the underlying mechanisms regulating Cap stability during PCV2 infection remain poorly understood. In this study, co-immunoprecipitation assays were used to demonstrate that PCV2 Cap protein can maintain its own stability by binding to the cellular Y-box-binding protein 1 (YBX1) during viral infection. Furthermore, binding domain mapping experiments revealed two specific regions essential for this interaction: the N-terminal arginine-rich motif (ARM) of PCV2 Cap (amino acid sequence: 1MTYPRRRYRRRRHRPRSHLG20) and the amino acid residues 189RRRR192 within the C-terminal domain (CTD) of YBX1. Virus rescue experiments further confirmed that substituting all arginine residues with alanine in the N-terminal ARM of Cap completely abolished its binding to YBX1. Analysis of viral replication capacity showed that YBX1 promotes PCV2 replication by stabilizing the viral Cap protein. Additionally, knockdown of YBX1 significantly reduced the protein levels of PCV2 Cap, and this effect was reversed either by treatment with the proteasome inhibitor MG132 or by restoring YBX1 expression. Collectively, these findings demonstrate that YBX1 promotes PCV2 replication by directly interacting with the ARM of PCV2 Cap, thereby inhibiting the proteasomal degradation of Cap during viral infection.