The Nuclear Localization Signal of Porcine Circovirus Type 4 Affects the Subcellular Localization of the Virus Capsid and the Production of Virus-like Particles.

Porcine circovirus 4 (PCV4) is a newly identified virus belonging to PCV of the Circoviridae family, the Circovirus genus. We previously found that PCV4 is pathogenic in vitro, while the virus's replication in cells is still unknown. In this study, we evaluated the N-terminal of the PCV4 capsid (Cap) and identified an NLS at amino acid residues 4-37 of the N-terminus of the PCV4 Cap, 4 RSRYSRRRRNRRNQRRRGLWPRASRRRYRWRRKN37 . The NLS was further divided into two fragments (NLS-A and NLS-B) based on the predicted structure, including two α-helixes, which were located at 4 RSRYSRRRRNRRNQRR19 and 24 PRASRRRYRWRRK36 , respectively. Further studies showed that the NLS, especially the first α-helixes formed by the NLS-A fragment, determined the nuclear localization of the Cap protein, and the amino acid 4 RSRY7 in the NLS of the PCV4 Cap was the critical motif affecting the VLP packaging. These results will provide a theoretical basis for elucidating the infection mechanism of PCV4 and developing subunit vaccines based on VLPs.