Mapping of the nuclear localization signals in open reading frame 2 protein from porcine circovirus type 1

Porcine circovirus type 1 (PCV1) contains two major open reading frames encoding the replication-associated proteins and the major structural capsid (Cap) protein. PCV1 Cap has an N-terminus carrying several potential monopartite or bipartite nuclear localization signals (NLS). The contribution of these partially overlapping motifs to nuclear importing was identified by expression of mutated PCV1 Cap versions fused to enhanced green fluorescent protein (EGFP). The C-terminus truncated PCV1 Cap-EGFP was localized in nuclei of PK-15 cells similar to the wild-type PCV1 Cap-EGFP, whereas truncation of the N-terminus rendered the fusion protein distributed into cytoplasm, indicating that the nuclear import of PCV1 Cap was efficiently mediated by its N-terminal region. Substitutions of basic residues in stretches ₉RR-RR₁₂ or the right part of ₂₅RRPYLAHPAFRNRYRWRRK₄₃ resulted in a diffused distribution of the fusion protein in both nuclei and cytoplasm, indicating that the two NLSs were responsible for restricted nuclear targeting of PCV1 Cap.