The tripartite motif protein 21 (TRIM21) belongs to the TRIM family, possessing an E3 ubiquitin ligase activity. Similar to other TRIMs, TRIM21 also contains three domains (named RBCC), including the Really Interesting New Gene (RING) domain, one or two B-Box domains (B-Box), and one PRY/SPRY domain. Notably, we found that the RING and B-Box domains are relatively more conservative than the PRY/SPRY domain, suggesting that TRIM21 of different species had similar functions. Recent results showed that TRIM21 participates in virus infection by directly interacting with viral proteins or modulating immune and inflammatory responses. TRIM21 also acts as a cytosol high-affinity antibody Fc receptor, binding to the antibody–virus complex and triggering an indirect antiviral antibody-dependent intracellular neutralization (ADIN). This paper focuses on the recent progress in the mechanism of TRIM21 during virus infection and the application prospects of TRIM21 on virus infection.

Porcine circovirus disease (PCVD) caused by porcine circovirus type 2 (PCV2) is widely distributed in pig farms. Up until now, nine genotypes of PCV2, PCV2a to 2i, have been identified in diseased pigs worldwide. This study analyzed 302 samples collected in the Jilin Province of China from 2016 to 2021, followed by genetic analysis of the PCV2 isolates. Meanwhile, the antigen epitopes, amino acid mutations, 3D structure of the PCV2 isolates and commercially available vaccine strains were evaluated and compared. The results showed that the predominant genotypes of PCV2 were PCV2b, followed by PCV2e and PCV2d in Jilin Province during 2016–2021. Although mutations were detected in the isolates, no recombination occurred in the PCV2 isolates, indicating a stable genotype of PCV2 in Jilin Province during these years. Moreover, the B cell epitopes in the Cap and Rep proteins of eighteen PCV2 isolates and T cell epitopes in the Cap of the isolates were changed compared to three currently used vaccine strains. The mutations in the Cap and Rep proteins did not affect their spatial conformation. Therefore, bivalent or multivalent vaccines with different genotypes of PCV2 might improve the protective effect of vaccines.

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, ⁴RSRYSRRRRNRRNQRRRGLWPRASRRRYRWRRKN³⁷. 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 ⁴RSRYSRRRRNRRNQRR¹⁹ and ²⁴PRASRRRYRWRRK³⁶, 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 ⁴RSRY⁷ 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.