The ORF3 of PEDV
As a unique feature to other coronaviruses such as TGEV and SeCoV, the sequence located between the PEDV S and E genes is responsible for encoding the nonstructural ORF3 protein. It consisting of 224 amino acids contains 4 transmembrane regions and forms a homologous tetramer structure. Notably, the ORF3 gene remains relatively conservative in wild strains, while the in vitro transmission of PEDV incurs deletion or mutation of it, thereby resulting in incomplete expression of the ORF3 protein (C. F et al., 2015; SJ, HK, DS, HJ, & BK, 2011). As an accessory protein, the ORF3 is generally believed not to play a role in the reproduction process of PEDV, therefore, the ORF3 gene is often replaced by a marker gene and other genes in reverse genetics. However, a series of investigations on the ORF3 have revealed that this is not the case. The bioinformatic analysis indicated that the complete ORF3 protein can form ion channels, the evidence supporting this stems from the fact that silencing the ORF3 gene would lead to a decrease of virus titer, suggesting that it may play a regulatory role in the process of PEDV infection (W. K et al., 2012). Moreover, Ye et al found that the ORF3 gene of PEDV can promote the formation of vesicle structure, prolong the S-phase of target cells, and enhance the proliferation of the attenuated strain expressing a truncated ORF3, indicating its implication in the replication process of PEDV (Y. S et al., 2015). Furthermore, Challika et al provided more direct evidence to demonstrate that the interaction of the PEDV ORF3 with the cellular vacuolar protein-sorting-associated protein 36 (VPS36) inhibits virus replication (K. C, Y, S, & A, 2019). These studies highlight the important roles of the ORF3 gene/protein in the PEDV infection.