Porcine aminopeptidase N
Porcine aminopeptidase N (pAPN), a protein of 963 aa widely distributed in the small intestine and kidney of pigs, can be decomposed into 2 subunits by trypsin, a 95 kDa of N-terminal and a 50 kDa of C- terminus, while the positions of 717-813aa contain 3 antigen neutralizing sites. The functions or roles of pAPN in the pathogenic mechanisms of PEDV or TGEV infection have been extensively studied in recent years. Through screening and identification, the main antigen functional regions of pAPN are determined to be located at the positions of 36-153aa, 349-591aa and 592-963aa, respectively (Bo-qi, Guang-xing, & Xiao-feng, 2009). Although the conclusion of the receptor role for pAPN in PEDV infection remains elusive, this suggestion has been supported by the facts that pAPN can bind to the S1 region of the S PEDV protein (BX, JW, & YJ, 2007), and the ST cells efficiently expressing pAPN through gene recombination can support the PEDV infection and proliferation, which also is closely associated with the distributed density of pAPN (N. E & C, 2010). However, other studies indicated that the presence of pAPN does not render Vero cells susceptible to PEDV infection (JS, DS, & BK, 2003) and the capability of pAPN to enhance the infectivity of PEDV is related to its aminopeptidase activity rather than its receptor role (S. K et al., 2016). Whether this discrepancy is only due to various cell lines used in these studies since the results obtained in ST cells can not be replicated in Hela cells, or whether the existence of other routes of PEDV infection results in variable consequences, remains to be further classified.