The S protein of PEDV
This S gene encoded spike (S) protein is a type I glycoprotein, while three S proteins of PEDV forming a rod-shaped functional spike trimer located in the outermost layer of virus particles, are the foundation for its multi-function, such as the binding of PEDV to other molecules, the entry of virus into target cells (L. C et al., 2015; F. F et al., 2017). Based on the homology analysis of its counterparts of other coronaviruses, the S protein of PEDV can be divided into two parts: S1 (1-789 aa) and S2 (790-1383 aa). The function of the S1 protein is to mediate the viruses to adsorb the viral receptors on the host cells, while the role of the S2 is to induce membrane fusion, thereby facilitating the viruses to invade the host cells (Wrapp D & JS, 2019; L. F, 2012, 2015). Moreover, the binding of the S1 with receptors on the host cell surface can incur the conformation change and fusion of the S2 with the cell membrane. Indeed, previous results have confirmed the interaction of the S protein with the target cell surface receptor(s), which is mediated by the S1 domain NTD (L. C et al., 2015; D. F et al., 2016). Besides these, the involvement of the S protein in trypsin-dependent PEDV propagation in cultured cells has been suggested (L. W, FJM, Q, PJM, & BJ, 2016). This effect of the S protein is considered to be exerted by proteolytic activation by trypsin or other proteases following its binding to receptor(s) (L. C et al., 2016; O et al., 2014), which is necessary for the membrane fusion, formation of syncytium, cell entry of the virus, thereby enhancing infectivity of PEDV (JE, DJ, & HJ, 2011). This is also supported by the S protein mutations influencing the proteolytic cleave of it (L. W et al., 2015; K. Y, C, V, RA, & KO, 2017). Additionally, a determinant role for the S protein in efficient viral release from target cells has also been proposed, while compelling evidence are required to substantiate this concept (O et al., 2014). It becomes clear that the receptor binding capability and the role in viral entry allow the S protein to determine PEDV invasion and release, tissue tropism, host range and cross-species transmission, even to affect trypsin-dependent PEDV proliferation (L. C et al., 2016; O et al., 2014; L. W et al., 2015). Therefore, the property of the S gene prone to mutation often leads to pathogenic alterations of this virus (C. F et al., 2015; H. Y et al., 2017). The S protein contains a variety of antigenic epitopes and plays an important role in inducing the body to produce neutralizing antibodies (Song D & B, 2012). In view of this vital role of the S protein, it is widely used for the development of PEDV subunit vaccine, genetic engineering vaccine and PEDV antibody detection kit (CY et al., 2019; L. H et al., 2018).