Sialic acid
It is recognized that a large number of the main receptors for many viruses belong to glycoprotein or glycolipid, with sialic acid or sialic acid derivatives at the end, such as those of pathogenic influenza A, B, C and parainfluenza virus (S. H et al., 2016). In this regard, the ability of PEDV binding to sialic acid, such as carbohydrates (Neu5Ac) and proteins especially glycoproteins or glycolipid molecules located at the cell surface, has also been confirmed in previous studies (K. F & G, 1993; P. G et al., 2012). Moreover, using glycoproteins as receptors is important strategy for viral intestinal pathogenicity because the properties of glycoproteins facilitate viral binding to mucin on the surface of epithelial cells in animal viscera (W. Jh, 2002). The effort of Daniel Wrapp et al. to parse the prefusion conformation of the PEDV S protein using cryo-electron microscopy at a resolution of 3.1 Å revealed the presence of the sialic acid-binding domain at the N terminal of the S1 subunit (Wrapp D & JS, 2019). Recognition of sugars as co-receptors for PEDV appears to be a strategy for adapting organisms to this class of diarrhea-causing viruses, suggesting that the binding of PEDV to sialic acid facilitates to survive in adverse intestinal conditions. Further in-depth investigation indicated that the S1-434, S253-533 fragments of PEDV weakly while the S19-638 fragment strongly binds to the pAPN expressing cells, suggesting the occurrence of recognition of pAPN as a receptor by the C-terminal region of the PEDV S1 protein and the capability of recognition of other cellular molecules like sugars by the N-terminal region of the S1 subunit (D. F et al., 2016). Thus, PEDV may use sugars as receptors or co-receptors, as similar to TGEV using Neu5Gc and Neu5Ac as co-receptors (P, CF, & S, 2006). The next works focusing on what exact sugar molecules are utilized by PEDV as co-receptors are of significance.