PROS1 and infection: Among these, the FP, HR1, HR2, and TM domains are the key functional regions of the S2 subunit for the occurrence of virus-cell fusion and formation of fusion pore (22), but little is known about the function of the regions between the S1/S2 site and S2′ site for the prefusion conformation of S protein and its mediated virus assembly, egress, or infection.