Spike Protein (SP)

Spike protein is also called S protein, it is an important protein in Coronavirus, its main function is to fusion the virus membrane with host plasma membrane. 

The spike protein (S protein) is a large type I transmembrane protein ranging from 1,160 amino acids for avian   infectious bronchitis virus (IBV) and up to 1,400 amino acids for feline coronavirus (FCoV). In addition, this protein is highly glycosylated as it contains 21 to 35 N-glycosylation sites. Spike proteins assemble into trimers on the virion surface to form the distinctive "corona", or crown-like appearance.

In SARS-CoV-2, spike protein composed of two subunits, S1 and S2 subunits. S1 subunit hypervariable region is closely related to the infection of coronavirus. S1 subunit contains two subdomains: N and C terminal domain. Both are able to function as "receptor binding domains" (RBD), which is responsible to recognize the cell surface receptor on host cell membrane. It's reported that spike protein of SARS-CoV-2 can infect the human respiratory epithelial cells by acting on human ACE2 receptor.