3.2. The spike protein
The spike protein of SARS-CoV-2 contains 1273 amino acids while the spike protein of SARS-CoV contains 1255 amino acids and that of the BatCoV RaTG13 contains 1269 amino acids (Table 1). The spike protein of SARS-CoV-2 and SARS-CoV has identity percentage of 76% and similarity percentage of 86% (Table.2). The identity and similarity percentages of the spike proteins of SARS-CoV-2 and RaTG13 are 97.4% and 98.4%, respectively (Table.2). The identity and similarity percentages of the spike proteins of SARS-CoV-2 and RaTG13 are higher than those of SARS-CoV-2 and SARS-CoV.
The spike protein of the coronaviruses consists of three polypeptide chains with two domains; S1 and S2. The S1 and S2 domains are responsible for binding the host cell receptors (S1) and for fusing of the virus with the membrane of the host cell. There is a hinge region between the S1 and S2 which is a target for the host cells proteases [21,22]. The spike protein of SARS-CoV-2 has a furin cleavage site in the hinge region. The furin cleavage site is composed of four amino acids (681- 684). The presence of the furin cleavage site may be responsible for the high transmission rate of SARS-CoV-2 compared to the other coronaviruses [23].