3.3. Orf3a
The accessory protein orf 3a of SARS-CoV-2 contains 275 amino acids and its gene (25393..26220) is located between the spike and the E protein genes. The orf3a proteins of SARS-CoV contains 274 while the NS3 of BatCoV RaTG13 is composed of 275 amino acids (Table.1). The amino sequence alignment of the orf 3a of SARS-CoV-2 and SARS-CoV showed that the sequence identity is 72.4% and the sequence similarity is 85.1%. The similarity percentage of the orf3a of SARS-CoV-2 and SARS-CoV was 90.2 rather than 85.1% depending on the report of Yashimito (2020) [12] which may be due to the different software programs used by the two studies. The orf3a (SARS-CoV-2) and NS3 (BatCoV RaTG13 ) are characterized by 97.8% identity and 98.9% similarity (Table.2).
The orf 3a plays different roles for the virus including: 1) viral envelope assembling; 2) host cell binding and infusion through interacting with the structural proteins (M, S, and E) and the accessory protein (7a) of SARS-CoV [24]. In the host organisms, the highest immunogenicity of the N-terminal of orf3a is well known with strong protective effect on the humoral immunity [25]. The orf3a ha a cysteine rich domain which possess a potassium ion channel activity through interacting with the S and E protein [24,26]. The C- terminal of the orf3a arrests the host cells cell cycle through depleting the cyclin D3 and it facilitates the apoptosis of the host cells through interacting with the M protein [27-29].