The 9b accessory protein in SARS-CoV has peculiar membrane-binding functions and is also seen to play a significant role in the suppression of our innate immune response by targeting the mitochondria. Antibodies against the 9b accessory protein were observed in individuals with SARS-CoV. 

Observed Functions of the 9b Accessory Protein (mainly in SARS-CoV)

The ORF coding the 9b accessory protein is present in the Nucleocapsid gene and is comprised of 98 amino acids. While the function of the 9b accessory protein in SARS-CoV2 is not exactly known, a relative idea of its function can be obtained by understanding its role in SARS-CoV as the 9b sequence is heavily conserved in other SARS isolates (indicating that it probably has a very similar function). the 9b sequence is heavily conserved in other SARS isolates.

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Accessory proteins play important roles as virulence factors affecting pathogenicity, release, and stability of SARS-CoV2 and aid in infection, replication, and transmission of the virus between individuals. The SARS-CoV2 accessory proteins are 3a, 3b, p6, 7a, 7b, 8b, 9b, and ORF14.

Accessory Proteins of SARS-CoV2

BARUAH, C., DEVI, P., & SHARMA, D. K.. (2020). Sequence Analysis and Structure Prediction of SARS-CoV-2 Accessory Proteins 9b and ORF14: Evolutionary Analysis Indicates Close Relatedness to Bat Coronavirus (Version 1). ChemRxiv. https://doi.org/10.26434/chemrxiv.12424958.v1

Sequence Analysis and Structure Prediction of SARS-CoV-2 Accessory
Proteins 9b and ORF14: Evolutionary Analysis Indicates Close
Relatedness to Bat Coronavirus

In regards to amino acid sequence and composition of proteins, SARS-CoV2 bears many resemblances and similarities with SARS-CoV but there are certainly significant differences as well:
- The 8a accessory protein is absent in SARS-CoV2 but is present in other SARS-CoV.
- The 8b accessory protein’s amino acid sequences is longer in SARS-CoV2 with 121 amino acids compared to the 84 amino acids in other SARS-CoV.
- The 3b accessory protein amino acid sequence is considerably shorter in SARS-CoV2 with just 22 amino acids compared to the 154 amino acids seen in other SARS-CoV.


Amino Acid Differences of Accessory Proteins between Sars-CoV2 and SARS-CoV

9b Accessory Protein of SARS-CoV2

Similar to the ORF 9b protein, the function of the ORF14 accessory protein is not exactly known but it is believed to play a potential role in the interaction between the host cells and the virus. This protein has been seen in SARS-CoV2, SARS-CoV, and SARS-like coronavirus Rp3 (a bat coronavirus). 


ORF14 Accessory Protein of SARS-CoV2

Similar to other accessory proteins, ORF3a plays an important role in the virulence of SARS-CoV2 including affecting the host antiviral response. ORF3a codes for a protein that is made of 274 amino acids and three regions: an ion channel, CBM (Caveolin Binding Motif), and TNF Receptor Associated Factor (TRAF). 

Structure of ORF3a Accessory Protein

Apoptosis, which is essentially cell death that is triggered by certain factors, is an important mechanism in the antiviral immune response as it allows the killing of infected cells which aids in limiting and controlling the infection as well as playing a regulatory role in inflammation. Multiple accessory proteins seem to affect apoptosis of host cells and contribute to viral pathogenesis by doing so, making apoptosis an overlapping and overarching concept regarding accessory protein of SARS-CoV2.

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