ORF7a

ORF7a is a type I transmembrane protein, localized mainly in ER, Golgi apparatus and in the cell surface. ORF7a is able to prevent virus tethering at the plasma membrane by BTS-2. Further in vitro experiments showed that ORF7a is able to induce apoptosis in a caspase dependent manner. ORF7a protein is very conserved between SARS-CoV-1 and SARS-CoV-2 with 85% sequence identity. The substitutions  may affect ORF7a interaction with other viral or host protei

Narrative

ORF7a is an accessory protein of coronaviruses and it is not essential for viral replication in vitro (Schaecher et al. 2007). ORF7a is a type I transmembrane protein, localized mainly in Golgi apparatus and in the cell surface (Nelson et al. 2005; Taylor et al. 2015). Besides, ORF7a colocalizes with calnexin (ER marker), showing that it is also localized at ER (Nelson et al. 2005).

ORF7a is an antagonist of bone marrow stromal antigen 2 (BST-2/CD317/tetherin) (Taylor et al. 2015). BST-2 is a pre-B-cell growth promoter that inhibits virus release by tethering budding virions to the host cell membrane (Sauter, Specht, and Kirchhoff 2010). A greater virion tethering to the cell membrane is observed when ORF7a is not present (Taylor et al. 2015). ORF7a is usually located in the Golgi apparatus and it relocates to the plasma membrane when BST-2 is expressed, colocalizing it (Taylor et al. 2015). ORF7a binds to BST-2 and reduces restriction activity of BTS2 by preventing its glycosylation (Taylor et al. 2015). Further in vitro experiments showed that ORF7a is able to induce apoptosis in a caspase dependent manner (Schaecher et al. 2007).

Structural analysis and comparison with SARS-CoV-1 ORF7a – SARS-CoV-2 ORF7a protein structure has a 15 amino acid (a.a.) N-terminal signal peptide, a 80-a.a. luminal domain, a 21-a.a. transmembrane domain, and a 5-a.a. cytoplasmic tail (Nelson et al. 2005). Residues Lys¹¹⁷, Asn¹¹⁸ and Lys¹¹⁹ form a motif that has been described as being recognized by COPII vesicular system implicated in the transport of proteins from endoplasmic reticulum (ER) to Golgi and they are required to exit the ER (Bickford, Mossessova, and Goldberg 2004).

ORF7a structure is composed of seven antiparallel β-sheets that altogether make a β-sandwich. The ORF7a is very conserved between SARS-CoV-1 and SARS-CoV-2 with 85% sequence identity. A total of 18 variations are verified, with 11 non-conservative substitutions and a deletion. Substitutions in the luminal domain are located in the protein surface. Therefore, the ORF7a fold is conserved, but these substitutions may affect ORF7a interaction with other viral or host proteins. Mostly conservative mutations are verified at ORF7a C-terminal (83 – 121), except by Ile¹¹¹Thr.