This β-structure region is predicted for the E protein in SARS, SARS-2, and other coronaviruses. However, no regions of the extended β-strand were detected, despite the predicted presence of a β-turn-β motif in the C-terminal region around a conserved Pro residue. Other residues (37%) are variously assigned to the coil, bend, or loops and turns. In the latter, E-TR (65 residues) showed 27 residues, strictly α-helical ( (accessed on 15 October 2022)), and 9 more were assigned to the 3–10 helices therefore, a total of 36 residues are ‘helical’ (63%). Information regarding the extramembrane domains of SARS E has been obtained by solution NMR using a DPC:SDS mixture or LMPG (1-myristoyl-2-hydroxy-sn-glycero-3-phospho-(1′-rac-glycerol)) detergent. The structure of E-TM (identical in SARS and SARS-2) is completely α-helical, as shown by studies in detergent dodecylphosphorylcholine (DPC), lipid membranes formed by 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and, more recently, using solid-state NMR and endoplasmic reticulum–Golgi intermediate compartment (ERGIC)-like membranes. Viruses bearing these mutations regained virulence, and, concomitantly, the E protein regained ion channel activity. Escape mutations were observed only in the TMD of the E protein, which is responsible for channel activity. In contrast, the wild-type E protein led to the inflammasome activation and elevated production of IL-1β, lung epithelial cell damage, and death. When these were introduced into a mouse-adapted SARS virus, they led to mice survival, which exhibited reduced lung edema and lower proinflammatory cytokine levels. The link between pathogenicity and channel activity was triggered after the discovery of channel-inactivating mutations at the transmembrane domain (TMD) of the E protein, N15A, and V25F. Our lab and others have shown that E protein forms ion channels, using full-length E (E-FL), its transmembrane domain (E-TM), or a truncated form (E-TR, residues 8–65). SARS E protein is 76 amino acids long and localizes to the endoplasmic reticulum–Golgi intermediate compartment (ERGIC) of infected cells, where the C-terminus is cytoplasm-oriented. In SARS, the envelope E protein is a well-known virulence factor. Due to its proximity to the channel, this β-structure domain could modulate channel activity or modify membrane structure at the time of virion formation inside the cell. Therefore, we conclude that E monomer–monomer interaction triggers formation of the β-structure from an undefined structure (possibly β-turns) in at least about 15 residues located at the C-terminal extramembrane domain. However, the latter only showed β-structure formation at the highest concentration tested, while having a weaker oligomerization tendency in detergents than in full-length E protein. The results were similar with a mixture of POPC:POPG (2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine/3-glycerol) and also when using an E-truncated form (residues 8–65). The full-length E protein at high protein-to-lipid ratios produced a clear shoulder at 1635 cm −1, consistent with the β-structure, but this was absent when the E protein was diluted, which instead showed a band at around 1688 cm −1, usually assigned to β-turns. Herein, we have studied the conformation of purified SARS-CoV-2 E protein in lipid bilayers that mimic the composition of ER–Golgi intermediate compartment (ERGIC) membranes. However, models of the extramembrane domains have only been obtained from solution NMR in detergents, and show no β-strands, in contrast to secondary-structure predictions. Structures for the latter are available in both detergent and lipid membranes. In SARS-CoV-2, the channel-forming envelope (E) protein is almost identical to the E protein in SARS-CoV, and both share an identical α-helical channel-forming domain. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the current COVID-19 pandemic.
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