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![[Figure 3]](cb5127fig3mag.jpg)
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Figure 3
(a) Cryo-EM map at 2.9 Å resolution of the full-length SARS-CoV-2 orf3a tetramer expressed in S. frugiperda and reconstituted in lipid nanodiscs (Kern et al., 2020  ; EMDB entry EMD-22136). The backbone chain of one of the protomers docked into the cryo-EM map is outlined in burgundy. The horizontal blue lines mark the limit between the upper TM domain and the lower cytoplasmic domain of the tetramer formed by multiple β-sheets. (b) Ribbon model of the SARS-CoV-2 orf3a tetramer derived from cryo-EM studies of the protein reconstituted in lipid nanodiscs. Each dimer possesses six TM helices and a cytoplasmic domain with predominantly β secondary structure. The two dimers in the tetramer are purported to be covalently joined by a disulfide bond formed by homologous Cys133 residues in each monomer (Marquez-Miranda et al., 2020). The molecular-modelling study by these authors revealed the presence of a chloride ion site in the channel lumen. (c) Ribbon models of the SARS-CoV-2 orf3a dimer: an ∼70 Å cylinder in a lateral view (left) and an end-on perspective as viewed from the extracellular side (right) derived from cryo-EM studies of the protein reconstituted in lipid nanodiscs (Kern et al., 2020). Each protomer of the dimer has three helices that can fully traverse a lipid bilayer (∼40 Å) and a 30 Å-long cytoplasmic domain with predominantly β secondary structure. The projection of the dimer onto the membrane plane is elliptic, with a major axis of ∼50 Å in width. The central ion path is flanked by TM segments 1 and 2, as seen in the centre of the end-on view. Images were produced using CCP4mg. |
![[Figure 3]](cb5127fig3.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
