Extended Data Fig. 1: Expression and cell-cell fusion of the S protein from Omicron BA.2.

(a) Schematic representation of a full-length Omicron BA.2 spike (S) protein. The sequence is derived from an Omicron BA.2 subvariant (hCoV-19/Denmark/DCGC-327158/2022). Segments of S1 and S2 include: NTD, N-terminal domain; RBD, receptor-binding domain; CTD1, C-terminal domain 1; CTD2, C-terminal domain 2; 630 loop, residues 620-640; S1/S2, the furin cleavage site at the S1/S2 boundary; S2’, S2’ cleavage site; FP, fusion peptide; FPPR, fusion peptide proximal region; HR1, heptad repeat 1; CH, central helix region; CD, connector domain; HR2, heptad repeat 2; TM, transmembrane segment; CT, cytoplasmic tail; and tree-like symbols for glycans. Positions of all mutations (from the amino-acid sequence of Wuhan-Hu-1) are indicated and those highlighted in red rectangles are also present in at least one of the previous VOCs. (b) Expression and processing of the full-length S constructs of various variants in HEK293 cells. S protein samples prepared from HEK293 cells transiently transfected with 10 μg of the full-length S expression plasmids were detected by anti-RBD polyclonal antibodies. Bands for the uncleaved S and S1 fragment are indicated. (c) HEK293T cells transfected with the untagged, full-length S protein expression plasmids were fused with ACE2-expressing cells. Cell-cell fusion led to reconstitution of α and ω fragments of β-galactosidase to form an active enzyme, and the fusion activity was then quantified by a chemiluminescent assay. No ACE2 and no S were negative controls. Error bars were generated with measurements on three biologically independent samples. All the experiments have been repeated at least twice with similar results.

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