Fig. 3: Discovery and characterization of the 540-subunit GI₉-F7 nanocage.

a, A cryo-electron micrograph showing GI₄-F7 and GI₉-F7 nanocages in the same preparation. Scale bar, 50 nm. b, Design model of GI₉-F7, constructed from 12 pentasymmetrons, 60 CCC homotrimers and 30 disymmetrons. A subunits, cyan; B subunits, magenta; C subunits, purple. c, Cryo-EM map of GI₉-F7 at 6.7 Å resolution. Scale bar, 71 nm. d, Comparison of a model derived from the cryo-EM map (grey) with the computational design model (other colours), aligned using a single asu (shown in cartoon). The three independent copies of the B:C interface in the asu are indicated. e, Alignment of int 1 (light grey), int 2 (medium grey) and int 3 (dark grey) from the cryo-EM model. f, Alignment of two neighbouring copies of AAB heterotrimers from the cryo-EM model to the design model. The two independent copies of the A:A (I3-01) interface, located in the pentasymmetron (int 4) and the disymmetron (int 5), are indicated. g, Alignment of the two A:A (I3-01) interfaces from the cryo-EM model, int 4 (light grey) and int 5 (medium grey). h, SDS–PAGE of antigen-bearing GI₄-F7 and GI₉-F7 nanocages. RBD–SpyTag (left lane) was conjugated to CCC–SpyCatcher in either GI₄-F7 or GI₉-F7 nanocages. Red arrowhead, conjugated CCC–RBD; black arrowhead, residual RBD–SpyTag; green arrowhead, A subunit from AAB and ABB and B subunit from BBB; blue arrowhead, B subunit from AAB and ABB. For gel source data, see Supplementary Fig. 5. i, Representative 2D class averages from negative-stain electron microscopy of RBD-conjugated GI₄-F7 or GI₉-F7 nanocages. j, Representative plot of Ca²⁺ flux induced by BCR signalling in RAMOS cells that stably express the SARS-CoV-2 spike-specific antibody COVA2-15 as an IgG BCR⁶⁰. Cell lines were stimulated with various antigens with 4 µg ml⁻¹ RBD after reading a 30 s baseline. Data are representative of two independent experiments.