Fig. 1: BUNV GPs exhibit a trimeric tripod-like architecture.

a Schematic of the M segment polyprotein constituted by Gn, NSm (non-structural protein) and Gc. The Gc protein is comprised of three sections: the head (blue), stalk (green) and floor (gold) regions. The fusion loop (fl; orange) resides in the floor region. Arrowheads indicate cleavage sites, black bars the predicted TMDs, numbers the amino acid residues, and ‘Y’ the glycans N60 (Gn, ♦) and N624 (Gc, *)^{6, 31, 32, 34}. b Cryo-ET tilt series were collected of BUNV virions (buffer: pH 7.3/no K⁺), then 3D tomographic reconstructions were calculated (14 tomograms). Slices through tomograms of two virions are shown (representative of 71 virions, full tomogram section in Supplementary Fig. 5d). c, d STA of over 20,000 GP spikes aligned through iterative refinements, generating a ~ 16 Å average (GS-FSC). Panels are sections through the electron density averages, showing a trimeric spike on top of the viral lipid bilayer (M; membrane). e, f Isosurface rendering of the GP trimer, identifying the three regions; head (light blue), stalk (light green) and floor (gold; where Gn also resides) on top of the viral membrane (grey). White arrowheads indicate a previously unresolved region in the floor. g, h The previously solved X-ray structures of the BUNV head (pdb: 6H3V; light blue) and SBV stalk (pdb: 6H3S; light green) domains⁶ were fitted into our model. The glycosylation site N624 is indicated (purple circle). i, j A model of the local lattice arrangement of BUNV GPs, emphasizing the two C3 symmetry axes; one forming a tripod, and one in the floor region where three neighbouring tripods connect. Scale bars: b = 100 nm, d = 5 nm; e, i = 2 nm.