Extended Data Fig. 1. Features of intermediate states.

a, Displacement measurements between dilated HA1 domains. Measurements are the position of the centroid of HA1 residues 45–310, with structures aligned on the invariant parts of HA2 (residues 76–98). b, An interaction can be seen between the glycan attached to Asn165 (orange) and Trp222 (green) on the neighbouring HA1 monomer (blue), crosslinking the HA1 domains. This is present in states I, II and III. c, Cryo-EM density (grey) and model, with HA1 in blue and HA2 in red, for the short helix of HA2 (residues 38–55) and the 30-loop (HA1 residues 22–37). Density extends beyond the top of the helix, indicating elongation of this helix by residues from the interhelical loop. d, Example of typical density (grey) for the same section of an HA2 helix from states I and IV (green).