Extended Data Fig. 3. Flexibility of the membrane-distal domains.

The membrane-distal regions of HA1 (roughly residues 45–310) can adopt a range of different orientations compared with the C3 symmetrized structure (state IV). The remainder of HA1 and HA2 adopt a structure with less evidence of flexibility. We examined the flexibility in ten maps generated by asymmetric classification. a, The generated models are shown aligned to the symmetrized version of the protein (yellow). b, Examples of the most-populated classes are shown on a molecular surface, with HA1 in blue and HA2 in red to emphasize the different locations of these domains when compared with the symmetrized version. c, The displacements of the locations of the centroids of each of these mobile domains were measured to the nearest symmetrized monomer, giving 30 data points. These adopt a range of locations, with the main flexibility being in a rotation angle around the threefold axis, with little lateral movement towards and away from the threefold axis. The angles of displacement vary from −15° to +25°.