Fig. 4: Cryo-EM structures of RBD-down S-proteins reveal differential stabilization of domain positions.

a, Cryo-EM reconstruction of u1S2q colored by chain. b, Rotated view (by ~90°) of a. c, Magnified view of the region containing the mutations, showing the proximity of the F855Y and N856I residue loop to the S2 residue L966 and S1 residue P589. d, Similar region to that shown in c but for the unmutated structure (PDB 6VXX). e, Magnified view of the region containing the mutations at a different angle than the one shown in c. f, The loop containing the A570L and T572I mutations, with the cryo-EM map shown as a transparent surface and fitted model shown in cartoon representation. g,h, Similar region to that shown in e but for the rS2d structure (PDB 6X29) (g) and the unmutated structure (PDB 6VXX) (h). i,j, Overlay of the three down state structures (i) and with the cryo-EM reconstruction for the down state for the u1S2q construct (j).