Figure 7. N55T provides a ‘fulcrum release’ to enable recognition of group 1 and group 2 HA stems.

(A) Overlay of the 09–1B12-UCA and mature 09–1B12 in complex with H3 (A/Perth/16/2009; see also Figures 1B,C, S1, Tables S1, S2) illustrates the angular change where the HC and LC pivot around the QxxV fulcrum (=flexibility around the fulcrum). (B) N55 forms more hydrogen bonds with S52, N57, and Q102 in the CDRH2 and CDRH3 loops (visualized using ChimeraX). By contrast, T55 shows substantially fewer interactions to better accommodate changes in the binding angles. (C) MD simulations on the Fabs alone (without HA) show the percentages of interactions between paired residues (S52, N/T55, N57, and Q102) during simulations. (D) The cryoEM map of 09–1B12-UCA + N55T and overlayed structures of 09–1B12-UCA + N55T/09–1B12 in complex with H1 (A/Michigan/45/2015) (3.5 and 3.3 Å, respectively). A rotational tilt is imposed by group 1 IAV N-glycans at positions N289, N278, and N33 from the neighboring protomer interacting with the antibody LC. (E) N55T alone enables this tilting around the QxxV fulcrum (which we term fulcrum release) as visualized by overlay of H3 + 09–1B12-UCA co-complex with H1 + 09–1B12-UCA-N55T co-complex. (F) Fulcrum release as seen in the mature antibody as visualized by overlay of H3 + 09–1B12 co-complex with H1 + 09–1B12 co-complex. Arrows indicate the flexible antibody tilting enabled by fulcrum release to accommodate the conserved group 1 IAV glycan positions. See also Figure S1, Tables S1, S2.