Membrane structure and fusion-triggering conformational change of the fusion domain from influenza hemagglutinin

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The N-terminal domain of the influenza hemagglutinin (HA) is the only portion of the molecule that inserts deeply into membranes of infected cells to mediate the viral and the host cell membrane fusion. This domain constitutes an autonomous folding unit in the membrane, causes hemolysis of red blood cells and catalyzes lipid exchange between juxtaposed membranes in a pH-dependent manner. Combining NMR structures determined at pHs 7.4 and 5 with EPR distance constraints, we have deduced the structures of the N-terminal domain of HA in the lipid bilayer. At both pHs, the domain is a kinked, predominantly helical amphipathic structure. At the fusogenic pH 5, however, the domain has a sharper bend, an additional 310-helix and a twist, resulting in the repositioning of Glu 15 and Asp 19 relative to that at the nonfusogenic pH 7.4. Rotation of these charged residues out of the membrane plane creates a hydrophobic pocket that allows a deeper insertion of the fusion domain into the core of the lipid bilayer. Such an insertion mode could perturb lipid packing and facilitate lipid mixing between juxtaposed membranes.

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Figure 1: CD spectra of the influenza HA fusion domain bound to DPC micelles (solid line) or lipid bilayers composed of POPC:POPG (4:1) (dashed line) at pH 5 (black) and pH 7.4 (red).
Figure 2
Figure 3: Structures of the influenza HA fusion domain in DPC micelles determined at pH 5 and 7.4 by 1H-NMR.
Figure 4: Representative ATR-FTIR spectra showing the effect of spin labeling on the structure of fusion domains bound to lipid bilayers.
Figure 5: Measured depths of 18 residues of the influenza HA fusion domain in lipid bilayers at pH 7.4 and 5.
Figure 6: Docking of the NMR structures to the EPR depth data.

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We thank K. Victor for his expert advice on the EPR measurements and fitting procedures. This work was supported by the National Institutes of Health.

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Correspondence to Lukas K. Tamm.

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Han, X., Bushweller, J., Cafiso, D. et al. Membrane structure and fusion-triggering conformational change of the fusion domain from influenza hemagglutinin. Nat Struct Mol Biol 8, 715–720 (2001) doi:10.1038/90434

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