FIG 5.

Negative-stain EM and 2D classification of Fab:BHA complexes under neutral and acidic conditions reveal structural similarities. (A) Electron micrographs and representative class averages of BHA at pH 7.5 and BHA that has been acidified for 10 min at pH 5.25. In response to acidic pH, BHA refolds into a postfusion conformation, forming small aggregates (rosettes) that are not amenable to 2D classification. (B) BHA bound by HC19 Fab at pH 7.5 maintains the prefusion form of the BHA trimer. After acidification for 10 min at pH 5.25, HC19 Fab appears to stabilize the BHA timer in a structure similar to that at neutral pH. (C) Both BHA bound by FI6v3 Fab at pH 7.5 and the Fab:BHA complex acidified for 10 min at pH 5.25 stabilize the BHA trimer and prevent refolding into a postfusion form. Bar = 20 nm.