FIG 3.

The R-K pair at the 205-72 pair destabilizes the HA prefusion structure at fusogenic pH. HA pseudoviruses were treated at acidic pH and digested with TPCK-trypsin. Nonreducing Western blots of TPCK-trypsin-digested Bris (A) and chimeric Mex.HA1-Bris.HA2 (B) HA pseudoviruses are shown. (Bottom) The undigested HA contents shown in the Western blots at different pHs were quantified. Full-length undigested HA (HA0) and HA fragments (disulfide-bonded HA1 and HA2 proteolytic fragments) were detected with rabbit antisera against the HA2 C helix.