FIG 6.

A histidine in the 205-72 pair is important for HA stability and virus infectivity. (A) HA pseudoviruses were treated with acidic pH and digested with TPCK-trypsin. Nonreducing Western blots were used to compare TPCK-trypsin-digested Bris HA pseudoviruses containing A-K, Q-K, Y-K, D-K, and F-K pairs. Full-length HA (HA0) and HA fragments (disulfide-bonded HA1 and HA2 proteolytic fragments) were detected with rabbit antisera against the HA2 C helix. (B) The contents of the undigested HA in panel A were quantified at different pHs. (C) Infectivities of HA pseudoviruses containing the A-K, Q-K, Y-K, D-K, and F-K pairs. (Bottom) The levels of mature HA (HA2) in pseudoviruses shown by Western blotting were detected with rabbit antisera against the HA2 C helix. Data are shown as the mean and standard deviation from three independent experiments. *, t test, P < 0.05.