The decrease of the intrinsic tryptophan fluorescence intensity of purified influenza (X31 strain) hemagglutinin (HA) was used to monitor the low pH-induced conformational change of this protein. The kinetics of the fluorescence decrease depended strongly on the pH. At pH optimal for fusion, the change in tryptophan fluorescence was fast and could be fitted to a monoexponential function. We measured a rate constant of 5.78 s-1 (t1/2 = 120 ms) at pH 4.9 using rapid stopped-flow mixing. Under suboptimal conditions (higher pH), the rate constant was decreased by an order of magnitude. In addition, a slow component appeared and the fluorescence decrease followed a sum of two exponentials. The kinetics of conformational changes were compared with those of the fusion of influenza virus with red blood cell membranes as assessed by the R18-dequenching assay. At optimal pH the HA conformational change was not rate-limiting for the fusion process. However, at sub-optimal pH, the slow transition to the fusogenic conformational of HA resulted in slower kinetics and decreased extent of fusion.
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