Conformational intermediates and fusion activity of influenza virus hemagglutinin

Three strains of influenza virus (H1, H2, and H3) exhibited similar charact eristics in the ability of their hemagglutinin (HA) to induce membrane fusi on, but the HAs differed in their susceptibility to inactivation. The exten t of inactivation depended on the pH of preincubation and was lowest for A/ Japan (H2 subtype), in agreement with previous studies (A, Purl, F. Booy, R . W. Doms, J, M, White, and R. Blumenthal, J, Virol, 64:3824-3832, 1990), W hile significant inactivation of X31 (H3 subtype) was observed at 37 degree s C at pH values corresponding to the maximum of fusion (about pH 5.0), no inactivation was seen at preincubation pH values 0.2 to 0.4 pH units higher . Surprisingly, low-pH preincubation under those conditions enhanced the fu sion rates and extents of A/Japan as well as those of X31, For A/PR 8/34 (H 1 subtype), neither a shift of the pH (to >5.0) nor a decrease of the tempe rature to 20 degrees C was sufficient to prevent inactivation, We provide e vidence that the activated HA is a conformational intermediate distinct fro m the native structure and from the final structure associated with the con formational change of HA, which is implicated by the high-resolution struct ure of the soluble trimeric fragment TBHA2 (P, A. Bullough, F, M. Hughson, J, J, Skehel, and D. C. Wiley, Nature 371.:37-43, 1994).