Antigenic determinants of influenza virus hemagglutinin. XI. Conformational changes detected by monoclonal antibodies.

At pH 5 influenza virus hemagglutinin undergoes an irreversible conformational change (J.J. Skehel, P. M. Bayley, E. B. Brown, S. R. Martin, M. D. Waterfield, J. M. White, I. A. Wilson, and D. C. Wiley (1982). Proc. Natl. Acad. Sci. USA 79, 968-972) which parallels the appearance of fusion activity of this molecule. This paper describes experiments which explore the conformational change using a panel of monoclonal antibodies which define four of the major antigenic sites of this protein. The results indicate that three of the major antigenic sites of hemagglutinin undergo changes when exposed to acid pH. These changes have little effect on the binding avidity of influenza virus to glycophorin, the major receptor present on the red blood cell surface. These findings have been used to postulate a mechanism where the molecule flexes around a central region resulting in rearrangement in space of its component domains on exposure to low pH.