ELECTRON-MICROSCOPY OF ANTIBODY COMPLEXES OF INFLUENZA-VIRUS HEMAGGLUTININ IN THE FUSION PH CONFORMATION

Activation of the membrane fusion potential of influenza haemagglutini n (HA) at endosomal pH requires changes in its structure. X-ray analys is of TBHA(2), a proteolytic fragment of HA in the fusion pH conformat ion, indicates that at the pH of fusion the 'fusion peptide' is displa ced by >10 nm from its location in the native structure to the tip of an 11 nm triple-stranded coiled coil, and that the formation of this s tructure involves extensive re-folding or reorganization of HA. Here w e examine the structure of TBHA(2) with the electron microscope and co mpare it with the fusion pH structure of HA(2) in virosomes, HA(2) in aggregates formed at fusion pH by the soluble, bromelain-released ecto domain BHA and HA(2) in liposomes with which BHA associates at fusion pH. We have oriented each HA(2) preparation for comparison, using site -specific monoclonal antibodies. We conclude that the structural chang es in membrane-anchored and soluble HA preparations at the pH of fusio n appear to be the same; that in the absence of a target membrane, the 'fusion peptide' of HA in virosomes associates with the virosome memb rane so that HA(2) is membrane bound at both N- and C-termini, which i mplies that inversion of the re-folded HA can occur; and that the stru ctural changes observed by X-ray analysis do not result from the prote olytic digestions used in the preparation of TBHA(2).