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

Activation of the membrane fusion potential of influenza haemagglutinin (HA) at endosomal pH requires changes in its structure. X-ray analysis of TBHA(2), a proteolytic fragment of HA in the fusion pH conformation, indicates that at the pH of fusion the 'fusion peptide' is displaced 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 structure involves extensive re-folding or reorganization of HA. Here we examine the structure of TBHA(2) with the electron microscope and compare it with the fusion pH structure of HA(2) in virosomes, HA(2) in aggregates formed at fusion pH by the soluble, bromelain-released ectodomain 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 changes in membrane-anchored and soluble HA preparations at the pH of fusion appear to be the same; that in the absence of a target membrane, the 'fusion peptide' of HA in virosomes associates with the virosome membrane so that HA(2) is membrane bound at both N- and C-termini, which implies that inversion of the re-folded HA can occur; and that the structural changes observed by X-ray analysis do not result from the proteolytic digestions used in the preparation of TBHA(2).