H-INDUCED MEMBRANE INSERTION OF INFLUENZA-VIRUS HEMAGGLUTININ INVOLVES THE HA2 AMINO-TERMINAL FUSION PEPTIDE BUT NOT THE COILED-COIL REGION()

Fusion of influenza virus with target membranes is induced by acid and involves complex changes in the viral envelope protein hemagglutinin (HA), In a first, kinetically distinct step, the HA polypeptide chain 2 (HA2) is inserted into the target membrane bilayer, Using hydrophobi c photolabeling with the phospholipid analogue 1-O-hexadecanoyl-2-O-[9 -[[[2-[I-125] iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl] xy]carb onyl]nonanoyl]-sn-glycero-3-phosphocholine, we identified the segment within HA2 that interacts with the membrane. The sole part of the HA2 ectodomain that was labeled with the membrane-restricted reagent is th e NH2-terminal fusion peptide (residues 1-22). No labeling occurred wi thin the long coiled coil region generated during the acid induced con formational transition (Bullough, P. A., Hughson, F. M., Skehel, J. J. , and Wiley, D. C. (1994) Nature 371, 37-43). These data strongly sugg est that the coiled coil region of HA2 does not insert into the lipid bilayer, This conclusion is at variance with the recent suggestion (Yu , Y. G., King, D. S., and Shin, Y.-K. (1994) Science 266, 274-276) tha t the coiled coil of HA may splay apart and insert into the target mem brane, providing a mechanism by which the viral and the target membran e may come in close apposition.