A SOLUBLE DOMAIN OF THE MEMBRANE-ANCHORING CHAIN OF INFLUENZA-VIRUS HEMAGGLUTININ (HA(2)) FOLDS IN ESCHERICHIA-COLI INTO THE LOW-PH-INDUCEDCONFORMATION

The extensive refolding of the membrane-anchoring chain of hemagglutin in (HA) of influenza virus (termed HA(2)) in cellular endosomes, which initiates viral entry by membrane fusion, suggests that viral HA is m etastable. HA(2) polypeptide residues 38-175 expressed in Escherichia coil are reported here to fold in vivo into a soluble trimer. The stru cture appears to be the same as the low-pH-induced conformation of vir al HA(2) by alpha-helical content, thermodynamic stability, protease d issection, electron microscopy, and antibody binding. These results pr ovide evidence that the structure of the low-pH-induced fold of viral HA(2) (TBHA(2)) observed crystallographically is the lowest-energy-sta te fold of the HA(2) polypeptide. They indicate that the HA(2) conform ation in viral HA before low pH activation of its fusion potential is metastable and suggest that removal of the receptor-binding chain (HA( 1)) is enough to allow HA(2) to adopt the stable state. Further, they provide direct evidence that low pH is not required to form the membra ne-fusion conformation but acts to make this state kinetically accessi ble in viral HA.