Effects of Exposure to Low pH on the Lateral Mobility of Influenza Hemagglutinin Expressed at the Cell Surface: Correlation between Mobility Inhibition and Inactivation

To investigate the possible role of viral glycoprotein mobility in membrane fusion, fluorescence photobleaching recovery was employed to study the effects of exposure to mildly acidic pH (required to convert many viral fusion proteins to the fusion-active form) on the lateral mobility of influenza hemagglutinin (HA) proteins expressed at the surface of transfected cells. HA proteins from two different strains were compared: X:31 HA, which is activated by a brief exposure to pH 4.9 but is irreversibly inactivated at longer exposure times, and HA from A/Japan/305/57, which is relatively stable to inactivation at this pH [Puri, A., Booy, F. P., Doms, R. W., White, J. M., & Blumenthal, R. (1990) J. Virol. 64, 3824-3832], The HA proteins from both strains, expressed in CV-1 cells using VS-40 vectors, exhibited relatively unrestricted lateral diffusion at the cell surface. The high mobility persisted following a brief exposure (1 min) to pH 4.9 to mediate conversion to the fusogenic state. Longer times (up to 15 min) of preincubation at pH 4.9 inhibited the lateral mobility of X:31 HA (the lateral diffusion rate was markedly reduced, followed by immobilization) but not of A/Japan HA, whose fusion activity is resistant to such treatment. Inhibition of the lateral mobility of X:31 HA due to preincubation at low pH was not specific to the CV-1 cells and was found also in a CHO cell line stably expressing this protein. The results presented demonstrate a close correlation between loss of mobility and inactivation of fusogenic activity, in accord with the notion that lateral motion of the HA proteins is required for fusion. We propose that the inactivation of X:31 HA following prolonged incubation at low pH involves a type of enhanced aggregation that leads to immobilization.