[Surface localization of amino acids in influenza virus hemagglutinin during functional transformation of virions by acidic pH].

Tritium planigraphy method was used to study the surface amino acids in influenza virus hemagglutinin (HA) at the functional transformation of virions by acidic pH. Influenza viruses undergo this transformation when virions penetrate a target cell. The mild bombardment of a whole virion by hot tritium atoms labels the surface-exposed HA amino acids without disturbing virion structure and infectivity. A comparison of the values of specific radioactivity of the HA subunits (HA1 and HA2) and analysis of the label distribution in individual amino acids in control (pH 7.4) and experimental (pH 5.0) viral samples has shown that: (i) The accessibility of a hemagglutinin molecule for tritium labeling is decreased at acidic pH; (ii) Compactization of apical HA1 hemagglutinin subunit increases at acidic pH; (iii) The HA1 compactization increases the exposition of the basal HA2 hemagglutinin subunit; (iv) The fusion peptide of the HA2 subunit (the N-terminal part) is not accessible for tritium label after the acidic exposition of virions. These data are in an agreement with the hypothesis that hemagglutinin fusion peptide is exposed only while contacting the target cell membrane at acidic pH.