Spin-labeling of influenza virus hemagglutinin permits analysis of the conformational change at low pH and its inhibition by antibody.

To study the conformational changes in the hemagglutinin (HA) molecule of A/seal/Mass/1/80 (H7N7) (Seal) influenza virus at low pH, a spin-labeling method was used. This method also permits study of antibody interaction with the HA. A synthetic nitroxide compound was used for spin-labeling of tyrosine residues of the isolated HA molecule. Electron spin resonance (ESR) spectra of the spin-labeled HA at various pH values indicated that a conformational transition occurred under acidic conditions, and around pH 5.8 the HA molecule has maximal flexibility. Since virus-induced hemolysis occurs optimally at pH 5.8-5.9, the HA molecule in the maximally flexible conformation is considered to mediate membrane fusion. The ESR spectra of the antibody-bound HA at various pH values revealed that monoclonal antibodies to different regions on the molecule may inhibit the conformational change by different modes. One antibody inhibited the changes in the HA that resulted in flexibility, while the other did not. These results support the assumption that monoclonal antibodies, which failed to inhibit hemagglutination of the virus yet neutralized viral infectivity, inhibited the fusion step in the viral replication process by interfering with a low pH-induced conformational change in the HA molecule (Kida, H., Webster, R.G. and Yanagawa, R. (1983) Arch. Virol. 76, 91-99).