Monoclonal antibodies detect M-protein epitopes on the surface of influenza virions

Various data obtained with activable hydrophobic probes, proteolytic treatments and anti M-protein polyclonal antibodies strongly suggest that M-protein of influenza A is an integral part of the lipid bilayer of native virions and somehow spans at the surface of the virions. Therefore we have looked for the presence of M-protein epitopes on the surface of influenza A virion by using four type A M-protein monoclonal antibodies. We developed a specific and sensitive competition ELISA where intact virions, dodecyl-sulfate disrupted virions and spikeless particles obtained after proteolytic treatment with caseinase C were used to test their ability to inhibit the reaction between these monoclonal antibodies and pure M-protein. Intact virions or SDS disrupted virions prevented three monoclonal antibodies from reacting with the M-protein. Spikeless particles also inhibited the specific binding of two of these antibodies, whereas the other fourth antibody was inhibited by contact with SDS disrupted particles only. Data presented show that at least three distinct M-protein epitopes were detected, of which at least two are exposed on the surface of intact virions. Of these two epitopes, one is inactivated by the proteolytic treatment. The third epitope could only react with its monoclonal antibody when the virus particles were solubilized with SDS. This work provides a clear demonstration that a substantial part of the M-protein spans the lipid bilayer and that the rest, protected by lipids, resists proteolytic enzymes and is prevented from binding with anti M-protein monoclonal antibodies.