Location of the influenza virus matrix protein M1 in the virion

The M1 protein is commonly believed to form a protein matrix underlying the lipid membrane of the influenza virus. Upon bombardment of virus particles with thermally activated tritium atoms (tritium planigraphy), the surface glycoprotein hemagglutinin and the matrix protein Mi were labeled, but not the nucleocapsid protein. Virion integrity proved to be the same in virus specimens before and after bombardment. Data on the M1 shielding in the virion and the flux attenuation coefficient of the lipid, bilayer obtained with liposomes showed that the M1 matrix protein is embedded in the inner leaflet of the lipid membrane at the distance of 2.5-5 nm from the outer virion surface.