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 s
pecimens before and after bombardment. Data on the M1 shielding in the viri
on and the flux attenuation coefficient of the lipid, bilayer obtained with
liposomes showed that the M1 matrix protein is embedded in the inner leafl
et of the lipid membrane at the distance of 2.5-5 nm from the outer virion
surface.