Preparation and characterization of influenza virus cores

Lipid-free influenza virus cores have been obtained by a three step procedure consisting of (a) treatment with proteolytic enzyme, (b) fixation with formaldehyde and (c) delipidization with saponin or deoxycholate. Several reagents proved efficient in removing the virus lipids as judged by morphological features and increased buoyant density, but only cores prepared by means of sodium deoxycholate have been characterized closely. Ultrathin sections revealed round bodies (about 65 nm in diam.) delineated by a single dense track and with an internal structure very similar to that of the complete virus particles. They contained both the nucleoprotein and the M-protein and no lipids. It is proposed to call the limiting structure which appears (3 to 4 nm thick in ultrathin sections) the core shell.