DIFFERENCE IMAGING OF ADENOVIRUS - BRIDGING THE RESOLUTION GAP BETWEEN X-RAY CRYSTALLOGRAPHY AND ELECTRON-MICROSCOPY

While X-ray crystallography provides atomic resolution structures of p roteins and small viruses, electron microscopy provides complementary structural information on the organization of larger assemblies at low er resolution. A novel combination of these two techniques has bridged this resolution gap and revealed the various structural components fo rming the capsid of human type 2 adenovirus. An image reconstruction o f the intact virus, derived from cryo-electron micrographs, was deconv olved with an approximate contrast transfer function to mitigate micro scope distortions. A model capsid was calculated from 240 copies of th e crystallographic structure of the major capsid protein and filtered to the correct resolution. Subtraction of the calculated capsid from t he corrected reconstruction gave a three-dimensional difference map re vealing the minor proteins that stabilize the virion. Elongated densit y penetrating the hexon capsid at the facet edges was ascribed to poly peptide IIIa, a component required for virion assembly. Density on the inner surface of the capsid, connecting the ring of peripentonal hexo ns, was assigned as polypeptide VI, a component that binds DNA. Identi fication of the regions of hexon that contact the penton base suggests a structural mechanism for previously proposed events during cell ent ry.