RESUMEN
Adenoviruses are double stranded DNA non enveloped viruses. Although these viruses are widely studied for gene therapy and anticancer applications, fundamental knowledge of these viruses, especially from a structural point of view is lacking. This is probably partly responsible for the limited success of the first clinical trials.With these viruses, one of the main conditions necessary to use adenoviruses for therapeutic application is structural modification of the virus. Indeed, one has to retarget the virus to specific tissues and/or remove all the immunogenic loops present on the outside of the virus in order to limit the host immune response. To make these changes rationally, the structure of the capsid has to be known at atomic resolution. Today, electron microscopy is the only tool that enables us to have access to the structure of the entire virus, but only at intermediate resolution. Because the atomic structures of the adenovirus major capsid proteins are known, one can combine these structures with the electron microscopy based envelope to calculate a model of the capsid at quasi-atomic resolution. These kinds of models are useful to visualize and understand the complexity of the virus. Nevertheless, the structure of the entire capsid at atomic resolution will really be necessary to use the virus in a safe way.
RESUMEN
The sub-viral dodecahedral particle of human adenovirus type 3, composed of the viral penton base and fiber proteins, shares an important characteristic of the entire virus: it can attach to cells and penetrate them. Structure determination of the fiberless dodecahedron by cryo-electron microscopy to 9 Angstroms resolution reveals tightly bound pentamer subunits, with only minimal interfaces between penton bases stabilizing the fragile dodecahedron. The internal cavity of the dodecahedron is approximately 80 Angstroms in diameter, and the interior surface is accessible to solvent through perforations of approximately 20 Angstroms diameter between the pentamer towers. We observe weak density beneath pentamers that we attribute to a penton base peptide including residues 38-48. The intact amino-terminal domain appears to interfere with pentamer-pentamer interactions and its absence by mutation or proteolysis is essential for dodecamer assembly. Differences between the 9 Angstroms dodecahedron structure and the adenovirus serotype 2 (Ad2) crystallographic model correlate closely with differences in sequence. The 3D structure of the dodecahedron including fibers at 16 Angstroms resolution reveals extra density on the top of the penton base that can be attributed to the fiber N terminus. The fiber itself exhibits striations that correlate with features of the atomic structure of the partial Ad2 fiber and that represent a repeat motif present in the amino acid sequence. These new observations offer important insights into particle assembly and stability, as well as the practicality of using the dodecahedron in targeted drug delivery. The structural work provides a sound basis for manipulating the properties of this particle and thereby enhancing its value for such therapeutic use.
