The overall aim of the project is to determine the structure of the adenovirus virion in atomic detail. A novel approach is mandated by its large size (about 1000 Angstroms diameter) and the complicated arrangement of its constituent 2700 polypeptides from 10 different protein species. The virion will be investigated by combining information from an X-ray crystallographic analysis of its major element, the coat protein hexon, with information from electron microscopy. Hexon is a trimeric molecule with three identical polypeptide chains of 967 residues (109,077 daltons each). The crystal structure of hexon at 2.9 Angstrom resolution is being refined by a combination of model-building using computer graphics, restrained least-squares refinement, and molecular dynamics simulated annealing methods to give a molecular model. The hexon structure will be extended to the diffraction limit of about 2 Angstroms resolution and its chemical characteristics analyzed in terms of charge, hydrogen bonding, and hydropathy to discover the nature of hexon's extraordinary stability. A unique structural feature in hexon, in which chains from different subunits are entangled, indicates that topology can play an important role in protein interactions. The structure of the immunologically distinct type 5 hexon will be determined, as will structures for hexons of types 40 and 41. Comparison of hexons from different serotypes will reveal the structural basis for their immunological differences and give the approximate location of antigenic determinants, reveal how the viral coat has differentiated, and indicate whether viral stability is correlated with type of illness. As hexon alone is an effective vaccine when used in place of the potentially oncogenic complete virus, short peptides could be used as cost-effective synthetic vaccines against adenoviral infections in humans and domestic animals. High resolution structures will permit exact modelling of interactions. The large size of hexon and its distinctive shape permit its recognition in electron micrographs of the virion and its fragments. This information has been used to develop a model for the arrangement of the 240 hexons in the capsid and to find relative positions of hexon molecules in two-dimensions to about 1 Angstrom. Hexon now will be modelled into the capsid to study hexon interactions in three-dimensions.
Work aim ed at understanding hexon's internal stability will be extended to the capsid to explore hexon-hexon interfaces and putative binding sites of a small protein that stabilizes the capsid. These studies will lay the foundation for drugs designed to disrupt the infective process by destabilizing or over-stabilizing the capsid. The structural studies on adenovirus will serve as a model for mammalian viruses and other complicated assemblies that are too large to be amenable to crystallography alone. Study of these biological systems requires an integration of investigative methods including electron microscopy, biochemistry, and molecular biology to probe their full range of physical and biological properties.
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