The Refined Crystal Structure of Hexon, the Major Coat Protein of Adenovirus Type 2, at 2·9 Å Resolution

Abstract

The crystal structure of hexon, the major coat protein from adenovirus type 2, has been refined at 2·9 Å resolution. Hexon is a homo-trimer (molecular mass 3×109,077 Da) and crystallizes in the cubic space group P 21 3, with a cell edge of 150·5 Å. There are four molecules in the unit cell so that the crystallographic asymmetric unit contains one subunit of the trimer. The electron density in most regions is well-defined and 880 amino acid residues, of the 967 in this unusually long polypeptide chain, have been located and fitted. The N terminus (1 to 43) and three internal stretches (192 to 203, 270 to 291 and 444 to 453) are not defined, and a stretch (168 to 207) with unclear side-chain density is modelled as poly(Ala/Gly). The current refined model, consisting of 6943 non-hydrogen protein atoms and 85 water molecules, yields an R-factor of 19·9% for 18,176 reflections in the resolution range 5·0 to 2·9 Å. The model has reasonable geometry with root-mean-square deviations from ideal bond lengths of 0·022 Å and angle-related 1-3 distances of 0·056 Å.The overall shape of the trimeric hexon molecule is unusual and may be divided into a pseudo-hexagonal base rich in β-structure, and a triangular top formed from three long loops containing some secondary structure. The base contains two similar pedestal domains, P1 and P2, each of which is a flattened eight-stranded β-barrel with the "jelly-roll greek key" topology characteristic of other viral coat proteins. P1 and P2 are related by an approximate 6-fold operation about the molecular 3-fold axis so that six barrels form the walls of the tubular hexon base. The hexon bases form close-packed p3 arrays on each facet of the icosahedral adenovirus virion. Unlike other vital capsids, the barrel axes are almost perpendicular to rather than parallel with the capsid surface. The hexon top, which consists of intimately interacting loops emerging from P1 and P2 in the base, has a triangular outline and so does not exhibit the pseudo-symmetry of the base.The structure of the hexon trimer shows how economically it meets the demands of its function as a stable protective viral coat, reveals the significance of the special features in its unusual amino acid sequence, and explains its biochemical and immunological properties. The molecule is hollow, with a large central cavity, and so has a high effective volume for its mass. Hydrophobic residues are clustered together in the interior of P1 and P2, and at the central depression in the top of the molecule. The latter is a "hydrophobic plug" that seals the central cavity and so shields the viral interior from the outside environment. Intra and intersubunit salt bridges, hydrogen bonds and hydrophobic contacts all contribute to the extreme stability of hexon. In the trimer, each hexon subunit is locked in position to confer additional stability. The interaction of the loops at the top mad the N-terminal arm at the bottom prevents any relative upward or downward displacement of a subunit with respect to its neighbors. The intricate topology of the subunits suggests why their correct folding into the trimeric molecule requires the initial formation of a complex of hexon polypeptides with the adenovirus 100 K protein.