THE REFINED CRYSTAL-STRUCTURE OF HEXON, THE MAJOR COAT PROTEIN OF ADENOVIRUS TYPE-2, AT 2-CENTER-DOT-9 ANGSTROM RESOLUTION

The crystal structure of hexon, the major coat protein from adenovirus type 2, has been refined at 2.9 Angstrom resolution. Hexon is a homo- trimer (molecular mass 3 x 109,077 Da) and crystallizes in the cubic s pace group P2(1), 3, with a cell edge of 150.5 Angstrom. There are fou r molecules in the unit cell so that the crystallographic asymmetric u nit contains one subunit of the trimer. The electron density in most r egions 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 2 91 and 444 to 453) are not defined, and a stretch (168 to 207) with un clear side-chain density is modelled as poly(Ala/Gly). The current ref ined 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 Angstrom. The model has reasonable geometr y with root-mean-square deviations from ideal bond lengths of 0.022 An gstrom and angle-related 1-3 distances of 0.056 Angstrom. The overall shape of the trimeric hexon molecule is unusual and may be divided int o a pseudo-hexagonal base rich in beta-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 beta-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-f old 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 icos ahedral adenovirus virion. Unlike other viral capsids, the barrel axes are almost perpendicular to rather than parallel with the capsid surf ace. The hexon top, which consists of intimately interacting loops eme rging 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 hex on trimer shows how economically it meets the demands of its function as a stable protective viral coat, reveals the significance of the spe cial features in its unusual amino acid sequence, and explains its bio chemical 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 lat ter is a ''hydrophobic plug'' that seals the central cavity and so shi elds the viral interior from the outside environment. Intra- and inter subunit salt bridges, hydrogen bonds and hydrophobic contacts all cont ribute to the extreme stability of hexon. In the trimer, each hexon su bunit is locked in position to confer additional stability. The intera ction of the loops at the top and the N-terminal arm at the bottom pre vents any relative upward or downward displacement of a subunit with r espect to its neighbors. The intricate topology of the subunits sugges ts why their correct folding into the trimeric molecule requires the i nitial formation of a complex of hexon polypeptides with the adenoviru s 100 K protein.