Refined atomic model of the four-layer aggregate of the tobacco mosaic virus coat protein at 2.4-A resolution.

Previous x-ray studies (2.8-A resolution) on crystals of tobacco mosaic virus coat protein grown from solutions containing high salt have characterized the structure of the protein aggregate as a dimer of a bilayered cylindrical disk formed by 34 chemically identical subunits. We have determined the crystal structure of the disk aggregate at 2.4-A resolution using x-ray diffraction from crystals maintained at cryogenic temperatures. Two regions of interest have been extensively refined. First, residues of the low-radius loop region, which were not modeled previously, have been traced completely in our electron density maps. Similar to the structure observed in the virus, the right radial helix in each protomer ends around residue 87, after which the protein chain forms an extended chain that extends to the left radial helix. The left radial helix appears as a long alpha-helix with high temperature factors for the main-chain atoms in the inner portion. The side-chain atoms in this region (residues 90-110) are not visible in the electron density maps and are assumed to be disordered. Second, interactions between subunits in the symmetry-related central A pair have been determined. No direct protein-protein interactions are observed in the major overlap region between these subunits; all interactions are mediated by two layers of ordered solvent molecules. The current structure emphasizes the importance of water in biological macromolecular assemblies

Structure and selectivity of a monovalent cation binding site in cubic insulin crystals.

Cubic insulin crystals contain a binding site for monovalent cations in a cavity of the crystal dyad in which the bound cation is ligated by protein atomic dipoles and water molecules. These types of interaction are analogous to interactions that occur in small cation-selective carrier and channel molecules. X-ray diffraction data collected from cubic insulin crystals containing Li+, Na+, K+, NH4+, Rb+, and Tl+ show that (i) the differences in cation size do not cause any large alteration in the protein structure around the cation, and (ii) the bound cation is co-ordinated by one or two water molecules, depending on its ionic radii. The relative binding affinities for cations at this dyad site were obtained from an x-ray diffraction analysis of competition experiments in which crystals were dialyzed in mixtures of Tl+ with Li+, Na+, NH4+, Rb+, or Cs+. These data show that this site provides very little discrimination between Na+, K+, Rb+, and Tl+, some selectivity against the small Li+ and the tetrahedrally shaped NH4+, and stronger selectivity against the larger Cs+. The capacity of this site to bind monovalent cations of different sizes may be accounted for by the small number of protein ligating groups and a change from two ligating waters with Li+ and Na+ to one ligating water with the larger cations