The 2 Å resolution structure of the sulfate-binding protein involved in active transport in Salmonella typhimurium

The crystal structure of the liganded form of the sulfate-binding protein, an initial receptor for active transport of sulfate in Salmonella typhimurium, has been solved and refined at 2.0 Å resolution (1 Å = 0.1 nm). The final model, which consists of 2422 non-hydrogen atoms, one sulfate substrate and 143 water molecules, yields a crystallographic R-factor of 14.0% for 16,959 reflections between 8 and 2 Å. The structure deviates from ideal bond lengths and angle distances by 0.015 Å and 0.037 Å, respectively. The protein is ellipsoid with overall dimensions of 35 Å × 35 Å × 65 Å and consists of two similar globular domains. The two domains are linked by three distinct peptide segments, which though widely separated in the amino acid sequence, are in close proximity in the tertiary structure. As these connecting segments are located near the periphery of the molecule, they further serve as the base or a “boundary” of the deep cleft formed between the two domains. Despite the unusual interdomain connectivity, both domains have similar supersecondary structure consisting of a central five-stranded β-pleated sheet sandwiched by α-helices on either side. The arrangement of the two domains gives rise to the ellipsoidal shape and to the cleft between the two domains wherein the sulfate substrate is found and completely engulfed. A discovery of considerable importance is that the sulfate substrate is tightly held in place primarily by seven hydrogen bonds, five of which are donated by main-chain peptide NH groups, another by a serine hydroxyl and the last by the indole NH moiety of a tryptophan side-chain; there are no positively charged residues, nor cations, nor water molecules within van der Waals' distance to the sulfate dianion. All the main-chain peptide units associated with the sulfate are in turn linked ( via the peptide CO group) to arrays of hydrogen bonds. Three of these arrays are composed of alternating peptide units and hydrogen bonds within the solvent-exposed part of three α-helices and two are linked to a histidine and an arginine residue. The sulfate-binding protein bears strong similarity to the structures of four other periplasmic binding proteins solved in our laboratory which are specific for l-arabinose, d-galactose/ d-glucose, leucine/isoleucine/valine and leucine. The similarity includes the ellipsoidal shape and the two globular domain structures, each domain consisting of a central β-pleated sheet flanked by α-helices. In all of the bi