THE THREE-DIMENSIONAL STRUCTURE OF SULFATE-BINDING PROTEIN FROM SALMONELLA TYPHIMURIUM
PFLUGRATH, JAMES WILLIAM
Doctor of Philosophy
The structure of the sulfate-binding protein, a component for the active transport of sulfate in Salmonella typhimurium, has been solved at 3.0 (ANGSTROM) resolution using multiple isomorphous replacement phases derived from 5 heavy atom derivatives. The mean figure of merit is 0.60 for 5626 reflections. Sulfate-binding protein is an elongated molecule with dimensions 35 x 35 x 60 (ANGSTROM). The polypeptide chain folds into two domains, which are designated the N domain and the C domain; the N domain contains the amino terminus, while the other domain contains the carboxyl terminus. The two domains are connected by three segments of polypeptide chain. Though these three segments are in close proximity in the tertiary structure, they are widely separated in the amino acid sequence. The connecting segments serve as the base of a deep cleft formed between the two domains. The N domain consists of a core of 5 strand (beta) sheet with two (alpha) helices on either side. The C domain is also composed of a core of 5 strand (beta) sheet with two (alpha) helices on one side, but 3 helices occur on the other side. The secondary structure is comprised of 42% (alpha) helices and 32% (beta) strands. Since the (beta) strands and (alpha) helices alternate along the chain except for two helices in the C domain, sulfate-binding protein is an (alpha)/(beta) protein. The overall structure of the sulfate-binding protein is similar to those previously determined for L-arabinose-, D-galactose- and leucine/isoleucine/valine-binding proteins. All have a two domain structure with each domain consisting of a core (beta) sheet flanked on either side by at least two helices. The sulfate binding site is in a cleft between the two domains. The amino-termini of three (alpha) helices, two from the C domain and a third from the N domain, interact with the sulfate anion through helix dipole moments. These interactions contribute to the stability of the anion-protein complex. The binding sites of the other binding proteins whose structures have been solved are also located in the cleft between the two domains. The location of the sulfate binding site is consistent with the proposal that binding proteins may undergo a hinge-bending so that one domain twists or rotates relative to the other upon ligand binding.