REFINED 1.89-ANGSTROM STRUCTURE OF THE HISTIDINE-BINDING PROTEIN COMPLEXED WITH HISTIDINE AND ITS RELATIONSHIP WITH MANY OTHER ACTIVE-TRANSPORT CHEMOSENSORY PROTEINS

The structure of the histidine-binding protein (HBP, M(r) = 26 100), i nvolved solely in active transport, has been determined by the molecul ar replacement technique and refined to 1.89-Angstrom resolution and t o an R-factor of 0.199. The structure is that of two protein molecules , each with a bound L-histidine, in the asymmetric unit. Replacement s olution was achieved by using a model of the crystal structure of the ligand-free, open-cleft form of the lysine/arginine/ornithine-binding protein which was modified so that the two domains are close to each o ther by bending the hinge connecting the two domains. The bound histid ine is held in place by 10 hydrogen bonds, 2 salt links, and about 60 van der Waals contacts. Elucidation of the HBP structure brings a tota l of eight different binding protein structures determined in our labo ratory, including those with specificities for monosaccharides, maltod extrins (linear and cyclic), aliphatic amino acids, and inorganic oxya nions. These structures comprise about a third of the entire family of periplasmic binding proteins which act as initial primary high-affini ty receptors of active transport in Gram-negative bacteria. Two of the binding proteins with specificities for glucose/galactose and maltode xtrins also serve in a similar capacity in chemotaxis. Though these pr oteins have different molecular weights (ranging from 26 000 to 40 000 ), amino acid sequences, and ligand specificities, their three-dimensi onal structures are similar overall. They are elongated (axial ratios of 2:1) and composed of two similar globular domains separated bp a de ep cleft wherein the ligand-binding site is located. These structures provide understanding of molecular recognition of a variety of ligands at the atomic level and functional roles of the binding proteins.