Conformational changes play a vital role in the biological function of many proteins. The wide spectrum of conformational changes observed in crystal structures can be broadly classified as small amplitude shear motion, large amplitude hinge bending motion, or some combination of the two (1, 2). Shear motion generally represents the sliding movement of secondary structural elements on other parts of the tertiary structure, whereas hinge-bending motion is characterized by a few localized torsional rotations that combine to produce dramatic changes in the protein as a whole.A classic example of the involvement of hinge-bending motion in protein function is found in the periplasmic receptors of the bacterial ABC transporter systems. Such systems use the energy of ATP to carry small ligands and ions across the cytoplasmic membranes of both prokaryotes and eukaryotes (3-5).A typical ABC system consists of an membrane-bound permease, an ATP-binding component, and, in most bacterial systems, a periplasmic receptor. Binding of a small molecule ligand to the periplasmic proteins favors their closure via large scale hinge-bending motions (6). These movements are required for productive interactions with the cognate membrane permeases and, in some cases, with membrane-bound chemotaxis receptors as well. The periplasmic sugar-binding proteins belong to a subfamily (pentose/hexose sugar receptors) of the larger family of periplasmic receptors (7). Crystal structures of several members of this subfamily have been reported in the closed, ligand-bound form, including allose-binding protein (ALBP 1 (8)), ribose-binding protein (RBP (9)), arabinose-binding protein (ABP (10)), and glucose-galactose-binding protein (GBP (11, 12)). Each consists of two similar Rossmann fold domains linked by a three-stranded hinge region. The binding site is located at the domain interface; extensive hydrogen bonding and hydrophobic interactions of the ligand with both domains of the protein stabilize the closed form. Although the periplasmic receptors can assume similar closed forms in the ligand-free state (e.g. Ref. 13), experimental data suggest that more open forms will predominate in the absence of ligand (6, 14 -16).The structures of three ligand-free forms of RBP provided the first picture of the conformational changes in this subfamily of receptors (17). The two domains of each open RBP were shown to move as nearly rigid bodies at the hinge that joins them; the observed structures were opened by 43, 53, and 64°w ith respect to the closed receptor. Most structural changes involved only a few torsional changes in the hinge segments, although some minor repacking was observed where domaindomain interactions were lost in the opened receptors. Further, the three structures represented discrete points along a conformational trajectory, thus describing the motion that should apply to ligand capture as well as ligand release into the permease. In each open form, the two domains had a similar set of packing interactions that were not present in the...

The periplasmic leucine-binding protein is the primary receptor for the leucine transport system in Escherichia coli. We report here the structure of an open ligand-free form solved by molecular replacement and refined at 1.5-Å resolution. In addition, two closed ligand-bound structures of the same protein are presented, a phenylalanine-bound form at 1.8 Å and a leucine-bound structure at a nominal resolution of 2.4 Å. These structures show the basis of this protein's ligand specificity, as well as illustrating the conformational changes that are associated with ligand binding. Comparison with earlier structures provides further information about solution conformations, as well as the different specificity of the closely related leucine/ isoleucine/valine-binding protein.

Maltose-binding proteins act as primary receptors in bacterial transport and chemotaxis systems. We report here crystal structures of the thermoacidostable maltose-binding protein from Alicyclobacillus acidocaldarius, and explore its modes of binding to maltose and maltotriose. Further, comparison with the structures of related proteins from Escherichia coli (a mesophile), and two hyperthermophiles (Pyrococcus furiosus and Thermococcus litoralis) allows an investigation of the basis of thermo-and acidostability in this family of proteins.The thermoacidophilic protein has fewer charged residues than the other three structures, which is compensated by an increase in the number of polar residues. Although the content of acidic and basic residues is approximately equal, more basic residues are exposed on its surface whereas most acidic residues are buried in the interior. As a consequence, this protein has a highly positive surface charge. Fewer salt bridges are buried than in the other MBP structures, but the number exposed on its surface does not appear to be unusual. These features appear to be correlated with the acidostability of the A. acidocaldarius protein rather than its thermostability.An analysis of cavities within the proteins shows that the extremophile proteins are more closely packed than the mesophilic one. Proline content is slightly higher in the hyperthermophiles and thermoacidophiles than in mesophiles, and this amino acid is more common at the second position of b-turns, properties that are also probably related to thermostability. Secondary structural content does not vary greatly in the different structures, and so is not a contributing factor.

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