The periplasmic maltodextrin binding protein of Escherichia coli serves as an initial receptor for the active transport of and chemotaxis toward maltooligosaccharides. The three-dimensional structure of the binding protein complexed with maltose has been previously reported [Spurlino, J. C., Lu, G.-Y., & Quiocho, F. A. (1991) J. Biol. Chem. 266, 5202-5219]. Here we report the structure of the unliganded form of the binding protein refined to 1.8-A resolution. This structure, combined with that for the liganded form, provides the first crystallographic evidence that a major ligand-induced conformational change occurs in a periplasmic binding protein. The unliganded structure shows a rigid-body "hinge-bending" between the two globular domains by approximately 35 degrees, relative to the maltose-bound structure, opening the sugar binding site groove located between the two domains. In addition, there is an 8 degrees twist of one domain relative to the other domain. The conformational changes observed between this structure and the maltose-bound structure are consistent with current models of maltose/maltodextrin transport and maltose chemotaxis and solidify a mechanism for receptor differentiation between the ligand-free and ligand-bound forms in signal transduction.

MBP has a two-domain structure joined by a hinge-bending region which contains the substrate-binding groove. The bound maltooligosaccharides have a ribbon-like structure: the edges of the ribbon are occupied by polar hydroxyl groups and the flat surfaces are composed of nonpolar patches of the sugar ring faces. The polar groups and nonpolar patches are heavily involved in forming hydrogen bonds and van der Waals contacts, respectively, with complimentary residues in the groove. Hinge-bending between the two domains enables the participation of both domains in the binding and sequestering of the oligosaccharides. Changes in the subtle contours of the binding site allow binding of maltodextrins of varying length with similarly high affinities. The fact that the three bound structures are essentially identical ensures productive interaction with the oligomeric membrane proteins, which are distinct for transport and chemotaxis.

The maltodextrin binding protein from Escherichia coli serves as the initial receptor for both the active transport of and chemotaxis toward a range of linear maltose sugars. The X-ray structures of both the maltose-bound and sugar-free forms of the protein have been previously described [Spurlino, J. C., Lu, G.-Y., & Quiocho, F. A. (1991) J. Biol. Chem. 266, 5202-5219; Sharff, A. J., Rodseth, L. E., Spurlino, J. C., & Quocho, F. A. (1992) Biochemistry 31, 10657-10663]. The X-ray crystal structure of the maltodextrin binding protein complexed with cyclomaltoheptaose (beta-cyclodextrin) has been determined from a single crystal. The structure has been refined to a final R-value of 21% at 1.8-A resolution. Although not a physiological ligand for the maltodextrin binding protein, beta-cyclodextrin has been shown to bind with a Kd of the same order as those of the linear maltodextrin substrates. The observed structure shows that the complexed protein remains in the fully open conformation and is almost identical to the structure of the unliganded protein. The sugar sits in the open cleft with three glucosyl units bound to the C-domain at the base of the cleft, in a similar position to maltotriose, the most tightly bound ligand. The top of the ring is loosely bound to the upper edge of the cleft on the N-domain. The sugar makes a total of 94 productive interactions (of less than 4.0-A length) with the protein and with bound water molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

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