Conformational changes of three periplasmic receptors for bacterial chemotaxis and transport: the maltose-, glucose/galactose- and ribose-binding proteins.

Small-angle X-ray scattering experiments were carried out for the maltose-, glucose/galactose- and ribose-binding proteins of Gram negative bacteria. All were shown to be monomers that decrease in radius of gyration on ligand binding. The results obtained for the maltose-binding protein agree well with crystal structures of the closed, ligand-bound, and open, ligand-free protein, suggesting that these are indeed the primary forms in solution. The closed form is stabilized by protein-sugar interactions, while the open conformation is stabilized by close contacts between the two domains. Since it is the proper special relationship of the domains in the closed form that is most important for interaction with chemotaxis and transport partners, the stabilization of the open form would help keep ligand-free molecules from interfering in function. The scattering results also provide evidence that a large conformational change takes place in association with ligand binding to the glucose/galactose- and ribose-binding proteins, and that the two changes are similar. Modeling suggests that the open forms resemble those found in the related leucine and leucine/isoleucine/valine-binding proteins, but are different from those observed for the maltose-binding protein and the related purine repressor.