MULTIPLE OPEN FORMS OF RIBOSE-BINDING PROTEIN TRACE THE PATH OF ITS CONFORMATIONAL CHANGE

Conformational changes are necessary for the function of bacterial per iplasmic receptors in chemotaxis and transport. Such changes allow ent ry and exit of ligand, and enable the correct interaction of the ligan d-bound proteins with the membrane components of each system. Three op en, ligand-free forms of the Escherichia coli ribose-binding protein w ere observed here by X-ray crystallographic studies. They are opened b y 43 degrees, 50 degrees and 64 degrees with respect to the Ligand-bou nd protein reported previously. The three open forms are not distinct, but show a clear relationship to each other. All are the product of a similar opening motion, and are stabilized by a new, almost identical packing interface between the domains. The changes are generated by s imilar bond rotations, although some differences in the three hinge se gments are needed to accommodate the various structural scenarios. Som e local repacking also occurs as interdomain contacts are lost. The le ast open (43 degrees) form is probably the dominant one in solution un der normal conditions, although a mixture of species seems likely. The open and closed forms have distinct surfaces in the regions known to be important in chemotaxis and transport, which will differentiate the ir interactions with the membrane components. It seems certain that th e conformational path that Links the forms described here is that foll owed during ligand retrieval, and in Ligand release into the membrane- bound permease system. (C) 1998 Academic Press Limited.